,transcriber's ,note3 ,italic text delimited by underscores1 bold text delimited by equal signs4 ,single underscore followed by text enclosed by braces indicates subscript4 ,caret followed by single character or text enclosed by braces text is superscript text4 ,,river ,,and ,,canal ,,engineering ,by the ,same ,author ,,hydraulics ,,with ,,working ,,tables4 ,second ,edition4 #afj illustrations1 xii #caa pp41 #hvo 7#aiaa74 #ab_/- net4 ,,punjab ,,rivers ,,and ,,works4 ,second ,edition4 #dg illustrations1 viii #fd pp41 folio 7#aiab74 #h_/- net4 ,,irrigation ,,works 7in the press74 #bg illustrations1 vi #acj pp41 #hvo 7#aiac74 #f_/- net4 ,,the ,,suction ,,caused ,,by ,,ships4 ,explained in ,popular ,language4 #b plates1 #bf pp41 #hvo1 sewed 7#aiab74 #a_/- net4 ,e4 ,f4 ,n4 ,,spon1 ,,ltd41 ,,london ,,river ,,and ,,canal ,,engineering ,,the ,,characteristics ,,of ,,open ,,flowing ,,streams1 ,,and ,,the ,,principles ,,and ,,methods ,,to ,,be ,,followed ,,in ,,dealing ,,with ,,them ,,by ,e4 ,s4 ,,bellasis1 ,m4,,inst4,c4,e4 ,,recently ,,superintending ,,engineer ,,in ,,the ,,irrigation ,,branch ,,of ,,the ,,public ,,works ,,department ,,of ,,india #gb ,,illustrations ,illustration ,london ,e4 ,f4 ,n4 ,,spon1 ,,ltd41 #eg ,,haymarket1 ,s4,w4 ,new ,york ,,spon ,,chamberlain1 #abc ,,liberty ,,street #aiac ,,table ,,of ,,contents ,,chapter ,i ,,introduction ,,article ,,page #a4 ,preliminary ,remarks #a #b4 ,rsum of the ,subject #a #c4 ,design and ,execution of ,works #c #d4 ,the ,hydraulics of ,open ,streams #d ,,chapter ,,ii ,,rainfall #a4 ,rainfall ,statistics #f #b4 ,available ,rainfall #i #c4 ,measurement of ,rainfall #ac #d4 ,influence of ,forests and ,vegetation #ad #e4 ,heavy ,falls in ,short ,periods #ae ,,chapter ,,iii ,,collection ,,of ,,information ,,concerning ,,streams #a4 ,preliminary ,remarks #ah #b4 ,stream ,gauges #ai #c4 ,plan and ,sections #ba #d4 ,discharge ,observations #ba #e4 ,discharge ,curves and ,tables #bc #f4 ,small ,streams #bd #g4 ,intermittent ,streams #be #h4 ,remarks #bf ,,chapter ,,iv ,,the ,,silting ,,and ,,scouring ,,action ,,of ,,streams #a4 ,preliminary ,remarks #bg #b4 ,rolled ,material #bi #c4 ,materials carried in ,suspension #ca #d4 ,methods of ,investigation #cc #e4 ,quantity and ,distribution of ,silt #ce #f4 ,practical ,formul and ,figures #cg #g4 ,action on the ,sides of a ,channel #dj #h4 ,action at ,bends #db #i4 ,general ,tendencies of ,streams #de ,,chapter ,v ,,methods ,,of ,,increasing ,,or ,,reducing ,,silting ,,or ,,scour #a4 ,preliminary ,remarks #dh #b4 ,increase of ,scour or ,reduction of ,silting #dh #c4 ,production of ,silt ,deposit #ea #d4 ,arrangements at ,bifurcations #ec #e4 ,a ,canal with ,headworks in a ,river #ed #f4 ,protection of the ,bed #eh ,,chapter ,,vi ,,works ,,for ,,the ,,protection ,,of ,,banks #a4 ,preliminary ,remarks #fj #b4 ,spurs #fa #c4 ,continuous ,lining of the ,bank #fd #d4 ,heavy ,stone ,pitching with ,apron #ga ,,chapter ,,vii ,,diversions ,,and ,,closures ,,of ,,streams #a4 ,diversions #gc #b4 ,closure of a ,flowing ,stream #ge #c4 ,instances of ,closures of ,streams #hj ,,chapter ,,viii ,,the ,,training ,,and ,,canalisation ,,of ,,rivers #a4 ,preliminary ,remarks #hd #b4 ,dredging and ,excavating #hd #c4 ,reduction of ,width #he #d4 ,alteration of ,depth or ,water-,level #hh #e4 ,training and ,canalising #hi ,,chapter ,,ix ,,canals ,,and ,,conduits #a4 ,banks #ib #b4 ,navigation ,canals #ic #c4 ,locks #if #d4 ,other ,artificial ,channels #ajj ,,chapter ,x ,,weirs ,,and ,,sluices #a4 ,preliminary ,remarks #ajb #b4 ,general ,design of a ,weir #aje #c4 ,weirs on ,sandy or ,porous ,soil #ajf #d4 ,various ,types of ,weirs #aaa #e4 ,weirs with ,sluices #aae #f4 ,falling ,shutters #aba #g4 ,adjustable ,weirs #abf #h4 ,remarks on ,sluices #abh ,,chapter ,,xi ,,bridges ,,and ,,syphons #a4 ,bridges #acb #b4 ,syphons and ,culverts #ace #c4 ,training ,works #acf ,,chapter ,,xii ,,drainage ,,and ,,floods #a4 ,preliminary ,remarks #ada #b4 ,small ,streams #ada #c4 ,rivers #adf #d4 ,prediction of ,floods #aej #e4 ,prevention of ,floods #aec #f4 ,lowering the ,water-,level #aed #g4 ,flood ,embankments #aef ,,chapter ,,xiii ,,reservoirs ,,and ,,dams #a4 ,reservoirs #afb #b4 ,capacity of ,reservoirs #afg #c4 ,earthen ,dams #agd #d4 ,masonry ,dams #aha ,,chapter ,,xiv ,,tidal ,,waters ,,and ,,works #a4 ,tides #aij #b4 ,tidal ,rivers #aib #c4 ,works in ,tidal ,rivers #aif #d4 ,tidal ,estuaries #aig #e4 ,works in ,tidal ,estuaries #aih ,,chapter ,,xv ,,river ,,bars #a4 ,deltaic ,rivers #bjc #b4 ,other ,rivers #bje ,,appendix ,a4 ,fallacies in the ,hydraulics of ,streams #bji 0 ,b4 ,pitching and ,bed ,protection #bab ,,index #bac ,,preface ,the object of this book is to describe the principles and practice adopted in the ,engineering of ,open ,streams4 ,if the book seems to be somewhat small for its object1 it will1 it is hoped1 be found that this is due to care in the arrangement and wording4 ,sources of information have been acknowledged in the text1 but special mention may be made of lectures given by ,professor ,unwin at ,coopers ,hill ,college1 of ,harcourt's large work on ,rivers and ,canals1 of the papers#a by ,binnie on rainfall1 by ,shaw on the closing of the river ,tista1 by ,harcourt on movable weirs and on estuaries1 by ,strange on reservoirs1 and by ,ottley and ,brightmore1 ,gore and ,wilson1 and ,hill on the stresses in masonry dams2 of the articles by ,bligh#b on weirs with porous foundations and by ,deacon#c on reservoir capacity1 of the ,indian ,government paper by ,spring on 8,river ,control on the ,guide ,bank ,system10 and of the ,punjab ,government paper containing ,kennedy's remarks on silting and scour in the ,sirhind ,canal4 ,the two papers last mentioned are not easily accessible1 and they contain matter of great interest4 ,the important points1 often obscured by masses of detail or figures1 have been extracted4#d ,silting and scour 7,,chap4 ,,iv47 had already been dealt with in ,hydraulics1#e but some further information has since come to light and the subject has been treated afresh and the matter re-written4 ,e4 ,s4 ,b4 ,,cheltenham1 #ast ,may #aiac4 ,,river ,,and ,,canal ,,engineering ,,chapter ,i ,,introduction #a4 ,preliminary ,remarks4--,river and ,canal ,engineering is that branch of engineering science which deals with the characteristics of streams flowing in open channels1 and with the principles and methods which should be followed in dealing with1 altering1 and controlling them4 ,it is not necessary to make a general distinction between natural and artificial streams2 some irrigation canals or other artificial channels are as large as rivers and have many of the same characteristics4 ,any special remarks applicable to either class will be given as occasion requires4 #b4 ,rsum of the ,subject4--,,chap4 ,,ii4 of this book deals with the collection of information concerning streams1 a procedure which is necessary before any considerable work in connection with a stream can be undertaken1 and often before it can even be decided whether or not it is to be undertaken4 ,,chap4 ,,iii4 deals with rainfall1 and describes how rainfall figures and statistics can be utilised by the engineer in dealing with streams4 ,,chap4 ,,iv4 explains the laws of silting and scouring action1 a subject of great importance and one to which the attention ordinarily given is insufficient4 ,the general characteristics of streams1 being due entirely to silting or scouring tendencies1 are included in this chapter4 ,,chap4 ,v4 describes how silting or scouring may be1 under some circumstances1 artificially induced or retarded4 ,,chap4 ,,vi4 deals with various methods of protecting banks against erosion or damage4 ,,chap4 ,,vii4 treats of diversions or the opening out of new channels1 and with the opposite of this1 viz4 the closing of channels1 a feat which1 when the stream is flowing1 is sometimes very difficult to achieve4 ,this chapter also deals with dredging and excavation4 ,,chap4 ,,viii4 discusses the subject of the training of streams1 a class of work which is generally undertaken in order to make them navigable or to improve their existing capacities for navigation1 but may be undertaken for other reasons4 ,the main features of this kind of work are the narrowing and deepening of the stream1 often the reduction of the velocity and slope1 and generally the raising of the water-level4 ,in this kind of work a channel may be completely remodelled and even new reaches constructed4 ,,chap4 ,,ix4 deals with artificial channels of earth or masonry1 and includes navigation canals4#f ,in ,,chaps4 ,x4 and ,,xi4 the chief masonry works or isolated structures--as distinguished from general works which extend over considerable lengths of channel--are dealt with1 and those principles of design discussed which affect the works in their hydraulic capacities4 ,general principles of design applicable to all kinds of works1 such as the thicknesses of arches or retaining walls1 are not considered2 they can be found in books on general engineering design4 ,,chap4 ,,xii4 treats of storm waters and river floods1 and shows how works can be designed for getting rid of flood water and how floods can be mitigated or prevented1 one of the chief measures1 the widening of the channel and the lowering of the water-level1 being the opposite of that adopted for training works4 ,embankments for stopping flooding are also dealt with4 ,,chap4 ,,xiii4 deals with reservoirs1 including the design of earthen and masonry dams4 ,,chaps4 ,,xiv4 and ,,xv4 deal with tidal waters1 river mouths and estuaries1 and works in connection with them1 viz4 the training of estuaries and the methods of dealing with bars1 the object being in all cases the improvement of the navigable capacities of the channels4 #c4 ,design and ,execution of ,works4--,after obtaining full information concerning the stream to be dealt with1 careful calculations are1 in the case of any large and important work1 made as to the effects which will be produced by it4 ,these effects cannot always be exactly foreseen4 ,sometimes matters can be arranged so that the work can be stopped short at some stage without destroying the utility of the portion done1 or so that the completed work can be altered to some extent4 ,in works for controlling streams there is1 as will appear in due course1 a considerable choice of types of work and methods of construction4 ,in practice it will generally be found that there are1 in any particular locality1 reasons for giving preference to one particular type or kind of work or1 at all events1 that the choice is limited to a few of them1 either because certain kinds of materials and appliances can be obtained more cheaply and readily than others1 or because works of a particular type have already been successfully adopted there1 or because the people of the district are accustomed to certain classes of work or methods of construction4 ,in out-of-the-way places it is often undesirable to avoid any type of work which cannot be quickly repaired or readily kept in order by such means as exist near the spot4 ,it is sometimes said that perishable materials1 such as trees1 stakes1 and brushwood1 cannot produce permanent results4 ,they can produce results which will last for a long time and which may even be permanent4 ,by the time the materials have decayed1 the changes wrought may have been very great1 deposits of shingle or silt may have occurred and become covered with vegetation1 and there may be little tendency for matters to revert to their former condition4 ,if the expense of using more lasting materials had had to be incurred1 the works might never have been carried out at all4 ,on the ,mississippi enormous quantities of work have been done with fascines4 #d4 ,the ,hydraulics of ,open ,streams4--,when any reach of a stream is altered1 say by widening1 narrowing1 or deepening1 so that the water-level is changed1 there will also be a change in the water-level1 a gradually diminishing change1 for some distance upstream of the reach4 ,also in the lowest portion of the reach the change will gradually diminish and it will vanish at the extreme downstream end of the reach4 ,in the next lowest reach there is no change4 ,thus if it is desired that the change in the water-level shall take full effect throughout the whole of a reach1 the change in the channel must be carried further down4 ,if a weir is built there is no change in the water-level downstream of it except such as may be due to loss of water in the reach upstream of it4 ,the above points are mentioned here because1 although they are really questions of hydraulics1 they are of much importance and of very general application4 ,matters connected with the hydraulics of open streams seem to lend themselves in a peculiar way to loosely expressed remarks and fallacious opinions4 ,the set of a stream towards a bank is sometimes supposed to profoundly affect the discharge of a diversion or branch4 ,its effect is simply that of 8velocity of approach10 which1 as is well known1 is quite small with ordinary velocities1 and is merely equivalent to a very small increase of head4 ,narrow bridges or other works are sometimes said to seriously 8obstruct0 a stream without any observations being made of the fall in the water surface through the bridge4 ,this fall is the only measure of the real obstruction4#g ,,chapter ,,ii ,,rainfall #a4 ,rainfall ,statistics4--,the mean annual rainfall varies very greatly according to the locality4 ,in ,england it varies from about #bj inches at ,hunstanton in ,cambridgeshire1 to about #bjj inches at ,seathwaite in ,cumberland2 in ,india1 from #b or #c inches in parts of ,scinde1 to #dej inches or more at ,cherrapunji in the ,eastern ,himalayas4 ,rain is brought by winds which blow across the sea4 ,hence the rainfall in any country is generally greatest in those localities where the prevailing winds blow from seaward1 provided they have travelled a great distance over the sea4 ,rainfall is greater among hills than elsewhere1 because the temperature at great elevations is lower4 ,currents of moist air striking the hills are deflected upwards1 become cooled1 and the water vapour becomes rain4 ,this process1 if the hills are not lofty1 may not produce its full effect till the air currents have passed over the hills1 and thus the rainfall on the leeward slopes may be greater than elsewhere1 but on the inner and more lofty ranges the rainfall is generally greatest on the windward side4 ,thus the rainfall may vary greatly at places not far apart4 ,an extreme instance of this occurs in the ,bombay hills1 where the mean annual falls at two stations only ten miles apart are respectively #cjj inches and #ej inches4 ,in temperate climates the rainfall is generally distributed over all the months of the year2 in the tropics the great bulk of the rain often falls in a few months4 ,the fall at any one place varies greatly from year to year4 ,to obtain really reliable figures concerning any place1 observations at that place should extend over a period of thirty to thirty-five years4 ,the figures of the mean annual fall will then probably be correct to within #b per cent4 ,the degree of accuracy to be expected in results deduced from observations extending over shorter periods is as follows3-- ,no4 of years #be #bj #ae #aj #e ,error per cent4 #c #c #e #h #ae ,these figures were deduced by ,binnie 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cix47 from an examination of rainfall figures obtained over long periods of time at many places scattered over the world4 ,the errors may1 of course1 be plus or minus4 ,they are the averages of the errors actually found1 and are themselves subject to fluctuations4 ,thus the #ae per cent4 error for a five-year period may be #af or #ac1 the #h per cent4 error for a ten-year period may be #h or #g1 with similar but minute fluctations for the other periods4 ,binnie's figures also show that the ratio of the fall at any place in the driest year to the mean annual fall1 averages #ea to #fh1 with a general average of #fj1 and that the ratio in the wettest year to the mean annual fall averages #ada to #age1 with a general average of #aea4 ,for ,india the general averages are #ej and #age4 ,these figures are useful as a means of estimating the probable greatest and least annual fall1 but they are averages for groups of places4 ,the greatest fall at any particular place may occasionally be twice the mean annual fall4 ,at some places in ,india1 in ,mauritius1 and at ,marseilles it has been two and a half times the mean annual fall4 ,the least annual fall may1 in ,india1 be as low as #bg of the mean4 ,in ,england the fall in a dry year has1 once at least1 been found to be only #cj of the mean annual fall4 ,the mean fall 7average for all places7 in the three driest years is1 from ,binnie's figures1 about #gf of the mean annual fall4 ,the figures given above1 except when a particular country is mentioned1 apply to all countries and to places where the rainfall is heavy1 as well as to those where it is light4 ,but in extremely dry places the fluctuations are likely to be much greater4 ,at ,kurrachee1 with a mean annual fall of only #ge inches1 the fall in a very wet year has been found to be #cgc times1 and in a very dry year only #jg times the mean annual fall4 ,in the ,united ,kingdom the probable rainfall at any place in the driest year may be taken as #fc of the mean annual fall4 ,for periods of two1 three1 four1 five and six consecutive dry years1 the figures are #gb1 #gg1 #hj1 #hb1 and #hce4 ,these figures are of importance in calculations for the capacity of reservoirs 7,,chap4 ,,xiii41 ,art4 #b74 ,when accurate statistics of rainfall are required for any work1 the rainfall of the tract concerned must be specially studied and local figures obtained for as many years as possible4 ,very frequently it is necessary to set up a rain-gauge1 or several if the tract is extensive or consists of several areas at different elevations4 ,sometimes there is only a year or so in which to collect figures4 ,in this case the ratio of the observed fall to that1 for the same period1 at the nearest station where regular records are kept1 is calculated4 ,this ratio is assumed to hold good throughout1 and thus the probable rainfall figures for the new station can be obtained for the whole period over which the records have been kept at the regular station4 ,the volumes of the ,british ,rainfall ,organisation contain a vast amount of information regarding rainfall4 ,for a large area there should be one rain-gauge for every #ejj acres1 for a small area more4 ,in the case of a valley there should be at least three gauges along the line of the deepest part--one at the highest point1 one at the lowest1 and one midway as regards height--and two gauges half way up the sides and opposite the middle gauge 7,ency4 ,brit41 ,tenth ,edition1 vol4 xxxiii474 ,some extra gauges may be set up for short periods in order to see whether the regular gauges give fair indications of the rainfall of the tract4 ,if they do not do so some allowances can be made for this4 #b4 ,available ,rainfall4--,the area drained by a stream is called its 8catchment area0 or 8basin40 ,the available rainfall in a catchment area is the total fall less the quantity which is evaporated or absorbed by vegetation4 ,the evaporation does not chiefly take place directly from the surface4 ,rain sinks a short distance into the ground1 and is subsequently evaporated4 ,the available rainfall does not all flow directly into the streams4 ,some sinks deep into the ground and forms springs1 and these many months later augment the flow of the stream and maintain it in dry seasons4 ,the available rainfall of a given catchment area is known as the 8yield0 of that area4 ,estimation of the available rainfall is necessary chiefly in cases where water is to be stored in reservoirs for town supply or irrigation4 ,the ratio of the available to the total rainfall depends chiefly on the nature and steepness of the surface of the catchment area1 on the temperature and dryness of the air1 and on the amount and distribution of the rainfall4 ,the ratio is far greater when the falls are heavy than when they are light4 ,again1 when the ground is fairly dry and the temperature high--as in summer in ,england--nearly the whole of the rainfall may evaporate2 but when the ground is soaked and the temperature low--as in late autumn and winter in ,england--the bulk of the rainfall runs off4 ,in the eighteen years from #ahic to #aijj the average discharge of the ,thames at ,teddington1 after allowing for abstractions by water companies1 was in ,july1 ,august1 and ,september #ab per cent4 of the rainfall--#fi inches--in its basin1 and in ,january1 ,february1 and ,march #fj per cent4 of the fall which was #ei inches4 ,the total fall in the year was #bfd inches4 ,some rivers in ,spain discharge1 in years of heavy rainfall1 #ci per cent41 and in years of light rainfall #i per cent4 of the rainfall 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxvii474 ,the discharge of a river is not always greatest in the month1 or even the year1 of greatest rainfall4 ,the table opposite gives some figures obtained by comparison of rainfall figures and stream discharges4 ,the case of the area of #bbjh acres near ,cape ,town is described in a paper by ,bartlett 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxxviii471 and it is shown by figures that part of the rainfall in the rainy season went to increase the underground supply which afterwards maintained the flow in the dry season4 ---------------------------------------- ---------------------------------------- --------------------- ,place4 ,catchment ,period over which ,total ,available ,remarks ,area4 ,observations ,rainfall ,rainfall4 ,extended4 ,observed4 ---------------------------------------- ---------------------------------------- --------------------- ,acres4 ,inches4 ,ratio to ,total4 ,nagpur1 ,central ,india #d1bbd ,june to ,september #dd #dj 7,monsoon period74 0 0 0 0 #cj #bg ,mercara1 ,south ,india #dh ,whole year4 #aai #cg ,gravelly soil overlying ,king ,william's ,town1 granite4 ,cape ,colony #fg1bjj 0 #bg #ba ,hills with forest and ,near ,cape ,town1 ,cape bush4 ,colony #aaj ,may to ,october #cae #ea ,bare hills4 ,near ,cape ,town1 ,cape 7rainy season74 ,colony #b1bjh 0 #dc #dj 0 ,melbury ,moor1 ,devonshire 444 ,whole year4 #ejg #ed ,newport1 ,monmouthshire 444 0 #dj #dj ,newport1 ,isle of ,wight 444 0 #cb #dj ,basin of ,nepean ,river1 #bhd sq4 ,n4,s4,w4 miles 0 #ddc #dd ,bare1 broken ground4 ,basin of ,cataract ,river1 ,n4,s4,w4 #gj sq4 miles 0 #ed #de 0 0 ---------------------------------------- ---------------------------------------- --------------------- ,the following statement shows how the available rainfall may vary from year to year4 ,the figures are those of a catchment area of #ej square miles on the ,cataract ,river1 ,new ,south ,wales 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxxi473-- ---------------------------------------- -------------------------- ,year4,rainfall4 ,available ,remarks4 ,rainfall4 ---------------------------------------- -------------------------- ,inches4 ,ratio to ,total4 #ahie #cda #hd ,heavy rain falling on saturated area4 #ahif #ccg #bh ,evenly distributed fall4 #ahig #ddg #di ,heavy rains in ,may4 #ahih #efd #de 0 0 ,february 7#ae ins474 #ahii #edi #dc 0 0 ,august 7#aae ins474 #aijj #bfa #ej 0 0 ,may and ,july4 #aija #cgd #aa ,evenly distributed fall4 #aijb #bii #jf #aijc #dag #bc ,no heavy fall4 ---------------------------------------- -------------------------- ,the manner in which the available rainfall may vary from month to month is shown in the following statement1 which gives the figures for #aije for the ,sudbury ,river in ,massachusetts3-- ---------------------------------------- - ,month4 ,rainfall4 ,available ,rainfall4 ---------------------------------------- - ,inches4 ,percentage of fall4 ,january #ec #dh ,february #bb #bd ,march #cb #adb ,april #bg #ajd ,may #ac #dj ,june #ej #af ,july #ee #f ,august #bg #h ,september #fi #ca ,october #ae #ah ,november #ba #bc ,december #dj #dj ---------------------------------------- - ,total #dbc ,average #cie ---------------------------------------- - ,rankine gives the ratio of the available rainfall to the whole fall as #aj on steep rocks1 #h to #f on moorland and hilly pasture1 #e to #d on flat1 cultivated country1 and nil on chalk4 ,these figures are only rough4 ,the figures for rocks and pastures are too high4 ,the loss from evaporation and absorption is not proportional to the rainfall4 ,it is far more correct to consider the loss as a fairly constant quantity in any given locality but increasing somewhat when the rainfall is great4 ,the available rainfall in ,great ,britain has generally been overestimated4 ,sometimes it has been taken as being #fj of the whole fall4 ,more commonly the loss is taken to be #ac to #ae inches4 ,this is correct for the western mountain districts1 where the rainfall is about #hj inches and the soil consists chiefly of rocks partly covered with moorland or pasture4 ,in other parts of the country1 especially where flat1 the loss is often #ag to #bj inches4 ,all the above figures are1 however1 general averages4 ,the proper estimation of the available rainfall at any place in any country depends a great deal on experience and judgment1 and on the extent to which figures for actual cases of similar character are available4 ,regarding the 8run-off0 from saturated land during short periods1 see ,,chap4 ,,xii41 ,arts4 #a and #b4 #c4 ,measurement of ,rainfall4--,a rain-gauge should be in open ground and not sheltered by objects of any kind4 ,the ordinary rain-gauge is a short cylinder4 ,this is often connected by a tapering piece to a longer cylinder of smaller diameter4 ,in this the rain is stored safely and is measured by a graduated rod4 ,the measurement can be made more accurately than if the diameter was throughout the same as at the top4 ,in other cases the water is poured out of the cylinder into a measuring vessel4 ,if the rain-gauge was sunk so that the top was level with the ground1 rain falling outside the gauge would splash into it and vitiate the readings unless the gauge was surrounded by a trench4 ,ordinarily the top of the gauge is from #a to #c feet above the ground4 ,when it is #a metre above the ground the rain registered is said to be on the average about #f per cent4 less than it should be1 owing to the fact that wind causes eddies and currents and carries away drops which should have fallen into the gauge4 ,the velocity of the wind increases with the height above the ground1 and so does the error of the rain-gauge4 ,devices for getting rid of the eddies have been invented by ,boernstein and ,nipher 7,ency4 ,brit41 ,tenth ,edition1 vol4 xviii471 but they have not yet come into general use4 ,the ,boernstein device is being used experimentally at ,eskdalemuir4 ,it would appear that much splashing cannot take place when the ground is covered with grass1 and that in such a case the top of the gauge could be #a foot above the ground1 thus making the error very small4 ,if the ground is at first level1 then rises and then again becomes level1 a rain-gauge at the foot of the slope will1 with the prevailing wind blowing up the slope1 register too much1 and a rain-gauge just beyond the top of the slope will register too little 7,ency4 ,brit41 ,tenth ,edition1 vol4 xxxiii474 #d4 ,influence of ,forests and ,vegetation4--,when the ground is covered with vegetation1 and especially forests1 the humus or mould formed from leaves1 etc41 absorbs and retains moisture4 ,it acts like a reservoir1 so that the run-off takes place slowly and the denudation and erosion of the soil is checked4 ,the roots of the trees or other vegetation also bind the soil together4 ,vegetation and forests thus mitigate the severity of floods and reduce the quantity of silt brought into the streams4 ,they also shield the ground from the direct rays of the sun and so reduce evaporation1 and thus1 on the whole1 augment the available rainfall4 ,forests render the climate more equable and tend to reduce the temperature1 and they thus1 at least on hills1 increase the actual rainfall to some extent4 ,if a forest is felled and replaced by cultivation1 the ploughing of the soil acts in the same way as the humus of the forest1 and the crops replace the trees2 and it has been stated that in the ,u4,s4,a4 the cultivation is as beneficial as the forests in mitigating floods and checking denudation of the soil 7,proc4 ,am4 ,soc4 ,c4,e41 vol4 xxxiv474 ,but when forests are felled they are not1 at least in hilly country1 always replaced by cultivation4 ,measures to put a stop to the destruction of forests or to afforest or reforest bare land may enter into questions of the rgime of streams or the supply of water4 ,on the ,rhine1 increase in the severity of floods was distinctly traced to deforestation of the drainage area4 ,it is usually said that forests act as reservoirs by preventing snows from melting4 ,this is disputed in the paper above quoted1 and it is stated that in the absence of forests the snow forms drifts of enormous depth1 and these melt very gradually and act as reservoirs after the snow in the forests has disappeared4 #e4 ,heavy ,falls in ,short ,periods4--,when rain water1 instead of being stored or utilised1 has to be got rid of1 it is of primary importance to estimate roughly--exact estimates are impossible--the greatest probable fall in a short time4 ,this bears a rough ratio to the mean annual fall4 ,the maximum observed falls in twenty-four hours range1 in the ,united ,kingdom1 generally from #je to #aj of the mean annual fall--but on one occasion the figure has been #bj1--;and in the tropics from #aj to #be4 ,actual figures for particular places can be extracted from the rain registers1 but the probability of their being exceeded must be taken into account4 ,the greatest fall observed in twenty-four hours in the ,united ,kingdom is #g inches1 and in ,india #cj inches in the ,eastern ,himalayas4 ,but much shorter periods than twenty-four hours have to be dealt with4 ,the following figures are given by ,chamier 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cxxxiv47 as applicable to ,new ,south ,wales1 and he considers that they are fair guides1 erring on the side of safety1 for other countries3-- ,duration of fall in hours #a #d #ab #bd ,ratio of fall to maximum daily fall #a ,the above figures are probably safe for ,england4 ,for ,india the case is far otherwise4 ,the following falls have been observed there3-- --------------------------------------- ,period4 ,fall4 ,rate per ,hour4,remarks4 --------------------------------------- ,inches4 ,inches4 #g hours #aj #adc #de hours #gg #ag #b hours #h #d #a hour #e #e #bj minutes #af #dh #aj minutes #a #f --------------------------------------- ,the falls of #a inch in ten minutes were frequently observed near the head of the ,upper ,jhelum ,canal1 a place where the annual rainfall is not more than #cj inches 7see also ,,chap4 ,,xii41 ,art4 #a74 ,in some parts of the ,eastern ,himalayas1 where #cj inches of rain has fallen in a day1 it is possible that #h inches may have fallen in an hour4 ,in ,england #d inches has fallen in an hour4 ,the heaviest falls in short periods do not usually occur in the wettest years1 and they may occur in very dry years4 ,nor do they always occur on a very wet day4 ,,chapter ,,iii ,,collection ,,of ,,information ,,concerning ,,streams #a4 ,preliminary ,remarks4--,the information which is required concerning streams depends on the character of the stream and on the nature of the work which is to be done4 ,for the present let it be supposed that the stream is large and perennial4 ,other kinds of streams will be dealt with in ,arts4 #f and #g4 ,in dealing with a large perennial stream it is nearly always necessary to know the approximate highest and lowest water-levels1 and these can generally be ascertained by local inquiry1 combined with observations of water marks2 but a higher level than the highest known and a lower level than the lowest known are always liable to occur1 and must to some extent be allowed for4 ,if navigation exists or is to be arranged for1 the highest and lowest levels consistent with navigation must be ascertained4 ,the highest such level depends chiefly on the heights of bridges4 ,a plan to a fairly large scale is also necessary in most cases4 ,if an embankment to keep out floods is to be made along a river which is so large that its flood-level cannot be appreciably affected by the construction of the work1 it may be necessary to obtain information only as to the actual flood-levels1 and as to the extent to which the stream is liable to erode its bank4 ,if a length of the bank of a stream has to be protected against scour1 it is necessary to know of what materials the bed and bank are composed1 and whether the channel is liable to changes and to what extent4 ,it is also desirable to know to what extent the water transports solids1 if any4 ,in some kinds of protective work these solids are utilised4 ,but in cases where the stream is to be much interfered with1 it is necessary to have full information concerning it1 not only as regards water-levels1 changes in the channel1 and transport of solids1 but as regards the longitudinal profile and cross-sections1 and the discharges corresponding to different water-levels4 ,the collection of some of this information1 particularly as to the water-levels and discharges at different times of the year and in floods1 may occupy a considerable time4 ,methods of ascertaining the quantity of silt carried in the water of a stream are described in ,,chap4 ,,iv41 ,art4 #d4 ,remarks regarding the other kinds of information required--the stream being still supposed to be large and perennial--are given in ,arts4 #b to #e of this chapter4 ,the degree of accuracy required in the information depends1 however1 on the importance of the work1 and sometimes the procedure can be simplified4 ,detailed remarks on gauges and on the instruments used and methods adopted for observing discharges and surface slopes1 are given in ,hydraulics1 ,,chap4 ,,viii4 and ,appendix ,h4 #b4 ,stream ,gauges4--,unless the stream being dealt with is an artificial one1 it is unlikely that the flow in the reach with which the work is concerned will be uniform4 ,the rise and fall of the water at one place cannot therefore be correctly inferred from those at another4 ,it will be desirable to have two gauges1 either read daily or else automatically1 recording the water-level1 one near each end of the reach concerned1 with intermediate gauges if the reach is very long4 ,if1 in or near the reach1 there is already a gauge which has been regularly read1 it may be sufficient to set up only one new gauge1 and to read it only for such a period of time as will give a good range of water-level1 and to compare the readings with those of the old gauge4 ,the readings of the new gauge for water-levels outside the range of those observed can then be inferred1 but if the stream is very irregular this may involve some trouble 7,art4 #d74 ,in the case of a large stream which shifts its course1 the reading of a gauge does not give a proper indication of the water-level4 ,in other words1 the distance of the gauge from the two ends of the reach is subject to alteration4 ,the case is the same as if the stream was stable and the gauge was shifted about4 ,in such a stream there ought1 if accuracy is required1 to be a group of two or more gauges for each point where there would be only one if the stream was not a shifting one4 ,also1 owing to erosion of the bank or the formation of a sandbank1 it may often be necessary to shift the gauge4 ,when possible it should be kept in a fixed line laid down at right angles to the general direction of the stream4 ,when shifted1 its zero level should be altered in such a way that the reading at the new site at the time of shifting is the same as it was before shifting4 ,when the gauge is moved back to the original site its zero should be placed at its original level1 though this may give rise to a sudden jump in the reading for the reason given in the first sentence of this paragraph4 #c4 ,plan and ,sections4--,making a survey and plan1 and laying down on it the lines for longitudinal and cross-sections1 and taking levels for the sections1 are ordinary operations of surveying4 ,if any land is liable to be flooded1 its boundaries should be shown on the plan and on some of the cross-sections4 ,unless the water is shallow1 it is necessary to obtain the bed levels from the water-level by soundings1 the level of a peg at the water-level having been obtained by levelling4 ,all the sections should show the water-level as it was at some particular time1 but the water-level will probably have altered while the survey was in progress1 and allowance must be made for this4 ,the pegs at all the cross-sections and on both banks of the stream--for the water-levels at opposite banks may not be exactly the same--may1 for instance1 be driven down to the water-level when it is steady1 and thereafter any changes in it noted and the soundings corrected accordingly4 ,in order to ascertain what changes are occurring in the channel it may be necessary to repeat the soundings at intervals and1 if there is much erosion of the bank1 to make fresh plans4 #d4 ,discharge ,observations4--,for a large stream it is necessary to observe the discharges by taking cross-sections and measuring the velocity4 ,if there is a sufficient range of water-levels1 it will be possible to make actual observations of a sufficient number of discharges4 ,if soundings cannot1 owing to the depth or velocity1 be taken at high water1 they must be inferred from those previously taken1 but this does not allow for changes in the channel1 which are sometimes considerable and rapid4 ,if there is not a sufficient range of water-level1 the discharges for some water-levels must be calculated from those at other water-levels4 ,in this case observations of the surface slope will be required1 and the discharge site should be so selected that no abrupt changes in the channel will come within the length over which the observations are to extend4 ,this length should be such that the fall in the water surface will be great enough to admit of accurate observation4 ,if the cross-section of the stream is nearly uniform throughout the whole of this length1 or if it varies in a regular manner1 being greatest at one end of the length and least at the other end4 the differences in the areas of the two end sections being not more than #aj or #ab per cent41 then the velocity and cross-section of the stream can be observed in the usual manner at the centre of the length2 but otherwise they should be observed at intervals over the whole length1 or at least in two places1 one where the section is small and one where it is great1 and the mean taken4 ,or the velocity can be observed at only one cross-section and calculated for the others by simple proportion and the mean taken4 ,the coefficient ,c can then be found from the formula ,c ,v_/7,,rs74 ,to find the discharge for a higher or lower water-level1 the change in the value of ,c corresponding to the change in ,r can be estimated by looking out the values of ,c in tables1 and the discharge calculated by using the new values of ,c and ,r and the new sectional area1 ,s remaining unaltered4 ,but if the channel is such that1 with the new water-level1 a change in ,s is likely to have occurred1 this change must be allowed for4 ,any such change will be due to the changed relative effects of irregularities1 either in the length over which the observations extended or downstream of that length4 ,the effect of irregularities in the bed is greatest at low water4 ,the effect of lateral narrowings is greatest at high water4 ,since a change of #aj per cent4 in ,s causes a change of only #e per cent4 in ,v1 it will usually suffice to draw on the longitudinal section the actual water surface observed and to sketch the probable surface for the new water-level4 ,if the whole channel is fairly regular for a long distance downstream of the discharge site1 no slope observations need be made nor need several sections be taken in order to find ,v4 ,the changed value of ,c should1 however1 be estimated in the manner above indicated4 ,for this purpose any probable value of ,s will suffice4 #e4 ,discharge ,curves and ,tables4--,ordinarily it will be possible1 by plotting the observed discharges as ordinates1 the gauge readings being the absciss1 to draw a discharge curve and from it construct a discharge table4 ,unless the channel is of firm material and not liable to change1 there are likely to be discrepancies among the observed discharges1 so that a regular discharge curve will not pass through all the plotted points4 ,if the discrepancies are not serious1 they can be disregarded and the curve drawn so as to pass as near as possible to all the points1 but otherwise trouble and uncertainty may arise4 ,the soundings should be compared in order to see whether changes have occurred in the channel4 ,if such changes do not account for the discrepancies1 the cause must be sought for in some of the recorded velocities4 ,if no sources of error in these can be found1 such as wind1 it is possible that the velocity has been affected by a change in the surface slope owing to some change in the channel downstream of the length4 ,failing this explanation1 the discrepancies must be set down to unknown causes4 ,with an unstable channel and where accuracy is required1 the sectional areas and velocities should be regularly tabulated or plotted so that changes may be watched and investigated4 ,to do this it may be necessary to take surface slope observations1 or to set up extra gauges which will show any changes in the slope4 ,if1 downstream of the discharge site1 there is any place where affluents come in and bring varying volumes of water1 or where gates or sluices are manipulated1 and if the influence of this extends up to the discharge site1 the water-level there no longer depends only on the discharge1 and a discharge table must be one with several columns whose headings indicate various conditions at the place where the disturbances occur4 ,in order to show how the gauge readings and discharges vary from day to day throughout the year1 a diagram should be prepared showing the gauge readings and discharges as ordinates1 the absciss being the times in days starting from any convenient date as zero4 ,such a diagram1 showing only gauge readings1 is given in fig4 #ef1 ,,chap4 ,,xii4 #f4 ,small ,streams4--,small streams will now be considered1 those1 for instance1 which are too small to be navigable and which occasionally run dry or nearly dry4 ,if the water of the stream is to be stored for water supply1 power or irrigation purposes1 full information as to discharges and silt carried will be required4 ,if the stream is small enough the discharges can be ascertained by means of a weir of planks4 ,the discharge is then known from the gauge readings4 ,cross-sections and large scale plans will not be required unless the stream is to be altered or embanked4 ,if the water1 instead of being stored1 is to be got rid of1 as in drainage work1 the only information required as to discharges is the maximum discharge4 ,large scale plans1 sections1 or information as to silt or water-levels 7except as a means of estimating the discharge7 will not be required unless the stream is to be altered or embanked4 ,in all these cases of small streams the information required is generally1 as has been seen1 less than in the case of large perennial streams1 but it is generally more difficult to obtain4 ,if the stream is ill-defined or its flow intermittent1 especially if it is also very small and the place sparsely inhabited1 it may be difficult to obtain any discharge figures except those based on figures of rainfall4 ,the method of obtaining such figures has been stated in ,,chap4 ,,ii4 ,the figures required are those of the annual and monthly fall when the water is to be stored1 and those of the greatest fall in a short period when the water is to be got rid of4 ,of course a plan of the catchment area is required4 #g4 ,intermittent ,streams4--,in the case of large streams whose flow is intermittent1 the information required will1 as before1 depend upon the circumstances4 ,such streams occur in many countries4 ,the difficulty in obtaining information is often very great4 ,to obtain figures of daily discharge a gauge must be set up in the stream and a register kept4 ,the chief difficulty in an out-of-the-way place is likely to be the obtaining correct information as to the maximum discharge4 ,information1 derived from reports or from supposed flood marks1 as to the highest water-level1 may be inaccurate1 and information based on rainfall figures may be extremely doubtful owing to the large size of the catchment area1 the absence of rain gauges1 and the difficulty1 especially if the rain is not heavy1 in estimating the available fall4 ,all sources of information must be utilised and1 whenever possible1 observations should be made over a long period of time4 #h4 ,remarks4--,very much remains to be done in collecting and publishing information concerning the ratio of the discharges to the rainfall4 ,by observing a fall of rain and the discharge of a stream before and after the fall1 it is possible to ascertain the figures for that occasion1 but they will not hold good for all occasions4 ,continuous observations are required4 ,the chief obstacle is the expense4 ,not only have measuring weirs and apparatus for automatically recording the water-level to be provided1 but the weirs would often cause flooding of land involving payment of compensation4 ,the most suitable places for making observations are those where reservoirs for water-works exist or are about to be made4 ,,chapter ,,iv ,,the ,,silting ,,and ,,scouring ,,action ,,of ,,streams #a4 ,preliminary ,remarks4--,when flowing water carries solid substances in suspension1 they are known as 8silt40 ,material is also moved by being rolled along the bed of the stream4 ,the difference between silt and rolled material is one of degree and not of kind4 ,material of one kind may be rolled and carried alternately4 ,the quantity of silt present in each cubic foot of water is called the 8charge0 of silt4 ,silt consists chiefly of mud and fine sand2 rolled material of sand1 gravel1 shingle1 and boulders4 ,when a stream erodes its channel1 it is said to 8scour40 ,when it deposits material in its channel1 it is said to 8silt40 ,both terms are used irrespective of whether the material is silt or rolled material4 ,a stream of given velocity and depth can carry only a certain charge of silt4 ,when it is carrying this it is said to be 8fully charged40 ,if a stream has power to scour any particular material from its channel1 it has power to transport it2 but the converse is not true4 ,if the material is hard or coherent1 the stream may have far more difficulty in eroding it than in merely keeping it moving4 ,and there is generally a little more difficulty even when the material is soft4 ,silting or scour may affect the bed of a channel or the sides or both4 ,the channel may thus decrease or increase in width or--if one bank is affected more than1 or in a different manner to1 the other--alter its position laterally whether or not it is altering its bed level1 and vice versa4 ,the cross-section of a stream is generally 8shallow10 i4e4 the width of the bed is greater than the combined submerged lengths of the sides1 and the action on the bed is generally greater than on the sides4 ,silting and scouring are generally regular or irregular in their action according as the flow is regular or irregular1 that is1 according as the channel is free or not from abrupt changes and eddies4 ,in a uniform canal fed from a river1 the deposit in the head reach of the canal forms a wedge-shaped mass1 the depth of the deposit decreasing with a fair approach to uniformity4 ,salient angles or places alongside of obstructions are most liable to scour1 and deep hollows or recesses to silt4 ,eddies have exceptionally strong scouring power4 ,immediately downstream of an abrupt change scour is often severe4 ,an abrupt change is one1 whether of sectional area or direction of flow1 and whether or not accompanied by a junction or bifurcation1 which is so sudden as to cause eddies4 ,the hole scoured alongside of an obstruction may extend to its upstream side1 though there is generally little initial tendency to scour there4 ,an obstruction is anything causing an abrupt decrease in any part of the cross-section of a stream1 whether or not there is a decrease in the whole cross-section1 e4g4 a bridge pier or spur4 ,most streams vary greatly at different times both in volume and velocity and in the quantity of material brought into them4 ,hence the action is not constant4 ,a stream may silt at one season and scour at another1 maintaining a steady average4 ,when this happens to a moderate extent1 or when the stream never silts or scours appreciably1 it is said to be in 8permanent rgime10 or 8stable40 ,most streams in earthen channels are either just stable and no more1 or are unstable4 ,waves1 whether due to wind or other agency1 may cause scour1 especially of the banks4 ,their effect on the bed becomes less as the depth of water increases1 but does not cease altogether at a depth of #ba feet1 as has been supposed4 ,salt water possesses a power of causing mud1 but not sand1 to deposit4 ,arts4 #b1 #c1 and #f of this chapter refer to action on the bed of a stream4 ,action on the sides will be considered in ,art4 #g4 ,weeds usually grow only in water which has so low a velocity that it carries no silt to speak of1 but if any silt is introduced the weeds cause a deposit4 ,the weeds also thrive on such a deposit4 #b4 ,rolled ,material4--,if a number of bodies have similar shapes1 and if ,d is the diameter of one of them and ,v the velocity of the water relatively to it1 the rolling force is theoretically as ,v#b ,d#b1 and the resisting force or weight as ,d#c4 ,if these are just balanced1 ,d varies as ,v#b1 or the diameters of similarly shaped bodies which can just be rolled are as ,v#b and their weights as ,v#f4 ,from practical observations1 it seems that the diameters do not vary quite so rapidly as they would by the above law1 the weights being more nearly as ,v#e4 ,let a stream of pure water having a depth ,d1 and with boulders on its bed1 have a velocity ,v just sufficient to move them very slowly4 ,any larger boulders would not be moved4 ,any smaller boulders would move more quickly4 ,similarly1 fine sand would be rolled more quickly than coarse sand4 ,if the velocity of the stream increases1 larger boulders would be moved4 ,streams are thus constantly sorting out the materials which they roll4 ,if the bed is examined it will be found that large boulders exist only down to a certain point1 smaller boulders1 shingle1 gravel1 coarse sand and fine sand following in succession4 ,if the water1 instead of being pure1 is supposed to contain silt1 this may affect its velocity--it is not1 however1 known to do so--but1 given a certain velocity1 it is not likely that the rolling power of the stream is much affected by its containing silt4 ,it is sometimes supposed that increased depth gives increased rolling power1 because of the increased pressure1 but this is not so4 ,the increased pressure due to depth acts on both the upstream and downstream sides of a body4 ,it is moved only by the pressure due to the velocity4 ,when sand is rolled along the bed of a stream there is usually a succession of abrupt falls in the bed4 ,after each fall there is a long gentle upward slope till the next fall is reached4 ,the sand is rolled up the long slope and falls over the steep one4 ,it soon becomes buried4 ,the positions of the falls of course keep moving downstream4 ,the height of a fall in a large channel is perhaps #f inches or #a foot1 and the distance between the falls #bj or #cj feet4 ,a fall does not usually extend straight across the bed but zigzags4 ,it has sometimes been said that the inclination of the bed of a stream1 when high1 facilitates scour1 the material rolling more easily down a steep inclined plane4 ,the inclination is nearly always too small to have any appreciable direct effect4 ,the inclination of the surface of the stream of course affects its velocity1 and this is the chief factor in the case4 ,a sudden rise in the bed of a stream does not necessarily cause rolled materials to accumulate there1 except perhaps to the extent necessary to form a gentle slope4 ,frequently even this slope is not formed1 especially if the rolled material is only sand4 ,the eddies stir it up and it is carried on4 ,the above remarks apply also to weirs or other local rises in the bed4 #c4 ,materials carried in ,suspension4--,it has long been known that the scouring and transporting power of a stream increases with its velocity4 ,observations made by ,kennedy have shown that its power to carry silt decreases as the depth of water increases 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cxix474 ,the power is probably derived from the eddies which are produced at the bed4 ,every suspended particle tends to sink1 if its specific gravity is greater than unity4 ,it is prevented from sinking by the upward components of the eddies4 ,if ,v is the velocity of the stream and ,d its depth1 the force exerted by the eddies generated on #a square foot of the bed is greater as the velocity is greater1 and is probably as ,v#b or thereabouts4 ,but1 given the charge of silt1 the weight of silt in a vertical column of water whose base is #a square foot is as ,d4 ,therefore the power of a stream to support silt is as ,v#b and inversely as ,d4 ,the silt charge which a stream of depth ,d can carry is as ,v4 ,v is called the 8critical velocity0 for that depth1 and is designated as ,v#j4 ,the full charge must be affected by the nature of the silt4 ,the specific gravity of fine mud is not much greater than that of water1 while that of sand is about #ae times as great4 ,moreover1 the particles of sand are far larger than the particles of mud4 ,if two streams of equal depths and velocities are fully charged1 one with particles of mud and the other with particles of sand1 the latter will sink more rapidly and will have to be more frequently thrown up4 ,they will be fewer in number4 ,from some observations referred to by ,kennedy 7,punjab ,irrigation ,paper1 ,no4 #i1 8,silt and ,scour in the ,sirhind ,canal10 #aijd71 it appears that in a fully charged stream which carried #a_/ccjj of its volume of a mixture of mud and sand of various grades1 sand of a particular degree of coarseness formed only #a_/ce1jjj of the volume of the water1 but that when the same stream was clear and was turned on to a bed of the coarse sand it took up #a_/ae1jjj of its volume4 ,it would thus appear that the full charge of silt is less as its coarseness and heaviness are greater4 ,this is in accordance with the laws mentioned above 7,art4 #b1 par4 #a74 ,see also ,,chap4 ,v41 ,art4 #b1 last paragraph4 ,it is probable that fine mud is carried almost equally into all parts of the stream1 whereas sand is nearly always found in greater proportion near the bed and1 as before remarked1 some materials may be rolled and suspended alternately4 ,the charge of mixed silt which a stream can carry is1 no doubt1 something between the charge which it can carry of each kind separately1 but the laws of this part of the subject are not yet fully known4 ,from the observations above referred to1 ,kennedy concludes that a canal with velocity ,v#j will carry in suspension #a_/ccjj to #a_/ejjj of its volume of silt1 according as it is charged with sand of all classes or only with the heavier classes4 ,let a stream be carrying a full charge of any kind of silt4 ,then if there is any reduction in velocity1 a deposit will occur--unless there is also a reduction of depth--until the charge of silt is reduced again to the full charge for the stream4 ,the deposit generally occurs slowly1 and extends over a considerable length of channel4 ,the heavier materials are1 of course1 deposited first4 ,if a stream is not fully charged1 it tends to become so by scouring its channel4 ,it is generally believed that a stream fully charged with silt cannot scour silt from its channel1 or bear any introduction of further silt4 ,this seems to be correct in the main1 but the remarks made in the latter part of the preceding paragraph must be taken into consideration4 ,it has been stated 7,art4 #b7 that a weir or a sudden rise in the bed does not necessarily cause an accumulation of rolled material4 ,it never causes a deposit of suspended material unless it causes a heading up and reduction of velocity to below the critical velocity4 #d4 ,methods of ,investigation4--,the quantity of silt in water is found by taking specimens of the water and evaporating it or1 if the silt is present in great quantity1 leaving it to settle for twelve hours--an ounce of alum can be added for every #aj cubic feet of water to accelerate settlement--drawing off the water by a syphon1 and heating the deposit to dry it4 ,the deposit is then measured or weighed4 ,it is best to weigh it4 ,if clay is filled into a measure1 the volume depends greatly on the manner in which it is filled in4 ,when silt deposits in large quantities in a channel1 or when heavy scour occurs1 the volume deposited or scoured is ascertained by taking careful sections of the channel4 ,illustration3 ,,fig4 #a4 ,silt is best classified by observing its rate of fall through still water4 ,a sand which falls at #aj feet per second is1 in ,india1 called class 7#a71 and mixed sand which falls at rates varying from #a to #b feet per second is called class #a_/b4 ,fig4 #a shows a sand separator designed by ,kennedy4 ,the scale is 4 ,it has a syphon action1 and the rate of flow can be altered by altering the length of the exit pipe4 ,suppose it is desired to measure the sand of class 7#aj7 and all heavier kinds4 ,the pipe is adjusted so as to give a velocity of #a foot per second to the upward flowing water1 which then carries off all silt of class 7#aj7 or finer4 ,all heavier silt falls into the glass tube4 ,it can be separated again by being mixed with water and passed through the instrument again1 the velocity of flow through the instrument being increased4 ,the quantity of silt present at various depths can be found by pumping specimens of water through pipes4 ,at each change of depth the pipe1 delivery hose1 etc41 should be cleaned4 ,allowance must be made for the velocity of ascent of the water up the pipe4 ,suppose this to be #ad feet per second4 ,then the velocity of sand of class 7#b7 would be #ab feet per second1 and the quantity of sand actually found in the water would have to be increased by one-sixth4 #e4 ,quantity and ,distribution of ,silt4--,the quantity of silt present in water varies enormously4 ,fine mud1 even though sufficient to discolour the water1 may be so small in volume that it only deposits when the water is still1 and even then deposits slowly4 ,in the river ,tay1 near ,perth1 the silt was found to be ordinarily #a_/aj1jjj of the volume of water1 and at low water only #a_/bh1jjj4 ,in the river ,sutlej at ,rupar1 near where it issues from the ,himalayas1 the silt in the flood season is extremely heavy4 ,out of #cfj observations1 made at various depths1 during the flood seasons of four successive years1 in water whose depth ranged up to #ab feet1 the silt was once found to be #ba per cent4 by weight of that of the water4 ,it was more than #ab per cent4 on four occasions1 and more than #jc per cent4 7or #c in #ajjj7 on sixty-four occasions4 ,generally about one-half of the silt was clay and sand of classes finer than 7#aj71 about one-third was sand of class #a_/b1 and the residue was sand of class #b_/c4 ,the sand of the river ,chenab is generally coarser than that of the ,sutlej4 ,there are very great differences in the degree of coarseness of river sand4 ,the sand in any river becomes finer and finer as the gradient flattens in approaching the sea4 ,sea sand has been found to be of class 7#bj74 ,in the ,sirhind ,canal1 which takes out from the ,sutlej at ,rupar1 the maximum quantity of suspended silt observed in the four flood seasons was #jg per cent41 on one occasion out of #bgj1 and it exceeded #jc per cent4 on twenty-five occasions4 ,about #hj per cent4 of the silt was clay4 ,in another part of the paper quoted1 it is stated that the silt suspended in the canal water averaged1 during the whole of one flood season1 about #a_/agjj of the volume of the water4 ,this would be about #a_/abjj by weight4 ,the silt deposited in the bed of the canal1 in a period of a few days1 was sometimes as much as #a_/ajjj of the water which had passed along1 and occasionally as much as #a_/ejj4 ,it was nearly all sand1 only about #c per cent4 being clay4 ,silt of classes finer than 7#a7 gave no trouble1 and were to be eliminated in future investigations4 ,in a canal1 as in a river1 the sand on the bed becomes finer the further from the head4 ,regarding the distribution of the silt at various depths1 in water #e to #ag feet deep1 the quantity of silt near the bed may1 when the charge is heavy and consists of mixed silt1 be #a to #c times that at the surface4 ,if the charge is fine mud1 there is likely to be as much silt at the surface as near the bed1 if sand1 there may be none at the surface and little in the upper part of the stream4 ,in all cases single observations are likely to show extraordinarily discordant results2 a number of observations must be made at each point and averaged4 #f4 ,practical ,formul and ,figures4--,a stream which carries silt generally rolls materials along its bed4 ,the proportion between the quantities of material rolled and carried is never known1 and this makes it impossible to frame an exact formula applicable to such cases1 but ,kennedy1 from his observations on canals fully charged with the heavy silt and fine sand usually found in ,indian rivers near the hills1 arrived at the empirical formula for critical velocities ,v #hd ,d#fd ,the observations were made on the ,bari ,doab ,canal and its branches1 the widths of the channels varying from #h feet to #ia feet1 and the depths of water from #bc feet to #gc feet4 ,the beds of these channels have1 in the course of years1 adjusted themselves by silting or scouring1 so that there is a state of permanent rgime1 each stream carrying its full charge of silt1 and the charges in all being about equal4 ,from further observations referred to above 7,art4 #c1 par4 #b7 it appears that this kind of silt forms about #a_/ccjj of the volume of the water1 and that on the ,sirhind ,canal1 sand coarser than the 7#aj7 class1 formed #a_/ce1jjj of the volume of water4 ,the formula gives the following critical velocities for various depths3-- ,d #a #b #c #d #e #f #g ,v#j #hd #acj #agj #bjd #bce #bfd #bib ,d #h #i #aj ,v#j #cah #cdc #cfg4 ,in ,indian rivers not near the hills the silt carried is not so heavy1 and the critical velocities are supposed to be about three-fourths of the above4 ,thrupp 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxi47 gives the following ranges of velocities as those which will enable streams to carry different kinds of silt4 ,it does not appear that the streams would be fully charged except at the higher figure given for each case4 ,d #aj #ajj ,v #ae to #bc #ce to #de 7,coarse sand74 ,v #ie to #ae #bc to #ce 7,heavy silt and fine sand74 ,v #de to #ie #ab to #bc 7,fine silt74 ,it cannot be said that the exact relations between ,d and ,v are yet known1 but it is of great practical importance to know that ,v must vary with ,d4 ,the precise manner in which it must vary does not1 for moderate changes1 make very much difference4 ,in designing a channel a suitable relation of depth to velocity can be arranged for1 and one quantity or the other kept in the ascendant1 according as scouring or silting is the evil to be guarded against4 ,the old idea was that an increase in ,v1 even if accompanied by an increase in ,d1 e4g4 simply running a higher supply in a given channel1 gave increased silt-transporting power4 ,in a stream of very shallow section this is probably correct1 for ,v increases faster than ,d#fd 7,hydraulics1 ,,chap4 ,,vi41 ,art4 #b74 ,in a stream of deep section a decrease in ,d gives increased silt-transporting power4 ,if the discharge is fixed1 a change in the depth or width must be met by a change of the opposite kind in the other quantity4 ,in this case widening or narrowing the channel may be proper according to circumstances4 ,in a deep section widening will decrease the depth of water1 and may also increase the velocity1 and it will thus give increased scouring power4 ,in a shallow section1 narrowing will increase the velocity more than it increases ,d#fd4 ,in a medium section it is a matter of exact calculation to find out whether widening or narrowing will improve matters4 ,if the water entering a channel has a higher silt-charge than can be carried in the channel1 some of it must deposit4 ,suppose an increased discharge to be run1 and that this gives a higher silt-carrying power and a smaller rate of deposit per cubic foot of discharge1 it does not follow that the deposit will be less4 ,the quantity of silt entering the channel is now greater than before4 ,owing to want of knowledge regarding the proportions of silt and rolled material1 and to want of exactness in the formul1 reliable calculations regarding proportions deposited cannot be made4 ,the channels in which the observations above referred to were made have all assumed nearly rectangular cross-sections1 the sides having become vertical by the deposit on them of finer silt2 but the formula probably applies approximately to any channel if ,d is the mean depth from side to side1 and ,v the mean velocity in the whole section4 ,if the ratio of ,v to ,d differs in different parts of a cross-section1 there is a tendency towards deposit in the parts where the ratio is least1 or to scour where it is greatest4 ,there is a tendency for the silt-charge to adjust itself1 that is1 to become less where the above ratio is less1 but the irregular movements of the stream cause a transference of water among all parts1 and this tends to equalise the silt-charge4 ,dubuat gives the following as the velocities close to the bed which will enable a stream to scour or roll various materials4 ,the bed velocity is probably less than the mean velocity in the ratio of about #f to #a in rough channels1 and about #g to #a in smooth channels3-- ,gravel as large as peas #gj feet per second 0 0 ,french beans #aj 0 0 0 0 #a inch in diameter #bbe 0 0 0 ,pebbles #a inch in diameter #ccc 0 0 0 ,heavy shingle #dj 0 0 0 ,soft rock1 brick1 earthenware #de 0 0 0 ,rock of various kinds #fj 0 0 0 and upward4 ,the figures for brick1 earthenware1 and rock can apply only to materials of exceedingly poor quality4 ,masonry of good hard stone will stand #bj feet per second1 and instances have occurred in which brickwork has withstood a velocity of #ij feet per second without injury so long as the water did not carry sand and merely flowed along the brickwork4 ,if there are abrupt changes in the stream1 causing eddies1 or if there is impact and shock1 or if sand1 gravel1 shingle1 or boulders are liable to be carried along1 velocities must be limited4 #g4 ,action on the ,sides of a ,channel4--,it has been seen that the laws of silting and scour on the bed of a channel depend on the ratio of the depth to the velocity4 ,the same laws probably hold good in the case of a gently shelving bank1 so that here again ,v ought to vary as ,d#fd4 ,the velocity near the angle where the slope meets the water surface seems to decrease faster than ,d#fd4 ,at all events1 silt tends to deposit in the angle and the slope to become steep4 ,when the slope is steep the law seems to be different1 the tendency for deposit or scour to occur on the bank depending on the actual velocity without much relation to the depth4 ,the velocity very near to a steep bank is always low relatively to that in the rest of the stream4 ,thus there is often a tendency for silt to deposit on the bank1 especially in the upper part1 and for the side to become vertical except for a slight rounding at the lower corner4 ,a bank may receive deposits when the bed may be receiving none1 and it may have a persistent tendency to grow out towards the stream4 ,the growth of the bank is generally regular1 the line of the bank being preserved1 but it may be irregular1 especially if vegetation1 other than small grass1 becomes established on the new deposits4 ,when scour of the sides of a channel occurs it may occur by direct action of the stream on the sides near the water-level1 or by action at or near the toe of the slope1 which causes the upper part of the bank to fall in4 ,such falling in is generally more or less irregular1 and the bank presents an uneven appearance4 ,the fallen pieces of bank may remain1 more or less intact1 especially if they are held together by the roots of grasses1 etc41 where they fell1 and prevent further scour occurring along the toe of the slope4 ,falling in of banks is most liable to occur in large streams and with light soils4 ,it may be caused by the waves which are produced by steamers and boats or1 especially in broad streams1 by wind4 ,the action on the banks at bends is discussed in ,art4 #h4 ,thus in designing a channel according to the principles laid down in ,art4 #f1 the question of action on the sides of the channel has to be dealt with as follows4 ,whether or not the velocity is to be low1 relatively to the depth1 i4e4 whether or not deposit on the bed is more likely to occur than scour1 care can be taken not to make it actually too low1 and not to make it actually too high1 particularly if the soil is light and friable4 ,with ordinary soils a mean velocity of #cc feet per second in the channel is generally safe as regards scour of the sides4 ,any velocity of more than #ce feet per second may give trouble4 ,a velocity of less than #a foot per second is likely to give rise to deposit on the sides4 ,in channels in alluvial soils the falling in of banks is sometimes said to occur more when the stream is falling than at other times4 ,this has been noticed on both the ,mississippi and the ,indus4 ,the cause has been said to be the draining out of water which had percolated into the bank1 the water in flowing out carrying some sand with it4 ,the effect of this cannot however be great4 #h4 ,action at ,bends4--,at a bend1 owing to the action of centrifugal force and to cross-currents caused thereby1 there is a deposit near the convex bank and a corresponding deepening--unless the bed is too hard to be scoured--near the concave bank4 ,the water-level at the concave bank is slightly higher than at the convex bank4 ,the greatest velocity instead of being in mid-stream is nearer the concave bank4 ,as the transverse current and transverse surface slope cannot commence or end abruptly1 there is a certain length in which they vary4 ,in this length the radius of curvature of the bend and the form of the cross-section also tend to vary4 ,this can often be seen in plans of river bends1 the curvature being less sharp towards the ends4 ,when once a stream has assumed a curved form1 be it ever so slight1 the tendency is for the bend to increase4 ,the greater velocity and greater depth near the concave bank react on each other1 each inducing the other4 ,the concave bank is worn away1 or becoming vertical by erosion near the bed1 cracks1 falls in1 and is washed away1 a deposit of silt occurring at the convex bank1 so that the width of the stream remains tolerably constant4 ,the bend may go on increasing1 and it often tends to move downstream4 ,illustration3 ,,fig4 #b4 ,in fig4 #b the deep places are shown by dotted lines4 ,along the straight dotted line there is no deep place4 ,such a line would be used for a ford4 ,at low water it becomes a shoal4 ,this is the chief reason why a tortuous stream at low water consists of alternate pools and rapids4 ,it is sometimes said that deep water occurs near to a steep hard bank4 ,such deepening is due to bends or obstructions which give the current a set towards the bank1 or it is due to irregularities in the bank which cause eddies4 ,in a straight channel with even and regular banks there is no such deepening4 ,when a bend has formed in a channel previously straight1 the stream at the lower end of the bend1 by setting against the opposite bank1 tends to cause another bend of the opposite kind to the first4 ,thus the tendency is for the stream to become tortuous and1 while the tortuosity is slight1 the length1 and therefore the slope and velocity1 are little affected2 but the action may continue until the increase in the length of the stream materially flattens the slope1 and the consequent reduction in velocity causes erosion to cease4 ,or the stream during a flood may find1 along the chord of a bend1 a direct route with1 of course1 a steeper slope4 ,scouring a channel along this route it straightens itself1 and its action then commences afresh4 ,short cuts of this kind do not1 however1 occur so frequently as is sometimes believed4 ,in some streams the bends acquire a horse-shoe shape and the neck becomes very narrow and short cuts may then occur4 ,otherwise they are not common4 ,v increases only as 7,s71 and if the country is covered with vegetation it is not easy for a stream to scour out a new channel4 ,the effect of bends on the velocity of a stream is not well understood4 ,in case of a bend of #ij the increased resistance to flow when the bend is absolutely sudden 7a sudden bend is known as an 8elbow07 amounts perhaps to ,v#b_/7b;g74 ,whether it is greater or less in the case of a gentle bend of #ij is not known4 ,in the case of a pipe there is a certain radius which gives a minimum resistance 7,hydraulics1 ,,chap4 ,v474 ,the increased resistance at a bend is due partly to the fact that the maximum velocity is no longer in the centre of the stream1 and partly to the fact that the velocities at the different parts of the cross-section have to be rearranged at the commencement of the bend and again at its termination4 ,thus the effect of a bend of #de is a good deal more than half of that of a bend of #ij4 ,two bends of #de1 both in the same direction1 with a straight reach between them1 will cause more resistance than a single bend of #ij with the straight reach above or below the bend4 ,if the two bends of #de are in different directions the resistance will be still greater4 ,a succession of sharp bends may produce a serious effect1 amounting to an increase in roughness of the channel4 ,a succession of gentle bends1 of any considerable angle1 cannot of course occur within a moderate length of channel4 ,when there is head to spare there is clearly no objection to bends1 except that the bank may need protection4 ,at a place where the bank has in any case to be protected1 e4g4 at a weir1 there is no objection to an elbow4 #i4 ,general ,tendencies of ,streams4--,since the velocity is greater as the area of the cross-section is less1 a stream always tends to scour where narrow or shallow1 and to silt where wide or deep4 ,the cross-section thus tends to become uniform in size4 ,suppose two cross-sections to be equal in size but different in shape4 ,the velocities of the two sections will be equal4 ,the tendency of the bed to silt will 7,art4 #f7 be greater at the deeper section and1 when silting has occurred on the bed1 the section will be reduced and there will be a tendency to scour at the sides4 ,thus the cross-sections tend to become also uniform in shape4 ,if a bank of silt has formed in a stream1 the tendency is for scour to occur4 ,there is also a tendency for silt to deposit just below the point where the bank ends4 ,hence a silt bank often moves downstream4 ,owing to the tendency to scour alongside of1 or downstream of1 obstructions 7,art4 #a71 it is clear that a stream constantly tends to destroy obstructions4 ,there is an obvious tendency for silt to deposit where the bed slope of a stream flattens1 and for scour to take place where it steepens 7,hydraulics1 figs4 #af and #ag1 pp4 #bd and #be71 and thus the tendency is for the slope to become uniform4 ,in a natural stream flowing from hilly country to a lake or sea1 the slope is steepest at the commencement and gradually flattens4 ,there is thus a tendency for the bed to rise except at the mouth of the stream4 ,this rising tends to increase the slope and velocity in the lower reaches1 and this again enhances the tendency1 described in the preceding article1 of the stream to increase in tortuosity4 ,when a silt-bearing stream overflows its banks the depth of water on the flooded bank is probably small and its velocity very low1 and a deposit of silt takes place on the bank4 ,when the deposit has reached a certain height it acts like a weir on the water of the next flood1 which flows quickly over it and1 instead of raising it higher1 deposits its silt further away from the stream4 ,in this way a strip of country along the stream gradually becomes raised1 the raising being greatest close to the stream4 ,the country slopes downwards in going away from the stream4 ,in other words1 the stream runs on a ridge4 ,if the bank becomes raised so high that flooding no longer occurs1 the raising action ceases1 but if1 as is likely in alluvial country1 the bed of the stream also rises1 the action may continue and the ridge become very pronounced4 ,some rivers have very wide and soft channels which are only filled from bank to bank in floods1 if then4 ,the deep stream winds about in the channel1 and the rest of it is occupied by sandbanks and minor arms4 ,the winding is the result of the velocity being too great for the channel4 ,the streams1 especially the main stream1 constantly shift their courses by scouring one bank or the other4 ,now and then the main stream takes a short cut1 either down a minor arm or across an easily eroded sandbank4 ,this is a very different matter from a short cut across high ground4 ,the sandbanks receive deposits of silt in floods1 but are constantly being cut away at the sides4 ,such rivers frequently erode their banks to an extraordinary extent4 ,the ,indus sometimes cuts into its bank #ajj feet or more in a day1 and it may cut for half a mile or more without cessation4 ,the tortuosity of such a stream increases as it gets nearer the sea4 ,the actual length of the ,indus in the #djj miles nearest the sea is #ci per cent4 greater than its course measured along the bank4 ,in the reach from the #fjjth to the #gjjth mile from the sea1 the difference is only #c per cent4 ,for a detailed description of some such rivers1 see ,punjab ,rivers and ,works4 ,sometimes general statements are made regarding silting or scour in connection1 for instance1 with a stream which is confined between embankments or training walls1 or has overflowed its banks or is held up by a weir4 ,it is impossible to say that any such condition1 or any condition1 will cause silting or scour1 unless the velocity depth and silt charge are known4 ,,chapter ,v ,,methods ,,of ,,increasing ,,or ,,reducing ,,silting ,,or ,,scour #a4 ,preliminary ,remarks4--,most important works which affect the rgime of a stream have some effect on its silting or scouring action1 but this is not generally their chief object4 ,such works will be dealt with in due course1 and the effects which they are likely to produce on silting or scouring will be mentioned4 ,in the present chapter only those works and measures will be considered whose chief object is to cause a stream to alter its silting or scouring action4 ,it does not matter1 so far as this discussion is concerned1 whether the object is direct1 i4e4 concerned only with the particular place where the effect is to be produced1 or indirect1 as1 for instance1 where a stream is made to scour in order that it may deposit material further down the stream4 ,the protection of banks from scour is considered in ,,chap4 ,,vi4 ,dredging is dealt with in ,,chap4 ,,viii4 #b4 ,production of ,scour or ,reduction of ,silting4--,sometimes the silt on the bed of a stream is artificially stirred up by simple measures1 as1 for instance1 by scrapers or harrows attached to boats which are allowed to drift with the stream1 or by means of a cylinder which has claw-like teeth projecting from its circumference and is rolled along the bed1 or by fitting up boats with shutters which are let down close to the bed and so cause a rush of water under them1 or by anchoring a steamer and working its screw propeller4 ,it is thus possible to cause a great deal of local scour1 but the silt tends to deposit again quickly1 and it is not easy to keep any considerable length of channel permanently scoured4 ,the system is suitable in a case in which a local shallow or sandbank is to be got rid of and deposit of silt a little further down is not objectionable4 ,it may be suitable in a case in which the bed is to be scoured while a deposit of silt at the sides of the channel is required1 especially if some arrangement to encourage silt deposit at the sides is used 7,art4 #c1 par4 #d2 also ,,chap4 ,,vi41 ,art4 #c74 ,holding back the water by means1 for instance1 of a regulator or movable weir1 and letting it in again with somewhat of a rush1 will1 if frequently repeated1 have some effect in moving silt on in the downstream reach4 ,regarding the upstream reach1 it has been remarked 7,,chap4 ,,iv41 ,art4 #c7 that a weir does not necessarily cause silt deposit4 ,if1 in a stream which does not ordinarily silt1 a regulator or movable weir causes1 when the water is headed up1 some silt deposit1 the cessation of the heading up not only removes the tendency to silt1 but the section of the stream1 at the place where the deposit occurred1 is less than elsewhere1 and there is thus a tendency to scour there4 ,if a regulator is alternately closed and opened1 no permanent deposit of much consequence is likely to occur4 ,a stream may be made to scour its channel by opening an escape or branch4 ,this causes a draw in the stream1 and an increase in velocity for a long distance upstream of the bifurcation 7,hydraulics1 ,,chap4 ,,vii41 ,art4 #f74 ,this procedure is sometimes adopted on irrigation canals4 ,the escape is generally opened in order to reduce the quantity of water passing down1 but it may be opened solely to induce scour or prevent silting4 ,the floor of the escape head is usually higher than the bed of the canal1 but this does not interfere with operations except at low supplies4 ,it may 7,,chap4 ,,iv41 ,art4 #b7 have some effect on the quantity of rolled material passed out of the escape4 ,if there is a weir in the river below the off-take of the canal1 and if the escape runs back to the river and thus has a good fall1 the scouring action in the canal may be very powerful4 ,if the main channel has a uniform slope throughout1 the slope of its water surface is greater upstream of the escape than downstream of the escape1 and there is thus an abrupt reduction of velocity and possibly a deposit of silt in the main channel below the escape4 ,this may or may not be objectionable4 ,in the case of an irrigation canal1 it is far less objectionable than deposit in the head of the canal4 ,the best point for the off-take of any escape or scouring channel depends on the position of the deposits in the main channel4 ,the off-take should be downstream of the chief deposits1 but as near to all of them as possible4 ,a breach in a bank acts of course in the same way as an escape4 ,a stream of clear water when sent down a channel will scour it if the material is sufficiently soft4 ,in the case of the ,sirhind ,canal1 it has already been mentioned 7,,chap4 ,,iv41 ,art4 #c71 that when the river water became clear after the floods the proportion of coarse sand1 i4e4 sand above the 7#aj7 class1 carried by the canal water was about #a_/ae1jjj by volume4 ,this was in the period from #bbnd ,september to #gth ,october4 ,from #hth to #bcrd ,october the proportion averaged #a_/cb1jjj from #bdth ,october to #hth ,november #a_/dd1jjj1 and from #ith to #bdth ,november #a_/he1jjj4 ,the reason of this reduction was that the comparatively clear water kept picking up the sand from the bed and moving it on1 the finer kinds being moved most quickly4 ,as the coarse sand left on the bed became less in quantity1 the water took up less4 ,it appears1 however1 that the water also picked up some clay which was left1 and that the total suspended silt in ,november was #a_/ijjj of the water4 ,all the observations mentioned in this paragraph appear to have been made at ,garhi1 #bf miles from the head of the canal4 #c4 ,production of ,silt ,deposit4--,works or measures for causing silt deposit may be undertaken in order to cause silt deposit in specific places where it will be useful1 or in order to free the water from silt4 ,sometimes both objects are combined4 ,if a stream can be turned into a large pond or low ground--a bank being built round it if necessary--it can be made to part with some or all of its silt whether rolled or suspended4 ,even if the pond is so large that the velocity becomes imperceptible1 the whole of the suspended matter will not deposit unless it has sufficient time1 but the matter which remains in the water is likely to be extremely small in amount4 ,the silting up of marshes1 pools1 borrow-pits1 etc41 is now being effected1 or should be effected1 in places where mosquitoes and malaria are prevalent4 ,in the upper or torrential part of a stream1 a high dam1 provided with a sluice and a high-level waste weir1 may be built across it4 ,the space above the dam becomes more or less filled with gravel1 etc4 ,this has been done in ,switzerland 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxi474 ,in the ,u4,s4,a4 long weirs have been built in order to stop the progress of detritus from gold mines4 ,such detritus was liable to choke up rivers and damage the adjoining lands4 ,the detritus from hill torrents can also be reduced by afforestation of the hill sides4 ,illustration3 ,,fig4 #c4 ,illustration3 ,,fig4 #d4 ,when a stream is in embankment--irrigation channels are frequently so--the bank can be set back 7fig4 #c71 and suspended silt will then deposit on the berms4 ,the object of this arrangement is generally to create very strong banks in low ground4 ,a similar plan can be adopted when the berm is only slightly below the water-level and even when it is only occasionally submerged4 ,in this case the deposit of a small bank of silt along the edge of the berm next the stream will prevent the access of fresh supplies of silt-bearing water to the parts further away4 ,gaps should be cut in the bank of silt at intervals1 and cross banks made to form 8silting tanks10 as shown in fig4 #d4 ,the inlets to the tank should be large1 and the outlets small1 so that the water in the tank may have little velocity4 ,it is not1 however1 correct to have the outlet so small--unless the water contain very little silt--that there is very little flow through the tank4 ,the tanks will generally be silted up most quickly by allowing a good flow through them1 even though only a small proportion of the silt in the water is deposited4 ,regular banks arranged to form tanks on the above principle can be made behind the original banks of a canal in cases where the original banks were not1 for any reason1 set back4 ,when a channel is made in low ground and the excavation is not sufficient to make the banks1 borrow-pits can be dug in the bed of the channel4 ,such pits should not be long and continuous1 but wide bars should be left so that a number of short pits will result4 ,these pits will trap rolled material as well as suspended silt4 ,the object in this case is to free the water from silt and to reduce the size of the channel and thus reduce the loss of water from percolation4 ,on the ,indus1 where it has a strong tendency to shift westwards1 long earthen dams or groynes are run out from the west bank across the sandbanks4 ,one object is to cause silt deposit1 and so increase the quantity of material which the river will have to cut away1 but whether this result is achieved is doubtful4 ,the sandbanks receive deposits in any case4 ,a groyne may increase the deposit on its upstream side1 but it cuts off the flood water from its downstream side and so reduces the deposit there4 #d4 ,arrangements at ,bifurcations4--,at a bifurcation1 as where a branch takes off from a canal1 it is possible to reduce the quantity of rolled material entering the canal by raising its bed or constructing a weir or 8sill0 in its head4 ,this arrangement may have great effect in excluding boulders1 shingle1 or gravel4 ,as regards rolled sand1 it has much less effect than might be expected 7,,chap4 ,,iv41 ,art4 #b74 ,if the canal is reduced in width 7fig4 #e7 there will be eddies below the bed level of the branch4 ,they will stir up the sand and some of it will enter the branch4 ,if the canal is not reduced in width1 eddies will be produced in the surface water1 and they will affect the bed4 ,the above remarks apply also to the case of a canal taken off from a river when there are no works in the river4 ,illustration3 ,,fig4 #e4 ,illustration3 ,,fig4 #f4 #e4 ,a ,canal with ,headworks in a ,river4--,in the case of a canal taking off from a river and provided with complete headworks1 it is possible to do a great deal more4 ,the case of the ,sirhind ,canal1 already referred to 7,,chap4 ,,iv41 ,arts4 #e and #f71 is a notable example4 ,the canal 7fig4 #f7 is more than #bjj feet wide1 the full depth of water #aj feet1 and the full discharge about #gjjj cubic feet per second4 ,in #ahic when the irrigation had developed1 and it became necessary to run high supplies in the summer--,july1 ,august1 and part of ,september--the increase in the silt deposit threatened to stop the working of the canal4 ,in the autumn and winter1 say from #beth ,september to #aeth ,march1 the water entering the canal is clear and much of the deposit was picked up by it1 but not all4 ,in the five years #ahic to #ahig inclusive1 the following remedial measures were adopted4 ,increased use was made of the escape at the twelfth mile4 ,this did some good1 but there was seldom water to spare4 ,in #ahic to #ahid the sill of the regulator was raised to #g feet above the canal bed1 and it was possible to raise it #c feet more by means of shutters4 ,this had little effect4 ,the coarsest class of sand was #d1 and the velocity of the water1 even of that part of it which came up from the river bed and passed over the sill1 was over #b feet per second1 so that all sand was carried over4 ,in #ahid to #ahie the divide wall1 which had been only #ei feet long1 was lengthened to #gaj feet1 so as to make a pond between the divide wall and the regulator1#h but probably the leakage through the under-sluices was often as much as the canal supply1 and the water in the pond was thus kept in rapid movement and full of silt4 ,the canal was closed in heavy floods4 ,this did some good1 but probably the canal was often closed needlessly when the water looked muddy but contained no excessive quantity of sand4 ,the above comments on the measures taken were made by ,mr ,kennedy when chief engineer4 ,the above measures did not reduce the silt deposits1 but the scour in the clear water season improved1 probably because higher supplies were run owing to increased irrigation4 ,the deposit in the upper reaches of the canal1 when at its maximum about the end of ,august of each year1 was generally more than twenty million cubic feet4 ,from the year #aijj a better system of regulation was enforced1 the under-sluices being kept closed as much as possible1 so that there was much less movement in the pond and much less silt in its water4 ,by #aijd the deposit in the canal had been reduced to three million cubic feet1 and no further trouble occurred4 ,during the period from #bjth ,september #aijh to #ajth ,october #aijh the quantity of silt in the canal above ,chamkour 7twelfth mile7 decreased from #ai1cbe1hjj cubic feet to #ab1dgg1fjj cubic feet4 ,the quantity scoured away was #f1hdh1bjj cubic feet4 ,during this period no silt entered the canal4 ,the quantity which passed out of the reach in question in suspension was #d1ahc1ffj cubic feet1 so that #b1ffd1edj cubic feet of material must have been rolled along the bed4 ,the rolled material was #fd per cent4 of the suspended material4 ,during this period the ,daher escape1 in the twelfth mile1 was open1 and the mean velocity in the canal just above the escape was about #d feet per second1 the depth of water being about #aj feet4 ,the velocity near the escape was thus greater than the critical velocity for mixed silt 7,,chap4 ,,iv41 ,art4 #f71 and even a long way up the canal it would be in excess of the critical velocity4 ,the water seems to have carried about #a_/ahjj of its volume of silt4 ,whether the above proportions of rolled to suspended matter would hold good in a fully charged stream flowing with the critical velocity it is not easy to say4 ,as silt deposits in the pond1 the velocity of the water in it1 along the course of the main current towards the canal1 increases and eventually the water begins to carry coarse sand dangerous for the canal4 ,in order to ascertain when this state of affairs has been reached1 two methods of procedure are possible4 ,one is to frequently test specimens of the water in the pond along the course of the main current and see when it contains more than #a_/ae1jjj of its volume of coarse sand4 ,this plan would be troublesome and liable to error1 and is rejected by ,kennedy1 who suggests that the depth and velocity of the water in the pond be frequently observed along the course of the main current4 ,as soon as the velocity exceeds the critical velocity for mixed silt1 it is time to close the canal and open the under-sluices and scour out the deposit from the pond4 ,the period in which most silt is believed to have been deposited in the canal is the spring and early summer1 say from #aeth ,march to #ast ,july4 ,this is the time when the snows are melting and the river water is clear4 ,it can then carry more sand than in the rains--#ast ,july to #aeth ,september1--when it is muddy4 ,kennedy also suggests that some under-sluices should be provided at the far side of the river1 i4e4 at the right-hand side of the weir4 ,it would then be possible1 by opening them1 to let floods pass without interfering with the pond4 ,the two spurs or groynes1 shown in the plan1 were constructed in #ahig so as to cause the stream to flow along the face of the canal regulator and not allow deposits to accumulate there4 ,the depth of silt deposited in a great part of the pond amounted at times to #h or #aj feet4 #f4 ,protection of the ,bed4--,it is possible to afford direct protection from scour to the bed of a stream by constructing walls across it1 but unless the walls are near together the protection will not be effective4 ,an arrangement used in some streams in ,switzerland consists of tree trunks secured by short piles and resting on brushwood4 ,but as long as the walls are not raised above the bed they cannot entirely stop scour1 unless extremely close together4 ,if raised above the bed they form a series of weirs4 ,the weirs must be so designed that the depth of water in a reach between two weirs is great enough to reduce the velocity down to the critical velocity1 or less4 ,the fall in the water surface at each weir being very small1 the discharge over the weir can be found by considering it as an orifice extending up to the downstream water surface1 and the head being the fall in the surface at the weir4 ,to stop scour of the bed by direct protection without raising the water-level1 the bed can be paved1 a plan adopted in artificial channels with very high velocities4 ,the paving can be of stones1 bricks1 or concrete blocks4 ,the ,villa system of protection1 which has been used in ,italy1 ,france1 and ,spain1 consists of a flexible covering laid on the bed4 ,prisms of burnt clay or cement are strung on several parallel galvanized iron wires1 which are attached to cross-bars so as to form a grid a few feet square4 ,the grids are loosely connected to one another at the corners1 and the whole covering adjusts itself to the irregularities of the bed 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cxlvii474 ,the special protection or paving required in connection with weirs and such-like works is considered in ,,chap4 ,x41 ,arts4 #b and #c4 ,,chapter ,,vi ,,works ,,for ,,the ,,protection ,,of ,,banks #a4 ,preliminary ,remarks4--,the protection of a length of bank from scour may be effected by spurs1 which are works projecting into the stream at intervals1 or by a continuous lining of the bank4 ,a spur forms an obstruction to the stream 7,,chap4 ,,iv41 ,art4 #a71 and when constructed1 or even partly constructed1 the scour near its end may be very severe1 even though there may be little contraction of the stream as a whole4 ,if the bed is soft a hole is scoured out4 ,into this hole the spur keeps subsiding1 and its construction1 or even its maintenance1 may be a matter of the greatest difficulty4 ,a high flood may destroy it4 ,if it does not do so1 it may be because the stream has1 for some reason1 ceased to attack the bank at that place4 ,a continuous lining of the bank is not open to any objection1 and is generally the best method of protection4 ,spurs made of large numbers of rather small trees1 weighted with nets filled with stones1 have been used on the great shifting rivers of the ,punjab which swallowed up enormous quantities of materials4 ,the use of spurs on such rivers has now1 in most cases1 been given up4 ,if ,l is the length of a spur measured at right angles to the bank1 the length of bank which it protects is about #g ,l--#c ,l upstream and #d ,l downstream1--but the spur has to be strongly built1 and its cost is1 in many cases1 not much less than the expense of protecting the whole bank with a continuous lining4 ,whatever method is adopted1 a plan1 large enough to show all irregularities1 should always be prepared1 and the line to which it is intended that the bank shall be brought marked on it4 ,illustration3 ,,fig4 #g4 ,sometimes natural spurs exist as1 for instance1 where a tree projects into a stream or has fallen into it1 and the holes between the spurs may be deep1 so that a continuous protection would be expensive4 ,or there may be trees standing in such positions that1 if felled1 they will be in good places for spurs4 ,in cases such as the above1 spurs may be suitable even in a stream with a soft channel4 ,regarding the use of spurs or groynes for diversion works or for reducing the width of a stream1 see ,,chap4 ,,vii41 ,art4 #a1 and ,,chap4 ,,viii41 ,art4 #c4 #b4 ,spurs4--,a spur may be made of-- 7a7 ,loose stone1 which may be faced with rubble above low-water level 7fig4 #g74 7b7 ,layers of fascines weighted with gravel or stones4 7c7 ,earth or sand closely covered with fascines4 7d7 ,a double line of stakes with fascines or brushwood laid between them 7fig4 #h74 7e7 ,a single line of stakes with planking or basket work on its upstream side1 or with twigs or wattle laid horizontally and passed in and out of the stakes1 as in fig4 #bj4 7f7 ,a single tree with the thick end of the trunk on the bank and with stakes1 if necessary1 to prevent the current from moving it4 7g7 ,a number of small trees heaped together and weighted with nets full of stones4 7h7 ,a layer of poles and over them a layer of fascines on which are built walls of fascine work so arranged as to form cells or hollow rectangular spaces which become filled with silt4 7i7 ,large fascines running out into the stream and having their inner ends staked to the bank while the outer ends float1 other fascines being added over them and projecting further into the stream1 and the whole eventually sinking4 ,illustration3 ,,fig4 #h4 ,combinations of the above are also used1 for instance1 7d7 or 7e7 may be used for the upper portion1 the foundation being 7a7 or 7c74 ,illustration3 ,,fig4 #i4 ,illustration3 ,,fig4 #aj4 ,illustration3 ,,fig4 #aa4 ,instead of running out at right angles to the bank a spur may be inclined somewhat downstream4 ,this somewhat reduces the eddying and scour round the end4 ,the ends of a system of spurs should be in the line which it is intended that the edge of the stream shall have 7fig4 #i74 ,the tops of short spurs are usually above high flood level4 ,sometimes spurs are made to slope downwards 7fig4 #aj71 and they then cause less disturbance of the water and less scour than if built to the form shown by the dotted line4 ,such spurs are sometimes combined with a low wall running across the bed of the stream1 the whole forming a 8profile0 of the cross-section to which it is intended to bring the channel4 ,regarding such walls1 see ,,chap4 ,v41 ,art4 #f4 ,when a spur is long it may have small subsidiary spurs 7fig4 #aa7 to reduce the rush of water along it2 or its end may have to be protected in the same manner as the advancing end of a closure dam 7,,chap4 ,,vii41 ,art4 #b74 ,the following is a curious case of misconception of the action of spurs4 ,in #aiji the river ,indus was eroding its right bank and threatening to destroy the town of ,dera ,ghazi ,khan4 ,a clump of date palms formed a promontory and resisted erosion to some extent4 ,a suggestion was made--by an engineer of eminence who had formerly been consulted in the case--to the effect that the date palms be removed1 the reason given being that they caused disturbance and scour4 ,on this principle spurs would have to be made not to protect a bank but to cause it to be eroded4 #c4 ,continuous ,lining of the ,bank4--,the lining or protection of a bank may be of stone or brick pitching 7figs4 #ab and #ac71 loose stone 7fig4 #ad71 fascines 7fig4 #ae71 turfing1 plantations1 brushwood1 or of other materials laid on the slopes4 ,before protecting a bank it is best to remove irregularities and bring it to a regular line4 ,this can generally be done most easily by filling in hollows1 but sometimes it is done by cutting off projections4 ,it is also necessary to make the side slope uniform4 ,where the slope is as shown by the dotted lines in figs4 #ab to #ad1 filling in can be effected1 but cutting away the upper part of the slope is also feasible4 ,such cutting away has been proposed as a remedy in itself in cases where the steep upper part of the slope was falling in1 but it is not much of a remedy4 ,stone pitching may rest1 if boats are required to come close to the bank1 on a toe wall of concrete1 as in fig4 #ac1i or otherwise on a foundation of loose stone1 as in fig4 #ab4 ,when concrete is used the bed is dredged to such a depth as will provide against undermining by scour4 ,sloping boards attached to piles are placed along the front face and the concrete is thrown in under water4 ,the slope of stone or brick pitching is usually from #b to #a to #a to #a1 but it may be as steep as to #a4 ,the earth behind the pitching must be well rammed in layers4 ,in order to prevent the earth from being eaten away by the water which penetrates through the interstices of the stone or brick1 a layer1 #c to #f inches thick1 of gravel or ballast is placed over the earth and rammed4 ,when loose stone is used1 dredging is not necessary1 but the stone is allowed to gradually sink down and more is added at the top4 ,a certain proportion of the stones should be of large size4 ,illustration3 ,,fig4 #ab4 ,illustration3 ,,fig4 #ac4 ,when fascining is used1 long twigs are made into bundles and tied up at every #b feet so as to form fascines about #d to #f inches thick1 and these are laid on the slopes and secured by pegs driven in at short intervals1 between the fascines4 ,illustration3 ,,fig4 #ad4 ,illustration3 ,,fig4 #ae4 ,sometimes the pitching or loose stone is not carried up to the top of the bank1 or even up to high flood-level1 and the bank above the pitching is protected by turfing--the pieces of turf being placed on edge normally to the slope if very steep 7fig4 #ad7 or laid parallel to the slope if it is not very steep--or1 above ordinary water-level1 by plantations of osiers or willows which obstruct the water and tend to cause silting1 and whose roots bind the banks together4 ,illustration3 ,,fig4 #af4 ,another method of using fascines is to lay them on the slopes with their lengths normal to the direction of the stream4 ,the upper end of a fascine is above low water1 and the lower end extends down to the bed of the stream4 ,sometimes large ropes made of straw1 or rough mats made of grass1 are laid on the slopes and pegged down1 or mattresses of fascines are laid on the slopes and weighted with stones4 ,illustration3 ,,fig4 #ag4 ,a deep recess in the bank 7fig4 #af7 can be filled in1 before the protection is added1 with earth well rammed4 ,on the ,adige the filling material consisted 7fig4 #ag7 of faggots filled with stones1 small cross dams being made at intervals1 as shown by the dotted lines1 to arrest flood water and cause it to deposit silt4 ,at the back of the berm1 poplar or willow slips were planted1 and these grew up and their roots held the bank together4 ,this system succeeded well4 ,a method of protection which is suitable when the water contains much silt is what is known in ,india as bushing4 ,large leafy branches of trees are cut and hung1 as shown in fig4 #ah1 by ropes to pegs4 ,they must be closely packed so as not to shake4 ,at first they require looking after1 but silt rapidly deposits and the branches become fixed and no longer dependent on the ropes4 ,if the work is carefully done1 the result is a smooth1 regular1 and tenacious berm1 as per dotted line in the figure4 ,illustration3 ,,fig4 #ah4 ,illustration3 ,,fig4 #ai4 ,another method1 used on canals1 is to make up the bank with earth and to revet it with twigs or reeds1 as shown in fig4 #ai4 ,the foundation must be taken down well below bed-level1 otherwise the work may slip4 ,this kind of work cannot be done except when the canal is dry4 ,if the bank consists of sand or of very sandy soil1 it must in any case have a flat side slope such as #c to #a4 ,if the sand is in layers alternating with firm soil1 it is a good plan to dig out some of the sand and to replace it with clods of hard earth4 ,staking 7fig4 #bj7 may be used1 the stakes being one or two feet apart from centre to centre1 and long twigs laid horizontally being passed in and out of the stakes1 or bushing filled in behind the stakes4 ,but bushing alone is cheaper and nearly as good4 ,illustration3 ,,fig4 #bj4 ,for protecting the banks of the ,indus it has been proposed 7,punjab ,rivers and ,works1 ,,chap4 ,,iv47 to use trees in exactly the same manner as bushing1 the trees being grown in several rows parallel to the river so that whenever the river1 by eroding its bank1 comes up to the lines of trees the first row will fall in4 ,the first row would be chained to the second1 which would take the place of the pegs used in bushing4 ,the other rows would remain as a reserve4 ,the ,villa system of bed protection 7,,chap4 ,v41 ,art4 #f7 has also been successfully used for bank protection on the ,scheldt1 and on the ,brussels-,ghent ,canal1 the prisms being about #aj #aj #d inches1 and having overlapping joints4 ,the bands of prisms are placed in position by a boat1 the bands unrolling over a drum4 ,the boat is provided with an oscillating platform carrying rollers at its end4 ,a thin layer of gravel is laid over the bank and is pressed down by the rollers before the prisms are laid on it 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cxxxiv41 and vol4 clxxv474 ,in the case of the river mentioned in ,,chap4 ,,xi41 ,art4 #c1 where extremely high velocities were met with1 cylindrical rolls of wire-netting were made1 each #ej feet long and #e feet in diameter1 and filled with boulders4 ,these rolls can be used for bank protection4 ,the netting was made by wires #f inches apart1 crossing each other at right angles and tied together at the crossings by short pieces of wire4 ,illustration3 ,,fig4 #ba4 ,on ship canals a berm 7fig4 #ba7 is frequently made a few feet below the water-level4 ,it serves as a foundation for the pitching1 which need not usually extend down to more than #e feet below the water-level4 ,below that the wash has little or no effect on the banks4 ,on ordinary navigation canals a similar berm is sometimes made--one or two feet in width and a foot or less below the water-level--and rushes are planted on it4 ,sometimes a bank has been protected by a kind of artificial weed1 consisting of bushes or branches of trees attached to ropes4 ,the end of the rope is fastened to the bank and the weeds float in the stream alongside the bank4 ,to protect a bank from ice1 which exercises an uplifting force on pitching1 use has been made of a covering of a kind of reinforced concrete consisting of slabs of concrete with wires embedded in it1 and fastened to the bank by wires1 #bj inches long1 running into the bank1 these wires being embedded in mortar so as to act like stakes4 ,illustration3 ,,fig4 #bb4 #d4 ,heavy ,stone ,pitching with ,apron4--,on the great shifting rivers of ,india a system of bank protection is adopted1 consisting of a pitched slope with an apron 7fig4 #bb74 ,the system is used chiefly in connection with railway bridges or weirs1 but it has been used in one instance1 that of ,dera ,ghazi ,khan1 for the protection of the bank near a town4 ,when1 as is usual1 the flood-level is higher than the river bank1 an artificial bank is made4 ,in any case the bank is properly aligned4 ,the pitching has a slope of #b to #a1 and consists of quarried blocks of stone loosely laid1 the largest blocks weighing perhaps #abj lbs4 ,the apron is laid at the time of low water on the sandbank or bed of the stream4 ,if necessary1 the ground is specially levelled for it4 ,it is intended to slip when scour occurs4 ,the following dimensions of the apron are given by ,spring 7,government of ,india ,technical ,paper1 ,no4 #aec1 8,river ,training and ,control on the ,guide ,bank ,system10 #aijd74 ,the probable maximum depth of scour can be calculated as explained in ,,chap4 ,,xi41 ,art4 #c4 ,if this depth1 measured from the toe of the slope pitching is ,d1 and if ,t is the thickness considered necessary for the slope pitching1 then the width of the apron should be #ae ,d1 and its thickness #abe ,t next the slope and #bh ,t next the river4 ,it will then be able to cover the scoured slope to a thickness of #abe ,t4 ,this thickness is made greater than ,t because the stone is not likely to slip quite regularly4 ,the thickness ,t should1 according to ,spring1 be #af inches to #eb inches1 being least with a slow current and a channel of coarse sand1 and greatest with a more rapid current and fine sand2 but since the sand is generally finer as the current is slower1 it would appear that a thickness of about #c feet would generally be suitable4 ,under the rough stone there should be smaller pieces or bricks4 ,along the top of the bank there is generally a line of rails so that stone from reserve stacks1 which are placed at intervals along the bank1 can be quickly brought to the spot in case the river anywhere damages the pitched slope4 ,for the special protection to banks required near weirs and similar works1 see ,,chap4 ,x41 ,arts4 #b and #c4 ,,chapter ,,vii ,,diversions ,,and ,,closures ,,of ,,streams #a4 ,diversions4--,when a stream is permanently diverted the new course is generally shorter than the old one1 and the diversion is then often called a cut-off4 ,the first result of a cut-off is a lowering of the water-level upstream and a tendency to scour there1 and to silt downstream of the cut-off4 ,fig4 #bc shows the longitudinal section of a stream after a cut-off ,a ,b has been made4 ,the bed tends to assume the position shown by the dotted line4 ,if both the diversion and the old channel are to remain open1 the water-level at the bifurcation will be lowered still more1 and the tendency to scour in the diversion will be reduced4 ,illustration3 ,,fig4 #bc4 ,if the material is soft enough to be scoured by the stream1 it is often practicable to excavate a diversion to a small section and to let it enlarge itself by scour4 ,this operation is immensely facilitated if the old channel can be closed at the bifurcation4 ,the question whether the scoured material will deposit in the channel downstream of the diversion must be taken into consideration2 also the question whether the diversion will continue to enlarge itself more than is desirable4 ,the velocity in the diversion will be a maximum if its section is of the 8best form10 i4e4 if its bed and sides are tangents to a semicircle whose diameter coincides with the water surface1 but this may not 7,,chap4 ,,iv41 ,art4 #f7 be the section which will give most scour4 ,in order to prevent the enlargement of the diversion taking place irregularly1 the excavation can be made as shown in fig4 #bd1 water being admitted only to the central gullet4 ,the side gullets should not be quite continuous1 but unexcavated portions should be left at intervals1 so that if the water in scouring out the channel breaks into the gullet1 it will not be able to flow along it until it has broken in all along4 ,illustration3 ,,fig4 #bd4 ,if a diversion is made1 not with the object of lowering the water-level but merely in order to shorten the channel1 the increased velocity caused by the steepened slope may be inconvenient4 ,in this case a weir or weirs can be added 7,,chap4 ,,viii41 ,art4 #d74 ,if the water contains sufficient silt to enable the abandoned loop to be silted up within a reasonable time1 it may be desirable to do this4 ,the silting up may1 for instance1 increase the value of the land4 ,the loop should be closed at its upper end4 ,water entering the lower end will cause a deposit there4 ,when the lower end is well obstructed by silt1 the upper end should be opened4 ,the set of the stream1 due1 for instance1 to a bend at the point where a diversion takes off has very little to do with the quantity of water which goes down the diversion4 ,the only effect of the set of the stream is a slight rise of the water-level as compared with the opposite bank4 ,similarly1 the angle at which the diversion takes off is only of importance in giving1 in some cases1 a velocity of approach whose effect is generally small4 ,the distribution of the water between the diversion and the old channel really depends on their relative discharging capacities4 ,if the required quantity of water does not flow down a diversion it can be dredged4 ,sometimes a long spur is run out to send the water towards the off-take of a diversion4 ,the effect of this is very small--it merely causes a set of the stream1--unless its length is so great that it amounts to something like a closure dam4 ,it is sometimes said that it is easier to lead a river than to drive it4 ,this remark is probably based on the fact that spurs1 such as those under consideration1 generally produce little effect1 whereas the excavation of a diversion or the deepening of a branch by dredging it1 is more likely to produce some result4 ,there is1 however1 no certainty about this4 ,sometimes too much is expected of such channels4 ,calculations are not always made as to the scouring power of the stream1 nor is account always taken of the fact that as the cut scours its gradient flattens4 #b4 ,closure of a ,flowing ,stream4--,the closure of a flowing stream by means of a dam is usually attended with some difficulty and sometimes with enormous difficulty4 ,there may be little trouble in running out dams from both banks for a certain distance1 but as soon as the gap between the dams becomes much less than the original width of the stream1 the water on the upstream side is headed up and there is a rush of water through the gap1 which tends to deeply scour the bed and to undermine the dams4 ,the smaller the gap becomes the greater is the rush and scour4 ,the closure is most easily effected at or near to the place where the stream bifurcates from another4 ,then1 as the gap decreases in width1 some of the water is driven down the other stream and it does not rise so much4 ,eventually all the water goes down the other stream1 and the total rise is only so much as will enable this other stream to carry the increased discharge4 ,if the closure is not effected near a bifurcation1 the rise of the water will go on even after the closure is completed1 and it will not cease1 unless the water escapes or breaks out somewhere1 until it has risen to the same level as that to which it would have risen if the closure had been at the bifurcation1 or perhaps not quite to the same level1 since there may still be a slight slope in the water surface and a small discharge which percolates through the dam4 ,sometimes in such a case it is possible to arrange for temporary escapes or bifurcations1 which will be shallow and therefore easily closed1 after the main closure has been completed4 ,a closure is1 of course1 far more easily effected where the bed is hard than where it is soft4 ,very often it is best to close temporarily at such a place or near a bifurcation1 even if the permanent dam has to be elsewhere1 and then to construct the permanent dam in the dry channel1 or in the still water1 and remove the temporary one or cut a gap in it4 ,generally the best method to adopt in a closure is to cover the bed of the channel beforehand--unless it is already hard enough--with a mattress or floor1 such that it cannot be scoured as the gap closes4 ,a floor may consist of a number of stones or sandbags dropped in from boats or by any suitable means1 and placed with care so that there shall not be gaps or mounds4 ,sandbags should be carefully sewn up4 ,a mattress may be made of fascines laid side by side and tied together1 floated into position1 weighted and sunk4 ,even a carpet made of matting or cloth and suitably weighted has sufficed in some cases4 ,if the scour is likely to be such that stones or sandbags will be carried away1 the stones may be placed in nets1 baskets1 or crates4 ,sandbags may also be placed in nets4 ,probably the long rolls of wire-netting filled with stones1 described in ,,chap4 ,,vi41 ,art4 #c1 would be better than anything1 and the diameter could be reduced somewhat4 ,the floor or mattress need not usually extend right across the stream4 ,it must cover a width much greater than--perhaps twice as great as--the width of the gap is likely to be when scour begins4 ,its length1 measured parallel to the direction of the stream1 must be such that severe eddies in the contracted stream will have ceased before the stream reaches its downstream edge4 ,it need not extend to any considerable distance upstream of the line of the dam4 ,the dams when started from the banks can generally be of simple earth or gravel1 or loose stones1 but before they have advanced far they will probably require protection at the ends by stones1 or by staking and brushwood1 or by fascines4 ,as soon as the dams have advanced well onto the mattress and their ends have been well protected1 it is best to cease contracting the stream from the sides and to contract it from the bottom by laying a number of sandbags across the gap so as to form a submerged weir4 ,in this way the rush of water is spread over a considerable width of the stream4 ,the weir is then raised until it comes up above water4 ,leakage can be stopped by throwing in earth1 or gravel1 or bundles of grass on the upstream side4 ,sometimes it is best to construct the mattress over the whole width of the stream1 and to effect the closure entirely by a weir1 carrying each layer right across before adding another4 ,the banks of the stream1 if not hard1 can be protected by sandbags1 stones1 staking or fascining4 ,the chief cause of failures of attempts to close flowing streams is neglect to provide a proper floor or mattress4 ,the stones or other materials may be of insufficient weight or not closely laid1 or the extent of the floor may be insufficient4 ,in a soft channel and deep water loose stones in almost any quantity may fail unless a mattress of fascines is laid under them4 ,another cause of failure is running short of materials1 such as sandbags4 ,allowance should be made for every contingency1 including making good any failure of parts of the work4 ,enormous sums of money have been wasted1 and vast inconvenience1 loss and trouble incurred1 in futile attempts to close breaches in banks1 or gaps in dams4 ,sometimes the gap is closed by sinking a barge loaded with stones1 or by sinking a 8cradle0 or large mattress made of fascines1 taken out to the site by four boats1 one supporting each corner1 and then loaded with stones and sunk4 ,another method is to run out a floating mattress of fascines from one side of the gap to the middle and sink it1 then to proceed similarly on the other side1 and so on4 ,an excellent plan1 when it can be adopted1 is to have more than one line of operations1 so that the heading up of the water is divided between them4 ,in ,india closures of streams having depths of #f or #h feet are effected by means of rough trestles made from trunks of small trees and placed at intervals in the stream like bridge piers1 one leg of the trestle inclined upstream and one downstream4 ,each pair of adjacent trestles is connected by a number of rough1 horizontal poles4 ,against these are placed bundles of brushwood4 ,earth is at the same time collected and is rapidly added at the last4 ,the chief danger is the undermining of the bed by scour4 ,this is prevented by driving in stakes and placing brushwood against them4 ,closures of small channels or of breaches in the banks of canals are effected by means of staking and brushwood4 ,where dangerous breaches are liable to occur1 it is a good plan to have a barge1 fitted up with a small pile-driver and carrying a supply of sheet piles1 ready at a convenient spot4 ,hurdle dykes1 first used on the ,mississippi1 were employed on the ,indus in #aijb to close partially the main channel of the river4 ,there were to be three dykes1 each dyke consisting of three lines of very long piles--some were #fj feet long1--driven into the bed of the stream1 which was to be protected with mattresses made of fascines and extending right across it1 with their heads above flood-level4 ,the idea was not to wholly stop the flow of the water1 but to obstruct it so much that silt would deposit1 the channel become choked up1 and the water find a course down another channel4 ,the work was begun in ,march #aijb1 and was in progress in ,may of the same year when an unusually early flood put a stop to it4 ,the dykes had at this time advanced considerable distances from the right bank of the stream1 but none had been completed4 ,two dykes out of the three were for the most part carried away4 ,the river1 however1 took a new course1 starting from a point far upstream1 the western channel became a creek1 and the remains of the dykes were soon embedded in silt4 ,in any case in which the provision of a proper mattress has been omitted1 or when the mattress has been destroyed1 or when a breach has occurred in an embankment1 whenever1 in short1 it is evident that the gap cannot be closed until some other escape for the water is provided1 it may be possible to provide such an escape by cutting partly through the dam or embankment on the downstream side at another place1 and thoroughly protecting the place and extending the protection downstream and away from the dam or embankment4 ,the water can then be let in1 and the closure of the old gap attempted4 ,if a closure is effected1 the protected gap can then be closed4 ,sometimes it may be desirable to make such a protected gap beforehand and with deliberation4 ,dams for closing streams which are dry can be made similarly to flood embankments 7,,chap4 ,,xii41 ,art4 #g74 ,sand does very well1 provided it is protected by a covering of clay or by fascining4 #c4 ,instances of ,closures of ,streams4--,in #aijd the ,colorado ,river broke into the ,salton ,sink--a valley covering #djjj square miles4 ,unsuccessful attempts were made to close the stream by two rows of piles with willows and sandbags between them1 by a gate #bjj feet long1 supported on #ejj piles1 and by twelve gates each #ab feet wide4 ,a 8rock-fill0 dam was then constructed on a mattress #ajj feet wide and #ae feet thick4 ,the river1 which was #fjj feet wide1 broke through1 but was stopped by the construction of three parallel rock-fill dams in the gap 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxi474 ,illustration3 ,,fig4 #be4 ,at the site of the railway bridge over the river ,tista in ,bengal1 it was necessary to close the main stream 7fig4 #be71 which flowed at the left side of the channel1 while the bridge had been built at the right4 ,the bed was of sand1 width #ejj feet1 depth #f feet1 and discharge #cgjj cubic feet per second4 ,the first attempt to close the stream was made at ,m ,n1 a floor of stone #bjj feet long1 #bj feet wide1 and #b feet thick1 being laid in the middle of the stream1 and dams of earth1 sandbags1 and stones being run out from each bank4 ,as the gap decreased in width the bed was torn up and the work failed4 ,the heading up was #c feet #i inches4 ,it was recognised later that the site should have been at the bifurcation higher up1 and that the stone floor should have been laid on a mattress4 ,in the next working season the dams ,c ,d and ,e ,f ,g were made4 ,the dam ,c ,d was of earth4 ,two walls1 each consisting of a double line of bamboos with the spaces between the lines filled with bundles of grass weighted with earth1 were run out #ej feet in advance of the earthwork near the lines of the toes of the slopes4 ,along the line of the upper wall a mattress of broken bricks #aj feet in width1 and #a foot thick1 was laid1 and was kept #ej feet in advance of the wall4 ,a total length of #ajjj feet of embankment was made in five months and pitched on its upstream side4 ,the end was strongly protected by a mass of stone4 ,the embankment ,f ,g was of earth4 ,the dam ,e ,f consisted of three lines of piles driven #aj feet into the bed4 ,a mattress weighted with stones extended for #bj feet upstream of the dam and #dj feet downstream4 ,a gap of #aej feet was left at ,d ,e1 and was not protected by a floor of any kind4 ,a channel1 parallel to ,f ,g and extending to ,k1 had been dug to a width of #bjj feet4 ,during the floods the heading up at ,d ,e was about #be feet1 and the water was #cj feet deep4 ,the line ,e ,f was greatly damaged and was repaired4 ,the cut ,f ,g ,k gradually enlarged1 and by the end of the floods more water was going down it than down the main stream4 ,the gap ,d ,e was finally closed by means of a line of bamboos and grass1 the bed being protected by a carpet1 #ajj #ej feet1 made of common cloth weighted with sandbags4 ,the success of the operations turned on the scouring out of the cut ,f ,g ,k4 ,it is remarkable that the gap ,d ,e did not become wholly unmanageable in the floods 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cl474 ,,chapter ,,viii ,,the ,,training ,,and ,,canalisation ,,of ,,rivers #a4 ,preliminary ,remarks4--,when a stream is trained or regularised it is generally made narrower1 but sometimes narrow places have to be widened4 ,deepening has also very frequently to be effected4 ,the object of training is generally the improvement of navigation1 but it may be the prevention of silt deposit4 ,some natural arms of rivers which form the head reaches of canals in the ,punjab are wide and tortuous1 and they are sometimes trained4 ,training often includes straightening or the cutting-off of bends1 as to which reference may be made to ,,chap4 ,,vii4 #b4 ,dredging and ,excavating4--,when a flowing stream is to be deepened1 the work is usually done by dredgers4 ,dredgers can remove mud1 sand1 clay1 boulders1 or broken pieces of rock4 ,the 8bucket ladder0 dredger is the commonest type4 ,the 8dipper0 dredger is another4 ,both these can work in depths of water ranging up to #ce feet4 ,the 8grab bucket0 dredger can work up to any depth and in a confined space4 ,the 8suction dredger0 drawls up mud or sand mixed with water4 ,a dredger may be fitted with a hopper or movable bottom1 by means of which it can discharge the dredged material--this1 however1 involves cessation of work while the dredger makes a journey to the place where the material is to be deposited--or it can discharge into hopper barges or directly on to the shore by means of long shoots4 ,for small works in comparatively shallow water the 8bag and spoon0 dredger1 worked by two men1 can be used4 ,when rock has to be removed under water it is blasted or broken up by the blows of heavy rams provided with steel-pointed cutters4 ,in widening a channel the excavation can be carried down in the ordinary way to below the water-level1 a narrow piece of earth1 like a wall1 being left to keep the water out4 ,if the channel cannot be laid dry1 the work can be finished by dredging4 ,regarding methods by which the stream is itself made to deepen or widen its channel1 reference may be made to ,,chap4 ,v4 #c4 ,reduction of ,width4--,if a channel which is to be narrowed is not a wide one1 the reduction in width can be effected by any of the processes described under bank protection 7,,chap4 ,,vi474 ,but in a wide channel1 reduction of the width by any direct process is generally impracticable4 ,the expense would generally be prohibitive4 ,earth1 if filled in1 is liable to be washed away unless protected all along4 ,reduction in the width of a large channel is nearly always effected either by groynes 7fig4 #bf7 or by training walls 7fig4 #bg74 ,spurs or short groynes for bank protection have been already described 7,,chap4 ,,vi41 ,art4 #b74 ,groynes for narrowing streams are made in the same way and of the same materials1 but are longer4 ,they are at right angles to the stream or nearly so4 ,groynes in the river ,sutlej have been mentioned in ,,chap4 ,v41 ,art4 #e1 and are shown in fig4 #f1 p4 #ee4 ,whether groynes or training walls are used1 the object is to confine the stream to a definite zone and to silt up the spaces at the sides4 ,these spaces when partly silted can be planted with osiers or with anything which will grow when partly submerged1 and this will assist in completing the silting4 ,illustration3 ,,fig4 #bf4 ,illustration3 ,,fig4 #bg4 ,a training wall can be made of any of the materials used for groynes4 ,in order to silt up the spaces between each wall and the adjacent bank of the stream1 other walls are run at intervals across them4 ,usually the training walls and cross walls are carried up only to ordinary water-level1 sometimes only to low-water level4 ,floods can thus spread out and submerge the walls and deposit silt4 ,if the walls are carried up too high it may be necessary1 in order to give room for floods1 to space them too far apart1 and this1 as will be seen below1 is objectionable4 ,the difference between training walls and groynes is one of degree rather than one of kind4 ,the material most commonly used is1 in either case1 loose stone--with pitching1 if desired1 above low-water level1--but it may be wattled stakes4 ,if the water of the stream contains silt at all stages of the supply1 gaps can be left in training walls so that silt deposit may occur at all times and not only in floods4 ,if the walls are of wattled stakes1 water will pass through them1 and it may not be necessary to leave any gaps4 ,groynes are frequently made with ,t-heads 7fig4 #bf71 and they are thus equivalent to training walls with long gaps in them4 ,the edge of the narrowed channel usually forms somewhat as shown in the figure4 ,if the groynes are placed so near together as to give a regular channel1 the cost is not likely to be much less than that of training walls4 ,the alignment of training walls or groynes should be such as will give the best channel consistent with economy in cost4 ,the best channel is generally that which is most free from sharp bends4 ,it is assumed for the present that no cuts or diversions of such lengths as to materially alter the gradient are to be made1 but that a certain amount of choice of alignment is afforded by the reduced width of the trained channel and by small diversions or easings of bends4 ,it is sometimes said that straight reaches are objectionable because the stream will tend to wander from side to side and cause shoals1 whereas in a bend there will be no such tendency4 ,the difficulty as to shoaling will be greatest at low water1 but it is likely to be serious only when the width between the training walls is too great4 ,if the width cannot be reduced to such an extent as to do away with the trouble1 it may be better to adopt a curved course4 ,the width between the training walls should generally be the same throughout1 whether the reaches are straight or curved1 but in view of the preceding remarks it may be desirable1 where a reach cannot be otherwise than straight and where shoaling is feared1 to give the straight portion a reduced width with of course a greater depth1 and similarly to reduce the width at reverse changes of curvature4 ,in curves which are at all sharp the curvature should be rather sharper in the middle of the curve than at the ends 7,,chap4 ,,iv41 ,art4 #h74 #d4 ,alteration of ,depth or ,water-,level4--,when the width of a stream is altered1 the depth of water--the gradient being supposed to be unchanged--must alter in the opposite manner4 ,a narrowing of the channel by training necessitates an increase in the depth of water1 and the same remark applies if an arm of the stream is closed4 ,the increase in depth may be effected either by raising the water-level or by lowering the bed--as may be convenient--or both4 ,if the bed is to be lowered and is of hard clay1 it may be necessary to dredge it and1 when this has been done1 training may be unnecessary4 ,if the bed is of soft mud1 a dredged channel is likely to fill up again1 and training alone will be the method to adopt4 ,if the bed is moderately hard1 say compact sand1 it may be suitable to train the channel first and then to dredge if necessary4 ,in any case1 shoals of hard material may have to be dredged or rocks1 whether these form shoals or lateral obstructions1 to be blasted or otherwise broken up 7,art4 #b74 ,in cases where it is desired to raise the water-level without any lowering of the bed1 training is of course necessary4 ,in any case in which the bed is likely to scour to a lower level than is desired1 or if the bed is to be raised1 the measures described in ,,chap4 ,v41 ,art4 #f1 may be adopted1 but they are hardly likely to be suitable and satisfactory in all cases4 #e4 ,training and ,canalising4--,the steps so far described1 together with any of those described in ,,chaps4 ,v4 and ,,vi41 exhaust the list of what can be done so long as only the cross-section of a stream is dealt with4 ,this is often called the 8regulation0 of a stream1 though 8training0 is a more satisfactory term4#aj ,illustration3 ,,fig4 #bh4 ,a mere alteration of the cross-section of a stream will not always afford a solution of the problem to be solved4 ,frequently a change of gradient is required4 ,the gradient can be steepened by means of straightenings1 or flattened by introducing weirs1 or perhaps by adopting a course somewhat more circuitous than was intended4 ,this extended scope of operations is known as canalising in the case of a river1 and remodelling in the case of a canal4 ,suppose that it is desired to alter the cross-section of a stream1 at ordinary water-level1 so as to reduce the width and increase the depth 7fig4 #bh74 ,if the mean depth is doubled1 the new width will be about equal to #a_/7cb7 of the old width 7,hydraulics1 ,,chap4 ,,vi41 ,art4 #b74 ,if this gives too narrow a channel1 it may be desirable to flatten the gradient4 ,if it gives too wide a channel1 the gradient can be steepened or a greater depth adopted4 ,while the width and depth of the stream will be fixed so as to be suitable for the navigation1 the ratio of depth to velocity should be so arranged1 if this is possible1 as to minimise trouble connected with silting or scour 7,,chap4 ,,iv41 ,art4 #f74 ,a remodelled channel is1 in short1 designed in exactly the same way as a new channel4 ,the depth of water exercises the greatest effect on the discharge1 and the gradient the least4 ,the weak point in a scheme which includes weirs is the difficulty of dealing with floods4 ,a scheme perfect in all other respects may be vitiated because of the obstruction1 caused by weirs1 to the passage of floods4 ,the difficulty is got over by means of movable weirs4 ,the whole subject of weirs is dealt with in ,,chap4 ,x4 ,training or canalising should not be effected in any reach of a stream without regard to other reaches4 ,a mere local lowering of the water-level by dredging may accentuate the effect of a shoal at the upper end of the reach4 ,when the water-level is raised by a weir or by narrowing the channel--though in the latter case the raising may not be permanent--it is generally best to commence the work from the upstream end4 ,the raising of the water-level will then not interfere with the execution of the rest of the work4 ,but in a case of widening1 where the water-level upstream of the work is lowered1 the work can conveniently be begun at the downstream end1 and the remark applies also to a case of straightening1 provided that the new channel is not so small that it at first causes no lowering4 ,in any case in which there is a doubt whether the whole of the scheme will be carried out1 the reach to be dealt with first can be decided on according to circumstances4 ,there is no general reason for selecting an upstream or downstream reach1 except that any raising or lowering of the water-level will extend upstream of the reach and not downstream of it 7,,chap4 ,i41 ,art4 #d74 ,training walls and groynes1 if made with stakes or fascines or any materials except stone1 require careful watching and maintenance4 ,,chapter ,,ix ,,canals ,,and ,,conduits #a4 ,banks4--,all banks which have to hold up water should be carefully made4 ,the earth should be deposited in layers and all clods broken up4 ,in high banks the layers should be moistened and rammed4 ,the dotted lines in fig4 #bi show two possible courses of percolation water4 ,the vertical height--from the water-level to the ground outside the bank1--divided by the length of the line of percolation is the hydraulic gradient1 as in the case of a pipe1 and this gradient is more or less a measure of the tendency to leakage4 ,a bank which has water constantly against it nearly always becomes almost water-tight in time4 ,the time is less or greater according as the soil is better1 and according to the amount of care with which the bank is made4 ,illustration3 ,,fig4 #bi4 ,the side slopes of banks vary with the soil4 ,generally they are #a to #a1 but they are sometimes #b to #a or even #c to #a if the soil is bad or sandy1 or if great precautions against breaches are to be taken4 ,leakage can sometimes be stopped by throwing chaff or other finely divided substances into the water at the site of the leak4 ,in other cases it is necessary to dig up part of the bank1 find the channel by which the water is escaping1 and fill it up by adding earth and ramming4 ,on some navigable canals in ,france it was at one time the custom to lay the reach dry1 when a bad leak occurred1 and to dig away the bank and lay slabs of concrete or puddle over the place4 ,this plan was abandoned1 and instead of it sheet piles are driven in4 ,they are then withdrawn one at a time and1 if any leakage occurs1 the space is filled with concrete4 ,the dimensions of a bank should depend to some extent on the head of water against it and on the volume of the stream whose water it holds up4 ,a breach is obviously more serious the greater the volume of the escaping water4 ,this volume depends on the size of the stream and on its velocity4 ,in navigation canals in ,england the bank on the side opposite the towing-path is usually #d to #f feet wide and #a feet above the water4 ,in irrigation canals in ,india the bank of a very large canal is #b feet above the water and #bj feet wide1 while that of a small canal with #f feet of water is #h or #aj feet wide and #a feet above the water1 and that of a small distributary channel with #c feet of water is #d feet wide and #a foot above the water4 ,the soil is often poor4 ,further remarks1 which apply to banks of special height or special importance1 are given under ,embankments 7,,chap4 ,,xii41 ,art4 #f74 #b4 ,navigation ,canals4--,a navigation canal is sometimes all on one level1 but generally different reaches are at different levels1 the change being made by means of locks4 ,a 8lateral0 canal--the most common kind--runs along a river valley more or less parallel to the river4 ,it is frequently cheaper to construct such a canal than to canalise the river4 ,a 8summit0 canal crosses over a ridge and connects two valleys4 ,a navigation canal requires a supply of water to make good the losses which occur by lockage1 leakage1 or absorption and evaporation4 ,a canal may be of any size1 according to the size of the boats which are to be used4 ,there is always room1 except in short reaches where the expense of construction has to be kept down1 for two boats to pass one another4 ,a lateral canal obtains water from the river or from the small affluents which it crosses4 ,for a summit canal it may be necessary to provide storage reservoirs4 ,the canal crosses the ridge where it is low1 and the reservoirs are made on higher ground4 ,reservoirs may be required also for other canals to hold water for use in dry seasons or in order to fill the canal quickly when laid dry for repairs4 ,in tropical countries weeds grow profusely in canals which have still or nearly still water4 ,traffic tends to keep them down1 but they have to be cleared periodically4 ,in designing a barge canal the chief considerations generally are that it shall not be in such low ground or so near a river as to be liable to damage by floods1 that it shall not traverse very permeable soil or gravel--this is often found near a river1--that the material excavated shall be as nearly as possible equal to that required1 at the same place1 for embankment1 and that as far as possible high embankments1 which are very expensive to construct and are more or less a source of danger1 shall be avoided4 ,the side slopes of the banks of a navigation canal depend on the nature of the soil4 ,they are generally #a to #a1 but the inner slope may be #b to #a4 ,the banks are generally #a or #b feet above the water-level1 the width of the bank on the towing-path side ranging from #h to #af feet1 but being generally #ab feet and the width of the other bank #d to #f feet4 ,the width of a canal is made sufficient for two boats to pass1 and the depth is #a to #b feet greater than the draught of the boats used4 ,in some cases the banks are protected by pitching for short lengths1 but generally they are merely turfed4 ,the sides near the water surface wear away1 so that the side slope becomes steeper in the upper part and flatter in the lower part4 ,the resistance of a boat to traction in a canal is given by the formula ,r r7#hdf7_/7b 7,a_/a771 where r is the resistance in a large body of water and ,a and a are the areas of the cross-sections of the canal and of the immersed part of the boat4 ,when ,a is six times a1 ,r is only #f per cent4 more than r4 ,in practice ,a is never less than six times a4 ,regarding methods of protecting banks1 see ,,chap4 ,,vi4 ,a ship canal is a barge canal on a large scale4 ,the speed of ships has to be strictly limited to avoid damage to the banks4 ,the ,manchester ,ship ,canal takes in the waters of the ,irwell and the ,mersey1 and conveys them for several miles4 ,it is thus a canalised river for part of its course4 ,below that it is a tidal stream1 the tide being admitted at its lower end where it joins the estuary of the ,mersey1 and passing out higher up where it leaves the estuary after skirting it4 ,this circulation of water is beneficial to the estuary4 ,the ,panama ,canal might have been constructed at one level1 but the cost of this1 and the time occupied1 would have been double that of making it a summit canal4 ,the water of the river ,chagres is to be impounded to form a lake of great extent that will not only supply water for lockage but will itself form part of the high-level reach of the canal1 and ships will be able to traverse it at greater speed than in the rest of the canal4 ,some ,indian irrigation canals have been constructed so as to be navigable4 ,the increase in cost has usually been enormously in excess of any resulting benefits4 #c4 ,locks4--,an ordinary lock is shown in fig4 #bi;,a4 ,the space above the head gates is called the 8head bay10 and that below the tail gates the 8tail bay40 ,the floor of the lock is often an inverted arch4 ,sometimes the floor is of cast-iron4 ,the 8lift wall0 is generally a horizontal arch4 ,the gates when closed press at their lower ends against the 8mitre sills02 and the vertical 8mitre posts0 at the edges of the gates meet and are pressed together4 ,the gate1 in opening and closing1 revolves above the cylindrical 8heel post0--which stands in the 8hollow quoin0 of the lock wall--and when fully open is contained in the 8gate recess40 ,a lock is always strongly built1 of masonry or concrete4 ,the walls have to withstand the earth pressure when the lock is laid dry for repairs4 ,the floor has to withstand the scouring action from the sluices4 ,regarding the upward pressure of the water when the lock is empty1 see ,,chap4 ,x41 ,art4 #c4 ,the lift or difference in the water-levels of the two reaches of a barge canal is generally from #d to #i feet1 but occasionally it is much more4 ,illustration3 ,,fig4 #bi;,a4 ,the gates of small locks are generally of wood and are counterbalanced4 ,those of large locks are of wood or steel1 and the weight is generally taken by rollers4 ,ordinary wood should not be used if the ,teredo navalis exists in the waters4 ,an iron gate1 if enclosed on all sides by plating1 is buoyant1 and the rollers and anchor straps which hold the upper ends of the heel posts are thus relieved of much weight4 ,the gates of the ,panama ,canal locks are #aaj feet long and #g feet thick1 and the height ranges from #dh feet to #hb feet4 ,the sluices for filling and emptying a lock are placed in the gates or in the walls4 ,the gates and sluices are generally worked by hydraulic power or by electricity4 ,locks are frequently arranged in flights4 ,there are1 in a few instances1 #bj to #cj locks in a flight1 the total lift being #aej to #bjj feet4 ,by this means the number of gates is reduced1 the tail gates of one lock being the head gates of the rest1 and there is a saving in labour in working the locks4 ,let ,l be the volume of water contained in a lock between the levels of the upper and lower reaches1 and let ,b be the submerged volume of a boat4 ,the 8lockage0 or volume of water withdrawn from the upper reach of the canal is shown in the following statement3-- ---------------------------------------- ----------------------------------- ,lockage4 ,reference ,number ,direction ,lock or ,lock or ------------------------ ,number of ,boats4 of ,travel4 ,locks ,locks ,single ,flight of of ,case4 ,found4 ,left4 ,lock4 m ,locks4 ---------------------------------------- ---------------------------------- #a #a ,down4 ,empty4 ,empty4 ,l - ,b ,l - ,b #b #a 0 ,full4 0 - ,b - ,b #c #a ,up4 ,empty4 ,full4 ,l ,b m,l ,b #d #a 0 ,full4 0 ,l ,b ,l ,b #e #bn ,up and ,going ,going n,l mn,l down down1 full4 down1 alternately4 ,going up1 empty4 empty4 ,going up1 full4 #f n ,down4 ,empty4 ,empty4 n,l-n,b n,l - n,b #g n 0 ,full4 0 7n-#a7,l 7n - #a7,l - n,b -n,b #h n ,up4 ,empty4 ,full4 n,ln,b 7mn-#a7,l n,b #i n 0 ,full4 0 n,ln,b n,l n,b #aj n ,down4 n ,up4 0 0 7#bn-#a7,l 7m#bn-#b7,l ---------------------------------------- ---------------------------------- ,in the case of a single lock1 if two boats are to pass through1 one descending and one ascending 7cases #b and #c71 the descending boat would be passed through first if the lock were full1 and the ascending boat first if empty2 in either case1 the total lockage is ,l1 or ,l_/#b for each boat4 ,this also appears from case #e4 ,cases #f to #aj show that if a long train of boats descends1 even though the lock is full for the first boat or if a long train ascends even the lock is empty for the first boat1 the total lockage is nearly ,l per boat4 ,thus in a single lock1 boats should pass up and down alternately so far as this may be possible4 ,in the case of a flight of m locks1 a single boat in descending uses no more water than if there were only one lock1 the same water passing from lock to lock1 but in ascending it uses more4 ,in the case of a number 7#bn7 of boats going up and down alternately 7case #e71 the lockage is m n ,l1 the lockage per lock per boat being ,l_/#b1 but in the case of a long train of boats descending followed by an equal train ascending 7cases #g and #h71 the lockage is less4 ,if n is supposed to be equal to m1 the average lockage per boat is as follows3-- m #a #b #c #d #e #f ,infinity ,lockage ,l_/#b ,l #g,l_/#f #e,l_/#d #ac,l_/#aj #d,l_/#c #c,l_/#b per boat ,thus in a case where n and m are very large1 the average lockage per boat1 when the boats pass up and down in trains1 is to the lockage per boat1 when the single boats pass up and down alternately through m single locks all at different places1 as #c is to m4 ,the reason for the difference1 which may appear puzzling1 is that when the locks are at different places they are worked independently of one another4 ,sometimes a lock is provided with intermediate gates which provide a short lock for short vessels4 ,in the ,manchester ,ship ,canal1 alongside each lock there is another of smaller size to be used for small vessels and thus save lockage4 ,at the ,eastham lock1 where the ,manchester ,ship ,canal descends into the estuary of the ,mersey1 there is1 below the tail gates1 an extra pair of gates opening towards the estuary1 so that the lock can be worked when the water of the estuary is higher than that in the canal4 ,water can be economised by means of a 8side-pond10 into which the upper portion of the water from a lock can be discharged and utilised again when the lock has to be filled4 ,if two locks are built side by side1 each acts as a side-pond to the other4 ,two flights of locks can be built side by side4 ,sometimes instead of a lock there is an inclined plane1 up or down which are drawn on rails caissons containing water in which the boats float4 ,the rails extend below the water-levels of the two reaches1 and the caissons can thus be run under the boats4 8,lifts0 have also been constructed by which the boats can be lifted bodily and swung over from one reach to the other4 #d4 ,other ,artificial ,channels4--,the method of calculating the discharges of channels in which water is to flow is a question of hydraulics4 ,the principles and rules to be followed1 in the design of earthen channels1 have been stated in ,,chap4 ,,iv41 ,art4 #f1 and in ,,chap4 ,,viii41 ,art4 #e4 ,the design of banks has been dealt with in ,art4 #a of this ,chapter4 ,for conveying water for the supply of towns1 or for other purposes1 masonry conduits are often used4 ,a usual form is shown in fig4 #cj4 ,the curving of the profile of the cross-section gives an increased sectional area and hydraulic radius1 and hence an increased discharge4 ,illustration3 ,,fig4 #cj4 ,,chapter ,x ,,weirs ,,and ,,sluices #a4 ,preliminary ,remarks4--,every structure which interferes at all with a stream causes an abrupt change in the stream 7,,chap4 ,,iv41 ,art4 #a74 ,at an abrupt change there are always eddies1 and these have a peculiar scouring effect4 ,this effect is greatest where the velocity of the stream is abruptly reduced as where1 for instance1 after being contracted by an obstruction1 it expands again or where it falls over a weir or issues from a sluice opening4 ,in all cases of this kind the protection of the structure from scour is of primary importance4 ,the site of a weir or other permanent structure should1 if the stream is unstable1 be in a fairly straight reach1 or at least not be immediately downstream of a bend4 ,this is because of the tendency of bends to shift downstream 7,,chap4 ,,iv41 ,art4 #h74 ,there is no particular advantage in selecting a narrow place4 ,a narrow place is likely to be deep or it may be liable to widen4 ,in a hard and stable stream there is no restriction as to site4 ,weirs are frequently constructed for purposes of navigation1 as mentioned in ,,chap4 ,,viii4 ,they are also used in streams which are not navigable in order that the gradient may not be too steep1 and in irrigation canals for the same reason4 ,they are used both in rivers and canals in order that the water-level may be raised and water drawn off by branch channels for purposes of manufactures1 water-power or irrigation4 ,upstream of a weir there is more or less tendency for silt to deposit1 but it by no means follows that there will be deposit 7,,chap4 ,,iv41 ,art4 #b1 last par41 and ,art4 #c1 last par474 ,when deposit of sand or mud is feared1 small horizontal passages1 known as 8weep holes10 may be left in the weir at the level of the upstream bed4 ,in the old ,nile barrages iron gratings were provided1 but they were needlessly large4 ,illustration3 ,,fig4 #ca4 ,an inherent defect of an ordinary weir is that it obstructs the passage of floods4 ,the obstruction may or may not be of consequence4 ,sometimes it is of great consequence4 ,attempts have been made to partially remedy the evil by placing the weir obliquely to the stream1 thus giving it a greater length4 ,at ordinary water-levels the flow over the crest of the weir is normal to its length1 or nearly so4 ,supposing that the water has to be held up to a given level1 the crest of the weir must be higher1 because of its greater length1 than if it were normal to the stream4 ,in a flood the water has a high velocity and flows over the weir in a direction nearly parallel to the axis of the stream1 so that the effective length of the weir is not much greater than if it were normal to the stream1 and1 its crest being higher1 it obstructs the flood as much4 ,oblique weirs are usually made as in fig4 #ca4 ,if made in one straight line1 there might be excessive action on the bank at the lower end4 ,if the weir is lengthened1 not by being built obliquely but by a widening of the stream at the site1 the crest has to be raised and nothing is gained4 ,the only arrangement by which a weir can be made to hold up water when a stream is low and to let floods pass freely1 consists in having part of the weir movable1 i4e4 consisting of gates1 shutters or horizontal or vertical timbers1 which can be withdrawn to let floods pass1 and can be manipulated to any extent so as to regulate the amount of water passing4 ,a familiar instance of a movable weir is the one which is usually placed across a mill stream1 the wooden gates working in grooves in the masonry4 ,above a weir in ,java1 #afb feet long1 there was a great accumulation of shingle in the bed of the river1 and the head of a canal taking off above the weir became choked4 ,the crest of the weir on the side away from the canal was raised #e feet and the crest sloped gradually down1 a length of #dc feet on the side next the canal remaining as it was4 ,this was quite successful4 ,it was practically a contraction of the river near the canal off-take1 and this must have caused scour1 so that the bed became lower than the floor of the canal head and the shingle was not carried in4 ,the shingle1 however1 is said to have been carried over the weir 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxv474 ,a lock is an adjunct to a weir1 used when navigation has to be provided for4 ,the lock may be placed close to the weir or it may be in a side channel1 the upstream end of the lock being about in a line with the weir4 ,locks have already been discussed in ,,chap4 ,,ix41 ,art4 #c4 ,frequently a 8salmon ladder0 has to be provided4 ,it consists of a series of steps or a zigzag arrangement so that the velocity of the water is not too great for the fish to ascend4 #b4 ,general ,design of a ,weir4--,unless the bed and sides of the channel are of rock1 a weir has side walls and rests on a strong floor or 8apron40 ,these need not extend far upstream1 but must extend some way downstream because of the scouring action of the water4#aa ,a common type of weir is shown in fig4 #cb4 ,the downstream face is made sloping1 so that the water may not fall vertically and strike the floor below the weir4 ,the thickness and length of the floor depend on the volume of water to be passed and on the height which it will fall and on the nature of the soil1 and are generally matters of judgment1 though rules regarding them1 applicable to certain special cases1 are given in the next article4 ,illustration3 ,,fig4 #cb4 ,the upper corners of the weir should be rounded4 ,this prevents their being worn away2 but the rounding of the upstream corner has another advantage4 ,if the corner is sharp1 the stream springs clear from it and the weir holds up the water higher1 especially in floods4 ,with small depths of water the difference is less1 and it vanishes when there is only a trickle of water4 ,thus a crest rounded on the upstream side holds up low-water nearly as well as a sharp-edged crest1 but lets floods pass more freely4 ,any batter given to the upstream face has a similar advantage4 ,the rounding is of more importance as the batter is less4 ,for similar reasons1 the upstream wing walls should be splayed or even curved so as to be tangential to the side wall1 and not built normally to the stream4 ,these advantages are sometimes lost sight of4 ,the downstream walls are splayed to reduce the swirl4 ,the body of the weir may be of rubble and the face-work of dressed stone4 ,in large weirs the stones are sometimes dowelled together4 ,where1 as in many parts of ,india1 stone is expensive1 brick is used for small weirs1 the crest and faces being brick on edge4 ,downstream of the floor1 unless the channel is of very hard material1 there is paving or pitching of the bed and pitching of the sides1 and these may terminate in a curtain wall4 ,the bank pitching may be of any of the kinds described in ,,chap4 ,,vi41 ,art4 #c1 and the bed paving as described in ,,chap4 ,v41 ,art4 #f1 but downstream of a weir the eddying is continuous and the lap of the water on the bank is ceaseless1 and good methods are necessary4 ,sometimes planking1 laid over a wooden framing or attached to piles1 is used instead of paving and pitching4 ,in case the height of a weir is great relatively to its thickness1 the danger of its being overturned must be considered4 ,to be safe against overturning1 the resultant of the pressure on the weir must pass through the middle third of its base 7see fig4 #fb1 ,,chap4 ,,xiii474 ,illustration3 ,,fig4 #cc4 #c4 ,weirs on ,sandy or ,porous ,soil4--,if the channel is very soft or sandy the weir may be built on one or more lines of wells4 ,the wells are not so much to support the weir as to form a curtain and prevent streams1 due to the hydraulic gradient ,a ,e 7fig4 #cc71 from forming under the structure and gradually removing the soil4 ,it is assumed in the case represented by the figure that the maximum head occurs when the downstream channel is dry4 ,any removal of soil from under the weir may cause its destruction4 ,the wells should be as close together as possible1 and the spaces between them carefully filled up with brickwork or concrete to as great a depth as possible1 and below that by piles4 ,instead of wells1 lines of sheet piling--cast-iron or wood--can be used4 ,a good fit should be made1 but it is not necessary that the joints should be absolutely water-tight4 ,the object is to flatten the hydraulic gradient by increasing the length travelled by the water from ,b ,e to ,b ,l ,g ,h ,e4 ,of course1 no flattening occurs at a point where the curtain is not water-tight1 but if only small interstices exist1 none but small trickles of water can pass1 and the interstices will probably soon be choked up1 just as the sand in a filter bed becomes clogged and has to be washed4 ,in any case1 no important stream could develop otherwise than round the toe of the curtain4 ,it has been stated that when a curtain is water-tight the water follows the line ,b ,l ,m ,g ,h ,k ,e1 but this requires proof4 ,another plan is to cover the bed and sides of the channel with a continuous sheet of concrete extending upstream of the weir from ,b to ,d--thus flattening the hydraulic gradient from ,a ,e to ,f ,e4 ,instead of concrete1 clay puddle can be used with pitching over it4 ,the choice between the different methods depends largely on questions of cost and facility of construction4 ,it has been said that a certain amount of leakage occurs under structures such as the ,okla weir 7,art4 #d71 which nevertheless remains undamaged4 ,there have1 however1 been cases in which failures of works have occurred1 especially when there has been a great difference between the water-levels of the upstream and downstream reaches1 from no other apparent cause than the passage of water underneath the works4 ,weirs in porous soils have been discussed by ,bligh 7,engineering ,news1 #bith ,december #aiaj71 who gives the following as safe hydraulic gradients 7s7 or ratio of the greatest head ,a ,b to the length ,b ,e3-- ,fine silt and sand as in the ,nile #a in #ah ,fine micaceous sand as in ,colorado and ,himalayan rivers #a in #ae ,ordinary coarse sand #a in #ab ,gravel and sand #a in #i ,boulders1 gravel and sand #a in #d to #a in #f ,these figures are probably quite safe enough even for the most important works and for those where the heading up is constant4 ,for small works or for regulators 7,art4 #e7 where the heading up is not constant1 steeper gradients are permissible4 ,much also depends on the condition of the water4 ,if it contains much silt1 all interstices will probably become choked up4 ,the hydraulic gradient in the case of the ,narora weir across the ,ganges was #a in #aa4 ,the weir failed after working for twenty years4 ,it was rebuilt with a gradient of #a in #af4 ,in the ,zifta and ,assiut regulators on the ,nile the gradients are #a in #afd and #a in #ba4 ,illustration3 ,,narora ,,weir ,,as ,,originally ,,built4 ,illustration3 ,,narora ,,weir ,,as ,,reconstructed4 ,illustration3 ,,foundation ,,of ,,the ,,zifta ,,regulator1 ,,river ,,nile4 ,regarding the upward pressure on the floor due to the hydrostatic pressure from the head ,a ,b1 there is a theory that the weight of a portion of the floor at any point ,p should be able to balance the pressure due to a head of water ,p ,r4 ,this1 supposing the masonry to be twice as heavy as water1 would give a thickness of floor equal to half ,p ,r4 ,according to ,bligh1 the theoretical thickness ought1 for safety1 to be increased by one-third4 ,practically the thickness need not1 in most cases1 be made even so great as is given by the theoretical rule4 ,on canals in the ,punjab it is certainly less4 ,water passing through soil or fine sand does not exert anything like the pressure which it exerts when passing through a pipe4 ,it acts in the same manner as in a capillary tube4 ,it is only in coarse sand or gravel or boulders that water flows as in a pipe4#ab ,if the tail water covers the floor1 the weight of a portion of floor is reduced by the weight of an equal volume of water4 ,if the foundation of any part of the floor is higher than ,b ,e1 the upward pressure on it is reduced because the water has to force its way upwards through the soil4 ,bligh also states as an empirical rule that in order to provide efficiently against scour the length of floor ,b ,e should be #d_/s 7,h_/#ac71 where ,h is the maximum head ,a ,b2 and he points out that in a case where this length is less--as it usually is--than that necessary to give a hydraulic gradient of the requisite flatness1 according to the rule previously quoted1 it is better to add an upstream floor ,b ,d1 which may be of puddle and therefore cheap1 than to add to the downstream floor a length ,e ,c which must be of masonry or concrete1 and that this arrangement1 by shifting the line of hydraulic gradient from ,a ,e to ,f ,e1 gives a reduced upward pressure on the downstream floor4 ,the length ,e ,n to which pitching1 if of 8rip-rap0 type1 should extend is given by ,bligh as #aj_/s 7,h_/#aj7 7q_/#ge71 where q is the maximum discharge in cubic feet per second passing over a #a-;foot length of the weir1 and ,h is the head ,a ,b4 #d4 ,various ,types of ,weirs4--,the type of weir shown in fig4 #cb may be varied by steepening or flattening the slopes of one or both faces4 ,flattening increases the cost but gives a greater spread for the foundations4 ,it may1 however1 be combined with a decrease in the width of the crest4 ,flattening of the downstream slope reduces the shock of the water on the floor1 but the slope itself1 especially the lower portion1 has to stand a good deal of wear1 and the length exposed to this is increased4 ,flattening the upstream slope facilitates the passage of floods4 ,the same result is obtained by making the crest slope upwards 7fig4 #cd74 ,in a small stream or in an irrigation distributing channel1 a weir may be a simple brick wall with both faces vertical and corners rounded4 ,illustration3 ,,fig4 #cd4 ,weirs in ,america are often built of crib-work filled with stones4 ,weirs are also made of sheet piling filled in with rubble1 and the top may be protected by sheet iron4 ,a weir made on the ,mersey in connection with the ,manchester ,ship ,canal works was so made4 ,there were three rows of piles and the filling in the back part was of clay4 ,sometimes the downstream faces of weirs used to be made curved 7figs4 #ce and #cf71 the object being to reduce the shock of the falling water1 but the advantage gained is not very appreciable1 and this type of weir is not very common4 ,illustration3 ,,fig4 #ce4 ,illustration3 ,,fig4 #cf4 ,the ,okla weir 7fig4 #cg7 across the river ,jumna near ,delhi was built about thirty-eight years ago on the river bed1 which consisted of fine sand4 ,the depth of water over the crest in floods is #f to #aj feet4 ,the material1 except the face-work and the three walls1 is dry rubble4 ,illustration3 ,,fig4 #cg4 ,when the reach of channel downstream of a weir has a bed-level much lower than that of the upstream reach--this is often the case in irrigation canals1--the work is known as a 8fall0 or 8rapid40 ,at a fall the water generally drops vertically1 and a cistern 7fig4 #ch7 is provided4 ,the falling water strikes that in the cistern and the shock on the floor is greatly reduced4 ,an empirical rule for the depth of the cistern1 measured from the bed of the downstream reach1 is ,k ,h ,h ,d1 where ,h is the depth of the crest of the fall below the upstream water-level1 and ,d is the difference between the upstream and downstream water-levels4 ,at some old falls on ,indian canals the water1 as it begins to fall into the cistern1 is made to pass through a grating which projects with an upward inclination from the crest of the weir at the downstream angle4 ,this splits up the water and reduces the shock1 but rubbish is liable to collect4 ,illustration3 ,,fig4 #ch4 ,in the usual modern type of canal fall in ,india the weir has no raised crest1 and the water is held up by lateral contraction of the waterway just above the fall4 ,the opening through which the water passes is trapezoidal 7fig4 #ci71 being wide at the water-level and narrow at the bed-level4 ,in a small channel there is only one opening1 but in a large canal there are several side by side1 so that the water falls in several distinct streams4 ,the curved lip shown in the plan is added to make the water spread out and cause less shock to the floor4 ,the dimensions of the openings are calculated so that however the supply in the canal may vary1 there is never any heading up or drawing down4 ,the detailed method of calculation for finding ,c ,f and the ratio of ,a ,b to ,b ,c is given in ,hydraulics1 ,,chap4 ,,iv4 ,in cases where it is only necessary for the notch to be accurate when the depth of water ranges from ,b ,c to three-fourths ,b ,c1 it will suffice to calculate as follows3--,let b be the bed width of the canal1 and let ,q be the discharge and ,b the mean width of the stream when the depth of water is ,b ,c4 ,decide on the number of notches1 and let ,w be the width of a notch calculated as if it were to be rectangular1 i4e4 by the ordinary weir formula4 ,increase the width to ,w #aje ,w4 ,then make the notch trapezoidal1 keeping the mean width ,w1 and making the bottom width w 7or ,c ,f71 such that w_/,w b_/,b4 ,the top width of the notch is of course increased as much as the bottom width is reduced4 ,illustration3 ,,fig4 #ci4 ,a rapid has a long downstream slope1 which is expensive to construct and difficult to keep in repair1 especially as the canals can only be closed for short periods4 ,rapids exist in large numbers on the ,bari ,doab ,canal in ,india1 the face-work consisting in many cases of rounded undressed boulders--with the interstices filled up by spawls and concrete--which stand the wear well4 ,rapids have again been used on the more modern canals in places where boulders are obtainable1 and where deep foundations would have given trouble in unwatering4 ,the upstream face of a rapid is vertical1 or has a steep slope4 ,illustration3 ,,fig4 #dj4 #e4 ,weirs with ,sluices4--,the long weirs built across ,indian rivers below the heads of irrigation canals generally extend across the greater part of the river bed4 ,in the remaining part--generally the part nearest the canal head--there is1 instead of the weir1 a set of openings or 8under-sluices0 7fig4 #dj7 with piers having iron grooves in which gates can slide vertically4 ,the piers may be twenty feet apart and five feet thick4 ,the gates are worked by one or more 8travellers10 which run on rails on the arched roadway4 ,the traveller is provided with screw gearing to start a gate which sticks4 ,when once started it is easily lifted by the ordinary gears4 ,the gates descend by their own weight4 ,the gate in each opening is usually in two halves1 upper and lower1 each in its own grooves1 and both can be lifted clear of the floods4 ,in intermediate stages of the river these gates have to be worked a good deal4 7,see also ,,chap4 ,v41 ,art4 #e47 ,usually the weir has1 all along its crest1 a set of hinged shutters1 which lie flat at all seasons1 except that of low water in the river4 ,illustration3 ,,fig4 #da4 ,the canal head consists of smaller arched openings1 provided with gates working in vertical grooves and lifted by a light traveller4 ,if the floor of the canal head is higher than the beds of the river and the canal1 it may be said to be a weir1 but otherwise the canal head is merely a set of sluices without a weir4 ,the barrage of the ,nile at ,assiut 7fig4 #da71 and the old barrages of the ,rosetta and ,damietta branches1 consist of sets of sluices without weirs4 ,at ,assiut there are piers five metres apart and gates working in grooves like those1 above described1 at ,indian headworks4 ,illustration3 ,,fig4 #db4 ,the 8dam0 across the ,ravi1 at the head of the ,sidhnai ,canal in the ,punjab1 also consists of sluice openings without a weir4 ,the piers are connected by horizontal beams 7fig4 #db71 against which1 and against a sill at their lower ends1 rest a number of nearly vertical timber 8needles10 fitting close together1 which can be removed when necessary by men standing on a foot-bridge4 ,in floods the needles are all removed and laid on the high-level bridge 7not shown in the drawing71 the foot-bridge being then submerged4 ,with needles the span between two piers can be greater than would be possible with a gate4 ,needles can be used up to a length of #ab or #ad feet1 excluding the handle which projects above the horizontal beam4 ,they can be of pine1 about #e inches deep in the direction of the stream1 and #d inches thick4 ,where a branch takes off from a canal in ,india there are usually no fixed weirs but two sets of piers--one in the canal and one in the branch1--with openings and gates like those at the canal heads1 or else with wider openings and needles4 ,these works are called regulators4 ,the gates are worked by travellers or by fixed windlasses or racks and pinions4 ,very small gates for distributaries are often worked entirely by screw gearing4 ,for the smaller branches the gates are replaced by sets of planks or timbers lying one above another and removed by means of hooks4 ,they are replaced by means of the hooks or by being held in position some little height above the water1 and dropped4 ,they are finally closed up by ramming4 ,in the case of either planks or needles1 leakage can be much reduced by throwing shavings or chopped straw into the water upstream of them4 ,needles can be provided on their downstream sides with eye-bolts just above the level of the beam against which their upper ends rest4 ,they can then be attached by chains or cords to the beam or to the next pier1 and cannot be lost when released4 ,they can be released by a lever which can be inserted under the eye-bolt4 ,by pushing the head of a needle forward and inserting a piece of wood under it1 a little water can be let through4 ,in this way1 or by removing needles here and there1 the discharge can be adjusted with exactness4 ,at a needle weir in an ,indian canal all the needles in one opening are reported to have broken simultaneously4 ,a possible explanation is that one needle broke and that the velocity thus set up in the approaching stream caused the others to break4 ,on another occasion when a canal was dry all the needles were blown down4 ,sometimes the beam or bar against which the upper ends of the needles rest is itself movable4 ,at ,ravenna1 in ,italy1 the bar between any two piers has a vertical pivot at one pier and can swing horizontally4 ,its other end is held by a prolongation of the next bar1 near to its pivot4 ,if the end bar of the weir is released1 each bar in turn is released automatically4 ,at ,teddington on the ,thames the oblique weir1 #dhj feet long1 has thirty-five gates1 which extend over half the length of the weir4 ,they are worked by travellers which run on a foot-bridge4 ,the openings do not extend down to the river bed1 but are placed on the top of a low weir4 ,the other half of the weir is fixed4 ,the gates are raised to let floods pass4 ,at ,richmond on the ,thames the arrangements are similar1 the gates being counterbalanced to admit of easy and rapid raising4 ,when raised they are tilted into a horizontal position so as not to obstruct the view4 ,in ,stoney's sluice gates a set of rollers is interposed between the gate and the groove4 ,the rollers are suspended from a chain1 one end of which is attached to the top of the gate and the other end to the groove4 ,the rollers thus move up or down at half the rate of the gate1 and some of them are always in the proper position for taking the pressure4 ,escape of water between the gate and the groove is prevented by a rod which is suspended on the upstream side of the gate close to its end1 and is pressed by the water against the pier4 ,stoney's sluice gates1 with spans ranging up to #cj feet1 have been used on the ,manchester ,ship ,canal for the sluices by which the water of the river ,weaver is passed across the canal1 and at locks for passing the flood waters of the ,irwell and ,mersey down the canal4 ,the gates are balanced by counterweights4 ,frame weirs1#ac used chiefly on rivers in ,france but also in ,belgium and ,germany1 are a modification of the needle and plank arrangements above described4 ,for the masonry piers there are substituted iron frames or trestles1 which are hinged at the floor-level so that1 when the timbers have been removed1 the frame can be turned over sideways and lie flat on the floor1 thus leaving the waterway absolutely clear from side to side of the stream4 ,the foot-bridge which rests on the frames is removed piece by piece4 ,the frames are raised again by means of chains attached to them4 ,in order that the frames may not be too heavy they are spaced #c to #d feet apart1 or very much nearer than when masonry piers are used4 ,horizontal planks can thus be used of shorter lengths than the needles1 and they can be made up into greater widths so that the leakage is less4 ,a further modification consists in placing the bridge platform above flood-level1 and in hinging the frames to it instead of to the floor4 ,the frame turns about a horizontal axis parallel to the length of the weir4 ,a weir of this kind can be used for greater depths of water than the ordinary frame weir4 ,in some cases the horizontal planks are connected together by hinges so that they form a 8curtain40 ,the curtain is raised by rolling it up by means of a traveller4 ,it admits of rapid and accurate adjustment of the water-level1 but there is considerable scouring action below a curtain when it is somewhat raised4 ,illustration3 ,,fig4 #dc4 #f4 ,falling ,shutters4--,in ,thnard's system1 first used in ,france1 a shutter 7fig4 #dc7 is hinged at its lower edge and is held up by a strut4 ,when the lower end of the strut is pushed aside it slides downstream and the shutter falls flat4 ,to enable the shutter to be raised again an upstream shutter1 which ordinarily lies flat and is held down by a bolt1 is released1 and it is then raised by the current to the extent permitted by a chain attached to it4 ,the downstream shutter is then raised4 ,thnard's system was not much used in ,france because the river had to fall to a level somewhat too low for navigation before the shutters could be raised4 ,the sudden jerk on the chain of the upstream shutter is also liable to do damage4 ,the system has been adopted on some of the long weirs which cross ,indian rivers downstream of the heads of irrigation canals4 ,to prevent damage by shock1 a hydraulic brake was designed by ,fouracres4 ,it consists of a piston which travels along a cylinder and drives water out through small holes4 ,the shutters are placed on the top of the fixed weir1 where they usually lie flat1 except in the low water season1 any adjustments of the river discharge being effected by means of the under-sluices4 ,in the ,chanoine system of falling shutters 7fig4 #dd71 used first in ,france1 the shutter is hinged at a point rather higher than the centre of pressure4 ,the hinge is supported by a vertical trestle1 which is hinged at its lower end and is supported by a strut which slides in a groove and rests against a stop4 ,when the water rises to a certain height above the top of the shutter1 it is turned by the force of the water into a horizontal position4 ,the struts can then be pushed sideways out of the stops by means of a 8tripping bar10 which lies along the floor parallel to the line of shutters and is worked from the bank4 ,the struts1 trestles1 and shutters then fall flat4 ,to close the weir the shutters are first raised into the horizontal position which they occupied before falling1 by means of a hook worked from a boat or by chains attached to a foot-bridge running across the river upstream of the weir4 ,they can then be easily closed by a boat-hook4 ,the water closes them of itself if it falls low enough4 ,illustration3 ,,fig4 #dd4 ,when the shutters fall a great rush of water occurs4 ,to obviate this a valve is made in the upper half of the shutter4 ,it consists of a miniature shutter on the same principle as the main shutter4 ,the pivot of the main shutter is made at such a height that the shutter will not turn over when only a small depth of water flows over it4 ,instead of this the valve comes into operation4 ,the valve also facilitates the raising of the shutter4 ,again1 instead of the tripping bar1 which would sometimes have to be of great length or be liable to damage owing to stones jamming in its teeth1 the shutter can be released by pulling the strut upstream so that it falls into a second groove1 down which it slides4 ,when a tripping bar is used1 its teeth can be so arranged that the shutters are released a few at a time1 first singly1 then in twos and threes4 ,sometimes there are gaps of a few inches between one shutter and the next1 and the gaps can be closed by needles if necessary4 ,chanoine shutters can be very rapidly lowered1 and they are used in ,france and in the ,u4,s4,a4 in places where sudden floods occur4 ,they are also used for navigation 8passes0 where most of the heavy traffic is downstream and where it is too heavy to be dealt with in a lock4 ,a foot-bridge across the stream or across the navigation pass is always an assistance1 but sometimes it cannot be used when there is much floating rubbish or ice4 ,with a foot-bridge the cost is greater than that of a needle weir4#ad ,illustration3 ,,fig4 #de4 ,in the ,bear ,trap weir 7fig4 #de7 the upstream shutter rests against the downstream one4 ,both are raised by admitting water from the upper reach1 by means of a culvert1 through an opening in the side wall1 and they are made to fall by placing this opening in communication with the downstream instead of the upstream reach4 ,this kind of shutter is only suitable for passes of moderate width1 and it is rather expensive on account of the culverts4#ae ,shutters with fixed supports are used on the ,irwell and ,mersey4 ,a fixed frame is built across the stream 7fig4 #df7 and the shutters are hinged to it4 ,when the water rises to a certain height above its top1 the shutter turns into a horizontal position1 but as this causes a severe rush of water the shutter is usually raised by a chain attached to its lower end and worked from the bank4 ,when in a horizontal position1 it is held there by a ratchet4 ,when the stream falls the ratchet is released and the shutter is closed by the stream4 ,this kind of shutter cannot be used where there is navigation4 ,on the weir #djjj feet long across the river ,chenab at ,khanki in the ,punjab1 the falling shutters1 #f feet high and #c feet wide1 are hinged at the base and held up by a tie-rod on the upstream side4 ,the trigger which releases the rod is actuated by means of a wire rope carrying a steel ball1 and worked by a winch from the abutment of the weir or from one of the piers1 which are #ejj feet apart4 ,a winch is fixed on the top of each pier1 and communication with the piers is effected by means of a cradle slung from a steel wire rope1 which rests on standards and runs across the weir4 ,the wire rope which carries the steel ball passes over a series of forks1 one on each shutter4 ,when one trigger has been released1 that shutter falls and the ball hangs loose4 ,a further haul on the rope causes it to actuate the trigger of the next shutter1 and so on4 ,if it is desired to drop only some of the shutters1 the rope is passed over the forks of those shutters only4 ,the shutters can be raised by means of a crane which runs along the weir on rails downstream of the shutters or1 if the water is too high to allow of this1 by a crane in the stern of a boat which is moored upstream of the weir and allowed to drop down4 ,illustration3 ,,fig4 #df4 ,illustration3 ,,fig4 #dg4 #g4 ,adjustable ,weirs4--,drum weirs1 invented by ,desfontaines1 have been used in ,france and ,germany4 ,two paddles 7fig4 #dg7 are fixed on a horizontal axis and can turn through about #ij1 the lower paddle1 which should be slightly the larger1 working in a 8drum10 which is roofed over and can1 by means of sluices1 be placed in communication with either the upper or lower reach of the stream4 ,according as the upper paddle is to be raised or lowered1 water is admitted from the upper reach above or below the lower paddle1 the water on its other side being at the same time placed in communication with the lower reach4 ,on the weirs first made on the ,marne1 the height of the upper paddle was #c feet #g inches1 and there were1 in a weir1 a number of pairs of paddles1 each being #d feet #aa inches wide4 ,by having sluices at both abutments communicating with both reaches1 and by opening or closing each of them more or less1 the various paddles can be made to take up different positions1 and thus perfect control over the discharge is obtained by simply turning a handle to control a sluice gate4 ,a weir has since been made with a single pair of paddles extending right across the opening 7#cc feet71 and the height of the upper paddle is over #i feet4#af ,the chief objection to drum weirs is the necessity for the hollow or drum1 which renders the work very expensive1 except when only a small depth of water is held up4 ,illustration3 ,,fig4 #dh4 ,the old sluice gates of the ,nile barrages were made segmental 7fig4 #dh71 turned on pivots in the piers1 and were raised by chains4 ,in some factories in ,bavaria and ,switzerland there are self-acting shutters which revolve on a horizontal axis at the lower edge1 and are counterbalanced by cylindrical weights which roll on ways in the side wall4 ,this arrangement is suitable when there is only one span1 which can1 however1 be as great as #cj feet4 ,an adjustable weir used at ,schweinfurt on the ,maine1 consists of a hollow iron cylinder1 #ei feet long and #aj feet in diameter1 running across the stream4 ,the cylinder is pear-shaped in cross-sections1 and can be made1 by means of mechanism1 to revolve1 the water passing over it4 ,another kind used at ,mulhausen on the ,rhine consists of a hollow iron cylinder #he feet long and #ih feet in diameter4 ,the whole cylinder can be raised by winches 7,min4 ,proc4 ,inst4 ,c4,e41 vols4 cliii4 and clvi474 #h4 ,remarks on ,sluices4--,in all kinds of sluice openings or regulators1 the principles of design as regards protection of the bed and sides1 splaying and curving of walls and piers1 thickness of floor1 and prevention of the formation of streams under the structure are the same as laid down for weirs4 ,in order that a pier may be safe from being overturned by the pressure of the water when the gates or timbers are down1 the resultant of its weight1 including that of anything resting on it1 and of the water pressure on it1 must pass through the middle third of its length4 ,this generally occurs when there is an arched roadway4 ,otherwise it must be arranged for by prolonging the base of the piers downstream1 and giving the downstream side a batter or steps4 ,the floor should usually be placed at a level somewhat lower than the mean bed-level of the stream4 ,the bed may possibly be lowered in course of time4 ,lowering the floor also gives a greater thickness of water cushion to take the shock of water falling over the gates or planks4 ,it is convenient to build1 on the floor1 a low wall or sill1 reaching up to the level of the bed or thereabouts1 and running across from pier to pier under the line of gates or needles4 ,the height of the gates or needles can thus be reduced1 and there is little chance of silt or stones collecting and interfering with them4 ,in the case of needles the wall must be strong enough to resist their horizontal pressure4 ,if ever the bed is lowered1 the wall can easily be cut down or removed4 ,sluices with gates are1 of course1 used in connection with works other than weirs or regulators1 as1 for instance1 in reservoirs or locks1 or generally for communication between any two bodies of water4 ,the gate may or may not be wholly submerged4 ,if it is not wholly submerged1 planks can be used4 ,needles can be used if the flow is always in one direction and never in the reverse direction4 ,in all cases protection downstream of the opening is required4 ,in designing a set of sluice openings or regulators1 it is sometimes the custom to make the total area of waterway the same as that of the stream in its unobstructed condition4 ,there is no particular reason why it should be the same4 ,in a description of the ,assiut ,barrage 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clviii41 p4 #cj71 it is mentioned that one of the reasons for placing the floor lower than the river bed was that the width of the waterway of the barrage was less than that of the river4 ,the bed has to be heavily protected in any case1 and the proper principle is to fix a velocity which is considered to be safe and1 the maximum discharge being known1 to determine the area of the waterway accordingly4 ,in the case of a very wide river like the ,nile1 with a well-defined channel1 it is inconvenient to make the distance between the abutments of a work much less than the width of the channel1 but so far as velocity is concerned1 the floor need not usually be lower than the bed4 ,the protection given to the channel on the upstream side of the barrage 7fig4 #da7 seems to be rather greater than necessary4 ,the thickness of the floor 7#i feet #aj inches7 seems excessive4 ,the thickness originally proposed was much less4 ,of the many kinds of apparatus described in this chapter each possesses some advantages and disadvantages4 ,gates require a bridge with powerful lifting apparatus1 and are suitable for large bodies of water and great depths4 ,comparing needles with planks1 the former can be worked by one man and admit of rapid removal1 and require far fewer piers4 ,planks require two men1 and are sometimes liable to jam1 but obstruct floating rubbish less than needles1 and in shallow water give rise to less leakage4 ,whether needles or planks are used1 masonry piers are most suitable where sand or gravel are liable to accumulate on the floor1 or where there is much floating rubbish4 ,the hinged frames are suitable in other cases4 ,falling shutters of the ,chanoine type admit of very rapid lowering1 and can be used without a foot-bridge4 ,the drum weir is perfect in action1 but its cost is high4 ,at any system of sluices the regulation should be so arranged as to minimise the chances of damage to the bed and banks where this is at all likely to occur4 ,if the gates are opened only near one side of the structure1 there will be a rush of water on that side1 and serious damage may occur4 ,the opening should be done symmetrically and1 as far as possible1 distributed along the whole length4 ,until experience has shown it to be unnecessary1 soundings should be taken at regular periods of time downstream of every important work where scour can occur4 ,when scour is found to have occurred at any particular part of the work1 the rush of water at such places should1 as far as possible1 be prevented1 and a chance given for silting to occur4 ,unless experience shows that damage is not likely to occur1 a stock of concrete blocks1 sandbags1 or other suitable materials should be kept on the spot ready for use4 ,life-buoys should be provided on any work where large volumes of water are dealt with1 especially if it is unfenced in any part1 or if any of the men employed are casual workers4 ,regarding works for preventing a river from shifting its course so as to damage or destroy a weir or similar work1 see ,,chap4 ,,xi41 ,art4 #c4 ,,chapter ,,xi ,,bridges ,,and ,,syphons #a4 ,bridges4--,bridges are of many kinds4 ,in this book only those parts of them are considered which are exposed to the stream4 ,if a bridge has piers1 there must be some disturbance of the water4 ,the disturbance will be least when the area of the waterway of the bridge is at least as great as that of the stream1 and when its shape is as nearly as possible the same4 ,for small streams1 a single span clearing the whole stream may be adopted1 especially when the channel is of soft material1 but for a large stream the cost of intermediate piers1 even with a certain amount of protection for them or with deep foundations1 will be more than counterbalanced by the smaller thickness of arch or depth of girder4 ,generally a bridge has vertical abutments which limit the waterway1 but it may have land-spans1 and in this case the stream as it rises can spread out4 ,piers and abutments should be so designed that abrupt changes in the section of the stream are1 as far as possible1 avoided1 the piers being rounded or boat-shaped at both ends and the abutments suitably curved 7fig4 #di74 ,boat-shaped piers1 besides presenting the neatest appearance1 cause the least amount of disturbance4 ,a bridge can be made safe against scour either by giving deep foundations to the piers and abutments or by adding a floor and1 if necessary1 pitching4 ,the former course is usually adopted and is the best4 ,but in a case in which the discharge of a stream is to be increased or has been underestimated1 it is often far easier to add a floor to an existing bridge than to increase the span of the bridge4 ,in order to increase the waterway the floor can be 8dished10 i4e4 made at a level lower than the bed of the stream#ag and gradually sloped up--the slopes being pitched--both upstream and downstream of the bridge1 to meet the bed4 ,illustration3 ,,fig4 #di4 ,in any case in which the water rises above the crown of the arch1 the bridge becomes a syphon1 and a floor is probably necessary unless the foundations are very deep1 or unless the rise of water above the crown is temporary4 ,illustration3 ,,fig4 #ej4 ,in the case of ,indian rivers which have soft channels1 and are ordinarily of moderate width but are subject to occasional floods when the width of the stream is multiplied several times and becomes very great1 it is the rule to make the span of a railway bridge far less than this greater width4 ,the stream during floods scours out a deep channel through the bridge with great rapidity1 and no heading up worth mentioning occurs4 ,the foundations of the piers are very deep1 being frequently #ej feet below the lowest point of the river bed which can be found anywhere within several miles of the bridge4 ,the span of the bridge can be arrived at by considering a general cross-section of the river as it is when in high flood1 and assuming that scour to the depth of the lowest point1 found as just explained1 will take place in one-third of the span of the bridge4 ,the span can then be so fixed as to give no heading up4 ,it is not assumed that there will be no increase in velocity through the bridge4 ,the velocity in the deep scoured portions will be increased4 ,the piers are protected by loose stone 7fig4 #ej74 ,the spans vary from #ajj to #bej feet4 ,the bridge over the river ,chenab at ,wazirabad had originally sixty-four spans of #ade feet each4 ,the number of spans has since been reduced to twenty-eight4 ,with a very long bridge1 the current of the shifting stream is more likely to strike the bridge obliquely1 though this is not the chief reason for reducing the length4 ,long spans1 say #bej feet1 have been found to be better than shorter spans2 the cost of the stone protection round the piers is of course less 7,government of ,india ,technical ,paper1 ,no4 #aec1 8,river ,training and ,control on the ,guide ,bank ,system10 by ,sir ,f4 ,j4 ,e4 ,spring1 ,c4,i4,e41 #aijd74 #b4 ,syphons and ,culverts4--,syphons are used to pass drainage channels or other streams under canals or other lines of communication4 ,in the case of a masonry syphon under a stream which may be dry while the syphon is full1 the weight of the arch and its solid load must be not less than the upward pressure of the water passing through the syphon4 ,the channel sometimes has a vertical drop at the upstream side 7fig4 #ea7 and a slope at the downstream side4 ,the slope enables any solid materials to be carried through1 and facilitates cleaning out and unwatering4 ,the drop at the upstream side does not give rise to any shock on the floor when the syphon is full1 but a slope is preferable if there is room for it1 and it causes less loss of head4 ,illustration3 ,,fig4 #ea4 ,a culvert which is liable to run full and which has a steep approach channel 7fig4 #eb7 may become suddenly drowned on the upstream side4 ,as soon as the water rises to the crown of the arch1 the wet border of the culvert increases and this reduces the velocity and discharge4 ,the water coming down the approach channel then rises abruptly1 and the increased section of the stream causes a reduced velocity of approach1 and this further reduces the discharge through the culvert4 ,the heading up continues until the difference in the upstream and downstream water-levels is great enough to readjust matters 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxxvi474 ,the possibility of this heading up occurring should be attended to in the design4 ,in the case of a culvert in a railway embankment where heavy floods have to be passed1 the culvert may be made bell-mouthed by a curved embankment constructed on its upstream side4 ,illustration3 ,,fig4 #eb4 #c4 ,training ,works4--,the object of the upstream and downstream protections already described 7,,chap4 ,x47 is to prevent damage to the structure owing to the disturbance caused by the structure itself4 ,when a river is given to shifting its course 7,,chap4 ,,iv41 ,art4 #i7 and cutting away its banks1 protection of another kind is required4 ,the stream1 if left to itself1 may cut away one bank upstream of the structure for a long distance1 and eventually damage1 or destroy by undermining1 the upstream pitching and the abutment itself4 ,this is known as outflanking4 ,if in the neighbourhood of the line ,a ,b 7fig4 #ec7 there is nothing for the river to damage1--if1 for instance1 the structure is a weir with a canal1 if any1 only on the opposite bank of the river1--and if the land is of no particular value1 the case could conceivably be met by protecting the abutment on all sides1 but even then there might be a chance of the erosion of the bank continuing until the stream had formed a connection at ,c with the downstream reach4 ,this1 of course1 in the case of a weir1 would render the work useless and might even destroy it4 ,illustration3 ,,fig4 #ec4 ,in the case of a bridge carrying a road or railway1 or of a syphon or aqueduct carrying a canal or other stream1 it is wholly inadmissible to allow the stream to cut away even as far as the point ,a for fear of its severing the line of communication4 ,thus in every case it is practically necessary to prevent any serious erosion of the bank upstream of the structure4 ,in ordinary cases it is sufficient to protect the bank ,c ,d by any of the methods given in ,,chap4 ,,vi41 ,art4 #c1 the protection being turned inwards1 as shown at ,d1 to prevent the end of it being damaged4 ,in the case of railway bridges across the great shifting rivers of ,india1 protection used at one time to be afforded by various systems of spurs4 ,this has now been abandoned in favour of ,bell's guide banks 7fig4 #ed71 which are found to be far more satisfactory4 ,these guide banks are discussed in the paper by ,spring quoted above 7,art4 #a74 ,the spaces behind the guide banks become filled with water1 at least during floods1 and are meant to be silted up4 ,an opening in the railway embankment should be provided at ,a1 and another on the opposite side of the river1 to ensure a constant flow of water 7,,chap4 ,v41 ,art4 #c71 but they should not be large enough to cause high velocity4 ,the chief danger to which a guide bank is liable is outflanking when the stream assumes the position shown4 ,to guard against this danger it is necessary to have very strong and massive heads to the guide banks4 ,when the bank of the eroding stream1 downstream of the guide bank head1 becomes a semicircle or thereabouts1 the stream takes a short-cut across the sandbank1 and to encourage this an artificial cut can be dug1 at the season of low water1 on any suitable line4 ,illustration3 ,,fig4 #ed4 ,if the guide banks were made with an increased width of opening at the upper end1 this would reduce the chance of outflanking but would increase the danger from a direct attack such as indicated1 in the figure1 on the left bank4 ,it has been suggested that the width at the upstream end should be less than at the bridge1 but this seems undesirable4 ,probably the form shown in fig4 #ed is the proper one4 ,the length of the guide bank upstream of the bridge is made about equal to the span of the bridge between the two guide banks4 ,if made less than this1 the river might cut into the line of railway4 ,the length of guide bank downstream of the bridge is generally #cjj to #ejj feet1 being greater as the velocity of the river is greater and the sand of its bed finer4 ,illustration3 ,,fig4 #ee4 ,the ,bengal ,dooars ,railway runs near the foot of the ,bhutan ,himalayas1 and crosses some broad river channels which1 after the excessively heavy rains which occur1 are filled by streams of very high velocities4 ,one such channel or set of channels 7fig4 #ee71 more than half a mile wide1 is provided with a bridge whose waterway consists of ten spans of #fj feet each4 ,the railway embankment across the remainder of the channel having been breached in many places in #aijc1 protection was afforded by ,t-headed spurs and other groynes1 the first arrangement1 which withstood the floods of #aijd1 being as shown in the figure4 ,the triangular apex of the ,a-shaped groyne1 south-east of the bridge1 was added in #aije because1 in its absence1 the water struck the bridge obliquely4 ,after the addition there was a great deposit of silt in the neighbourhood of the four ,t-headed spurs4 ,next year the river1 in a great flood1 rose over the top of the railway embankment near these spurs1 and finally caused a breach #fjj feet wide4 ,the embankment was afterwards raised4 ,the velocity through the bridge seems to have approached #ah feet per second4 ,the bridge had at first no floor4 ,a floor was added1 but was much damaged by the floods 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxiii474 ,the level of the floor is not given1 but it would seem to have been desirable to make it at a very low level4 ,the rising of the stream over the railway embankment was attributed to the silting up near the ,t-headed spurs4 ,the addition of the triangular portion above referred to would seem to have somewhat assisted this process4 ,if all the trouble could have been foreseen1 it might have been best to build an additional bridge #bjjj feet south-east of the existing bridge4 ,the groynes were composed of the wire-network rolls1 described in ,,chap4 ,,vi41 ,art4 #c1 piled pyramid fashion4 ,,chapter ,,xii ,,drainage ,,and ,,floods #a4 ,preliminary ,remarks4--,arts4 #b and #c of this ,chapter deal with the calculation of flood discharges1 ,art4 #b dealing with small streams1 in which the water has to be got rid of1 and ,art4 #c with large streams4 ,the remaining articles discuss the methods of predicting floods and of preventing them from doing damage4 ,when the discharge figures have been arrived at in any case1 the necessary masonry works can be designed in accordance with the principles described in ,,chaps4 ,x4 and ,,xi4 ,for remarks regarding the design of channels and banks1 see ,,chap4 ,,ix41 ,art4 #d1 and also ,art4 #f of the present ,chapter4 ,in ,england1 land near a stream or flooded area is said to be 8awash0 when the flood water rises to within #c feet of the surface of the ground4 ,the drainage of such land is apt to be unsatisfactory4 ,if land is flooded or awash1 it may be desirable to shift the outfall of a branch drain to a point lower down in the main outfall4 #b4 ,small ,streams4--,in dealing with small streams1 such as branch drains or natural streams not far from their sources1 the engineer is concerned only with their maximum discharges4 ,he has to design culverts1 bridges or syphons to pass the streams under roads or other works1 or to design channels or waste weirs for them4 ,in a settled country there may be already some works in existence on the same stream1 and these may form a guide1 or it may be possible to obtain local information as to the height or volume of floods4 ,even in such a case rainfall figures will be most useful4 ,in districts where there is no settled population1 and in any case where the stream is ill-defined1 and the flow fitful1 the rainfall figures may afford the only1 or at least far the best1 means of estimating the discharge4 ,the rainfall to be considered in all these cases is the maximum likely to fall in a short period of time4 ,the catchment areas dealt with are small1 say #e square miles or less4 ,it must be assumed that the fall of rain extends to all parts of the catchment area1 and that its duration is sufficient for the water from all parts of it to reach the site of the work4 ,the different valleys or divisions of the catchment area should be considered separately1 and regard must be had1 not only to the area of each division1 but to its length and declivity measured along the course of the stream which drains it4 ,on these two factors depend the time taken by the rain water to reach the site of the work4 ,the rate at which the rain water flows over the ground into rills or small subsidiary streams may be taken to be mile per hour in flat land1 and #a mile per hour on steep hill sides4 ,the velocity of the current in the rills and larger streams is generally #b to #d miles per hour4 ,it can1 when necessary1 be calculated roughly from the size and slope of the stream4 ,to be on the safe side1 the highest probable figure can be taken4 ,the time taken by the water to flow from the furthest points of the catchment area to the site of the work having been arrived at as above1 the next thing is to estimate the probable maximum intensity of the rainfall during that time over the whole catchment area4 ,the only figures immediately available will be the mean annual rainfall1 or perhaps the maximum fall in twenty-four hours1 but it has been shown 7,,chap4 ,,ii41 ,art4 #e7 how the probable maximum fall over a shorter period may be estimated4 ,the next thing to be calculated is the 8run-off10 i4e4 the probable proportion of the rainfall which will at once run off4 ,this may be less than the proportion which will eventually become 8available10 because some of it may go to feed the underground supply from which springs are fed4 ,the proportion running off a small area in a short time would1 under most circumstances1 be rather difficult to estimate1 but in the case under consideration1 only the probable maximum figure is required4 ,this occurs when the ground is saturated4 ,under these circumstances the ratio of the run-off to the total fall may be somewhat as follows3-- ,steep rocky hillsides #gj to #ij ,ordinary hills #ej 0 #ge ,undulating country #dj 0 #ej ,flat country #cj 0 #ce ,the figures can be increased when the surface is specially hard or frozen1 and decreased when it is soft1 sandy1 covered with woods or vegetation1 or cultivated4 ,whether or not the above procedure is necessary in its entirety depends chiefly on the size of the proposed work and on the degree of inconvenience likely to arise from any wrong estimation of the discharge4 ,in designing syphons to carry torrents across the ,upper ,jhelum ,canal in the ,punjab1 the discharge from a catchment area of #gi square miles was found to be about #djjj cubic feet per second4 ,this is at the rate of about #ejjj cubic feet per second per square mile1 and is equivalent to a run-off of #gh inches in an hour4 ,the catchment area was among low hills1 not far from the ,himalayas1 and the declivities of the rills were very steep4 ,the superintending engineer1 ,mr ,r4 ,e4 ,purves1 states#ah that the discharge observations were reliable1 and that falls of rain of an inch in ten minutes occur not infrequently1 even though the fall in twenty-four hours might not exceed #b or #c inches4 ,in order to account for the discharge in the case under consideration1 it would at first seem to be necessary to assume not only that a fall at the rate of #gh inches per hour had occurred1 but that the whole of it had run off4 ,it is not1 however1 necessary to assume quite so much4 ,the ground being saturated1 the rain falling in a period of five minutes might be reaching the discharge site with little loss4 ,a suddenly increased fall at the rate of #f inches per hour might then occur1 and the water travelling more quickly and with hardly any loss1 would overtake that already passing the site4 ,this case seems to show that for a very small catchment area the whole of the fall1 and more1 must be allowed for4 ,the ,chief ,engineer of the ,punjab did not accept the above figures4#ai ,he remarked that observations taken under great difficulties as to time and place are liable to error1 and he considered that an allowance of rainfall at the rate of #dh inches per hour--a rate which had been observed elsewhere--and a run-off of #ge of the fall would be sufficient4 ,he accepted a discharge of #bjjj cubic feet per second for catchment areas of less than #e square miles1 assuming the run-off to be #ge of the fall1 but afterwards increased the figure to #bdjj cubic feet per second4 ,the chief engineer did not overlook the fact that in the designs for the drainage aqueducts a free-board of #e feet had been allowed1 and perhaps this led to an acceptance of an estimated discharge less than would otherwise have been accepted4 ,it does not seem to be at all certain that the figure put forward by ,mr ,purves was far wrong4 ,when the original project estimate for the ,upper ,jhelum ,canal was framed1 the irrigation engineers had had no experience of small and steep catchments1 and no one had suspected that the discharge per square mile would be anything like the above4 ,the sums of money provided for works for the passages of torrents had to be increased in ratios varying from #be to #a to #f to #a4 ,the following statement shows the figures for other small catchment areas in the neighbourhood of the ,upper ,jhelum ,canal3-- ------------------------------------- ,catchment ,discharge per ,run-off4 ,area4 square mile4 ------------------------------------- ,sq4 miles4 ,cub4 ft4 per sec4 ,inches4 4#gi #ejjj #gh #adg #chbe #ehb #bif #bbad #cdf ------------------------------------- ,in the south-east of ,new ,south ,wales flood discharges of #ace and #hd cubic feet per second have been found for catchment areas of #ia and #be square miles respectively in broken country4 #c4 ,rivers4--,it is possible to apply the methods of the preceding article to large catchment areas1 but the results would be quite unreliable4 ,if the calculations were made so as to err on the side of safety1 the resulting discharges would often be enormous4 ,the following table shows some figures based on actual flood discharges4 ,none of the localities have excessive rainfalls1 though most are liable to occasional very heavy falls4 ,in mountainous districts in the ,north of ,england and in ,scotland the flood discharges per square mile of catchment area have been found to vary from #fd to #cbj cubic feet per second4 ---------------------------------------- ------------------------- ,flood ,discharge ,reference ,country4 ,locality4 ,catchment per sq4 mile ,remarks4 ,number4 ,area4 of ,catchment ,area4 ---------------------------------------- ------------------------- ,sq4 miles ,cub4 ft4 per sec4 #a ,india4 ,upper #e to #aj #afac ,jhelum4 #b 0 ,nagpur4 #ff #dhj #c ,south ,near ,cape #cde #gh ,africa4 ,town4 #d 0 ,near ,port #ce #fdj ,estimated4 ,elizabeth #e ,new ,south,south-,east #di #cg ,wales4 ,district4 #f ,india4 ,upper #ef #ajjj ,jhelum4 #g 0 0 #agd #eej #h ,new ,south,south-,east #dah #aab ,wales4 ,district4 #i ,india4 ,kali ,nadi #beic #ea ,estimated ,stream4 or more roughly4 ---------------------------------------- ------------------------- ,the tendency of the figures in column #e of the table is to decrease as the catchment area increases4 ,this tendency has long been known1 and attempts have been made to found on it formul for calculating flood discharges4 ,one such formula is ,q c ,m#c_/d where ,q is the flood discharge in cubic feet per second and ,m is the area of the catchment in square miles4 ,the formula is roughly correct1 c being a constant for catchment areas of not dissimilar characters and with rainfalls not differing much4 ,but for other cases there is no knowing how c may vary1 and this renders the formula practically useless4 ,the author of another such formula quotes cases ,nos4 #e and #h in the above table1 and the two cases mentioned at the end of ,art4 #b as agreeing fairly well with the result of his formula4 ,the tendency just mentioned is due to the fact that every river is composed of tributaries which have their own small catchment areas but are1 when measured to the general outlet or point where the discharge is under consideration1 of very different lengths1 to the improbability of heavy rainfall occurring over all these small areas at such times as to cause the different flood waves to arrive simultaneously at the outlet1 and to the facts that in the case of the longer tributaries the flood waves flatten out 7,hydraulics1 ,,chap4 ,,ix41 ,arts4 #c and #d7 so as to arrive more gradually1 and that1 unless rain is also falling all along their courses1 these longer tributaries undergo losses from evaporation and absorption4 ,but occasionally it happens that the various flood waves do arrive at the outlet more or less simultaneously1 and that the rainfall continues so long and is so widely distributed--though not necessarily of the same intensity as that which caused the flood--that the flood waves do not flatten out and that losses in the channels do not occur4 ,floods can thus vary to an extraordinary degree in severity1 and formul are quite useless4 ,this is why floods occur surpassing all previous records1 as1 for instance1 the recent floods in ,paris4 ,however severe a flood may be1 it can never be said that the maximum has1 even probably1 been attained unless it can be shown that the rainfall has been so heavy1 so long continued1 and so distributed that anything worse is not likely to occur4 ,the best method of estimating the flood discharge of a large perennial stream is to ascertain1 by local inquiry1 the height to which it is known to have risen1 and to take cross-sections of the channel and calculate the discharge 7,,chap4 ,,iii41 ,arts4 #d and #e74 ,in designing works1 allowance can be made for a flood exceeding any known before4 ,this method applies also to a case in which a river is formed by the junction of two or more large tributaries4 ,it is possible that the tributaries have not1 within the memory of man1 been in high flood simultaneously4 ,if so1 the chances of this occurring are no greater and no less than if the stream was composed merely of a number of small affluents4 ,remarks regarding intermittent streams are given in ,,chap4 ,,iii41 ,art4 #g4 ,since an acre contains #dc1efj square feet1 and a twelfth of this is #cfcj1 it follows that a fall of #d inches of rain1 of which #a inch runs off1 in an hour1 gives a discharge of #cfcj cubic feet per hour1 or about #a cubic foot per second4 ,this is #fdj cubic feet per second for a square mile4 ,the figures in column #e of the above table show that the run-off was1 in the cases quoted1 generally far less than #a inch4 ,in case ,no4 #d it was #a inch1 and in case ,no4 #b it was #c_/d inch4 ,in the case of the ,kali ,nadi 7,no4 #i in the table7 an aqueduct to carry the ,lower ,ganges ,canal over the stream was being designed4 ,the flood discharge1 estimated from the supposed flood-level and cross-section of the stream was 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 xcv47 #bf1ceb cubic feet per second4 ,the discharge1 estimated by assuming a fall of #f inches of rain in twenty-four hours over the catchment area--then believed to be #cjbe square miles--and a run-off of #be of the fall1 was #aad1iej cubic feet per second4 ,this figure was rejected on the ground that the rainfall would not be continuous over so large an area as #cjbe square miles4 ,an allowance of #g cubic feet per second per square mile was made and1 a fresh survey having shown that the catchment area was only #beic square miles1 a discharge of #ah1jjj cubic feet per second was allowed for4 ,the aqueduct was built1 about the year #ahge1 with five arched spans of #ce feet each1 the total area of the waterway being about #cjjj square feet4 ,the length of the piers and abutments was #bab feet1 the width of the canal carried over the aqueduct being #aib feet4 ,in #ahhd the aqueduct was partly destroyed by a flood whose discharge was about #dd1jjj cubic feet per second4 ,in ,july #ahhe it was wholly destroyed by a flood whose discharge was estimated at #acb1dge cubic feet per second1 but was probably more4 ,the discharge must have been more than #ea cubic feet per second per square mile4 ,the aqueduct was rebuilt with a waterway of about #ae1jjj square feet4 ,below the aqueduct there was a bridge which had been standing for a hundred years4 ,its waterway was only #aadf square feet4 ,it was not much damaged by the flood of #ahhd1 but much of the water passed round it1 breaking through the embanked roadway or pouring over it4 ,it is understood that the bridge was destroyed by the flood of #ahhe4 ,this case shows the necessity for making every possible allowance in calculating flood discharges for important works4 ,the smallness of the discharge1 as calculated from the cross-section of the stream1 was probably owing to its being dry when the survey was made1 so that the velocity could not be observed1 but it is probable that such a discharge as wrecked the aqueduct had never before passed down the stream4 #d4 ,prediction of ,floods4--,at any place high up on the course of a stream1 the occurrence of a flood can often be predicted when rain storms--often accompanied in the tropics by lightning--can be seen to be occurring towards the sources of the stream4 ,for any station lower down the stream and for precise information in any case1 the readings of gauges higher up the stream can be telegraphed4 ,if the station is at a great distance from the gauge and if there is railway communication1 the readings can be sent by post4 ,in order to be able to predict the time of the arrival of a flood at the lower station the reading of a gauge there1 and also of that at the upper station1 should be taken at frequent intervals4 ,in the case of large rivers and distances of hundreds of miles1 the interval may be six or even twelve hours1 but in other cases it should be much less4 ,if the readings are plotted1 as in fig4 #ef1 oblique lines can be drawn to connect the saliences and depressions1 and the time taken by each change can thus be readily seen4 ,when the upper part of the stream is formed by two or more important tributaries there should be a gauge in each4 ,as to what constitutes a flood1 the gauge diagram of a river 7fig4 #ef7 is generally such that a line can be sketched as shown dotted4 ,the rises above this line are floods4 ,the maximum flood discharge of a ,northern ,indian river is estimated roughly as being #ajj times the low-water discharge4 ,leslie's rule for floods in the ,british ,isles is that if all the daily discharges of a stream during the year are ranged in order of magnitude1 the discharges of the upper quarter are considered to be floods4 ,illustration3 ,,fig4 #ef4 ,in ,india it is sometimes arranged that a telegram shall1 in the low-water stage of the river1 be sent from the upper station when a rise of #b feet occurs in twenty-four hours or any less period1 with a further telegram for any such subsequent rise4 ,the telegram states the exact reading on the gauge and whether the water is rising steady or falling4 ,this is given as indicating the procedure that may be followed where the telegraph has to be used1 but when long and frequent telegrams are not desirable4 ,the advancing end of a flood wave may1 while the wave is rising and being formed1 travel rapidly1 but when the wave has been formed it travels at the ordinary rate of flow of the risen stream4 ,the advancing end of a trough may1 while it is being formed1 travel rapidly1 but after formation it travels at the ordinary rate of the fallen stream 7,hydraulics1 ,,chap4 ,,ix41 ,arts4 #c and #d74 ,thus the rate at which a change in water-level travels down a stream depends at first on the amount of the rise or fall1 but afterwards on the water-level of the risen or fallen stream4 ,by taking the above facts into consideration and noting the actual times obtained from the diagram1 it will be possible to arrive at the probable time that will be taken by any change4 ,it will also be possible to predict the height of the flood4 ,if it is worth while1 an empirical formula can be got out4 ,if there are tributaries1 each with a gauge1 the matter will be more difficult4 ,probably the floods in the tributaries will arrive at different times1 but even in such cases empirical formul have been arrived at1 especially in ,france1 and are mentioned in various volumes of the ,proceedings of the ,institution of ,civil ,engineers4 ,in all cases predictions are liable to be more or less upset if rain falls in the tract between the upper and lower gauges4 ,in very dry weather the speed of a flood wave may be somewhat reduced1 and the height to which it rises will almost certainly be reduced4 ,the full effect of a change will not be felt at the lower station unless the change at the upper station is maintained for a sufficiently long period4 ,a short wave or trough flattens out4 ,thus in any empirical formula or system of prediction1 the time over which the change extends at the upper gauge must be taken into account1 or else there must be several upper gauges and the readings of all of them be taken into account4 ,in mountainous districts landslips sometimes occur and block the valley of a stream which then forms a lake4 ,the water gradually rises and eventually flows over the dam and sweeps it away causing a flood1 which is of great suddenness and height but decreases very quickly in height as it travels down the valley4 ,in a case which occurred in the ,himalayas in #ahhh the inhabitants of the valleys1 from the dam to the point where the river debouches from the hills1 were compelled by ,government to vacate all habitations below the probable level of the flood1 and no loss of life occurred4 ,similar floods1 but on a smaller scale1 may be caused by the bursting of ordinary reservoir dams4 ,in some continental rivers ice may obstruct the stream and cause floods4 #e4 ,prevention of ,floods4--,the extended use of field drains has1 in recent years1 done much to increase the severity of floods in ,england and other countries4 ,one method of mitigating or preventing floods is the construction of reservoirs for storing the water4 ,reservoirs locally known as 8washes10 formed by setting back the embankments1 exist on the ,fen rivers4 ,one wash1 on the ,nene1 below ,peterborough1 is #ab miles long and half a mile wide and is filled1 in floods1 to a depth of #g feet and holds #a inch of rainfall over the river basin1 and this is found to be sufficient4 ,reservoir construction is1 however1 in most cases1 impracticable owing to the expense4 ,to store the water which is given by #a inch of rain in the basin of the ,thames1 a reservoir would be needed #ej feet deep and covering about #g square miles4 ,it might cost #g1jjj1jjj4 ,the afforestation or reforestation of river basins 7,,chap4 ,,ii41 ,art4 #d7 is also occasionally undertaken1 but is not generally practicable4#bj ,the most practicable methods for preventing flooding are lowering the water-level of the stream and constructing embankments along it4 ,these will be considered in the next two articles4 #f4 ,lowering the ,water-,level4--,the water-level of a given length of stream can be lowered by lowering the bed1 widening the channel or straightening the channel4 ,the efficiency of these processes is in the order named4 ,as stated in ,,chap4 ,i41 ,art4 #d1 the alteration to the channel must in any case be continued to some point downstream of the reach under consideration4 ,let the channel be supposed to be of 8shallow0 section with sloping sides4 ,let ,w be the mean width1 ,d the depth1 and ,s the slope4 ,let it be required to lower the water-level by an amount equal to ,d_/#e4 ,this can be effected by lowering the bed by about #be per cent4 of ,d1 or by increasing the width by about #ej per cent41 or by increasing the slope by about #ajj per cent4 ,if the bed is lowered1 ,v is not affected1 and the mean width is reduced4 ,increase in ,w reduces ,d1 and therefore reduces the hydraulic radius and the velocity4 ,hence the large amount of widening necessary4 ,when ,s is increased the velocity1 if ,r remains the same1 is affected only as ,s 7,hydraulics1 ,,chap4 ,,vi41 ,art4 #b71 but the depth of water is reduced and ,r therefore reduced4 ,dressing the sides of a channel1 so as to make it smoother1 produces the same effect as a slight widening4 ,it does not1 of course1 follow that lowering the bed is always the best plan and straightening the worst4 ,any one of the processes may be more or less impracticable because1 for instance1 of the hardness of the material to be removed1 or the expense--including compensation--of removing obstructions4 ,a particular kind of widening consists in digging a new channel and keeping both the new and the old channel open4 ,if a channel contains a weir1 or a local raised portion of bed forming a kind of submerged weir1 or a contracted place or narrow bridge1 the upstream water-level can be lowered by simply removing or reducing the obstruction4 ,the lowering of the water-level will be greatest at the site of the obstruction1 and will be zero at some point far upstream 7,hydraulics1 ,,chap4 ,,vii41 ,art4 #e74 ,if the raised portion forms a long shoal1 its removal--supposing its height above the general bed to be the same--will have more effect than if it were short4 ,if the height of the raised portion is small compared to the depth of water1 or the amount of contraction small compared to the width of the stream1 the removal may have much less effect than might appear 7,,chap4 ,i41 ,art4 #d74 ,in soft soils one advantage of the straightening system for lowering the water-level is that short-cuts can be dug to a small section1 and left to enlarge themselves 7,,chap4 ,,vii41 ,art4 #a74 ,another advantage is that after any diversions have enlarged themselves to the size of the rest of the channel--or have originally been so excavated--the whole channel may scour1 and the water-level continue to fall4 ,this1 of course1 should be allowed for if likely to occur4 ,the same thing may occur in the case of the removal of a weir1 shoal1 or contracted piece of channel4 ,the scour will act at first close to the site of the obstruction1 but it may work upstream4 ,in widening or deepening a channel for the purpose of mitigating floods1 it is a good plan to begin work at the downstream end1 because the lowering of the water-level will extend upstream beyond the reach in which work is done1 and this may facilitate work further upstream4 ,as regards any tendency for a widened reach to silt up again1 any such silting is not likely to be great in a short period of time1 and need not prevent the carrying on of work in various reaches1 if this is convenient4 #g4 ,flood ,embankments4--,a flood embankment may be close to the edge of the river or it may be set back4 ,if set back it need not follow all the windings of the stream4 ,the setting back of an embankment gives an increased waterway to the stream during floods1 and therefore a lower flood-level1 but the effect of this is trifling in cases where the depth of the water on the flooded land is small1 especially if such land is covered with vegetation1 or is otherwise much obstructed4 ,setting back is generally necessary in cases where the stream is liable to erode the banks to any considerable extent4 ,in such a case the embankment should not be so near to the river as to be in much danger from erosion1 but the ground1 as already stated 7,,chap4 ,,iv41 ,art4 #i71 generally falls1 in going away from the river1 so that when an embankment is set well back it is in lower ground1 more expensive and more liable to breach4 ,the most suitable alignment is a matter of judgment1 and depends largely on the extent to which the river is likely to shift4 ,embankments should1 where possible1 be made in straight or properly curved reaches4 ,a flood embankment1 at least at its upstream end1 should terminate in ground which is above flood-level4 ,the top of an embankment should be1 in the case of a large river1 #b or #c feet above the high flood-level of the river4 ,it should1 of course1 be graded parallel to the general high flood-level1 but neither the gradient nor the height of the flood is usually known with accuracy 7,,chap4 ,,ii41 ,arts4 #a and #b74 ,there is generally a record or mark of some high flood1 and this is taken provisionally as the flood-level4 ,or the level is calculated approximately from the flood readings on the nearest river gauge4 ,if experience shows that the embankment is too low1 it is raised4 ,the cross-section of an embankment depends on the soil1 on the extent of damage which results if a breach occurs1 on the funds available1 and on the value of the land which the embankment has to occupy4 ,where an affluent enters the river it will probably be necessary to run out branch embankments4 ,sometimes cross embankments are run from the main embankment to high land4 ,their object is to localise the damage if a breach occurs4 ,along the back of the embankment there may be a drain and it can be made to discharge its water1 when the river is not in flood1 through the embankment by means of sluices or by pipes closed by flap valves which will not allow flood water from the river to pass through4 ,there may be sluices in the embankment for the purpose of irrigating the land at the back4 ,the immediate effect of the construction of flood embankments along a river is to raise the water-level1 because the floods can no longer spread out over the country1 but this effect will not be great if the sectional area of the flood water was small or its velocity low4 ,the river may or may not tend1 after the construction of flood embankments1 to raise or lower its bed4 ,it has already been remarked that questions of silting or scouring cannot be answered in a general manner4 ,in the case1 however1 of floods spilling over a piece of country1 the depth of the flood water is generally small and the country more or less obstructed4 ,some deposit of silt generally occurs4 ,the construction of an embankment reduces the area of the flood water1 and thus generally reduces the silting and leaves more silt in the river proper4 ,the depth and velocity in the river are increased4 ,everything depends on which is increased most4 ,most likely the stream is of shallow section and the velocity is increased most 7,,chap4 ,,iv41 ,art4 #f1 par4 #f71 and the increased silt-supporting power may make up for the increased charge of silt4 ,sometimes when a main embankment is set far back1 a subsidiary embankment of smaller section is constructed closer to the stream4 ,this is often objectionable4 ,the smaller embankment is liable to breach1 and the water then rises suddenly instead of gradually against the main embankment1 which is thus endangered to some extent1 especially as it is dry instead of being soaked4 ,it is often said that one effect of embanking a reach of a river is to increase the severity of floods further downstream4 ,the importance of this is generally exaggerated4 ,the narrowing of the flood stream in the embanked portion causes the flood to travel more quickly and rise higher in that particular reach4 ,at a place further downstream the same effect is produced1 but in a less degree and only because of the increased velocity and consequent reduction in the flattening out of the flood wave1 especially when the rise is soon succeeded by a fall4 ,when there is a gradual rise lasting for a considerable time--and this is most likely to cause a high flood--there is no rise of the flood-level downstream of the embanked reach1 except such as is due to the increase in the discharge of the stream consequent on the absorption and evaporation being less than before1 owing to the reduced area of flooding in the embanked reach4 ,in the case of a long-continued rise1 such as that just mentioned1 it is the reach immediately upstream of the embanked reach which will1 to some extent1 share in the increased height of the floods4 ,an embankment may suitably have side slopes of #d to #a on the river side and #c to #a on the land side1 with a top #aj feet wide and #c feet above high flood-level4 ,on the ,irrawaddy the top width is generally #h feet4 ,for very high and very low embankments it is #aj feet and #c feet respectively4 ,in ,holland #a foot above high flood-level was at one time supposed to be the rule1 but in practice it was usually #d feet4 ,with sandy soil the riverward slope prescribed was #f to #a4 ,such flat slopes are not necessary if fascining or stiff soil is used as a protection4 ,on the ,rhine the top width of embankments consisting of gravel and sand has been made about #ae feet1 but the side slopes were #a to #a and #a to #a4 ,the embankments had spurs to keep off the current4 ,sand1 protected as above1 makes a good embankment1 and rats do not burrow into it4 ,of course1 if a breach occurs in an embankment consisting mainly of sand1 it will enlarge very quickly4 ,in some cases an embankment has a core wall of sand or of clay puddle4 ,in ,holland1 on sandy soil1 a trench #h feet wide is made and taken down to the clay4 ,embankments require to be made with great care4 ,the earth should be deposited in layers4 ,in ,holland1 horses are driven up and down over each layer4 ,in some parts of ,india the earth for embankments is brought from the borrow pits by scoops drawn by bullocks4 ,the earthwork is of so excellent a character1 owing to the earth being trodden down1 that no settlement has to be allowed for4 ,where the soil is sand the top and faces of the embankment should be of good stiff soil1 if it can be obtained1 for a thickness of #i inches or a foot1 or else the face next the river should be protected by fascining 7,,chap4 ,,vi41 ,art4 #c7 for #b feet above1 and several feet below high flood-level4 ,such protection may be necessary in any case where waves are liable to occur4 ,in ,holland embankments are turfed1 and trees and shrubs are not allowed to grow4 ,in the ,punjab the growth of all kinds of jungle is encouraged4 ,it binds the soil together and protects it from the wash of waves and from winds which blow away sand and dust1 and so wear the embankment slowly away4 ,in embanking a long reach of a river it is convenient to begin from the upstream end1 because otherwise floods may get behind the finished part of the embankment and1 becoming impounded in a 8pocket0 formed by the embankment and high land1 rise to an abnormal height and1 unless gaps in the embankment have been left or are subsequently made1 cause breaches4 ,during high floods pegs should be driven in at frequent intervals1 to mark the high flood-levels4 ,if a higher flood occurs1 the peg is shifted4 ,the levels of the pegs can be observed at leisure4 ,when a breach occurs in an embankment1 the first thing to do is to protect the ends so that the breach shall not lengthen4 ,if the water passing through a breach becomes pocketed1 the embankment may have to be cut to let it out4 ,regarding the stoppage of leakages1 see ,,chap4 ,,ix41 ,art4 #a4 ,regarding the closure of breaches1 see ,,chap4 ,,vii41 ,art4 #b4 ,for a description of flood embankments along the great shifting rivers of ,northern ,india1 see ,punjab ,rivers and ,works4 ,note to ,art4 #e4--;,floods can sometimes be mitigated by sinking pits in the flooded area so that the flood water comes in contact with permeable strata and is absorbed by them4 ,,chapter ,,xiii ,,reservoirs ,,and ,,dams #a4 ,reservoirs4--,the object of a reservoir is to store water for town supply or for irrigation or other purposes4 ,reservoirs for the water supply of towns are divided into 8impounding reservoirs0 and 8service reservoirs10 the latter being of comparatively small size1 and their object being to store1 near to the town1 a supply sufficient for a short period4 ,instead of one impounding reservoir there may be several1 formed by various dams and one discharging into another4 ,when a reservoir is mentioned without qualification1 an impounding reservoir is meant4 ,a reservoir is generally made by blocking up a valley by means of a dam of earth or masonry4 ,the site of the dam should be selected at a place where the valley is narrow4 ,the lowest portion or 8bottom water0 of a reservoir is usually not drawn upon1 because it is less pure than the rest1 and it has to be left1 in dry weather1 for the fish4 ,it is not included in calculating the capacity of the reservoir4 ,in ,great ,britain1 when the water of a stream is impounded1 8compensation water0 has to be given back to the stream lower down4 ,this compensation water is generally given in the form of a constant supply1 and amounts to perhaps a quarter of the available supply4 ,it has to be included in calculating the daily supply taken out of the reservoir4 ,the advantage to the stream in having this addition to it during dry weather is very great4 ,it has already been seen 7,,chap4 ,,ix41 ,art4 #a7 that in an earthen bank which has to retain water the leakage generally decreases rapidly and the bank becomes almost impermeable4 ,the same is true of the surface of a valley1 in the case of most ordinary soils1 provided that it is kept submerged4 ,any portions which become exposed to the sun and weather are likely to crack and give rise to percolation4 ,thus a reservoir formed by the construction of a dam resting on the surface of the ground may be more or less water-tight according to circumstances4 ,there are many which are sufficiently water-tight4 ,but in most cases the dam--or an impervious core-wall--is carried down to an impervious stratum4 ,a masonry dam is carried down to rock4 ,in the case of dams of considerable height the soil should be examined by borings4 ,if there is an inclined stratum not well connected with that below it1 unequal settlement of the dam may occur2 and this may also happen if there is a thick stratum of clay1 owing to its compressibility4 ,except for very high dams--those1 for instance1 more than #aaj or #abj feet in height measured from the ground to the water-level--an earthen dam is cheaper than a masonry dam4 ,it is also more easily raised and strengthened--though this operation has also been effected on masonry dams--in case1 for instance1 of the silting up of the reservoir1 a process which is slow in ,england1 but not so slow when water containing much silt is dealt with1 as in the case of irrigation reservoirs in ,india4 ,sometimes a dam consists of a wall of masonry or concrete with earth behind it as a support4 ,whatever kind of dam is used1 its construction always demands very great care4 ,serious disasters1 with much loss of life1 have occurred owing to failures of dams4 ,a reservoir with an earthen dam is provided with a waste weir for the purpose of passing off flood water1 which might otherwise overtop the dam and destroy it4 ,generally the waste weir is a continuation of the line of the dam4 ,its crest has to be below the high-water level of the reservoir1 but not lower than can be helped1 and its length has therefore to be considerable4 ,sometimes it is provided with grooved piers between which planks are placed in the season when floods are not likely to occur4 ,in connection with irrigation reservoirs in ,western ,india1 it has been pointed out by ,strange 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 cxxxii47 that a long high-level waste weir is best suited to cases in which the replenishment of the reservoir is uncertain1 and that in cases where it is nearly certain1 the high-level weir prevents the water-level in the reservoir being quickly lowered in the case of an accident or for the purpose of effecting repairs1 impounds the earliest floods1 which are most charged with silt1 and causes the water area to be a maximum1 and therefore gives all floods the maximum time in which to deposit silt4 ,he accordingly suggests that the crests of waste weirs in these reservoirs should be shortened and lowered and provided with falling shutters 7this had been done in one reservoir and has since been done in another71 and that sluices be added with sills at a still lower level than the lowered crest4 ,these proposals seem to be entirely reasonable1 though of course it would be necessary to have skilled supervision over the working of the sluices4 ,sometimes the waste weir is made in a separate place1 being separated from the dam by a saddle4 ,a masonry dam may act as its own waste weir1 the flood water flowing over the crest and down the rear slope2 but in cases where heavy floods are liable to occur it is usual to provide a separate waste weir by cutting away the side of the gorge either close to the dam or at some other place4 ,while a dam is in course of construction arrangements must be made to deal with flood water4 ,generally the construction of some part of the dam has to be deferred to let the water pass4 ,in the case of a masonry dam it does not much matter what part is thus deferred provided the usual procedure of stepping the work back is followed4 ,in the case of an earthen dam it is best to defer a portion1 not in the lowest ground where the dam is highest1 but to one side of it1 thus allowing the highest part of the dam to be brought up continuously4 ,temporary embankments and weirs can be constructed to cause the water to traverse the desired route without doing damage4 ,stepping of the earthwork should be avoided as far as possible4 ,if it has to be adopted1 the steps should be small4 ,sometimes the flood water is conveyed away by means of a 8by-wash10 by an entirely different route4 ,illustration3 ,,fig4 #eg4 ,in ,indian reservoirs the discharge over the waste weir may at times be great4 ,the waste weir is sometimes in the position shown in fig4 #eg1 a e being the weir4 ,in such a case a special hydraulic problem arises4 ,in a case where a stream whose velocity is ,v issues from a reservoir or takes off at right angles from a larger stream there is 7,hydraulics1 ,,chap4 ,,ii41 ,arts4 #ai and #bj7 a fall in the water of about ,v#b_/b;g4 ,the same thing occurs downstream of a weir1 at least when there is a clear fall which is vertical or nearly so1 so that the water after falling has no horizontal velocity4 ,the water has to be started afresh on its course4 ,in the case represented by the figure1 the width of the channel is often restricted because of high ground beyond f1 and the velocity in the channel may be very high4 ,suppose the channel below e f to be of brickwork with vertical sides1 and to have a #bj-;foot bed1 a slope of #a in #ejj1 and a depth of water of #aj feet4 ,the velocity may be #ae feet per second1 and ,v#b_/b;g is #cdi feet4 ,if the water has a clear fall over the weir at e1 allowance must be made for a depth of water of #acdi feet1 not #aj feet1 in the channel at e4 ,ordinarily the length a e will be much greater1 relatively to e f1 than shown in the figure4 ,suppose that a e is #cjj feet and that the slope of the floor of the channel is carried on at #a in #ejj from e f up to a1 b1 c1 and d1 following in each case the lines marked on the figure which represent the directions of flow4 ,the length f a will be about #caj feet1 and the floor level at a will be about #fb feet higher than at e f4 ,the water-levels below the weir will be in each case #acdi feet above the floor4 ,this should be allowed for in the design4 ,it is true that the stream on first starting into horizontal motion below the weir moves more or less at right angles to it1 and has thus a large sectional area and a low velocity2 but it very soon has to turn parallel to the weir and acquire the full velocity of #ae feet per second1 and there must be the requisite extra head to give this velocity4 ,if the weir is drowned1 the water on passing over it may have a high horizontal velocity1 but it will be at right angles to the axis of the channel1 and its effect will be wasted in eddies4 #b4 ,capacity of ,reservoirs4--,a reservoir depends for its supply on the yield of a particular valley or valleys which form its catchment area1 and the capacity of the reservoir or reservoirs can be altered by altering the height or number of the dams4 ,the need for a reservoir is entirely owing to the inequality in the distribution of the rainfall4 ,if the rain fell in equal quantities week by week1 the daily fluctuations could probably be equalised by the service reservoirs4 ,the impounding reservoir could be quite small4 ,actually1 a reservoir is needed to 8equalise0 the flow--that is1 to give a steady flow for an intermittent one4 ,the smaller the reservoir1 the sooner it will go dry in a drought and the sooner it will overflow in wet weather and cause waste of the water4 ,in other words1 the larger the reservoir the better it will fulfil its function of equalising the flow and the greater the degree to which the catchment area will be utilised4 ,illustration3 ,,fig4 #eh4 ,in the ,british ,isles the distribution of the rainfall which is most trying for a reservoir1 occurs when the rain is heavy during the winter and very light in summer4 ,fig4 #eh shows a diagram for a reservoir in the driest year1 when the rainfall is 7,,chap4 ,,ii41 ,art4 #a7 #fc of the mean annual fall4 ,the distribution of the fall is supposed to be unfavourable as just described4 ,the lower part of the figure shows the water-level at the end of each month1 the reservoir being supposed to have vertical sides so that the quantity of water in it is proportional to the depth of water4 ,the upper part of the figure shows the water impounded 7available fall multiplied by area of catchment7 in full lines1 and the consumption in a dotted line4 ,the distance between the two lines in any month is the same as the rise or fall of the reservoir in that month4 ,there is supposed to be no overflow1 and the total consumption of water in the year is equal to the quantity impounded in the year1 so that the levels of the reservoir water surface on #ast ,january and #cast ,december1 as shown by the horizontal lines ,a1 ,b at the left and right of the figure1 are the same4 ,deacon1 who has investigated the subject1 has found 7,ency4 ,brit41 ,tenth ,edition1 vol4 #cc1 8,water ,supply07 that1 in order to satisfy the above conditions1 the capacity of the reservoir must be #cj per cent4 of the water impounded during the year1 or about #aaj days' consumption4 ,on #ast ,january the reservoir must be about two-thirds full4 ,at the end of ,february it is ready to overflow4 ,at the end of ,august it is just becoming dry4 ,the daily consumption is supposed to be steady throughout the year4 ,as an instance1 suppose the catchment area to be #ajjj acres1 the mean annual fall #fj inches1 with a loss from evaporation and absorption of #ad inches4 ,the available rainfall of the year is 7see last column of table below7 #bch inches1 or #aihc feet4 ,the water impounded and consumed during the year is #ajjj #dc1efj #aihc #fbe #eci1ifb1jjj gallons4 ,the reservoir capacity must be #c_/ajths of this1 or #afa1ihh1fjj gallons4 ,this is represented by the height ,c ,e4 ,if the mean available rainfall in ,january and ,february is #fc inches1 or #ebe feet1 the water impounded during those months is #ajjj #dc1efj #ebe #fbe #adc1ica1jjj gallons1 and the consumption is #eci1ifb1jjj_/f #hi1iic1ffg gallons4 ,the difference1 #ec1icg1ccc gallons1 represents the addition ,a ,c1 to the reservoir4 ,similarly1 the light summer rainfall causes the depletion ,a ,e1 and the heavy rainfall in the last four months of the year the addition ,e ,b4 ,if the height of the reservoir above ,a ,b were less than ,a ,c1 there would be overflow at the end of ,february2 and if the depth below ,a ,b were less than ,a ,e1 the reservoir would go dry before the drought ended4 ,if the capacity of the reservoir were increased either at the top or bottom1 the cost would be increased and nothing would be gained4 ,it is not meant that the highest and lowest levels of any reservoir designed as above would always1 in the driest year1 exactly correspond with the points of overflow and going dry1 but they would do so nearly4 ,deacon states that such a reservoir would fail only once in fifty years1 and then only for a short time4 ,the reservoir considered above does not1 as already remarked1 fully utilise the yield of the catchment area4 ,in a wetter year there would be overflow and the yield from the reservoir would not be much increased4 ,in order to equalise the flow of the two driest years the capacity of the reservoir must be increased1 its yield being also increased1 and so on for larger groups of years4 ,by collecting information for large numbers of places in the ,british ,isles1 ,deacon has prepared diagrams and tables which show the capacities and yields of reservoirs4 ,the following table gives the figures for the case where the rainfall is #fj inches and the loss by evaporation and absorption #ad inches3-- ---------------------------------------- ---------------------- ,number of ,column #b ,driest ,net ,capacity ,column #c ,consecutive of ,reservoir or ,number,ratio of ,years the for a ,daily of ,days' ,rainfall,available ,flow of ,catchment ,yield of ,supply to ,mean ,rainfall4 which ,area of #ajj ,reservoir4 contained ,annual is to be acres4 in the ,fall4 ,equalised4 ,reservoir4 ---------------------------------------- ---------------------- ,gallons4 ,gallons4 ,inches4 #a #aff1jjj1jjj #a1dge1jjj #aac #fc #bch #b #beh1jjj1jjj #a1hae1jjj #adb #gb #bib #c #cbi1jjj1jjj #a1ihg1jjj #afe #gg #cbb #d #cij1jjj1jjj #b1ajc1jjj #aij #hj #cdj #e #dda1jjj1jjj #b1ahg1jjj #bja #hb #ceb #f #dhg1jjj1jjj #b1bee1jjj #baf #hce #cfa ---------------------------------------- ---------------------- ,the figures in the fifth column are those given in ,,chap4 ,,ii41 ,art4 #a4 ,the figures in the last column show the corresponding available falls1 after deducting the loss of #ad inches4 ,it will be seen that1 owing to this deduction1 the available falls for the shorter periods are reduced in a greater ratio than the figures in the fifth column4 ,in arranging for the supply of towns in the ,british ,isles it is usual to design the reservoirs so as to equalise the flow of the three driest consecutive years4 ,existing reservoirs1 old and new1 usually contain from #adj to #agj days' supply1 but some contain less4 ,the above table shows that for the assumed fall of #fj inches and loss of #ad inches1 the capacity of a reservoir1 to allow for a six-year dry period1 has to be #di per cent4 more than for a three-year dry period1 while the daily supply from it is only #ac per cent4 greater4 ,the following statement gives ,deacon's figures for mean annual rainfalls ranging from #cj to #ajj inches4 ,the columns marked ,r show the reservoir capacities in millions of gallons1 and those marked ,s the daily yields of the reservoirs in thousands of gallons4 ,the figures for other falls can be interpolated4 ,for a fall of1 for instance1 #ej inches1 the figures1 whether of ,r or ,s1 are practically a mean between those for falls of #dj and #fj inches4 ---------------------------------------- ----------------- ,number ,f #cj4 ,f #dj4 ,f #fj4 ,f #ajj4 of ,years whose ,supply is to be ---------------------------------------- --- ,equalised4 ,r4 ,s4 ,r4 ,s4 ,r4 ,s4 ,r4 ,s4 ---------------------------------------- ------------- #a #ce #cjj #gi #fie #aff #adge #cde #cjdj #b #he #dgj #adj #ijj #beh #ahae #die #cfjj #c #abj #efj #aij #ajej #cbi #aihg #faj #cijj #d #aej #fbj #bcj #aaaj #cij #bajc #gaj #dajj #e #age #fej #bfj #aagj #dda #bahg #hjj #dbcj #f #aie #fhj #bij #abbj #dhg #bbee #hhg #dcbj ---------------------------------------- ------------- ,in all cases the loss is supposed to be #ad inches annually4 ,if it is #ae or #ac inches1 the reservoir capacity is less or more by about five1 ten1 or fifteen million gallons1 according as the number of years in column #a is #a1 #c1 or #f4 ,and the daily yield is less or more by about #ej1jjj gallons4 ,with a low rainfall the advantage of a large reservoir is somewhat increased4 ,the capacity of the six-year reservoir for a fall of #cj inches is #fc per cent4 more than that of the three-year reservoir1 but the supply is #bb per cent4 greater4 ,the figures given above for reservoir capacities are suitable for the ,british ,isles4 ,they assume that the distribution of the rainfall is the least favourable that is at all likely to occur4 ,deacon states that the figures do not relieve the engineer of the exercise of judgment4 ,as regards the ,british ,isles1 the chief questions on which judgment has to be exercised are whether to equalise the flow of three years or of another number1 and how much to allow for loss4 ,as already stated1 three years is the period usually taken4 ,the figures are suitable for most places in ,europe1 but in some places1 e4g4 on the ,mediterranean coast1 the distribution of the rainfall is somewhat less favourable than in the ,british ,isles4 ,in other parts of the world1 and notably in or near the tropics1 the distribution of the rainfall must be specially studied1 and a diagram be prepared on the same principle as in the case of fig4 #eh4 ,the diagram should be extended to cover the desired number of years4 ,in hot countries loss by evaporation from the surface of the reservoir should be allowed for4 ,in ,india during the hot dry months this loss may be half an inch in twenty-four hours4 ,in the article above quoted it is shown that if1 as commonly happens1 the consumption of water is1 in summer1 greater than the mean1 and in winter less1 the conditions are still more trying for the reservoir2 and that in the case where the summer consumption is #ac per cent4 greater than the mean1 the capacity of the reservoir which impounds the water of the driest year must be #cc per cent41 instead of #cj per cent41 of the total supply impounded during the year4 ,it would then contain #aba days' instead of #aaj days' mean supply4 ,in the table on page #agj the number of days' supply is #aac4 ,from this it appears that the tables from which extracts have been given are calculated on the basis of a constant consumption4 ,this1 however1 in the case where the number of years whose supply is equalised is greater than one1 makes1 owing to the increased size of the reservoir1 no practical difference4 ,the calculations for the great reservoirs in ,radnorshire for the supply of the city of ,birmingham are as follows 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxx474 ,the ratio of the mean fall in the three driest years to the mean annual fall was taken as #hj instead of #gg4 ,there is some difference of opinion as to the best figure3-- ,mean annual fall determined from readings of various gauges #fe inches ,mean fall of three driest years #eb 0 ,deduct loss from evaporation and absorption and losses during floods #ae 0 ,available rainfall #cg 0 ,this multiplied by #dd1jjj acres1 the area of the catchment1 gives #ajb million gallons per day4 ,of this1 #bg1jjj1jjj gallons is compensation water1 leaving #ge1jjj1jjj gallons for ,birmingham4 ,capacity of reservoirs1 #ag1bej1jjj1jjj gallons1 or #afi days' supply4 ,illustration3 ,,fig4 #ei4 #c4 ,earthen ,dams4--,before an earthen dam is made1 any soft soil on the site should be removed and the ground downstream of the site should be drained4 ,a few trenches1 running parallel to the axis of the dam1 can be dug so as to give the dam a hold1 though there is never any danger of its being moved horizontally by the thrust of the water4 ,if the ground has a side-long slope it should be benched as shown in fig4 #ei4 ,the front slope of an earthen dam is generally about #c to #a1 and the rear slope about #b to #a4 ,the top has a width of to the greatest depth of water held up1 and is #e to #aj feet above the highest water-level4 ,the borrow pits from which the earth for the dam is got should not be near enough to it to in any way affect its stability4 ,illustration3 ,,fig4 #fj4 ,in ,england1 and generally in other countries1 an earthen dam has a core-wall 7fig4 #fj7 which is carried down to an impervious stratum1 and is keyed into it to a depth of a foot or more in the case of hard rock and several feet in the case of clay4 ,on this core-wall the impermeability of the dam chiefly depends4 ,the core-wall may be of clay puddle1 concrete1 or masonry4 ,in ,england it is generally of clay puddle4 ,the core-wall sometimes extends down to a depth of #ajj or #bjj feet4 ,its top is horizontal and about level with the highest water-level4 ,it is desirable not to make the foundation stepped1 but to let it follow the profile of the impervious stratum4 ,the wall is keyed at its ends into the sides of the valley or gorge4 ,a core-wall of concrete or masonry is1 in a high dam1 necessarily a comparatively thin structure1 and it may be subjected to great strains by unequal pressures of the earth which surrounds it4 ,it is therefore to some extent liable to crack4 ,a core-wall of concrete used for the water-works of ,boston1 ,u4,s4,a41 is #ajj feet high1 #h feet thick at the base1 and #d feet thick at the top4 ,a clay-puddle wall1 being plastic and moist1 at least during the period immediately succeeding the construction of the work1 is not very liable to crack4 ,the top width of a puddle wall may be #d to #aj feet1 and the batter of the sides from #a in #bj to #a in #h4 ,the clay used for the wall above the ground-level should contain about #cc per cent4 of sand and stones4 ,this diminishes its shrinkage if it dries4 ,it should not be given too much water in mixing4 ,it should be thoroughly mixed and worked up and trodden down4 ,the clay puddle and the earth of the dam should be carried up uniformly4 ,the allowance for settlement may be #a_/cj to #a_/ej4 ,the earth should be deposited in thin layers1 moistened and rammed1 and all clods broken4 ,in ,india and some other countries1 instead of the earth being rammed1 cattle or sheep are driven over it repeatedly4 ,this makes earthwork of most excellent quality1 and the settlement1 if any1 is very small4 ,in cases where1 owing to a fissure in the rock below the bottom of the puddle trench1 water comes through under the puddle1 it is usual to carry it away in a pipe running vertically in a groove up the side of the trench and then horizontally till it emerges from the dam4 ,such water1 and any other leakage1 can often in ,great ,britain be used as part of the compensation water4 ,there is1 however1 a certain chance1 when there are water-bearing fissures in the rock below the bottom of the trench1 that some percolating stream of water may wash away the puddle1 and it is preferable to use a concrete core-wall in such cases1 carrying it up to about ground-level and keying it into a much thicker wall of puddle which is carried up to the water-level4 ,it has been suggested that the clay puddle or other impervious layer should be placed1 not vertically and in the middle of the dam1 but lying on the upstream face of the dam1 so as to keep out water from the whole dam instead of from only half of it4 ,objections to this1 if clay puddle is used1 are that vermin may bore holes in it1 and that1 with some clays1 it would slip4 ,these objections might be overcome to some extent by laying a pitching of concrete blocks over the puddle4 ,other objections1 applying also when masonry or concrete is used1 are that the superficial area and cost are increased1 and that cracks would occur from settlement of the earth and from changes of temperature when the water in the reservoir was low4 ,a good many cases have occurred in which an impervious layer laid on the slopes has failed from one cause or another4 ,in ,france it is usual to rely on such a layer--concrete is used--and to dispense with a core-wall4 ,the practice of having a vertical wall appears to be the best1 and is the most widely adopted4 ,when puddle is used the weight of the mass above it forces it to completely fill the trench1 and when once it is in position and covered up it is not at all likely to be damaged4 ,illustration3 ,,fig4 #fa4 ,the outer portion of a high embankment sometimes slips 7fig4 #fa71 and precautions should be taken against this4 ,a slip may occur if the site of the dam has not been carefully selected as to geological formation1 or if there is unequal settlement owing to the work having been done at different times4 ,one cause of slips is sudden and partial changes in the degree of saturation1 and another cause is excessive saturation4 ,some clays when wet require extremely flat side slopes1 and will not stand even at #e to #a4 ,the outer parts of the embankment are not required for stopping percolation 7this will be further considered in the following paragraph71 and1 though they must be carefully laid and consolidated1 they should be of porous material1 and the part on the downstream side of the dam must be well drained4 ,a series of surface drains may be arranged and filled with loose stone and gravel4 ,there is also a distinct advantage in using heavy material such as small stone for the lowest portion of the outer parts on both sides of the dam4 ,when good material cannot be obtained1 the side slope on the downstream side of the dam may be flattened4 ,a side slope starting at the top with #c to #a and becoming #d to #a lower down1 and finally #e to #a at the base1 is a very good form for prevention of slipping and generally for the safety of a dam4 ,the part on the side next the reservoir is not likely to slip4 ,it becomes soaked1 but it has the pressure of the water against it and is pitched4 ,in ,madras1 where reservoirs are very numerous1 the slope on the side next the water is generally only #a to #a4 ,the different parts of an earthen dam fulfil two distinct functions4 ,some parts1 which may be called the staunches1 have to stop the percolation of water from the reservoirs4 ,other parts1 which may be called the supports1 have merely to hold up the staunches4 ,in the ,british type of dam the portion nearest the core-wall on either side 7fig4 #fj7 is generally made of earth specially selected for impermeability4 ,the distance to which it extends from the wall depends partly on the quantity of such earth available4 ,in any case it has to be very carefully made and consolidated1 to avoid unequal pressures on the core-wall1 or unequal settlement which might cause it to part from the wall4 ,one of its functions is to keep the core-wall moist when the water-level in the reservoir falls4 ,whether it is also to be considered as a staunch or a support might at first appear to be of no consequence1 but it is of importance as affecting the question of drainage4 ,the support on the downstream side of the dam must1 as has just been seen1 be made of porous material and be well drained2 but obviously a staunch must not be porous1 nor can it be penetrated by drains4 ,the question must be decided in each case according to judgment4 ,in a discussion which took place on the above-mentioned paper by ,strange1 at the ,institution of ,civil ,engineers1 much diversity of opinion was expressed among eminent engineers as to the desirability of draining the downstream half of the dam1 i4e4 the part downstream of the core-wall4 ,by some it was urged that drainage is necessary to lessen the chance of the earthwork slipping4 ,others contended that any drain which penetrates the dam must facilitate the percolation of water from the reservoir4 ,it is clear that some of the speakers regarded the dam downstream of the core-wall as being partly staunch1 and some as being wholly support4 ,if for any reason there seems a chance of water leaking through the core-wall1 it is desirable to regard the earth-filling next to it as staunch4 ,in ,western ,india a kind of puddle is made by mixing three parts of 8black cotton soil0 with two of sand4 ,the object of the puddle wall is only to prevent water from finding its way along the surface of the ground4 ,it is carried down only to a fairly water-tight stratum and is carried up only to #a foot above the ground4 ,above that the mass of the dam is made of black cotton soil as a staunch1 with more porous material on both sides of it4 ,in order to afford full protection against waves and their splashes1 the pitching on the upstream face of a dam should extend up to a height of #e feet1 measured vertically1 above the highest water-level4 ,in the case of a dam in which the 8fetch0 or distance over which the waves have been in process of formation exceeds two miles1 the above height should be slightly increased4#ba ,the pitching is usually of stones roughly squared at their outer ends and laid on a layer of broken stones4 ,the water from a reservoir is usually drawn off by means of pipes which are laid inside a masonry culvert built under the dam4 ,the pipes can thus be inspected4 ,the culvert is blocked at its upstream end by a thick masonry wall through which the pipes pass4 ,accidents which have happened in the past have been due to weakness of the culvert or to water finding its way along the outside of the masonry4 ,the culvert can be made of proper strength1 and it should have a thick coating of clay puddle which is worked into the clay-puddle core-wall of the dam4 ,if the core-wall is of masonry or concrete1 the masonry of the culvert is properly joined to it4 ,in many cases the culvert and pipes are taken through a cutting or tunnel and not under the dam4 ,at the upstream end of the culvert there is a masonry tower--access to it is obtained from the top of the dam by a foot-bridge1--and from it valves for opening and closing the pipes are worked4 ,if the reservoir is for the water supply of a town1 it is arranged1 by means of a vertical pipe1 that the draw-off can be at various levels so that the surface-water can always be used4 ,in the case of some of the towers at the reservoirs whence ,birmingham is now supplied1 the vertical pipe consists of a number of steel cylinders with gun-metal faces which are so accurately made that the joint is water-tight when one cylinder merely stands on another4 ,the draw-off is obtained from a given level by lifting a particular number of cylinders4 ,sometimes the tower is made of reinforced concrete4 ,when it is lofty it should be strong enough to resist a strong wind1 blowing when the reservoir is empty4 #d4 ,masonry ,dams4--,for heights much exceeding #aaj or #abj feet a masonry dam may be cheaper than an earthen dam2 and in case a flood occurs while work is in progress the masonry might suffer little injury1 while earthwork might be swept away completely4 ,masonry dams are usually built of random rubble masonry with faces of dressed stone4 ,such masonry weighs about #adj lbs4 per cubic foot1 and is ordinarily quite safe when subjected to pressures of #bj tons per square foot1 but in a masonry dam a high factor of safety is necessary1 and #ae tons per square foot may be allowed4 ,in a wall of such masonry with both faces vertical1 the pressure1 owing to the weight of the wall1 will reach the above limit when the wall has attained a height of about #bbj feet4 ,in a masonry dam1 although the masonry is always of the best quality1 it is a rule to calculate the dimensions so as to give no tension on any part of the masonry4 ,any crack or opening of a joint1 occurring perhaps before the masonry had hardened1 would let in water1 and its pressure would tend to gradually extend the crack and eventually to overturn the portion above the crack4 ,fig4 #fb shows the upper part of a masonry dam4 ,the lines with arrows show the vertical force due to the weight of the masonry above ,a ,b1 the horizontal force due to the water-pressure on it--acting at two-thirds of the depth1--and the resultant of these two4 ,in order that there may be no tension on the masonry1 the resultant must always fall within the middle third of the thickness of the dam4 ,in order to prevent its falling outside the middle third1 the downstream face must be splayed out1 and the splay will go on increasing somewhat4 ,suppose1 now1 that the reservoir is laid dry4 ,it will be found that in the case of a dam more than #ajj feet high the pressure due to the weight of the wall alone will fall outside the middle third--to the upstream side of it1 of course--of the thickness of the wall1 and a slight splay must be given to the upstream side4 ,the vertical pressure of the water on this splayed part must be taken into consideration4 ,the limit of pressure1 #ae tons per square foot1 may eventually be reached owing to the height of the dam1 and additional splay may have to be given for this4 ,when the outside splay becomes considerable a further allowance is made for it1 because the stress at the edge of a horizontal section is tangential to the face4 ,in order that the tangential stress may not exceed #ae tons per square foot1 the vertical stress at the outer edge of a horizontal section of the dam must not exceed about #ab tons4 ,by following the above rules the section of the dam can be calculated1 beginning from the top and working downwards4 ,the resulting profile of the dam is somewhat as shown in fig4 #fc4 ,if a masonry dam is designed on the principles given above--that is1 so as to be safe as regards crushing and overturning--it will be safe as regards shearing or sliding horizontally1 but a test calculation can easily be made for this4 ,illustration3 ,,fig4 #fb4 ,illustration3 ,,fig4 #fc4 ,calculations of the above kind do not1 of course1 enable all the stresses in a solid mass of masonry to be found4 ,great stresses are caused by expansion and contraction owing to changes in temperature4 ,others are caused by the connections of the dam with the rock on which it rests and with the sides of the gorge4 ,the method of calculation described above indicates a suitable form for the profile of a dam4 ,the large factor of safety adopted allows for other stresses4 ,the sections of the oldest dams1 made in ,spain1 were somewhat as shown in fig4 #fd1 and contained about twice as much material as was necessary4 ,the object of the calculations is to save this needless expenditure4 ,masonry dams designed on the above principles have been constructed for heights ranging up to nearly #cjj feet1 measured from the foundation to the top4 ,the foundation is always on hard rock free from fissures4 ,generally a foundation trench is cut4 ,the ends of the dam are carried into the rock on the sides of the gorge4 ,they should not1 however1 if the sides of the gorge are steep1 be built in with mortar1 but be allowed to expand and contract vertically1 a water-tight joint being made by means of asphalt 7,ency4 ,brit41 ,tenth ,edition1 vol4 #cc1 8,water ,supply074 ,this obviously reduces the straining4 ,a dam should be built in cool weather1 so that any stresses to which it will eventually be subjected owing to changes in temperature will be chiefly compressive4 ,the upstream face should be as water-tight as possible4 ,there should not1 however1 be too sudden a change in the character of the masonry from the face work to the inside work4 ,if there are any springs1 they must be carefully connected to pipes and carried outside the dam4 ,no water must be permitted to get under or inside the dam1 either from springs in the sides of the gorge or from the water in the reservoir4 ,many existing dams leak slightly where they join the sides of the valley1 and most have developed some vertical cracks normal to the face4 ,illustration3 ,,fig4 #fd4 ,out of some hundred high masonry dams which have been erected1 only three are known to have failed4 ,of these1 the ,puentes dam was partly founded on piles2 and in two1 the ,habra and ,bouzey dams1 the rule of the middle third was not attended to4 ,another dam1 not so high1 the ,austin dam1 in ,texas1 ,u4,s4,a41 failed seven years after construction4 ,it was #fe feet high and founded on limestone1 the width of the base being #ff feet4 ,springs in the bed and sides of the gorge had1 during the construction of the dam1 given much trouble1 and had1 after its completion1 forced their way through the underlying rock4 ,at the time of failure #aa feet of water was passing over the dam1 which sheared in two places1 a length of #ddj feet of it being pushed forward for #dj or #ej feet without overturning1 but subsequently breaking up4 ,the dam was founded in a trench cut in the rock4 ,the rock on the downstream side of the foundation trench appears to have been worn away by the water1 so that there was no longer a trench 7,scientific ,american1 #bhth ,april #aijj74 ,the above1 however1 does not seem to be sufficient to account for failure4 ,the horizontal water-pressure on a #a-;foot length of the dam would be #ahj1jjj lbs4 and the weight of masonry to be moved perhaps #cbj1jjj lbs4 ,it seems probable that water from upstream found its way under the dam and exercised a lifting force on it and so caused it to slide4 ,if a masonry dam1 instead of being straight1 is made curved on plan1 with its convexity upstream1 it acts as an arch1 and its thickness can1 in the case of a fairly narrow gorge1 be greatly reduced4 ,this type of dam is a suitable one to use when the sides of the gorge are of firm and solid rock and there is no doubt about their being able to stand the thrust without yielding4 ,several dams of very considerable size have recently been built in this way4 ,the thickness of the upper part of the dam and the ratio of the versed-sine of the arch to the span can be decided on by the methods used for arches in general4 ,the lower part of the dam is made thicker4 ,the lowest part cannot act as an arch1 because it is attached to the foundation4 ,it is1 however1 assisted by the portion above it1 which acts as an arch1 and thus need not be so thick as in a 8gravity0 section4 ,the ,bear ,valley dam1 which is #fd feet high1 is only #c feet thick at the top4 ,the thickness increases gradually to #h feet at #dh feet from the top4 ,the chord of the curve is #bej feet and the radius of curvature #cce feet4 ,if the gorge is wide1 the thickness of the arch comes out so great that nothing is saved by adopting the curved form4 ,but in such a case1 and in any case1 a dam can be made slightly curved so as to offer a greatly increased resistance to overturning4 ,it need not act as an arch1 and can be prevented from so acting1 in order that excessive stresses may be avoided1 by letting the ends of the dam1 after they have entered the grooves cut in the sides of the gorge1 stop short of the ends of the grooves4 ,illustration3 ,,fig4 #fe4 ,during the last few years much attention has been given to the investigation of the stresses to which a masonry dam is subjected4 ,some investigations have been theoretical and others practical1 models of india-rubber and other substances having been used for experiment4 ,the investigations show that generally the stresses in a model of a dam are very much the same as would be expected1 but that there is a tensile stress1 previously overlooked1 near the point ,m 7fig4 #fe71 where the dam rests on its foundation4 ,the tension is on the foundation1 on the line ,m ,n1 and is due to the horizontal thrust of the water4 ,it is natural that in an elastic model this stress should manifest itself by deformation4 ,in the case of an actual dam resting on rock1 matters are different2 but this tensile stress deserves consideration4 ,for the present let it be supposed that there is no trench1 the dam merely standing on the rock4 ,suppose that the rock has only the thickness ,m ,r4 ,there is tension in ,m ,n1 and probably compression in ,n ,r4 ,it is assumed that1 along the base ,m ,p1 there is perfect union between the dam and the rock4 ,the tension to which the rock is occasionally subjected owing to changes of temperature may exceed any tension due to the water-pressure1 but it is conceivable that the tension occurring from both causes might cause a crack at ,m ,n1 and that this might extend to ,r4 ,this implies a minute sliding of the dam and of the rock below it1 movement taking place on the plane ,r ,q4 ,the thrust of the water is now resisted by the rock downstream of ,p ,q4 ,the dam1 with the rock ,m ,r ,q ,p adhering to it1 tends to rotate about the point ,p4 ,the tendency to rotate will be enhanced if water enters ,m ,r1 and still more if it enters ,r ,q4 ,no rotation can1 however1 take place unless the rock at ,m ,r is splintered away4 ,the rock would also have to fracture at ,p ,q4 ,it has been suggested that the upstream face of the dam be made curved as shown by the dotted line4 ,this would shift the chief tension to m n1 and the dam1 with the rock beneath it and the weight of the water above the curved portion1 would obviously offer an increased resistance to rotation about ,p4 ,the cost of the dam would of course be increased4 ,the danger of a crack forming at ,m ,n seems to exist only when there is a thin upper stratum of rock not firmly connected to rock below4 ,when this condition is believed to exist1 a masonry dam1 if built at all1 should have the upstream face curved as above described4 ,in the case of any existing dam of great height1 when the above condition is suspected to exist1 the reservoir might be laid dry1 and if any crack at ,m ,n is discovered a curved portion could be added2 but in this case the union between the new and the old work would be imperfect1 and the curve should start from high up on the upstream face of the dam4 ,it has been suggested that asphalt or some impervious material be laid on the rock to prevent water from entering any crack4 ,it would1 however1 not only have to be laid upstream of the dam1 but to extend under part of the dam1 and thus weaken it to some extent4 ,in the case 7fig4 #fc7 in which the dam is founded in a deep trench1 the building up of the upstream triangular space and uniting the material both to the dam and to the side of the trench1 might be of some use1 but a crack might form in it4 ,it would be desirable to add a curved portion1 as above described1 on the top of the rock if sound1 or to remove the unsound rock and widen the trench and then add the curved portion4 ,adding material to the downstream triangular space1 and uniting it well1 would also increase the resistance of the dam to overturning1 not so much because of the additional weight1 as because of the raising of the point about which the dam would have to revolve in overturning4 ,several recent dams have been built of cyclopean concrete1 blocks of rock as heavy as #aj tons being sometimes used in the work4 ,such blocks are laid on one of their flat faces4 ,in the ,u4,s4,a4 some reservoirs have been made with walls of reinforced concrete1 backed by earth embankments 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxxxix471 and also of cyclopean masonry reinforced with steel rods4 ,another kind of dam which has been used in the ,u4,s4,a4 is the rock-fill dam with a core--corresponding to the puddle wall in an earthen dam--of steel plates riveted together and made water-tight and inserted into the rock at each side4 ,in the case of the ,east ,canyon ,creek ,reservoir1 ,morgan1 ,utah1 the dam is #aaj feet high4 ,the steel plates vary in thickness from #c_/h-;inch at the bottom to -inch at the top1 and are embedded in asphaltum concrete and rest on a concrete base4 ,the dry-stone work of the dam is hand-packed on both faces1 and also on both sides of the core4 ,the rest is thrown in4 ,the upstream face is #a to #a1 and the downstream face #b to #a4 ,the waste weir is at one end of the dam and is continued by a flume1 so that the water falls clear of the dam4 ,the outlet is a tunnel in the rock4 ,,chapter ,,xiv ,,tidal ,,waters ,,and ,,works #a4 ,tides4--,the tides or 8tidal waves0 are caused by the attraction of the moon and the sun4 ,the phenomena are complex1 and a full discussion of their causes need not be given here4 ,when the tide rises it is said to 8flow10 and it is called the flood tide2 when it falls it is called the ebb tide4 ,the period between one tide and the next1 e4g4 from high water to high water1 is about twelve hours1 twenty-five minutes4 ,at a spring tide the range of the tide is greater than usual2 at a neap tide less4 ,where there are channels1 as1 for instance1 the seas which surround the ,british ,isles1 the tidal waves run up them as the tide rises in the neighbouring ocean1 and run back as it falls4 ,at some places1 as ,southampton1 the tide comes in from two directions1 and there is a double tide4 ,the times and levels of high and low water at various places have been ascertained by observation1 and are recorded4 ,the levels are1 however1 liable to be affected by winds4 ,a wind blowing towards the shore raises the level of both high and low water2 a wind blowing off shore lowers both levels4 ,a severe storm in the ,north ,sea has caused a double tide at ,london ,docks1 by accelerating the ,north ,sea tidal wave4 ,in a funnel-shaped estuary1 especially if it faces the direction of the tidal wave in the sea1 the tide in going up the channel increases in velocity1 and the momentum of the water causes it to rise higher and higher as the width decreases4 ,at the upper end of the ,bristol ,channel the range of the tide is double the range in the sea outside the channel4 ,the ,bay of ,fundy is another place where a similar phenomenon occurs4 ,when a river or estuary is shallow and the range of the tide is great1 so that its rise is rapid1 the flood tide in some cases advances in the form of a wave or 8bore10 causing a sudden rise in the water-level and a sudden reversal of the flow of the stream4 ,a bore is most pronounced at spring tides4 ,that of the ,severn is well known4 ,in the case of a tide running along a coast or up an estuary1 the water of the flood tide1 after it has ceased to rise1 continues for a short time1 owing to its momentum1 to flow in the same direction as before4 ,the same thing happens when the ebb tide ceases to fall4 ,the tide also acquires special velocity1 just as a river does1 round any projecting headland4 ,the rise and fall of the tide are least rapid near the turns of the tide4 ,if the time from the beginning to the end of the flow be divided into six equal parts1 the proportional rise of the water will be approximately as follows4 ,and similarly with the fall during the ebb4 ,time #a #b #c #d #e #f ,rise of water #jfg #be #e #ge #icc #a ,tidal waters are frequently charged1 more or less1 with silt1 obtained from the shore or from shallows near it1 either by currents or tidal waves sweeping along it1 or by the action of ordinary waves4 ,tidal waters flowing up and down the lower portions of rivers render them to an enormous degree more capable of carrying navigation and1 especially if they become enlarged and form estuaries1 more capable of being altered by training works4 ,a tide-gauge is constructed on the same principle as a self-registering stream-gauge4 ,the rise and fall of the water are reduced1 by mechanism1 to a convenient range1 and are recorded on a band carried on a drum1 which is caused to revolve by clockwork4 ,another kind1 which depends on the use of an inverted syphon filled with air and a syphon of mercury1 is described in ,min4 ,proc4 ,inst4 ,c4,e41 vol4 clxiv4 ,illustration3 ,,fig4 #ff4 #b4 ,tidal ,rivers4--,let ,a ,b 7fig4 #ff7 be the surface of the lower part or mouth of a river1 supposed to be of uniform width1 and let ,b be the mean sea-level4 ,as the tide rises to ,d the water of the river is headed up and assumes the line ,a ,d4 ,when the tide falls to ,f there is a draw1 the river surface taking the line ,a ,f4 ,if the rise of the tide ,b ,h is so great that the discharge of the river cannot keep pace with it1 so as to fill up the whole space between ,a and ,h to the level of ,h1 there will be a flow of sea water from ,h to some point ,m1 and of river water from ,a to ,m4 ,the point ,m will be lower than ,a and ,h4 ,if the tide now turns and the water-level ,h begins to fall1 there will still be a flow along ,h ,m4 ,for a brief period it will be due to momentum1 but it will continue until1 by the rise of the water-level at ,m and the fall at ,h1 the surface has assumed the form indicated by the dotted line ,a ,n ,j4 ,while this is happening1 the point corresponding to ,m--where the concave curve of the upland water meets the convex curve of the tidal water--rises higher and shifts seaward4 ,the character of the two curves remains the same1 but they become flatter and the surface ,n ,j nearly level4 ,thus the time of high tide at ,m is later than at ,h4 ,it is later for each point passed in going up the river from ,h towards ,a4 ,eventually a point ,a is reached where there is no tide1 that is1 no rise or fall4 ,far below this point1 between ,a and ,b1 there is a point above which there is no upward current but only a slackening of the downstream flow4 ,at ,h a diagram showing the rise and fall of the tide is symmetrical1 at ,n the rises and falls are less than at ,h1 and the periods of their occurrence later4 ,in going up the river the duration of the flood tide decreases and that of the ebb tide increases4 ,the flood tide attains its greatest velocity soon after its commencement1 the ebb tide towards its close4 ,the distances to which the tidal influence extend are of course greater the greater the range of the tide and the flatter the slope of the river4 ,the discharge of the river of course varies4 ,the greater the discharge the more the rise of the river tends to keep pace with that of the tide and the less the distance to which the tidal influence extends4 ,on a longitudinal section of the river1 the high-water line will be shown as ,a ,n ,h4 ,this is merely done for convenience4 ,it is never high water at all points simultaneously4 ,to show the actual state of affairs at various stages of the tide1 series of lines must be drawn as in fig4 #fg1 where the firm lines show the flood1 and the dotted lines the ebb tide4 ,the flow in the tidal reach of a river is the same as if the water was alternately headed up by a movable weir and then allowed to flow freely and be drawn down4 ,if the water carries silt1 the tendency for deposit to occur is 7,,chap4 ,v41 ,art4 #b7 no greater than if there was no heading up or drawing down4 ,the tendency depends chiefly on whether there is1 on the average1 any reduction in velocity or increase in depth as compared with the non-tidal upstream reach1 and whether the water in that reach is fully charged with silt4 ,if both the answers are in the negative1 no deposit due to river silt is likely to occur in the tidal reach4 ,illustration3 ,,fig4 #fg4 ,if the sea water is charged with silt1 it will of course carry silt into the river as it flows up1 but the whole volume of water which enters has to flow out again4 ,on the whole1 the tendency for silt to be deposited in the river is due only to the period of 8slack tide0 near the time when the flow ceases4 ,the tendency is seldom marked4 ,if the sea water carries silt and the river water is clear1 the latter assists of course in removing any deposit--that is1 it tends to keep the channel clear4 ,if the river channel is soft and if the sea water carries no silt1 it may1 in passing up and down the river1 become charged with silt and return to sea still carrying it4 ,it thus has a scouring effect on the channel1 and may deepen or widen it4 ,if1 owing1 for instance1 to the flattening of the bed slope in its lower reaches1 the river tends to deposit its own silt in its tidal reach1 the sea water may prevent this deposit4 ,thus1 as regards silting in the tidal reach of the river1 the tidal water of the sea has little prejudicial effect if it is silt-laden1 and a beneficial effect if it is not4 ,silt is likely to deposit in the tidal reach of a river of uniform width1 only in a case in which the river water carries much silt1 and the slope is flat or cross-section great compared to that of the upper reach4 ,sea water is heavier than fresh water by about #bd per cent41 and this1 to some extent1 prevents their mixing4 ,at all stages of the flood tide the tendency at the point where the fresh water meets the salt water is for the fresh water to accumulate towards the surface and the sea water towards the bottom4 ,when the tide begins to flow up the river there may be a low-level salt water current moving landward and a high-level fresh water current moving seaward1 but this is quite a temporary state of affairs4 ,the surface slope is landward1 and the water moving seaward is not moving in obedience to the surface slope4 ,it is only moving as a result of momentum previously acquired4 ,the low-level current may have some extra velocity and extra scouring power1 but this cannot be much1 because the mean landward velocity of the whole stream must1 owing to the internal resistances caused by the two currents1 be less than it would be if there were not two currents4 ,moreover1 the state of affairs is temporary4 ,the two kinds of water mix eventually1 and their temporary separation has no considerable effect on the general tendency of the river in the tidal reach to scour or to silt4 ,a body of water included at any moment between any two cross-sections of the tidal portion of a river may not reach the sea during the next ebb tide4 ,in this case it will flow back up the channel with the next flood tide1 and so be kept moving up and down1 getting nearer1 however1 to the sea at each tide4 ,de ,franchimont has shown 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clx47 how a diagrammatic route-guide can be prepared for any tidal river to show pilots or captains of vessels the best times for starting on voyages up or down the river1 and for passing each point on it4 #c4 ,works in ,tidal ,rivers4--,if any works are required in the tidal portion of a river1 the principles to be followed in designing them are the same as if the river was non-tidal4 ,all that has been said in ,,chap4 ,,viii41 ,arts4 #a to #c1 applies to them4 ,the river may be straightened or trained or dredged4 ,generally training and dredging are combined4 ,any dredging in the portion of the river nearest the sea will not1 of course1 alter the water levels near the mouth1 but it will alter them further up4 ,the tide will come up in greater volume and will rise higher and extend further up4 ,the ebb will be facilitated1 and the low-water level will be lowered4 ,if any narrowing of the channel near its mouth is effected by training walls for the purpose of lowering the bed1 the effect on the volume of tidal water entering the river must be taken into consideration4 ,if the narrowing is confined to a reach near the mouth1 and if the resulting deepening is not sufficient to counteract the effect of the narrowing1 the volume of tidal water reaching the unnarrowed portions of the channel will be reduced1 and this may be injurious4 ,its scouring action may be insufficient4 ,the proper course may be to continue the narrowing upstream4 ,if this is done1 then it is obvious that the width of channel in which deep water is to be maintained at high water1 or which is to be kept free from deposit1 is reduced in about the same proportion as the volume of tidal water is reduced1 and no harm is likely to result4 ,any weir or similar structure which abruptly stops the flow of the tide up a river checks it of course for a long distance back1 perhaps to the mouth4 ,old ,london ,bridge used to obstruct the tide1 and its removal increased the range of the tide1 and was beneficial4 ,tidal rivers generally widen out to some extent near their mouths1 and are thus rather estuaries than rivers4 ,the works in such rivers are more fully discussed in ,art4 #e4 #d4 ,tidal ,estuaries4--,if1 instead of a river of uniform width1 there is an estuary whose width increases steadily towards the sea so that it is funnel-shaped1 the conditions described in ,art4 #b are modified4 ,an estuary is formed first by the waves of the sea1 which wear away the angles at the mouth of the river and allow the tide to enter in greater volume1 and then by the flow and ebb of the tides4 ,the slope of the bed of the estuary is usually much flatter than that of the river1 and the water surface is as shown in fig4 #fg4 ,the tidal movements extend further upstream than in the case of a river1 not only because of the greater difficulty experienced by the upland water in filling up the wide channel of the estuary1 but because of the momentum of the tidal water driving its way up the funnel-shaped channel 7,art4 #a74 ,the capacity of the estuary is of course much greater than is required for the discharge of the upland water alone4 ,if the sea-level remained always at one height and if the upland water contained silt1 it would tend to deposit in the estuary and would certainly deposit in it to some extent4 ,the action of the sea water is the same as described in ,art4 #b1 scouring if it is clear when entering1 of less account if it is not clear4 ,owing to the funnel shape of the estuary1 the tide rises higher at its upper end than if the estuary were replaced by a river channel1 and the tide also extends further up4 ,this may partly or wholly compensate for the greater tendency of silt to deposit in an estuary as compared with a river channel4 ,the ebb tide in an estuary does not always follow exactly the same course as the flood tide4 ,of course the lowest parts of the estuary are filled first and emptied last1 but the channels are not all continuous4 ,a channel open at its lower end may have a dead end at its upper termination1 and vice versa4 ,also1 at sharp bends in the channels1 the momentum of the water may cause differences in the paths traversed by the flowing and ebbing currents4 ,wherever there is a deep channel the water from the adjacent sandbanks tends1 towards the close of the ebb1 to flow cross-wise into the channel1 and in doing this it to some extent washes down the banks into the channel4 #e4 ,works in ,tidal ,estuaries4--,estuaries1 when shallow1 offer great facilities for training4 ,it used at one time to be said that any change which reduces the volume of tidal flow must be injurious4 ,it would be injurious to restrict the mouth of the estuary1 unless it were exceptionally wide1 and leave the rest untouched4 ,if the whole estuary is narrowed1 and a suitable funnel shape preserved1 the width to be kept open is1 relatively to the size of the mouth1 no greater than before1 and the tide may flow up as far as before1 and rise to as high a level4 ,the narrowing1 if properly arranged1 will improve the shape of the estuary and cause an increased scour4 ,the effect of the upland water is also greater in the narrower channel4 ,improvements to estuaries are not1 however1 restricted to training4 ,there is always one or more deep channels1 and the best of these can be selected and improved by dredging4 ,the channel should be one along which both the flood tide and the ebb tide will run4 ,the above remarks as to training do not apply to a case in which there is a bar outside the mouth of the estuary4 ,training might check the scour at the bar4 ,bars are treated of in ,,chap4 ,,xv4 ,if an estuary is not funnel-shaped1 if1 for instance1 it widens out very rapidly1 the tidal flow is much less effective in keeping the channel open4 ,in this case1 training works1 which would give the necessary funnel shape1 are indicated rather than dredging4 ,if an estuary is narrow at the entrance1 the flow is much less powerful1 unless the narrow part is of greater depth1 but even then the force of the tide is reduced owing to the change in the shape of the channel4 ,the bed of an estuary may be of such soft or sandy material that a dredged channel would be likely to be quickly filled up again by the slipping in of material at the sides 7,art4 #d74 ,in such a case an untrained channel can only be kept open to its full depth by constant dredging1 and probably the best course is to construct a trained channel1 although it may be more expensive than in the case of a harder channel1 because of the depth to which the foundations of the walls must be sunk into the soft bed4 ,also1 if the bed of the estuary is constantly shifting1 a dredged channel alone will not succeed1 and training must be resorted to4 ,again1 the bed may be of such hard material that training walls would not cause it to scour4 ,in this case a channel should be dredged and need not be trained4 ,for the great body of intermediate cases in which the deep channel can be formed either by dredging or training1 both methods can be adopted4 ,a common plan is to train the upper part and to dredge the lower part where the estuary is wider and the training walls would be more exposed to the waves4 ,illustration3 ,,fig4 #fh4 ,when an estuary is thus partly trained1 the deepening due to the training does not extend far beyond the point where the walls terminate4 ,the deposit of material along the sides of the estuary may1 however1 extend some distance further down in places where the tide can no longer have free play4 ,this occurred in the ,seine estuary 7fig4 #fh74 ,the authorities of ,havre1 which lies at one side of the estuary not far from its mouth1 feared that if the training walls were brought further down1 the deposits might extend to their neighbourhood4 ,the reduction in the capacity of the estuary1 due to the deposits1 caused it to become filled up more quickly1 and the time of high water at ,havre was advanced4 ,the dotted lines show a good arrangement of training walls proposed by ,harcourt4 ,there is no doubt that it is always feasible to carry training walls right through an estuary1 or at least down to a point where deep water is reached1 and if a proper funnel shape is given to the channel the reduction of the tidal flow and silting up of the spaces behind the walls need not cause any trouble4 ,training the complete estuary was carried out in the case of the ,tees1 where1 however1 the estuary was not of great length1 and was not of a good shape for keeping itself open4 ,any affluents entering the estuary can be provided with separate trained channels4 ,difficulty may1 however1 arise if there are towns which would be shut off from the estuary by the silt banks4 ,generally the line selected for the trained or dredged channel should1 though it must be as short and direct as possible1 coincide as nearly as possible with that which the water naturally tends to keep open4 ,this may be toward one side of the estuary or the other1 according to the direction from which the tidal wave approaches4 ,in the case of the ,dee1 the best line was not adopted1 attention having been chiefly given to the question of silting up the spaces outside the walls and so reclaiming land1 a matter which should always be treated as of quite secondary importance4 ,training walls in estuaries are generally built only up to half-tide level4 ,were it not for the expense they might be built up to high-water level4 ,in the ,seine estuary the walls were made of blocks of chalk4 ,whether a trained channel will keep itself open or will need periodical dredging depends1 of course1 on the amount of silt in the water and on its velocity and depth4 ,the question must be worked out and calculated as in the case of a non-tidal river4 ,the estuary of the ,mersey differs from most others4 ,towards the mouth1 near ,liverpool1 it is narrow and it widens out further inland4 ,the tides1 running through the narrow portion1 to fill up the large inland basin and to empty it again1 keep the narrow part scoured to a great depth4 ,it was proposed to train the wide portion for the ,manchester ,ship ,canal4 ,the training would1 no doubt1 have succeeded1 but1 owing to the silting up of the greater part of the estuary1 the scouring near ,liverpool would have been very greatly reduced and serious damage done to that port4 ,,chapter ,,xv ,,river ,,bars #a4 ,deltaic ,rivers4--,when a river flows into a tideless sea its silt deposits and forms a shoal or bar4 ,this shoal may in time extend and rise up to the water-level4 ,the current of the river makes its way through it in various directions1 and in this way a delta is formed and constantly extends seawards4 ,this flattens the slope of the lower portions of the river1 and causes raising of the bed in the reaches upstream1 and this again may cause the water to break out further upstream and form fresh channels to the sea4 ,the bars at the mouths of deltaic rivers are generally formed with great rapidity1 and they are apt to form a complete hindrance to navigation4 ,they are sometimes partly scoured away by floods in the river1 but in this case the scoured material may deposit on the outer slopes of the bar4 ,if a river which carries silt has no delta1 it is probably because there is a littoral current1 which prevents the silt from depositing4 ,on the other hand1 if a river brings down very heavy sediment1 a delta may be formed even when tidal flow is not wholly absent4 ,this occurs in the case of the ,ganges4 ,the bars at the mouths of deltaic rivers cannot usually be kept down by dredging except at great expense4 ,the usual method of dealing with them is to run out two parallel jetties1 in continuation of the river banks1 so as to bring the mouth of the river out to the bar4 ,the river then scours a channel through the bar and1 if the walls are not too far apart1 the depth will probably become as great as in the river and sufficient for navigation4 ,the river1 however1 tends to at once form a new bar further out4 ,the rapidity with which the new bar forms will be greater or less as the specific gravity of the materials carried by the river is greater or less1 and as the strength of any littoral current is less or greater4 ,clay is spread far out while sand quickly sinks4 ,all deposits are1 however1 swept away if there is a strong littoral current4 ,the steeper the slope of the bed of the sea away from the bar the longer the new deposit will take in forming a fresh bar4 ,also the less the discharge of the river the less the deposit will be4 ,the branch of a deltaic river selected for improvement by having the bar at its mouth removed1 should be one which has a small discharge and whose mouth is in a position where there is a strong littoral current4 ,in the case of the ,rhone1 the branch selected was the eastern one1 whose mouth was not exposed to any littoral current4 ,moreover1 the other branches of the river were closed1 and this increased the discharge of the branch which was left open4 ,the work did not succeed4 ,in other cases1 the parallel jetty method has succeeded1 and notably in the case of the ,mississippi4 ,in this case willow mattresses weighted with stones were used4 ,the question of keeping down the discharge does not1 however1 appear to have always received sufficient attention4 ,in the case of the ,mississippi the 8,south pass0 was selected for improvement4 ,in order to remove a shoal its upper end was narrowed and its discharge reduced4 ,the upper ends of the other 8passes0 were then obstructed so as to restore the discharges of all the passes to their former amounts4 ,the wisdom of this step is questionable4 ,it is desirable to keep down the discharge of the branch which is to be improved to the lowest limit consistent with free navigation4 ,if the width of the river near its mouth is greater than is desirable for the width between the jetties1 the latter are sometimes made to converge though their outer ends are made parallel4 ,in the case of the ,mississippi the jetties were made with a slight curve to the right4 ,it would seem desirable always to make the jetties with quite a considerable curve4 ,the jetty which was convex to the channel could then probably be shortened4 ,in a case where there is a littoral current1 say to the right1 the curve of the jetties could be to the right1 so that the stream on issuing would tend to merge into the current and assist it4 #b4 ,other ,rivers4--,it often happens that the materials--sand1 gravel1 and shingle--of which a sea beach is composed shift gradually along the shore4 ,this is known as 8littoral drift40 ,it is by some supposed to be due to the action of the tides1 and by others to the action of waves1 the drift taking place in the direction of the prevailing winds1 excluding those which are off shore4 ,the latter cause is the more probable4 ,most rivers have bars at their mouths4 ,in the case of deltaic rivers the bar1 as already stated1 is caused by the heavy silt carried by the river1 though it may be assisted by littoral drift4 ,in the case of non-deltaic rivers flowing into tideless seas1 the quantity of silt is not enough to form a bar1 and the same is generally true in the case of tidal rivers where the volume of tidal water is usually much greater than that of the upland water4 ,in both these classes of rivers the formation of the bars is due chiefly to littoral drift or to sediment brought in by the sea water4 ,the bar1 as in the case of deltaic rivers1 may be partly scoured away by a flood in the river1 and the scoured material may deposit on the seaward slope of the bar4 ,generally1 the navigation channel across a bar of this kind can be kept sufficiently deep by dredging1 but sometimes jetties1 like those mentioned in the preceding article1 have been constructed1 and in this case there is the great advantage that the bar is not liable to form further out4 ,if littoral drift tends to accumulate1 the jetties1 or at least the one on the side whence the drift comes1 can be lengthened4 ,this was done1 as mentioned by ,harcourt 7,rivers and ,canals1 ,,chap4 ,,ix471 in the case of the rivers ,chicago1 ,buffalo1 and ,oswego1 which flow into the ,great ,lakes of ,america4 ,the same writer states that the jetties at the ,swine mouth of the tideless river ,oder were made to curve to the left1 the convex or left-hand jetty being the shorter1 but that this exposed the mouth to littoral drift coming from the left4 ,the river1 upstream of the jetties1 had a slight curve towards the left1 but this could have been corrected or1 at all events1 the jetties made to curve to the right4 ,a case 7fig4 #fi7 where parallel jetties were recently constructed in a tidal sea is that of the mouth of the ,richmond ,river1 ,new ,south ,wales 7,min4 ,proc4 ,inst4 ,c4,e41 vol4 clx474 ,illustration3 ,,fig4 #fi4 ,in the case of a bar at the mouth of an estuary1 parallel jetties would be too far apart4 ,in such cases converging breakwaters 7fig4 #gj7 are sometimes made1 especially if the tidal capacity of the estuary is small4 ,the entrance is generally #ajjj to #bejj feet wide4 ,if made narrow1 it would reduce the tidal flow too much4 ,the space inside the breakwaters adds to the tidal capacity1 and thus induces scour at the bar4 ,the case is similar to that of the ,mersey estuary 7,,chap4 ,,xiv41 ,art4 #e71 the breakwaters assisting scour at the bar1 though perhaps slightly interfering with the tidal flow in the estuary4 ,illustration3 ,,fig4 #gj4 ,converging breakwaters also tend to stop littoral drift1 and the space inside them acts as a harbour of refuge in storms and as a sheltered place where dredgers can work 7,rivers and ,canals1 ,,chap4 ,,xi474 ,they have to be heavily built and are very expensive1 and they are generally adopted only when there is an important seaport1 and when they can be put to all the uses above indicated4 ,,appendix ,a ,fallacies in the ,hydraulics of ,streams 7,,chap4 ,i41 ,art4 #d1 and ,,chap4 ,,vi41 ,art #b74--;,in an inundation canal in ,india the supply during floods was excessive4 ,orders were given that a flume be made at the head1 as shown in fig4 #ga4 ,the sides were to be revetted1 as shown in fig4 #ai 7,,chap4 ,,vi41 ,art4 #c72 the length1 excluding the splayed parts1 was to be #bjj feet1 and the floor was to be a mattress well staked or pegged down4 ,the order stated that 8by this means we cannot get into the canal much more than its true capacity40 ,with #i feet of water1 a surface fall of #d inches in #cjj feet would give a velocity of some #fe feet per second1 and a further fall of about #h inches would be required at the head of the flume to impress this velocity on the water4 ,the flume would reduce the depth of water in the canal by #a foot1 i4e4 from #i feet to #h feet4 ,this would not be in anything like the proportion desired4 ,moreover the flume1 unless the bed was extremely well protected1 would be destroyed4 ,the above is a case of exaggerating the effect of an 8obstruction40 ,illustration3 ,,fig4 #ga4 ,again1 on a branch canal it was observed that 8wherever cattle crossings exist there is a deep silt deposit which practically blocks the branch40 ,the deposit exists because the sides of the channel are worn down4 ,a wide place always tends to shoal 7,,chap4 ,,iv41 ,art4 #i74 ,if the deposit obstructed the flow of water there would be a rush of water past it1 and it could not exist4 ,the ,gagera branch of the ,lower ,chenab ,canal--the left-hand branch in fig4 #gb--was found to silt4 ,it was proposed to make a divide wall 7fig4 #gb7 extending up to full supply level4 ,the idea is unintelligible4 ,the silt does not travel by itself but is carried or rolled by the water4 ,as long as water entered the ,gagera branch1 silt would go with it4 ,the authorities1 who had apparently accepted the proposal1 altered the estimate when they received it1 and ordered the wall to be made as shown dotted and of only half the height4 ,this was done4 ,the idea seems to have been that the wall would act as a sill and stop rolling silt4 ,this is intelligible1 but see ,,chap4 ,,iv41 ,art4 #b1 last paragraph4 ,moreover1 there was a large gap1 ,a ,b1 in the wall4 ,the work is said to have proved useless1 and proposals have been made to continue the wall from ,a to ,b4 ,in this form it is conceivable that it may be of use4 ,illustration3 ,,fig4 #gb4 ,in a river1 the rises and falls at different places are not1 of coarse1 the same1 even when they are long continued4 ,in the river ,chenab1 at the railway bridge at ,shershah1 the rise from low water to high flood is generally a foot or two more than the rise at a point #be miles upstream4 ,it has been suggested that the railway embankments1 which run across the flooded area1 cause a heading up of the stream4 ,if this were the case1 to any appreciable extent1 there would be a 8rapid0 through the bridge1 which1 if it did not destroy the bridge1 would at least be visible and audible4 ,the exaggerated ideas which often prevail regarding the tendency of a river1 when in flood1 to scour out a new channel1 have been mentioned in ,,chap4 ,,iv41 ,art4 #h4 ,spring1 in his paper on river control1 admits1 when mentioning ,dera ,ghazi ,khan1 that there was little danger1 but in mentioning the ,chenab ,bridge at ,shershah he quotes1 without disputing it1 an opinion of the opposite kind 7,government of ,india ,technical ,paper1 ,no4 #aec1 8,river ,training and ,control on the ,guide ,bank ,system074 ,for some other fallacies1 see ,hydraulics1 ,,chap4 ,,vii41 ,arts4 #i and #ae4 ,,appendix ,b ,pitching and ,bed ,protection 7,,chap4 ,,vi41 ,art4 #c1 and ,,chap4 ,x41 ,art4 #b74--;,any scour upstream of a weir is merely due to the eddies formed upstream of the crest 7,hydraulics1 ,,chap4 ,,ii41 ,art4 #g71 and is not serious4 ,and1 similarly1 as to scour upstream of a pier4 ,a hole formed alongside a pier or obstruction1 if there is no floor1 may work upstream4 ,the chief use of a floor extending far upstream is to flatten the hydraulic gradient 7,,chap4 ,x41 ,art4 #c74 ,for pitching of the sides1 monolithic concrete is not very suitable1 because it may settle unequally and crack4 ,for heavy pitching1 concrete blocks can be used4 ,they can rest on a layer of #c to #f inches of rammed ballast or gravel4 ,the toe wall1 as shown in fig4 #ac1 page #fe1 is sometimes dispensed with1 the pitching being merely continued to a suitable depth below the bed1 and the bottom edge being at right angles to the slope instead of horizontal4 ,the portion below the bed may be of concrete4 ,,index ,abrupt changes in streams1 #bh4 ,alterations in a channel1 upstream effect1 #d4 ,aqueduct1 ,kali ,nadi1 #adi4 ,available rainfall1 #i4 ,bank protection1 #fj4 -- -- artificial weeds1 #gj4 -- -- berms1 #gj4 -- -- bushing1 #fh4 -- -- fascining1 #ff4 -- -- heavy stone pitching with apron1 #ga4 -- -- on the ,adige1 #fg4 -- -- reinforced concrete1 #gj4 -- -- rolls of wire-netting1 #gj1 #adj4 -- -- staking1 #fi4 -- -- trees1 #fh4 -- -- twig revetment1 #fh4 -- -- ,villa system1 #fi4 ,banks1 #ib4 -- ,bell's guide1 #acg4 -- continuous lining of1 #fd4 -- dimensions of1 #ic4 -- guide1 #acg4 -- side slopes of1 #ib4 ,barrage of the ,nile1 ,assiut1 #aag4 ,bars1 river1 #bjc4 ,bed1 protection of1 #eh4 -- -- ,villa system1 #ei4 ,bell's guide banks1 #acg4 ,bends1 effect of1 #dd4 -- short cuts of1 #dd4 ,bengal ,dooars ,railway1 bridge and floods1 #aci4 ,bifurcation of a channel1 #ec4 ,birmingham water supply1 #agc4 ,borrow-pits in bed of channel1 #ec4 ,breakwaters1 converging1 #bjg4 ,bridges1 #acb4 -- foundations or floor1 #acb4 -- on ,indian rivers1 #acd4 -- piers and abutments1 #acb4 -- protection of1 #acg4 -- at ,wazirabad1 #acd4 ,british ,rainfall ,organisation1 #i4 ,canalisation of rivers1 #hd4 ,canals1 #ib4 -- fall1 #aac4 -- headworks1 #ed4 -- navigation1 #ic4 -- rapid1 #aac4 -- ,ship1 #ie4 ,catchment area1 8yield10 #i4 ,channel1 alterations in1 #d4 ,chanoine falling shutters1 #aba4 ,chenab ,river at ,shershah1 #baj4 ,choice of types of work1 #c4 ,closures of streams1 ,colorado ,river1 #hj4 -- -- cradle for1 #gh4 -- -- ,tista ,river1 #ha4 ,collection of information concerning streams1 #ah4 ,colorado ,river1 closure of1 #hj4 ,conduits1 #ib1 #ajj4 ,cradle for closing streams1 #gh4 ,culverts1 #ace4 -- flooding of1 #acf4 ,dams1 culvert of1 #ahj4 -- design of masonry1 #aha4 -- earthen1 #agd4 -- masonry #aha4 -- -- construction of1 #ahe4 -- -- design1 #aha4 -- -- failures of1 #ahe4 -- -- stresses in1 #ahe4 -- pitching of1 #agi4 -- reservoir1 #afb4 -- ,sidhnai ,canal1 #aag4 -- tower for culvert1 #ahj4 ,dee estuary1 #bja4 ,dera ,ghazi ,khan1 ,indus at1 #ga4 ,discharge curves1 #bc4 -- observations1 #ba4 -- tables1 #bd4 ,divide wall1 ,gagera branch canal1 #bji4 ,drainage1 #ada4 ,dredging and excavating1 #hd1 #hh4 ,drift1 littoral1 #bje4 ,eddies1 scouring power of1 #bh4 ,embankments1 #aef4 -- design of1 #aeg4 -- ,holland1 #aei4 -- ,irrawaddy1 #aei4 -- ,rhine1 #aei4 -- slips in1 #agg4 ,estuaries1 ,dee1 #bja4 -- ,mersey1 #bjb4 -- ,seine1 #bjj4 -- tidal1 #aig4 ,fall1 canal1 #aac4 ,fallacies in hydraulics1 #e4 ,falling shutters1 ,chanoine1 #aba4 -- -- ,fouracres1 #aba4 -- -- ,khanki1 #abd4 -- -- ,thnard's1 #aba4 ,flood discharge1 estimating1 #adh4 ,floods1 #ada1 #adf4 -- ,bengal ,dooars ,railway1 #aci4 -- formul for1 #adg4 -- prediction1 #aej4 -- prevention1 #aec4 ,flowing stream1 closure of1 #ge4 ,ford in a river1 #dc4 ,forests and vegetation1 influence of1 on rainfall1 #ad4 ,formul for floods1 #adg4 ,groynes or spurs1 #eh1 #fj1 #fa4 -- on the ,indus1 #ec4 ,guide banks1 #acg4 ,headworks of a canal1 #ed4 ,holland1 embankments1 #aei4 ,hurdle dykes1 #gi4 ,hydraulics of open streams1 #d4 -- -- fallacies in1 #bji4 ,important works1 precautions at1 #acj4 ,indian rivers and bridges1 #acd4 ,indus1 groynes on1 #ec4 ,information concerning streams1 collection of1 #ah4 ,inundation canal1 flume in1 #bji4 ,irrawaddy1 embankments1 #aei4 ,irrigation channels in embankment1 #eb4 ,irwell1 weir on1 #abd4 ,jetties1 in continuation of river banks1 #bjd4 -- ,mississippi1 #bje4 -- ,richmond ,river1 ,new ,south ,wales1 #bjf4 ,kali ,nadi1 aqueduct1 #adi4 ,khanki1 falling shutters at1 #abd4 ,leakage1 stoppage of1 #gh4 ,lockage1 #ih4 ,locks1 #if4 -- in flights1 #ih4 ,mersey estuary1 #bjb4 ,mississippi jetties1 #dh4 ,narora weir1 #aji4 ,navigation canals1 #ic4 ,needles1 regulator1 #aag4 ,new ,south ,wales1 available rainfall1 #ab4 ,obstruction1 effect of1 #e4 ,obstructions in streams1 #bh4 ,okla weir1 #aab4 ,open streams1 hydraulics of1 #d4 ,perishable materials1 use of1 #d4 ,permanent rgime of streams1 #bi4 ,pitching1 #fd4 ,prediction of floods1 #aej4 ,prevention of floods1 #aec4 ,protection of banks 7see ,bank protection74 ,rainfall1 #f4 -- available1 #i1 #bf4 -- -- ,new ,south ,wales1 #i4 -- -- ,sudbury ,river1 ,massachusetts1 #ab4 -- -- various countries1 #aa4 -- ,british ,organisation1 #i4 -- 8catchment area10 8basin10 #i4 -- distribution of1 #g4 -- driest year1 #g4 -- evaporation1 #i4 -- heavy falls in short periods1 #ae4 -- influence of cultivation1 #ae4 -- -- of forests and vegetation1 #ad4 -- local figures1 #h4 -- measurement of1 #ac4 -- observations1 period of1 #g4 -- statistics1 #f4 -- variation of1 #f4 ,rain-gauge1 #h1 #ac4 ,rapid1 canal1 #aac4 ,regulators1 #aah4 ,reservoirs1 #afb4 -- capacity1 #afg4 -- compensation water1 #afb4 -- leakage1 #afc4 -- waste weir1 #afd4 ,rsum of the subject1 #a4 ,rhine1 embankments1 #aei4 ,richmond river1 jetties1 #bjf4 -- weir at1 #aai4 ,river bars1 #bjc4 -- training groynes1 #he4 -- -- walls1 #he4 ,rivers1 deltaic1 #bjc4 -- floods in1 #adf4 -- non-deltaic1 #bje4 -- tidal1 #aib4 -- training and canalisation1 #hd1 #hi4 ,run-off in small streams1 #adc4 ,salt water1 effect of1 #bi4 ,sand separator1 #cd4 ,sandbanks1 #dg4 ,scour 7see ,silt and scour74 ,seine estuary1 #bjj4 ,set of stream1 effect of1 #e4 ,ship canals1 #ie4 ,shutters1 self-acting1 ,switzerland and ,bavaria1 #abg4 ,silting and scouring1 #bg4 ,silt and scour1 action at bends1 #db4 -- -- -- on sides of channel1 #dj4 -- -- effect of regulator or movable weir1 #di4 -- -- in the ,sirhind ,canal1 #cb1 #ed4 -- -- increasing or reducing1 #dh4 -- -- materials carried in suspension1 #c4 -- -- methods of investigation1 #cc4 -- -- practical formul and figures1 #cg4 -- -- production of1 #dh4 -- -- rolled materials1 #bi4 -- -- sand separator1 #cd4 -- -- scrapers or harrows1 #dh4 -- clay and sand1 #cf4 -- deposit1 production of1 #ea4 -- in river ,sutlej1 #ce4 -- in river ,tay1 #ce4 -- quantity and distribution of1 #ce4 -- upstream of weirs1 #ajc4 ,sidhnai ,canal1 dam1 #aag4 ,sirhind ,canal1 silt and scour in1 #cb1 #ed4 ,slips in embankments1 #agg4 ,sluice gates1 ,stoney's1 #aai4 ,sluices1 #ajb4 ,small streams1 floods in1 #ada4 ,soundings1 #ba4 ,spurs or groynes1 #eh1 #fj1 #fa4 ,stream gauges1 #ai4 -- diversions of1 #gc4 -- general tendencies of1 #de4 -- information concerning intermittent1 #be4 -- -- -- small1 #bd1 #be4 -- overflow1 #df4 -- shifting1 #bj1 #dg4 ,sudbury ,river1 ,massachusetts1 available rainfall1 #ab4 ,surface slope observations1 #bb4 ,survey of a stream1 #ba4 ,sutlej ,river1 silt in1 #ce4 ,syphons1 #acb1 #ace4 ,tay ,river1 silt in1 #ce4 ,teddington1 weir at1 #aai4 ,tidal estuaries1 works in1 #aih4 -- river1 diagrammatic route-guide1 #aif4 -- rivers1 #aib4 -- works in1 #aif4 ,tide-gauges1 #aib4 ,tides1 #aij4 ,tista ,river1 closure of1 #ha4 ,training of rivers1 #hd4 ,types of work1 choice of1 #c4 ,upper ,jhelum ,canal1 syphons to carry torrents1 #add4 ,velocities which enable a stream to scour1 #dj4 ,villa system of bank protection1 #fi4 -- -- bed protection1 #ei4 ,waste weir1 hydraulic problem1 #afe4 ,water supply1 ,birmingham1 #agc4 ,waterway1 area of1 in regulators1 #abi4 ,wave1 travel of1 down a stream1 #aeb4 ,waves1 effect of1 #bi4 ,wazirabad1 bridge at1 #acd4 ,weeds1 artificial1 #gj4 -- growth of1 #bi4 ,weirs1 #ajb4 -- adjustable1 #abf4 -- ,bear ,trap1 #abc4 -- drum1 #abg4 -- frame1 #abj4 -- general design of1 #aje4 -- ,narora1 #aji4 -- oblique1 #ajc4 -- ,okla1 #aab4 -- on sandy or porous soil1 #ajf4 -- ,richmond1 #aai4 -- silt deposit1 upstream of1 #ajc4 -- types of1 #aaa4 -- waste1 #afd4 -- with sluices1 #aae4 ,wire-netting for bank protection1 #gj1 #adj4 ,works1 design and execution of1 #c4 ,zifta ,regulator1 #aji4 ,,printed ,,by ,,neill ,,and ,,co41 ,,ltd41 ,,edinburgh4 ,,footnotes3 #a ,min4 ,proc4 ,inst4 ,c4,e4 #b ,engineering ,news4 #c ,encyclopdia ,britannica4 #d ,the paper by ,spring--in size it is a book--will repay perusal by engineers engaged on railway bridges over large shifting rivers4 ,london ,agents1 ,constable ,co4 #e ,hydraulics with ,working ,tables4 ,spon1 #aiab4 #f ,irrigation canals are dealt with in ,irrigation ,works 7,spon1 #aiac74 #g ,see also ,appendix ,a4 #h ,the regulator runs across the canal head2 the under-sluices are a continuation of the weir1 between the divide wall and the regulator4 #i ,see also ,appendix ,b4 #aj ,on ,indian canals the term 8regulation0 is applied to the control of the discharge at the regulators or off-take works4 #aa ,see also ,appendix ,b4 #ab ,irrigation ,works1 ,,chap4 ,i41 ,art4 #d4 #ac ,min4 ,proc4 ,inst4 ,c4,e41 vols4 lx4 and lxxxv4 #ad ,rivers and ,canals1 ,harcourt4 #ae ,rivers and ,canals1 ,harcourt4 #af ,rivers and ,canals1 ,harcourt4 #ag ,the foundations of piers and abutments should be deep enough to allow of this4 #ah ,report on the ,revised ,estimate1 ,upper ,jhelum ,canal4 #ai ,revised ,estimate of the ,upper ,jhelum1 ,upper ,chenab1 and ,lower ,bari ,doab ,canals4 #bj ,see also ,note on p4 #afa4 #ba ,the height of a wave is supposed to be #ad7;fetch71 but this allows nothing for splashing4 ,transcriber's ,note3 ,obvious printer errors corrected silently4 ,inconsistent spelling and hyphenation are as in the original4