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GOVERNMENT OF INDIACENTRAL WATER COMMISSION
MANUAL ONIRRIGATION AND POWER CHANNELS
NEW DELHIMARCH 1984
MANUAL ONIRRIGATION AND POWER CHANNELS
Prepared by
CENTRAL \NATER COMMISSION
Published by
Central Board of Irrigation and PowerMalcha Marg, Chanakyapuri, New Delhi-11 0021
The economy and the well-being of the rural masses depends onagriculture of which irrigation is the most important single input. The canalsystem is a vital element for the success of an Irrigation Project. Canals arealso vital components of many run-of-the-river hydro-electric power projects.Although India has a long and enviable record of successful design and cons-truction of irrigation and hydro-power projects, it was felt desirable to com-pile useful information in the form of a manual for the benefit of thoseengaged in the planning, designs, construction, operation and maintenanceof canals. This manual on Irrigation and Power Channels deals, in brief, withthe planning and design of both lined and unlined canals.
Chapter I introduces the importance and advantages of irrigation andthe additional uses of canal waters.
Chapter II gives in brief the planning and investigations necessarybefore detailing an irrigation scheme. The field data to be collected, prelimi-nary and detailed surveys to be done, different types of canal alignmentsgenerally adopted, and other features regarding the L-Section and X-Sectionsare brought out.
Chapter IlIon design of unlined canals gives in brief some of theimportant farmulae used in the design of stable channels. Recommendationsof I.S.!. in their various Codes have also been mentioned with respect to pro-vision of side slopes, inspection paths, free-board; bed width to depth ratio,allowable velocities, berms, dowels, etc.
Chapter IV on design of lined canals describes different types oflining in use and their suitability in different situations. I.S.I. recommendationspertaining to various aspects of the design of lined canals have also beenincluded. Necessity and details of under-drainage arrangements are alsodescribed in details.
Chapter V on power channels describes the speCial considerations to betaken into account while designing power channels.
Chapter VI gives in brief the various types of problems faced duringconstruction, operation and maintenance of the cana Is.
Appendix I gives some examples on design of unlined canals, whileAppendix II gives examples on design of lined canals. A number of plates havebeen included to simplify the calculations and save time.
The Manual gives the broad guidelines for planning and design of asystem. A satisfactory and successful design depends upon the individual skilland practical experience of a Designer and should cater to the site require-ment. I hope this Manual will be of assistance to the engineers in the field ofIrrigation and Power in planning and design of canal systems.
I would like to bring on record the useful work done by Shri P.C.Lau, Deputy Director, Shri P. Sen, former Director, and Shri R.P. Malhotra,Director (B.C.D.-II) in preparation of this Manual. Contributions of Shri R.Rangachari, Member (JRC): Shri K. Madhavan, Chief Engineer (Designs)and Shri N.K. Agrawal, former Member (D&R) in the preparation of thisManual are acknowledged.
Comments and suggestions for improvement of the Manual are welcome.
PRIT AM SINGH
ChairmanCentral fVater Commission
Chapter II
Chapter III
Appendix I
Chapter IV
Appendix II
Chapter V
Chapter VI
S
S.R.VVa
W
Area of Cross-section of Channel in Square Metre
Bed Width in MetresCoefficient of DischargeCubic Metres per SecondCanal Bed LevelCritical Velocity RatioDepth of Water in MetresLacey's Silt FactorFree Board in MetresFull Supply Level
Full Supply DepthGround LevelInspection PathCoefficient of Rugosity
Wetted Perimeter in MetresDischarge in cumecs
Discharge in Cumecs/Meter WidthHydraulic Mean Depth in MetreSide Slope RatioLongitudinal Surface Slope of WaterService Road
Mean Velocity in Metre/sec.Kennedy's Critical Velocity in Metre/sec.Water Surface Width in Metres
A
B
Cd
CumecsC.B.L.
C.V.R.D
f
FF.S.L.
F.S.D.G.L.LP.N
P
QqR
Introduction
1.1 India is potentially one of the richest agricul-tural countries in the world. The total arable land inIndia is estimated to be 147 million hectares which isthe third largest in the world, ranking behind only theU.S.S.R. with 225 million hectares and the U.S.A.with 194 million hectares. This is owing to the factthat the proportion of arable land to total geographi-cal area is the highest in India being 45 percentagainst 10 percent for U.S. S.R. and 25 percent forU.S.A.
Irrigation has been practised in India from veryearly times from wells, tanks, inundation canals andsmall channels. Major projects were built by theBritish starting in the middle of the 19th Century, andthe rate of development though, moderate, was steady.
1.2 Irrigation is defined as the natural or artifi-cial application of water to soil for the purpose ofsupplying the moisture essential or beneficial to plantgrowth.
Water is tlOrmally supplied to the plants by naturethrough the agency of rain. A second agency is throughthe flood waters of rivers which spread and inundatelarge areas during the flood season and recede backleaving the land well irrigated for cultivation of a cropduring the dry season. These natural processes areusefully supplemented by canals to maximise thebenefits to cultivators.
1.3 This manual is an attempt to cover broadguidelines, keeping in view the practices recommendedby ISl in their various published Cod.es and the prac-tices adopted by reputed Agencies and Organisationslike U.S.B.R., U.S. Army Corps of Engineers, etc.,which may be of some help to the designers. TheIrrigation distribution system consists of various com-ponents like regulation structures, cross drainagestructures, bridges, lined or unlined channel sections,etc. In this manual, only the channcI part has beendealt in detail.
1.4 The source of all water used for irrigation is'precipitation', i.e., the water received on the earthfrom the atmosphere in the form of rain, snow andhail etc. The prOC~3Sof utill3ation of this water in-volves th~ construction of engineering works of appre-
ciable magnitude, and is termed 'artificial Irrigation.'Artificial Irrigation may be divided into 'lift' irrigationand 'flow'· irrigation according to whether the water islifted to the fields by some mechanical/man ual meansor flows by gravity from the source to the field.
In order to provide artificial irrigation, irrigationschemes are planned and executed utilising the avail-a?le natural source. of ~ater. The schemes mainly con-SISt of storage or diverSIon structures and distributionsystems. In this publication an attempt has beenmade to cover the latter, comprising open channels.
1.5 . In addi.tion to the surface water, ground waterreserVOIrS provId~ ,!ery usef~l ~ources which .are beingmore and more utilised for lfflgatlOn purposes thesedays. Augmentation Tube Wells have been installedto promote conjunctive use of surface and sub-surfacewater~ to !neet. the increased demand of irrigation andto mamtalJ1 sUitable ground water conditions. Due tointroduction of high yielding crop varieties and multi-ple cropping such conjunctive use of water has severaladvantages. Some of these are:
(1) Water loss due to seepage from canals andwater courses, irrigated areas, etc., is utilisedagain there by mitigating to some extent thewaterlogging problem.
(2) Compared to surface water, ground water isrelatively free from surface pollutants, and hasthe advantage of being less susceptible tochanges in quality, chemical composition andtemperature variation.
(3) puring the period whe.n canal water supply ismadequate or not avaJlable, the farmers coulddraw supply from tube wells to irrigate theirfields in time.
In North India, Tube Wells have been found to beso useful and advantageous that their number hasbeen increasing rapidly.
1.6 It is hardly necessary to emphasise the import-ance and advantages of irrigation development whenthe pressing necessity of stepping up food production
for the growing population of the country has automa-tically brought it to the forefront. Even so, some oftae salient advantages of development of irrigation arerecapitulated below:
(i) Protection from Famine
This is the most important function of irrigation inthis countly. Though most parts of India receiveconsiderable rainfall, it is mostly seasonal and oftenerratic. The subsistence level of the farmers beingvery low, and their surplus wealt~ even i? no~malyears practically nil, failures of raInS even 111 a smgleyear would lead to large scale distress ~nd mor~ t~anone successive failures to acute famll1e causing lm-poverishment and starvation in the affected area, butfor the protection offered by artificial irrigation.
(ii) Improvement in Yield and Value of the Crops
The yield of practically all crops increases by arational and scientific and assured application ofwater. The optimum quantity of water, i.e., delta togive best results for a certain crop in a certain a~eacan be determined experimentalIy. Any quantitylesser than or in excess of optimum reduces yield.When permanent. regular and timely supply of' wateris assured, hybrid crops natnrally replace inferiorcrops to increase yield.
Almost all the large irrigation systems in India arevery well planned, financially sound and pay revenue tothe State. More important. perhaps. is the increase inthe wealth and prosperity of the cultivators. With theintroduction of irrigation land value appreciates andit is beneficial to both the land holder and the State,8S the latter can legitimately enhance the irrigationcess on such land.
(iv) Generation of H.E. Power
On projects primarily designed for irrigation,power generation can often be obtained at comparati-vely low cost by utili sing the available head.
(v) Inland NavigationIrrigation canals can be us~d for inland navigation
to mitigate the load of transport on Road System andconsumption of petroleum products.
(vi) Effect of Health
In case suitable drainage f-ystem is not providedfor irrigated areas, the direct influence of a canal irri-gation project is to increase the sub·soil water leveland water logging in lower areas and the incidence ofmalaria could occur in the area. The indirect effectof reducing the risk of crop failures and increasingfood production improves the nutrition of the peopleconsiderably. This leads to increased resistance todisease.
(vii) Domestic Water Supply
In large dry areas the canals are the only sourceof water for domestic purposes. The rise in sub-soilwater level in dry area could assist in meeting dem-ands of domestic water supply by pumping the groundwater.
(viii) Improvement of CommunicationsOn all important channels and upto distributaries,
an unsurfaced driving road/service road is providedprimarily for purpose of inspection and control. Th?seroads are sometimes the only motorable roads avaIla-ble into the interior for essential purpose in commandarea. This net work of canals with roads improve thecommunications in remote areas of command.(ix) Canal Plantations
Trees are planted along banks of channels, nearmasonry works and. in any open land available withthe canals. They add to the timber wealth of thecountry and play their part in checking soil erosionsand keeping ecological balance in the area.
(x) Industrial Water Supply
Often the requirements of the neighbouring indus-try or the Thermal Power Stations for cooling etc.could also bl; met with from these channels.
Planning and
2.1 Preliminary Surveys
. When the proposal for extending canal irrigationlOtO a new area IS taken up for consideration, the firststeps IS to make a thorough reconnaissance survey ofthe area. During this survey all the factors which effectirrigation development must be carefully observed andnotes made from judgement and from quick measure-ments which can be taken on the spot. The follov ..•ingare the main factors to be considered:
(i) Spring Levels in the Area
The spring levels in the area should be ascerta inedand noted as they have an important bearing on thedesirability of canal irrigation. If the spring ~levels arealready high, it would be a factor against though notn.ccessarily decisive to the introduction' of canal i rriga-tlOIl for the following reasons:
(a) The seepage from the canal system, if cons-tructed, may raise the water-table still furtherby making the area prone to waterlogging.
(b) Uncertainty of Demand: In areas where thewater-table is high, irrigation demand may beslack. At the same time, such areas may bewell adopted to development of cash cropswhich cannot be grown without irrigation.
(c) Rise of water· table may increase the incidenceof malaria in the area and the opinion ofpublic health authorities on this point shouldbe obtained.
(ii) Rainfall Figures in (he Area
The available rainfall data of the area should beobtained and studied with respect to the percentage ofyears in which the rainfall deDarfs from the normalduring monsoon and in winter: This will give a goodidea of the likelihood of irrigation demand in thearea.
(iii) Type of Soil
The type of soil should be judged by visual inspec-tion and by enquiry. Subsequently, soil surveys shouldpe ~arried ou t before finalisinf,?; the scheme,
Investigations
(iv) Topography of the Area
A canal system operates essentially by gravity flowsystem. It has to be so laid out in an area that allthe channels run on ridges so that they may irrigate oneither side by flow. The topography of the area maybe judged keeping in view the above mentioned re-quirements. If the project is considered worth furtherinvestigations contoured maps are prepared.
(v) Crops Already SOH,'/J in the Area
The crops grown in the area should be noted andAgriculture Department should be consulted for exist-ing and proposed cropping pattern and cropping cal-endar. Water requirement of each crop should befinalised considering the local practice, useful rainfall,and type of soil in the area. The water demand ini-tially will depend on the crops grown/proposed andfortnightly water requirements. The intensity of irri-gation in the area should be assessed from records toarrive at reasonable intensity of irrigation in the area.
(vi) Extent of Existing Well and Pond Irrigation in theArea
(a) The limited resources available in the countryshould be considered and deployed suitablywhere there is maximum need and providesthe greatest benefit. If an area has an extensivesystem of irrigation from open wells and pondswhich is sufficient for its protection againstfailure of rains. it should naturally receive alow priority for the introduction of canalirrigation.
(b) Since it may not be possible to meet the de-mand of irrigation during low supplies/closuresfrom the sources, i.e., river/reservoir, the pcssi-bility of conjunctive use of ground water tomeet the demand of sowing, transplant ation/maturing of crops during Jaw supplies/closureshould be examined. This will, on ncrasirnreduce the capacity of channels and s~ve inconstruction cost and encourage the optimumutilisation of available water resources. Thiswill incidently keep the rise of sub-soil waterlevel within permissible limit and give more~ime for maintenance of distribution system,
(vii) Quality of Cultivators and their Views of IrrigaiiQn
Intelligent, hardworking cultivators would adaptthemselves to irrigated cultivation, avail of its advant-ages and make economical, efficient use of water. Thereverse would be the case if they are otherwise. Thiscan make all the difference to the success or failure ofan irrigation project.
After reconnaissance, all the information gatheredshould be carefully analysed. If the result indicatesfeasibility of the project, more detailed surveys andcollection of data would then be carried out.
2.2 Detailed Survey
2.2.1 In settled areas the work of detailed investi-gations is very much facilitated with the help of re-venue records and mouza maps which are available foreach village. The following statistics of area for eachvillage should be obtained from revenue records-totalarea of land, cultivated area, culturable waste, area ofeach crop grown in the last few yeaTs, and the areairrigated under existing arrangements from ponds andwells .. This information, will be required in comput-ing gross and culturable commanded areas and indetermining the intensity of irrigation and the area tobe irrigated.
2.2.2 Crops and Crop Calendar
After examining the records of eX\stlOg cultivatedareas and the crops, the proposed cropping patternand crop calendar should be evolved considering theexisting soil and climatic conditions in the area inconsultation with the Agricultural Department.
Water requirement of each crop should be workedout by suitable methods like Penman's formula or bycarrying out experiments considering the type of soil,temperature, rain fall in the area, etc. Net fortnightlywater requirements are then worked out consideringeffective useful rain fall for each crop and the capacityof canal assessed. on the peak demand.
2.2.3 The village settlement maps called "Shajra"maps are prepared at the time of settlement to thescale of 1:4000. These maps show the boundary andnumber of each field, location of inhabited areas,culturable and barren land, groves, tanks and a fewother distinguishing features.
These sheets are then to be completed fortopographical detail not available on them butessential for planning of an irrigation scheme. Thefirst necessity is to mark out the contours aftercarrying out levelling in the area. The contours shouldpreferably be at 0.5 m intervals. In other areas sim-ilar contoured plans to a scale of 1:15000 and a contourinterval of 0.50 m will be prepared abinitio b~ usualmethods of engineering survey-ch&h~ ~ud cQ:~~wass.orplane table as CQnyeni~llt,
2.2.4 The trial pits/augerholes at intervals of 500m or at suitable intervals depending on the variationof soil strata along the canal alignment should betaken upto the designed bed level. The strata metwith should be brought out alongwith the L-Section ofthe canal. Transmission losses should be estimatedas per the strata met with in various reaches to arriveat realistic figure.
The distribution system should be economical fromconsiderations of capital investment and cost of an-nual maintenance. It should provide adequate irriga-tion facilities to the areas under the command of chan-nel. The following broad principles may be consideredfor the layout of distribution system.
(i) Water should be taken directly as far as possi-ble to the area of the command, subject to pra-tical considerations of lnaximum permissibleheight of the bank filling and cutting. Thealignment of the canals will have to be thatwhich would result in the greatest saving ofboth captial and maintenance l'Osts and alsoin the loss of head and transmission losses.
In an undulating country a straight align-ment of canal for any length may not be possi-ble as it would involve heavy filling and cuttingresulting in both heavy capital and maintenanceinvestments. The economically shortest routeis to be kept in view which can be attained byan alignment (which could be arrived at bytrial and error) between:
(a) A line strictly following the falling con-tour line where balanced filling and exca-vation is feasible.
(b) A straight line from head to tail in arolling country involves heavy filling anddeep cutting. In a flat country saving inloss of head throughthe straight alignmentis an important factor. The transmissionlosses assume more significance in case ofunlined canals than in case of lined canals.
(ii) An irrigation channel must run on a watershed or ridge and where that is not possible itshould run as near a ridge as possible. Inridge position the canal will be able to com-mand on both sides and will avoid interferingwith natural drainage. Where the main canalsare not aligned along the ridge, branch/distri-butaries could be aligned along subsidiaryridges which the canal would cross during itscourse.
(iii) The alignment of an irrigation channel/canalshould p~ ~entral in its comwano all far a~
possible and the length of the off-take canalsshould be minimum.
Water should be carried in bulk as far aspossible and whenever the canal bifurcates thetotal wetted perimeter increases and resultsin greater losses. ,Generally, the cost ?f onecanal is not proportIOnate to the capaCity ~sthe two channels are costlier than one for theIrcombined capacities. Canals with a biggercapacity can be. designed with a fla.tter slopeand will be ultImately beneficIal III a flatcountry.
The drainage line should not be blocked asthe canal itself may get damaged and result Inflooding and water logging of t~e surroun.dingareas. When irrigation channel IS on t~e ndge!water shed interference with drainage IS avoId-ed but if it is not located on ridge and it h~~ tocross various drainage lines adequate provlSlonto pass the drainage water safely across thechannel is necessary.
In addition to the above, the foll?wing practi-cal points should also be consIdered whtlefixing the alignment of canals.
Proper location of cross drainage works ismost important as the cost of C.D. Worksin a canal system is a expensive item. Thecrossing should be located at places wherethe waterway is sufficient, the catchmentis minimum, the foundations are good andthe material of construction is availablenearby.
The alignment of canal sho.uld ~voiddifficult country, i.e., one haVIng ndges,rocky, sandy and alkaline strata and alsoreligious centres such as mosques, templesand burial grounds.
When a canal passes near a village or animportant town, it sho~l~ pass on thelower side even at addltlOnal cost. Incase it passes' on the high~r side of t~etown it should be at a conSIderable dIS-tance and may be lined as the seepagefrom canal may create difficulties.
Irrigation channels are generally alig~ed with re-ference to the contours of the country III one of thefollowing different ways:
(i) as contour channels,
(ii) as Ridge channels, and
(iii) as side-slope channels.
A contour channel is carried on an alignmentwhich conforms generally to that of the contours ofthe country traversed by giving, such slope along itslength as is necessary to produce the required velocityof flow. As the line of flow of surface drainage is atright angles to the ground contours such a channelcuts across th~ natural drainage lines of the countrytraversed. Such an alignment does not imply anexact conformation with the contours of the naturalground, as it is usual, when traversing undulatingcountry which displays any marked natural featuresto shorten the line by crossing ridges in deeper cuttingand valleys in higher bank than the general average.Thus in such country a contour channel would be ofshort~r length than the corresponding falling contourlaid out along the ground surface without taking anyshort cuts.
A ridge channel is one alignment along any naturalwatershed. There will clearly be no drainage crossingsuch a line. The watershed bc:ing the highest ridge inthe doabs, water from a channel on the watershed canflow by gravity to fIelds on either side, directly orthrough some otber irrigation channels.
The main canal has to take-off from the river, thelowest point in the cross-section, and it must mountthe ridge Or the highest point in the area in as short adistance as possible. This is possible because the canalrequires much less bed slope than is usually availablein the country near its head reaches. It is within thisreach that all the important cross drainage works onthe canal have to be constructed. In its head reaches acanal is also generally in heavy cutting and this com-bined with the C.D. works makes this portion of thecanal very expensive. The alignment in this reach is tobe determined by studying the various alternatives,and comparing the advantages and disadvantages andthe costs of each.
2.3.3 Side Slope Channels
A side stope channel is one alignment at right anglesto the contours of the country traversed and not on awatershed or valley line. ~uch a line would be parallelto natur~l r~n-off of.dramage and thus such an align-ment aVOIds mterceptwg cross-drainage.
2.4 The main aim of these arrangements of thelayout of a distribution system is to secure in the mosteconomical way effective water distribution combinedw~th adeq~ate ,:ommand of the area to be irrigatedWIth as little mterference as possible with naturaldrain~ge. The above statement of ai~s at once suggestan ahgnment ~tong any watershed WIthin the irrigablearea as secunng command of all the ground upto thenext valleys on either side of the alignment without
any interference with drainage and it may be taken asaXIomatic that the watersheds laying in any irrigatedtract should be occupied by distribution channels un-less there are strong reasons against such an arrange-ment. Side slope channels haVe the advantage of notintercepting cross-drainage but their course mustfollow the shortest route to the nearest valley and suchchannel will be along a line of steepest possible slopeexcept in very flat areas. Only the smaller of the dis-trihutary channels should be so located. Contourchannels are carried on slopes, the main watersheds ofwhich are not commanded. Such channels with minorexceptions only irrigate on one side of them from theupper boundaries of the irrigated area.
One of the 3 ways of aligning the canal may beadopted but there may be situations in which it maybe necessary to deviate the canal off the adoptedcourse for short reaches. For example, the watershed·or the contour might take a sharp loop between. somepoints, the alignment may cut across "abadi", or alarge number of drainages may cross th.e a1Jgnment.In the case of watershed or contour taklllg a sharploop, by aligning the canals stra~ght considerable l~ngthcan be saved. It will, however, Intercept the dralllageof the pocket of land be~ween .the wa~ersh~d and thecanal, in such reaches. ThIs dramage Will either have
to be passed across the canal by suitable C.D. workor the feasibility of diversion to adjacent catchmentcould be done to avoid the C.D. work. The alignmentof canal may have to be diverted so as to bye-pa~stowns or vi'llages located on them. Number of cross-drainage works may b(" reduced by shifting alignmentas far as economically and technically feasible.
Curves in unlined canals shall be as gentle aspossible as they lead to disturbance of flow and atendency to silt on the inside and to scour on the out-side of the curve. The curves are usually, simple cir-cular curves. At the velocities, permissible in unlinedchannels, the super-elevation of water surface is verysmall. The situation will be different in lined channelswhere higher vel0cities are permissible. In any reachof a lined channel, with hyper-critical velocity, curvesare best avoided; in case they have got to be provided,the water surface profile must be very carefully workedout.
As per I.S.I. recommendations, radii of curvesshould be usually 10 to 15 times the bed width subjectto minimum given in Table 1.
Dischargem3{sec
Radii Min.m
Dischargem3{sec
Radii Min.m
80 & above
Below 80 to 30
Below 30 to 15
Below 15 to 3
Below 3 to 0.3
Less than 0.3
1500
1000
600300
150
90
280 & above
Below 280 to 200
Below 200 to 140
Below 140 to 70
Below 70 to 40
900760
600
450
300
The above radii are not applicable to unlined canals located in hilly reaches and in highly per-meable soil.On lined canals where the above radii cannot be provided, proper super-elevation shall beprovided.For navigation channels further modification may have to be made.
From the headworks where water enters the maincanal to the outlet where it enters the watercoursethere are continuous losses which have to be accountedfor in the designs. These losses are quite considerableroughly forming 25 to 50 percent of the water di-verted. There are losses in the water course and on thefield also, but they are taken to be covered in theoutlet discharge factor.
The losses in the canals take place on account oftwo causes: (1) evaporation and (2) seepage. Theev.aporation losses are usually a small proportion ofthe total losses in earthern channels and are generallyneglected. The seepage loss is influenced by the natureand porosity of the soil; the depth, turbidity and tem-perature of water; the age and shape of the canalsection; and the position of the ground water level.The seepage losses in unlined channels as suggested byEtcheverry and Harding are given in Table II.
Seepage LossCumecs/millionm20fwettedPerimeter
Impervious clay loam
Medium day loam under laidwith hard pan at depth of notover 0.60 to 0.90m below level
Ordinary clay loam silt soil orlavash loam
Gravelly or sandy clay loam,cemented gravel, sand and clay
Sandy loam
Loose sandy soils
Gravelly sandy soilsPorous gravelly soil
Very gravelly soil
2.70 to 3.60
3.60 to 5.205.20 to 6.10
7.00 to 8.808.80 to 10.70
10.70 to 21.30
In the case oflined canals seepage losses may beassumed as 0.60 cumec/million m2 of wetted perimeter.
A capacity and design statement along with com-mand statement should be drawn up for each channelto enable its L-Section to be prepared. Specimen formsfor the above statements are given in Plates 1 and 2.
For this purpose the locations of outlets and theboundaries of areas to be irrigated by each outletshould be marked on the contoured survey plan of thearea showing the alignment of the channel and itsirrigation boundary. The irrigation boundaries usuallyfollow the drainage lines between two ridges. In mark-mg the boundaries of irrigated areas, the folloWingpoints should be kept in vIew:
(i) The area allotted to each outlet should notrequire more than 0.09 cumecs or less than0.03 cumecs.
(ii) The length of main water course should notexceed 3 km.
(iii) Water course should not be required to crossnatural drainages.
(iv) The crossing of national highways and roadsshould be aVOIded as far as pOSsible.
The disadvantages of large sized outlets are:
(i) Cultivators find it difficult to manage or main-tain the field channel.
(ii) Disputes arise due to greater number pf share-holders on one ou tIet.
(iii) 'Warabandi' troubles arise particularly forshareholders of small holdings.
The disadvantages of small sized outlets are:
(i) There is greater loss of water by absorption.~I: outlet With 0.015 cumecs discharge mayIrngate only If3rd the area irrigated by anoutlet of 0.03 cumecs.
(ii) The cost is comp'aratively mOre owing to thegrea ter number of outlets, V. R. Culverts an dW.C. Culverts required.
(iii) Low.v.elocity in water course causes silting upreq l.lIr1I1gfrequent clearance.
(il') G:eater number of outlets cause trouble in dis-tnbutJOn of water and regime of the channel.
Havin.g .marked the boundaries of the areas thatcould be.lrnga~ed by :a~h o~tlet, the next step is tofix . the. 1l1tenslty of Irngatl.on: The intensity of irri-gatIOn IS go~e~ne~ by the eXlst1l1g protection by welland pond lrf1gatl~n, types of crops sown or likelyto develop after. lr:troduction of canal irrigationperfor.~ance of eXlstmg channels in similar areas andquantItIes of water available,
. . "Duty" of .can.al water can be defined as the arealfflgated by unIt dIscharge which in metric units will
mean the number of hectares irrigated by 1 cumec forthe specific number of days cal~ed the base period forthe duty. Duty for a channclIs usually calculated onthe head discharge. Duty based on dIscharge passedthrough the outlet and thus excluding all losses in thecanal ~system, is called the outlet discharge factor.
The commanded and uncommanded area and thewater levels required at the head of water courses orfield channels and supply channels should be markedout. This is more a process of trial and error.
A good deal of judgement which can-be acquiredby practice is required in fixing the water level at thehead of water course and marking c~)lnman.de~ areaswhich will neither pitch the F.S.L. III an IrngatlOnchannel unnecessarily high in an attempt to commandisolated high patches, nor lowe:: it too much at thecost of irrigation on which the whole SUCl'ess of theproject depends. It has to be Iemembered that suchhigh patches are levelled up III course of tIme by thecultivators themselves.
For determining the F.S.L. in a distributarychannel, the water levels required. at thc head of watcrcourses all along the length ~f channel s.hould beplotted on the ~-~ection. on whIch G.L. sprmg levelsand other field InformatIOn have already been plotted.The F.S. Line should thus be so marked at the pre-determined slope, as to give a good average workmghead into the water courses. This procedu.rc .readtlydetermines the location of falls ~m the d~str~blltarychannel as well. The F.S.Ls. determmed at dlstr~butaryheads should similarly be plotted on the L~SectlOns ?fbranch and main canals and then ~.S. Imes fixed IIIthe same manner as for (1Jstnbutanes. If ~owever,the consideration of balancmg depth enables hIgher fullsupply to be fixed, it may have to be done. Usually,.it is found sufficient to work at the command atoff taking channels upto the fust fall, for the purposeof fixing' F.S.Ls. in the main and branch canals. But,when there is a reverse slope Jl1 the country, the F.S.L.at the head should be dete!mincd after considering thecommand from head to tatl.
In working out the F.S.Ls. in channels, thefollowing working heads are usually allowed:
Head over fleld
Slope in water courseMin. working head from majordistributary to a minorMin. working head at outletMin. working head from branch tobranch or to major distributary
Min. workirg head from main canal
0.0751'01 in 5000
0.30m
0.15m
O.SOm
0.60m
A control point on a regulating bridge should beprovided below the off take. A minimum working headof 0075m should be allowed at these points.
2.9 Plotting of L-Section
(a) It is essential that an L-Section of channelshowing all salient information observed in the fieldsuch as natural surface levels and sub-soil water levelsfull supply levels, canal bed levels, water surface slopes:bed WIdths, free board, broad details of hydraulic dataof ou.tlets, regulators, bridges, drainage crossings,offtaklllg channels etc. and nature of soil where thecanal is passing through b~ giving test pit or augerhole data at about 500 m intervals be prepared. Forthe L-Section a longitudinal scale of about 1:10,000 to1:20,000 and vertical scale of about 1:100 are recom-mended. A specimen L-Section of canal is shown inPlate No.3.
(b) While plotting L-Section designed dischargebed gradient, bed widlh, full supply depth should beindicated. Designed discharge sholl1d account for offtake channel discharges and losses in canal. Waterlevels should be assessed after considering head lossesin various structures, i.e., regulators, falls, bridges,cn. works etc.
The following main types of structures will berequired on a canal:
(ii) Regulating structures
(iii) Communication structures/bridges.
The rust two types of structures are more essentialfor the safety and maintenance of the canals and areusually provided on all canals irrespective of itscapacity. The regulating structures could be useful inequitable distribution of water in the command area.Communication works are necessary to provide inter-communication in the command area lying on eitherside.
(i) Cross Drainage Works: Generally the alignmentof th~ .canals crosses the natural drainage valleys andprovIsIOn has to. be made to. take the drains safelyacross the canal WIthout damagll1g it. The usual kindof cross drainage works are:
(1) Su per passage
(2) Drainage syphon
(3) Canal Syphon
(4) Level crossing
(5) Aqueduct.
PLANNING AND INVESTIGATIONS
In effect there are three ways of crossing of thedrainage water and the canal.
To pass the drainage over the irrigationchannel the bed level and the water level ofthe drai~ is higher than the canal. This typeis named as a super passage.
To pass the drainage into the irrigation channelso that the drainage and irrigation water mixup. This type is called a level crossing.
To pass the drainage under the canal. Thistype is called a drainage syphon or a syphonaqueduct. The full supply level, the bed levelof canal and the HFL and bed level of thedrainage reach for the most suitable alignmentof canal will determine the type of structures.The particular type to be adopted cculdfurther be determined by the magnitude of theflood discharge of the nallah and thc designdischarge of the canal.
(ii) Regulating Sauctures : These structures arerequired to maintain the level and the discharges atthe designed figure. Some of the works may be auto-matic and some require regulation, establishment andmanipUlation. The Control could be from upstreamor by downstream, i.e., "Demand Management".
(b) Cross regulator
(c) Head regulator of distributaries/minors
(d) Escapes
(e) Outlets for water courses
(f) Silt ejectors.
Falls are provided in a canal when the fall of theterrain is more than that of the canal and the water levelbecomes in excess of the level required in the canal toreduce maximum permissible velocity and earth work.
This structure dissipates the excess energy and thefilling reach is minimised.
Cross regulators are required to maintain the fullsupply level in the canal. These are provided at inter-vals across the canal and below major off-take pointsso that when the canal is running at less than fullsupply discharge water can be raised to feed the off-take canal/channel to take its authorised discharge atdesign full or part supply levels.
Head regulators are provided for regulating thedischarges to feed the distributaries.
Escapes are provided for discharging the excesswater out of the canal during periods of low demandin the command. They are generally provided up-stream of cross regula tors near a natural valley so asto release the water from the canal and avoid floodingin the command area during flood/rainy season.
Outlets for water courses are provided from minorsand distributaries according to the demand in thlZcommand. Canals taking off from the head worksgenerally carry silt along with water. The silt loadand size of silt requires critical examination. In casesilt is of coarse or medium coarse, this gets depositedin the canal and reduces its capacity. Ivfedium coarseand coarse silt is also harmful for flelds in case thisfl11ds its way to the fields through the water courses.Power channels also need to consider the safe silt sizepermitted for the turbines. Silt ejectors are providedin the head reach so as to remove the silt to permissi-ble limits to save the canal from silting and fields!irrigated areas from losing the fertility. Silt ejectorsare provided in power c\laJ1llels to eject the silt fromwater to save turbines from getting damaged.
(iii) Communication Works: These are mostly roadbridges and are provided for all existing and futureanticipated roads to provide communications facilityin the command area. In addition to this some moreroad bridges are also provided so as to providecommunication facilities to towns and villages. Theseare generally provided at interval of about 6 km. Butin case of distributaries, road bridges are generallyprovided at intervals of 2 to 3 km dependingon local conditions to facilitate the movement ofmaterial and people.
------sL. R.D. OF AN OFF CRITICAL DISTANCE FPOlI LOSS OF NEAD DEPTH
NO. TAKE OR POINT IN N.S. CRITICAL FIELD
OUTLET POINT TOHEAD OFOUTLET
---_ ..__ . ------I 2 3 4 5 6
--1------_._-_.
PLATE ~.u. I
COMMAND STATEMENTOF
WATEflLEVELAT HEAD OFOUTl.ET(COLUMN3~·5.j·61
WORKINGHEADtQOLUMN $-71
._-----------------_.\I
- -------_._-------------.Q.APACITY a DESIGN STATEMENT
..QE
."._-~
------SL. BELOW KM. R.O.OF AN GROSS GROSS CULTURAL OUTLET OULEr LOSSES TOTAL TOTALNO. OFF TAKE AREAlaA.1 C""''''''''''EO COMMANDED AREA TO Bf. IRRIGATED
OISCHARGE DISCHARGE IN REACH l.OSSES DISCHARGE...£HANNEL DIMENSIONS
OR OUTLET IHECT.1 AREA AREA FACTOR IN CUMECS _.IG.C.A.I ICC.A.1 RABI SUGAR RICE FULL FULL DESIGN LACEy'S', ,tHECT.1 IHECT.1 IHEcn BED SlON: BED WIDTH SUPf'LY SUPPLY BED LEVEL D"CHAflGt: OR n HT. OF BANK
DEPTH LEVEL --_ ...-I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19
-- " -_._--,
.-PLATE NO.2: CAPACITY & DESIGN S1' TEMENT
·------------··-----l
VELOGI;~--r-" -- ._- -"-
IN MlSEC. II REMAlll(S
-20-t-------
(') G> !"' ..,(') "'~ " !n c 3:
0 !" -; QlZ C r r -; »l; z Z z
0 '""' r G> ~Z "' '"~ < z
'" ...... r'"'0 ~3.z.92.15 92.65 1·135
150 92·59 1.435
300 92·48 1.255
450
600
750
00
10:50
1200
1350 91.59
1500
1650 91.72
1800 91.4El 91.16
1950 91.41 9/.10 91.50
2100 91.15 91.04 91.54
2250 92.85
w-OUI00g
'" Cl (') .., !'" " (:) »(') is
~r» G> r c:
" '" r
'" '" »'" '" -;
'" 0
"1\ CS
SWS
OL1
CS -
\/ I
3 l- iEs I I/ ..- I I
7 AclRS I4
II
8S-
~ ES
r RXC'l;U VRSl>0iTiz CS-i-" <rI)
(UI OL00 CS
S
VRS
I L
VRS CS6
I VRB OL
VRS
ES1)
o ;u:c:: l> I'T1 UI()()():I:;U
i-x~0in~in:'n~~~al:TJ ~
I'T1Z()m(J)
rrI
(j)
()~l> -lZo~Z
o'TI
:s:l>
Z
Design of Unlined Canals
The flow of water specially in earthern channelspresents a complicated problem, which cannot besolved with mathematical precision. The unlinedchannel is an open channel excavated and shaped tothe required cross-section in natural earth or fill,without special treatment to the sub-grade. Wherea channel is constructed partly or entirely in fill or innatural earth of unstable characteristics it maybecome necessary to prevent seepage and percolationby compacting the sub-grade.
The motive force offlowing water in open channelsis entirely due to the slopes of the channel. The waterflows from higher to a lower point by. virtue ofgravity. The flowing water has to overcome theresistance in the channel caused by numerous factorssuch as surface tension, atmospheric pressure at surfaceand friction at side and bottom of the channel. Thefrietien on the channel section clepends on thematerial formir.g the wetted perimeter" the irregularityof the wetted perimeter, variation in the size andshape of the cross-sections, curves, weeds and variouskinds of growth, scouring, silting, suspended materialand the bed load. In practice the resultant of all theseforces is expressed by a coefficient of roughness deter-mined by experiments.
3.2 Flow Characteristics of Open Channels
The discharge passing in an open channel IS ex-pressed by the continuity formula:
Q=AxV
Q = Discharge in cumecs
A = Cross-sectional area in sq mV = Mea~ velocity of flow in m/sec
The mean velocity is given by the Chezy's formula:
V = CV R.S
R = Hydraulic mean radius in m
8 = Sine of slope of water surface
This is a practical and fundamental formula forthe un!form flow of water in open channels and it isthe ?asls of all modern velocity formulae. Since co-~fficler:rt 'c' represents the retarding influences variousInvestIgators have determined its value on the basis ofexpernnents and observed data. The most widelyused values of 'c' are given by : .
(a) Kutter's Formula
23 t. 0.00155 1, --S-+NC=--------. 1+ (23+ _0.0~155 ) :
The values of 'N' as recommended by I.S.I. aregiven in Table Ill.
For some conditions value of 'C' from Kutter'sformula are given in Plates 4 and 5.
(b) Manning's Formula
1V = -- fi.2/3 81/2II
The Chczy's 'C' r.nJ Manning's 'n' are related bythe Equation:
1C = -~ R1/6n
In normal ranges of slopes and hydraulic meanradius the values of Manning's 'n' and Kutter's "N"generally seem to be numerically very close for practi-cal purposes.
87
1+~V~
Value of 'N'
Normal
1. Earth, straight and uniform
(a) Clean, recently completed 0.016 0.018 0.020(b) Clear, after weathering 0.018 0.022 0.025(c) Gravel, uniform section, clean 0.022 0.025 0.030(d) With short grass, few weeds 0.022 0.027 0.033
2. Earth, winding and sluggish
(a) No vegetation 0.023 0.025 0.030(b) Grass, some weeds 0.025 0.030 0.033(c) Dense weeds or aquatic plants in deep channels 0.030 0.035 0.035(d) Earth bottom and rubble sides 0.030 0.035 0.040(e) Stony bottom and weedy ban ks 0.025 0.035 0040(I) Cobble bottom and clear sides 0.030 0.040 0.050
3. Dragline excavated or dredged
(a) No Vegetation 0.025 0.028 0.033(b) Light brush on banks 0.035 0.050 0.060
4. Channels not maintained (Weeds and brush uncut)
(a) Dense weeds, high as flow depth 0.050 0.080 0.120(b) Clean bottom. brush on sides 0.040 0.050 0.080(c) Same, highest stage of flow 0.045 0.070 0.110(d) Dense brush, high stage 0.080 0.100 0.140
Where 'M' is the roughness coefficient.of "M" for various surfaces are as below:
Type of Channel
Smooth plaster, planed timber
Timber, ashlar, neat brickwork
Rubble inasonry
Plaster, very smooth earth
Earth channels in usual conditions
Earth channels in bad conditions
0.06
0.16
0'46
0.851.30
1.75
The values of "e" as per Bazin's form ula are givenin Plate 6.
3.3 Stable Channel
A stable channel may be defined as one in whichneither SGo\Fin~ I1Qr &tHing ta~es p\qce. A satisfactory
design of stable channel depends largely upon the ex-perience and judgement of t~e de~igner. An attempthas, therefore, been made by InvestIgators to obtain acritical velocity offlow at which there will be neitherscouring nor silting. .F.ormulae that have been pro-posed for stable condItIOns of the channel expresseither a relationship between the C.V. and depth offlow or the relationships between the width of channeland depth of flow. The various formulae available fordesign of stable channels are given below:
(a) Kennedy Formula
The general form of equation based on the obser-vation on stable channels is :
Yo "'" Critical velocity in mjsec (Non.siltin~anG NOI1,sco~dl1~)
D = Depth C'f flow in metres
m = Critical velocity ratio
Mr. R.G. Kennedy made his observations on theUpper Bari Doab Canal system, one of the oldest in.the Punjab. On this systeI~1 he selected a n~mber otsites on various channels which had not reqUIred anysilt clearance for the last thirty years and were thusconsidered stable. By plotting his observations .ofvelocity and depth on stable reaches, Kennedy obtam-ed his well known equation.
Vo = 0.55 D00f4
This equation is applicable to channels. flow~ng insandy silt of the same grade as that w]:uch eXists onthe Upper Bari Doab canal system on whIc.h the obser-vations were made. Kennedy later recogmsed that thegrade of silt played a part in this rela~ionship andintroduced another factor in the equatIOn, called byhim as "the critical velocity ratio" and given thesymbol 'm'. The equation was then written as
V = 0.84111 DO'64
Sands c()arser than the standard were assigned valuesof m from 1.1 to 1.2 and those finer from 0.9 to 0.8.Kennedy made no correlation between wate~ surfaceslope of regime chanmls and the mean. velOCity. or thevertical depth. He relied on Kutter s equatIOn togive slopes to the channels.
The values of Va for different depths are shown inPlate No.7
The following equations were obtained by obser-vations at other places:
V = 0.391 DO'55 Godavari Delta
V = 0.53 DO'6E Krishna Western Delta
V = 0.567 DO'57 Lower Chenab Canal
V = 0.283 DO'73 Egyptian Canals
A general feature of these formulae ~as that. thevalue of 'c' was proportional to the SIze of the bedmaterial.
In actual practice, the design can be. carried. outwith the help of Garret's diagrams whIch p;ovlde a
hical solution of Kennedy's and Kutter s equa-gtraps Garrets diagrams are given in Plate Nos. 8 andIon. .' f 11 •9. The procedure for theIr use IS as 0 ows .
(i) Follow the given discharge cu.rve till it meetsthe given ho~izontal ~lope Ime and mark offtheir point of IDter-sectlOns.
(ii) Draw a vertical line through the point ofintersection. This will intersect several bedwidth curves.
(ifi) Choose the point of intersection of the verticalline on any bed width and read the correspond-ing value of water depth (D), and critical velo-city Va' on the right hand ordinate.
(iv) Calculate the actual velocity of flow (V) forthe chosen bed width and the correspondingvalue of the water depth.
(v) Determine the critical velocity ratio 'm'( :: ) taking V as calculated and Va as read.
(vi) If the value of VIVo is not the same as given,repeat the procedure with other values of thebed width till the C.V.R. approximates to thegi ven val ue.
These diagrams are drawn for side slopes oft:l onthe ass~lmption that irrigation channels after sometime acquire this slope due to silt deposition on theberms, irrespective of the initial side slope provided.These curves are drawn for N = 0.0225. They canalso be utilized for any value of N with the help of amonogram provided at the top of the curves. Whenusing for any other value of N, the vertical line drawnat the intersection of the discharge and slope line hasto be shifted, either to the left or to the right side to theextent given in the monogram. Further procedure fordetermining bed width and depth remains unaltered.
Lacey used Kennedy's data and substituted 'F'"Lacey's silt factor" for 'm' and found that '1' workedout to be square root of the critic81 velocity ratio
(:: ) for velocities not exceeding more than Im/sec
and if the hydraulic mean radius 'R' is substituted for'D' the variation in exponent as found on the differ-ent canal system disappears. The concept of 'R' inplace of' D' by Lacey, according to him, is becauseeddies which keep the silt in suspension are generatedfrom bed and sides both normal to the surfaces of gene-ration and not from bed only as stated by Kennedy.He,therefore, assumed that hydraulic mean radius (R)as variable. However, it has been observed that thereis hardly any difference in values of R & D in widechannels. On the basis of the above arguments Laceyderived a set of equations by plotting mean value6 ofobserved data.
• •. (i)
•.. (it)
A=Area of the channel section in m2
V=Velocity ('·fflow in m/sec.f=Lacey's silt factor.
Lacey's regime flow equation isV=10.8 R 2j3 S 1/3
From the above equationsperimeter-discharge relation:
P=4.825YQ
... (iii)(i) and (ii) he derived
P is wetted perimeter in mQ is discharge in cumecs
From this equation it was thus established thatwetted perimeter of a channel in regime was depen-dent on the discharge only and was Independent ofthe grade of silt denoted by I'·
Fi om these equations a number of other impor~antrelationships have been obtained which are gtvenbelow:
f5/3
S=3316Q J/~V=0.4382(Qj2) 1j6
R=0.47 ( ]. yf2( q2 )1/11.35 7- .ql/l
A=2.28jTJ2Silt Factor 'f'
According to Lacey, the silt factor' f' will b~ d~-pendent on the average size of boundary ~atenalll1the channel and its density. Its value vanes as thesquare root of the mean diameter of the particles.
f=-1.76Y n1r
m,.=average size of particles in mm.
Designs based on the Lacey's equations can besolved with the help of graphs in plates 10, 11,12and 13 representing graphically the equations.
3.4 Selection of Method of Design
From the designer's view point, the whole of Indiacan broadly be divided into two regions, the northernstates of India comprising of the Punjab, Haryana,V.P., Bihar, West Bengal, Rajasthan and some partsof M.P. where canals draw their supplies from riverscarrying heavily sediment laden water. These canalscarry sediment similar to that forming their bed andbanks. On the other hand, the canals in South India
though not devoid of sediment, carry much lessersediment and the boundary material of canal sectionis dissimilar to the sediment in transport. On thebasis of above dissimilarity the following practices ofdesign are recommended:
(i) For channels taking off from reservoirs carry-ing silt free water and flowing through hardsoils or disintegrated or fresh rock, it appearsquite desirable from the point of view of eco-nomy to design the channel for as high avelocity as the soil can withstand withouterosion, consistent with the slope av~ilablefrom the command point of view and econo-mies of the channel section. Too steep a slopeshould normally be avoided. Lacey's designis generally not applicable to design of suchchannels. The permissible velocity shouldfirst be fixed and the channel section shouldbe determined from Chezy's formula withKutter's value of C & N. The Kennedy orGarrets' diagrams may be used to facilitatedesigns and avoid calculations by trial anderror method.
Different sections are possible depending. upon the choice of depth. The channel section
should be deep and narrow particularly forsmall channels like distributaries and minors.Practical bed width-depth ratios as given inpara 3 5 may be used for general guidance.Departure may however, be made where foundnecessary to provide shallow and wide sectionto avoid cutting in hard soil below the ground.The criteria of balancing depth may not bedesirable to be adhered to in such cases.
(ii) For the canals carrying heavily sediment ladenwater the design practice are mostly based onthe Lacey's and Kennedy's formula. Lacey'smethod of design should be used for design ofnew channels flowing through alluvial soils.Greatest care should be exccrciscd in selectingthe value of 'f'. The section may then bedetermined either by calculations as illustratedin the example in Appendix-II or read fromLacey's diagram. It should be examined tosee that the mean velocity is neither too highnor too low for the type of the soil along thealignment of the channels. If necessary, thevalue of 'I' may be correlated to the Kennedy'sC V.R. Lacey's design cannot, however, beused for silt bearing channels carrying lowdischarges, the limit of which is given by thestraight line on the left side of Plate-9 beyondwhich the discharge curves are discontinued.In such cases water surface slope may beworked from Lacey's equations and dimensionsdetermined from Kutter's formula. Thismethod is also usually adopted for design ofchannels with low silt factor in cohesivesoils.
Wi) In case of existing channels requiring remodel-. ling which have not been originally designed
with Lacey's equation, it will not generally befound convenient and economical to adoptLacey's design. Lacey-cum-Kutter or Kennedy-cum-Kutter formula may be used.
3.5 Bed Width-Depth Ratio
Problem of selecting a section often arises in caseof channels or reaches of channels in which morethan one water surface slope can be adopted withoutaffecting the command. This results in more thanone section with different bed-width-depth ratios forthe same discharge, value of n and critical velocityratio as may be seen from the Table IV.
TABLE IV
Q n CVR Slope Bed F.S.(Cumecs) per width Depth
1000 m m
28317 0.0225 Lo 0.22 9.75 2.5328.317 0.0225 1.0 0.20 12.04 2.3028.317 0.0225 1.0 0.18 16.15 1.98
28.317 0.0225 1.0 0.16 28.96 1.42
The same difficulty has been experienced in chan-nels taking off from reservoirs and carryil~g relativelysilt free water. The CVR is not the dommant factorin such cases and the channel· may be design~d forpermissible velocity which the soil can stand withoutscouring.
According to Ellis the best channel section for thesame cross-section area and slope, posses. water atmaximum velocity with greatest hydrau!Jc !!1eanradius. For any trapezoidal channel of fixed se.ctlOnalarea (A), if',' is the ratio of horizontal to vertical. ofthe side slopes and '8' is the length of th~ two SIdeslopes. per unit depth which equals 2 V,- + 1 and'M' is the ratio of bed width to depth.
.• jM", .;-Hydraulic Mean Radius (R) = 'V M'!- S v A
and it is maximum when M = S - 2,Accordingly for a channel of side slope t : 1, for thebest discharging section, the value of M works outto be 1.236 or say M = H·For least absorption, M works out to be .4. Ell,is alsostated that in case of all sizes of.cha.nnels m whIch t~erelationship of velocity to ?ept.h .ISgiven by Kennedy sequations, the absorption IS mmlInum for val!les of Mbetween 1.5 and 3.2. For val~es <;>fM varyIng from1.2 to 5 the loss due to absorptIOn IS never more than5 per cent in excess of minimum.
The experience of the working of irrigatio~ chan-nels in alluvial soil has however, shown. the eXistenceof certain bed-width-depth ratio dependmg upon the
discharge or in other words size of channel. TheKennedy depth formula gives maximum depth thatcan be allowed in an earthen open channel. As forexample, a channel situated in a soil which can standmean scouring velocity of 1 m/sec. cannot have fullsupply depths of more than 2.56 metres as may be seenfrom Plate No.7. This however, has no relationshipwith the shape of channel i.e. bed-width-depth ratio.Whenever therefore, there is a free choice for selectingdimensions of bed width and depth the ratios whichmay be obtained from Plate No. 14 may generally beused for guidance.
3.6 Most Efficient Cross-Section
Considering purely from the stand point of hydr-aulics of a channel, the most efficient cross sectionsis the one which with a given slope and area has themaximum carrying capacity. Chezy's formula indi-cates that for a given water surface slope, the velocityand consequently, the discharging capacity of thesection is maximum when the hydraulic mean radius(R) is maximum. The lraximum R is given by thecircular section. Since channel in earth have usuallytrapezoidal section, maximum R is given by halfhexagonal described about the semi circle with itscentre at the water surface. Such a section however,requires great depths for larger discharges. 1hevelocity induced in such section may also become sogreat as to cause erosion of the sides and scour in thebed of the channel. The non-scouring mean velocitiesas recommended by various authorities includingCWPRS, Pune are given below:
(1) Very light, loose sand toaverage sandy soil. 0.3 to 0.6 mfsec.
(2) Sandy loam, black cottonsoil & similar soil. 0.6 to 0.9 m/sec.
(3) Ordinary soils. 0.6 to 0.9 m/sec.
(4) Firm loam, clay loam,alluv ial soil, soft clay andmurum . 0.9 to 1.1 m/sec.
(5) Hard clay or grit. 1.0 to 1.5 m/sec.
(6) Gravel and shingle. 1.5 to 1.8 m/sec.(7) Cemented gravel conglomite,
hard pan. 1.8 to 2.4 m/sec.(8) Soft rock. 1.4 to 2.4 m/sec.(9) Hard rock. 2,4 to 3.7 m/sec.
(10) Very hard rock or cementconcrete. 4.5 to 7.6 m/sec.
3.7 Side Slopes
The side slopes to be adopted depend upon thenature of the soil in which the channel is to be eXCa-vated, depth of cutting and fheight of embankmentand shall be designed with the following conditions, inalluvial soils.
(a) Sudden draw down conditio!ls for innerslopes.
(b)· The canal running full with plenty of rainfaIlfor outer slopes.
The minimum slopes usually all owed arc given inTable V
]. Very light loose sand 1.5:1 to 2:1 (cutting)to average sandy soil. 2:1 to 3:1 (embankment)
2. Sandy loam, loamy 1:1 to 15:1 (cutting)soil and similar soils 2:1 (embankment)
3. Sandy soil or gravel. 1: 1 to 2:14. Murum, hard soil 0.75:1 to 1.5:1
etc.5. Rock 0.25:1 to 0.5: 1
The slopes given above are suitable for averageheight upto 3 metres. For greater heights, it is pre-ferable to provide ledges 1.5 to 3 111wide at inter-vals of 3 m instead of flattening the slopes. Thelatter method is, however, slightly economical.
In aIluvial and sandy soils tbe side slopes in cuttinggenrally assume side slopes of t : 1 on silting up, incourse of time. This slope is therefore, assumed indesigning the section for computation of hydraulicmean radius, area and perimeter even though in ex-ecution actually flatter slopes are adopted.
In hard clays and rocky soils, however, it is notnecessary to follow the above practice. The side slopeactually provided in excavation may be taken into ac-count for working out the sec:tional area of channel.
3.8 Free Board
Free board in general will be governed by consider-ation of the following aspects:
(i) Soil Characteristics.(ii) Size of canal and its locations.(iii) Hydraulic gradient.(iv) Water surface fluctuations.(v) Percolation.(vi) Service road requirements.
According to U.S. Bureau of Reclamation practicesfree board may be determined by the formula:
F= 1 X J C.1J1.812 \j
where, F= Free board in metres.C= Coefficient.1)= Depth of water in canal in metres.
The coefficient 'c' varies linearly from 1.5 wherethe canal capacity is 0.7 cumecs to 2.5 for canal of 85cumecs or more. However, greater free boards andbank heights than those within these rane:es may beused in exceptional ci rcumstances. '"
As per I.S.I. reco'1lmendations a minimum freeboard of 0.50 m for discharges less than 10 cumecsand 0.75 m for discharges greater tban 10 cumecs is tobe provided. The free board shall be measured fromthe F.S.L. to tbe level of the top of bank. The heightof dowel porti<:)ll should not be used for free boardpurposes. It IS observed t~at the free-board given bythe formula of U.S.B R. IS very much higher thanthose recommended by I.S.I.
The minimum values adopted for top width of thebanks are as follows :
Discharge(cumec)
Minimum Bank top widthInsp~ction Bank Non-inspection
(m) (m)
0.15 to 757.5 to 10.0
10.0 to 15.015.0 to 30.030 and above.
Width differel?t than those recommended above maybe D.sed when Justified by speCIfic conditions FordistrIbutary can~ls .carrying less than 1.5 cume~s andminor cana~s, It IS generally 110t economical to Con-struct a serVIce road on top of bank as thi~ u 11. q . I sua yIe ulres more matefla s than theexcavatiol1 prov·dI S h . dIes.n uc cases, serv~ce roa may be provided on naturalground surface adjacent to the bank. Howeve th
t f 'd' r, eImpor ance 0 prO'll Ing adequate service roads wherethey are needed should always be kept in view.
~he banks sh.ould invariably cover the hydraulicgradIent. The WIdth of the non-inspection bank shouldbe checked to see that Cover for hydraulic grad' t'provided. 'Ien IS
Berm is a horizontal space initially left at groundle~el . beltweb~n ~he ftoe of bank and excavation. Thepnnclpa 0 jectlves or providing berms are:
(a) ~o provide v.:ider waterway above F.S.L. tolIm~t. the heIght. of rise of water at the time ofpositive fluctuatIOns.
(b) To affo~d less ~ervious silt lining on the sidesof.the silt carryll1g channels to decrease ab _phon losses. sor
(c) To recede the banks away from the directaction of water to prevent breaches.
(d) To provide a SCOpe for future widening ofcanal.
(e) To provide space for borrowing earth duringconstruction at times of necessity and later onfor maintenance when the berms are formed.
(f) To break the flow of rain water down the bankslope. Berms are to be provided in all cuttings,when the depth of cutting is more than 3 m.Where a canal is constructed in a deep throughcut requiring waste banks, berms should beprovided between the canal section cut andthe waste bank.
The following practice is recommended:(i) When the F.SL. is above G.L. but the bed
is below G.L., that is, the canal is partly incutting and partly in filling, berm may bekept at N.S.L. equal to 2D in width where,D is F.S.D.
(ii) When the F.S.L. and the C.B.L. are bothabove the G.L. that is, the canal is infilling, the berm may be kept at the F.S.L.equal to 3D in width.
(iii) When the F.S.L. is below G.L., that is thecanal is completely in cutting, the bermmay be kept at the F.S.L. equal to 2D inwidth. In embankments, adequate bermmay be provided so as to retain the mini-mum cover over the hydraulic grade line.
For a channel running in embankment. the perco-lation line through the bank though actually curved, isassumed straight and sloping at 1:4 to 1:6 gradientdepending upon the bank material. It is taken forordinary loamy soil as 1:6, average clayey soil 1:5 andgood clayey soil 1:4. A soil having hydraulic gradientflatter than 1:6 is not suitable for canal banks. Whenthe height of bank is such that percolation line inter-sects the outer slope above G.L., an extra layer ofearth is put to provide a minimum cover of 0.30 m.The surface of this cover is either kept parallel to thehydraulic gradient or is in horizontal steps.
For embankments more than 5 m height, the trueposition of saturation line shall be worked out andstability of slope should ,be checked as in earth dams.
Dowel is generally constructed on the inner side ofthe bank, while in those places, where rains are heavy,dowels are provided on both sides of the bank to pre-vent their cutting. Water is then led out at intervalsthrough artificial gutters. The height of the dowel isgenerally kept 0.5 m with side slopes 1.5:1 and topwidth as 0.5 m.
Typical cross-section s of unlined canal are shownin Plate 15.
Some typical examples of design of unlined canalare given in Appendix-I
Example I
Design of Channel section by Kennedy, Chczy &Ku tter formulae.DATA
Q = 30 cumecsN = 0.02S = 1/8000Side Slope t : 1
DESIGN
Against Q = 30 cumecs the value of bid ratio fromcurve from plate No. 12 works out=7.5
Let the depth of chaJ1]~el = 2.15 ill
b = 2.15x7.5 = 16.125 m = say 16.0 mArea (l6+1.08)x2.15=36.72 m2
Pw 16+4.81 = 20.81 m
A 36.72---=-----=176 mPw 20.81 .
23 + ---~---+ (~~_l?..?-)(
23 +()0155 ) X ~1+ --S- ".1-1\-
1=23+-0:-020· +000155 X 8000
1+(2H-.00155 x 80(iO)~-
y' 12623+50+ 12.401+ (23+ 1240) x 0.02
1:f3-85.40
-i~5-3-2- 55.8
V C V RS-----
55.8 X V 1.76 x 1/8000558x1.33
----- = 0.84 m/sec.89.4Q A X V = 36.72 0.840 = 30.84 cumecs
Critical velocity = 0.55 D 0.64= 0.58 x 2.15°'64= 0.90 m/sec
9.v·R. ;;: ~:~~ = Q.9~ O,K.
Example II
Design of channel section for 30 cumecs discharge.using Lacey's formulae.
DATA
Q = 30 cumecsf = 1.0Side slopes = t: 1DES'GN
R 0.47 X (}9:- yf3
047 X (+ )1/3
= 1.46 m
PlV or P = 4.825 V-Q= 4.825 V 30' = 26.43 m
D = Pw == V Pw2-6.944 RPw------- --------3-:-472"--··------
= 26.43 - ".1698:-54 -6.944-->::1.46><26.43 = 1.64 In-----------3472 -----
B = PlV - 2.236 D26.43 -- 2.236 X 1.64 = 22.76 say 22.8f 5/3 = 1_. _
-33T6- Ql;6 3316x(30)1/61
=---' -3316XT:""i61-5836 i.e. 17 cm/km
V ( -9:{02- r/6 =0.77m/sec.
Example IIIDesign of channel sections for 30 cumecs dischargeusing Garrett's di agramDATAQ= 30 cumecsSide slope = t : 1Longitudinal slope = 1/8000 or 12.5 cm/l<:rn.N = 0.02From Garrett's diagramB = 16 mD = 2.15 mVo = 0.9 111jScC.
MEAN COEFFICIENTS n OF ROUGHNESSMEAN
:£ RAD.R RAD-R6z METRES .009 .010 .011 .012 .013 .015 .017 .020 .025 .030 .035 .040 METRESI1J..JII..0 .025 52 45 40 35 31 25 21 17 12 9 8 6 .025I-
% .050 63 55 48 43 39 32 27 21 16 12 10 8 ·050:> ·1 75 66 59 53 48 40 34 27 21 16 13 II .1a:: .wO .2 87 17 69 62 57 48 41 34 26 21 17 15 .2Il.0NO ·4 99 88 80 72 69 57 49 41 32 26 22 19 ·4oln0% ·6 104 93 84 77 71 53 45 25 22-0: 61 36 29 ·6• • I III 100 90 83 77 67 59 50 40 33 28 25 Iw 2 118 107 98 90 84 74 65 56 46 39 34 30 20..0 4 124 113 104 97 90 79 71 62 51 44 39 35 4..Jl/l 10 130 119 110 102 96 85 77 67 57 50 45 40 10
30 135 124 114 107 100 90 82 73 62 55 50 46 30
t .025 55 47 41 37 33 27 22 17 13 10 8 7 ·025t!) .050 66 58 51 45 40 33 28 23 17 13 II 9 ·050zw -I 78 68 61 55 50 42 35 28 21 17 14 12 ·1..JII.. ·2 90 80 70 64 59 49 42 35 27 22 18 15 ·20I- ·3 95 85 76 70 63 54 47 39 30 24 21 17 '3z:> ·4 99 89 80 73 67 57 50 42 32 27 22 20 ·4a:: ·6 105 94 85 78 72 62 54 45~g 36 30 25 22 ·6.,.In I III 100 90 83 77 67 59 50 40 33 28 25 ,ON 2 117 106 97 89 83 73 65 56 45 38 34 30 2o!:0- 4 123 III 102 95 88 78 70 61 50 43 38 34 4. . 6 125 114 105 97 91 81 72 63 53 46 40 36w 60..0 10 128 117 108 100 93 83 75 66 55 48 43 39 10..Jl/l 30 132. 121 112. 104 98 87 79 70 60 52 48 43 30
MEAN COEFFICIENTS n OF ROUGHNESSMEAN
RAD·R RAD·R
:i METRES .009 .010 .011 .012 .013 .015 .017 .020 .025 .030 .035 .040 METRESl-t!) .025 57 50 43 38 34 28 23 18 13 II 9 7 ·025zLLJ ·050 69 59 52 47 42 34 29 23 17 13 II 9 ·050..JII.. ., 80 70 63 56 50 42 36 30 22 17 14 12 .,0 ·2 90 80 72 65 60 50 43 35 27I- 22 18 16 '2Z '3 96 86 77 70 64 54 47 39 30 25 21 18 '3:>Q:O ·4 100 89 81 74 67 58 50 42 33 27 23 19 ·4wo0..0 .6 104 94 85 78 72 62 54 46 36 30 25 22 '60--z I III 100 90 83 77 67 59 50 40 33 28 25 I0-0- 2 116 106 97 90 83 72 64 55 45 38 33 29 2I; II
w 4 121 III 102 94 87 77 69 60 50 42 37 33 40..0 G 124 113 104 97 90 80 71 .62 52 45 40 36 6..JVl 10 127 115 106 99 92 82 73 64 54 47 42 38 10
30 130 119 110 102 96 86 77 68 58 51 46 42 30'--- -
:r.....(!)ZIJJ..JIJ..o•...Z::>
ffio0.011')0NOOVOz0-0-• II,.~o_Ilfl
------,-,- - -COEFFICIENTS rI OF ROUmiNESS Mt::AN
.0~101.f1~liRAD·n
5009 .010 .011 .012 .Ol~ .015 .017 .020_ ,..025 :(j~() MEnlES(j C C C G G C C C C C C
5 34 29 25 22 20 17 14 II 9 7 6 5 .02544 38 33 30 27 22 19 16 12 9 8 7 .0558 50 44 40 36 30 26 21 16 13 II 9 .1'1'2 63 56 51 46 39 34 28 21 18 15 13 .282 72 64 58 53 45 39 33 25 21 11 \5 .389 79 71 64 59 50 44 37 29 23 2D 17 ·499 88 80 72 67 57 50 42 33 28 23 20 .6III \00 90 83 77 67 59 50 40 33 28 25 I121 109 100 92 85 74 66 57 46 38 33 29 (·5012'/ 115 106 96 91 80 71 61 50 42 37' 32 2136 124 114 106 99 87 78 68 56 48 42 37 :3142 1:10 120 III 104 93 83 73 61 52 46 41 4149 137 12'7 119 III 100 90 80 61 58 51 46 6158 145 135 127 120 108 98 88 75 66 59 53 10!64 151 141 133 126 114 1()4 94 81 7'l. 64 59 1516'7 15.'5145 137 130 118 108 98 85 75 108 62 20\72 160 150 142 135 123 113 103 90 81 '74 68 :30- ---
MEANRAO.Rr.U::TRE
.02
.05
.1
.2
.3
.4
.6Q
I-50t34
G10i~20?P
- ._. - ._-.025 40 35 30 26' 24 20 '"I 13 10 8 7 5 ·mm.05 52 44 39 34 31 26 22 18 13 1\ 9 7 .05·1 65 57 50 44 40 :>4 29 24 18 14 12 10 ·1.2 79 69 62 56 51 43 37 3'0 23 "'::1'9 16 13 ·2·3 87 77 . 69 62 51 "·8 42 35 27 22 18 16 '3·4 93 83 74 67 62 53 46 38 30 25 21 18 ·4.6 102 90 82 74 69 59 52 43 34 28 24 21 ·6
JO I. III 100 90 83 77 67 59 50 40 33 28 25 I.0 '·5 118 10'1 97 90 83 73 65 55 45 38 53 28 1·50 102 94 87 77 59 4·8 35 31 20 2; 123 III 68 41t\I j 129 117 108 100 93 83 74 64 53 45 40 35 3~ \ 104 97 86 77 68 56 49 43 38 4- 4 133 121 11'2
" 6 136 126 117 109 102 91 8? 72 61 53 47 42 6IQ 143 131 122 1\4 107 96 87 78 66 58 52 47 1015 141 135 126 118 III . 10{) 91 82 70 62 56 51 1520 150 137 128 120 113 103 94 84 72 64 58 53 203Q 152 140 131 123 116 105 97 87 76 68 G2 57 30
--~_.
,--- - -fJll';tlN G()EFr-!C~ENTS i1 OF OOUGHf\lESS MEANRAD.r~ RA[).R
fJE'fRES 01)09 .010 .011 .(Wl .eml .015 .Oi'f .020 .025 .030 .055 .040 METRE~
:l!:l- .025 47 40 35 31 28 22 19 15 II 9 7 6 .O2~l!lz .05 59 50 44 40 35 29 25 20 15 12 10 8 .05w..J
26 16 13 II -(lL .1 72 62 55 50 45 ?J;7 32 190
~4 74 66 60 54 46 39 '32 25 20 17 14 ·2I- '2zo -3 91 81 73 66 60 51 44 37 28 23 19 17 -3'='0 ·4 97 86 77 70 64 55 48 40 31 25 21 18 ·40::0wO .6 104· 92 83 76 70 60 53 45 35 29 25 21 '6':::~
i' III 100 90 83 77 67 59 50 4··0 33 28 25 I0__
°d H, 117 105 96 8e: B2 72- 64 54 44 34 32 28 1'50 2 120 109 100 92 35 15 67 57 47 40 34 30 2II 4 128 116 107 99 92 32 n 64 53 46 40 36 4
IJJ56 49 43 39 6d. G 131 1\9 110 102 96 85 n 67
9 10 135 123 1/4 106 100 89 31 71 60 53 4'7 43 10If)
15 137 126 ilG 109 102 92 83 "/4 63 55 50 46 15
--- 00 141 12:9 120 112 106 ~5 37 78 67 59 54 50 30____ ._"_-'==-_.__-=--~~_'_"'_=_. ___ ....-c- _"_,- _____
HYDRAULIC VERY SMOO SMOOTHMEAN DEPTH(R) TH CEMEN BOARDS BRIOK\IN METERS.) &- PLANED OONO. GLAZE.(
BOARDS. EARTHENWARE PIPES.
-----
M=0.1085 M=O,29
ASHLAR EARTH C1<N1<LS~EARTH OANALSMASON- IN GOO coNOt- IN ORDINARYRY. TION AND CA- OONDITION.
NALS PITCHEDWITH STONE.
EARTH OANALSEXCEPTIONALLYROUGH.
"::54 ~"::35 l"' ~017 I
23 ---~t-~;-------I----~~RCHEZY'S FORMULA:- V= C v""RS
----1- '27 I 20 16 BAZIN'S FORMULA:- C~ 1:57.6
-, - - ---- --- -------------- 1.81+. M
30 22 18 V'lr
0.10 73 60 36f---------- ------- .-------.._'.- ..-.--. --
0.15 75 62 40
I e21.00 82
-- ----"------ .. --- --
1.50 83
2.00 84
2.50 84
3.00 85
3.50 85
21~:-:-.....~~f~~-~~~~~~~:-'~~:.••-~-:~!.....-:-:------------ ---------.---l- ----J---------
I '55 41 I 33 I 27------1------!56 43 34 I 28
---- --- ------------l---------58 45 36 I 30
----------
PLATE NO·6:VALUES OF C FROMBAZIN'S FORMULA
;60 47
-- "_. - "---. -
63 51
66 55
68 57
69 59
70 60
ii
..----------, -----------iI
0 0.00
0·00 0.00
1'0 o.~?---
2'0 I (}857
3'0 11'110
010 I ~~~ZO i 0.25 i 0·30
.122 .163 I 0.~225· .254
DEPTH FROM a TO 3.95METRES
0.3;;-' 0'40 10.45 10'50 ! 0.55 I 0·60 0'65
.2DO 0.306 0 330 '353 .375! ·397 I .417
.667 ,662 ·698 ~ ~750 L'748 .761
·949 ·963 ~;;;;-I ,989)',0021"014 '027
" I'1'192 1·205 1·215 1,·226 1·237 1,·248 1·260
_.
~0'70 0'75 ·0'80 0'85
.437 .457 ·476 ·495
I-0514 I O~
I0·844 I'Tl3
~801'816 0·830
I1·039 1·051 1·063 1·075
~~'~1'271 1.282 1'292 1·303
I1,314 1'325 I
I[,I
I'1 _
.~~~-~18
·882 i '896 .911
1·154 ..~-- 1·158 i
SLOPE IN eM./ KM...., ..., UI • (JI (1\ 21 CD <DCJ)0 0 Ul 0 0 0 0 0 0
z"0.05 0
0.02 0NN(Jl
0.03
0;04
0.05
"1J 0.07'510.20
r
~rn 0.10z 0
0 CJ)0.1250:z:
CD J> 0.150 fo,e::0G) 0.20
Q mZ» z 0.025;0 0.0275;0 0
rn c 0.0303: 0.75-i rrl I..• 0 I(j)
0 0-35- 0'.0»Q 0·45::0 0'50:t:- 0.55& 0.60
0.650;>00.75
1.00 i3·00
1.250
1.500I
L:750 5.00I
IIII! "- !
I:- ~ N0 N U1 -"j
"0 0(JI 0 r.1l
i 0<i II 9--_.__ .-
9 9 0 0 ------0 P 9 9 P 9 9 9 0 0 9 ~
~I
~ N r'¢ Q-J .Il> ~ QI ~}\ '-l" en l:l .:,,: ~ ~ c:a 0>N lj) {;'l 0 0 :""1 @ UI -...! N '", Oi:l N Cl)
01 --.j OJ .t> (J)r,.'! vi Ul t..'1 0 lJ! N l)l();I
L -----------
a:; 10 C CDCO C7l 10 l\l It' 0 ~ It' 0 It'~ 10 l\l CD It' l\l ~ I(:C\i
"" (11 I{\ 0 10
>0(11 0) C1' ci CO It' r::: CO CO "7 It' It' "t0 0 0 0 (; 0 0 0 0 0 0 0 0 0
I , , , I
0 0 10 00 It' It" l\l qN ..•
" 2~.OO
" " 2.e.00" "-24·0(;
22.00
20.00
19.0018.00
17.00
16.00
15.00 ~/4.00
<{0::
/3-00 (9<{
12.00 0/1.00 
-lI
Q
en
CD
0Ol GO r-..0 0 0
ooo0::UJa.(/)I-0::«a.Z
UJa.9(/)
J·o0.80·6
0-10·080·06
I"J·Z
1'3 01'4::~~t-- ~ I I III
- ~I_ -r-- _r--- -y -
-- ----I--~1--",,--- - --J..!.:9 ---U:.S. --~--r-
-r--r-~5=-as
---... -_ I__ - - :::::-I"-. =+:---=:::::r---~:::-~=+=--r-- -Ft-- '---I --_ i___ r---t-1--_- f--
-r--..u·4 --
o'OZ I
IO'OJP 0 p p op-:w
i\) W UI ':"'<00
-_---r--- I_ 1"-_ -
- ..::- I"-r--::::::--_ -~-t--_-- ---r-=:----- --___ r---r--
r--- ---'-- -I-"'-r--. _ I
: r ~-r--._ i !I-';-+--!i
I I I II ' I, I I
; ! !I -1__
!
•..i !
f==~:::;~~~A'5r-- - - W!!i /'4I---- -_ f..---- __ ' •---;----- .------ .- -- /'0- --<.. o·g--...L- --'8-- ."--'-t--_ ~
-.:....~6
-1----_ "'..11\
'--r--~-.J_lQ,4
I i I
I I !
I IUl ~ "- CDI'\) w 0 N W
~ ~ 0 N W 06 0 0 0 0 0 00 0 0 0 0 0 0 00
DISCHARGE IN CUM EC
PLATE NO. II: LACEY'S REGIME SLOPE DIAGRAMDISCH.ARGES 0'1 TO 600 CUMECSILT FACTOR tf) 0·4 TO 1·6
, " \ \ ,\ I '. \\~~ I' " ~f\\",, ~,\\ ~~l~~....•.... , , \.-~:-,,\\\\ \
~ \ \ \\*~\'( \ \ ~ r--\ \ \ ~\\ \I !
\ ~" , ,\,,, ~\ \ ?t!~', \ \ ,~. \ ~ ,
\ .-AP-:\', ' , l\ ,
'I' \ \ ~I \ \ \ \ ~\ \ \'\\/'\ " \ , "'I\\'J.~": \ \ \ " ,:...~\\
\ \ 'Y-'\'. \,,\, I
I \ y~' \' ," \ ,I \ ~\ \ \ \ ,\ \ \\ X \ \ \ , ,\,\\ \'~/\ \ \" \\'
\ \ \ \ " ,\ '\ ,..\ \ \ "~\ I""\ 1\\",' '9\ \ \ '\.x ,\ ~o·I \ \ \ ,-/\.\ '\..K\; , 0'&\ \ \ X \ y~'.>"'(,...,
\ \ /~~/\\~"'\ \);- ~\ ~, \ 1\)'/\ '~\\~O~"/ /v ~\ ~\'.\~(.f!)
V/' /1 \,/ \. ):-A"\ \~\\',( V"I, X '. \ X \ \ \ \ , 'O.AV /\/ y \, ~ \ I \ :xf->,
1 I V \, X I \ Iv'\ \ \ \\f / i/} ,/\ \.~[\\\ \\\\~.~y r /~ ,l)( \ \ \,X '\'! ! ;! ! A \ x\ '.\\'.\v jI ~ I X \"\~\\~0.2
I'l CD I \' \ \o 0 J \ \\\\\1\\
~ I WI \1\1\o I I \ \ \ \ \I I 0 \ \ \I 0) \ \ \ I \ \
j/ ~ \ \ \ \ \\I I
o ; I I I ~.,, I I v..:.
I f!/PI I ~10
ID I'6, 6
·•z c0 \
U5 ·(/)
o~z:E-00
l&.l)00::0 l&J
0 z~Gl ~~0
z...Ja:<t.0 >0:: ,.I1-:
0 Z ,••• l&J
,U ·
QlO
0il ~
~
~0:0~w~~0::::>(J)U
0 Z~<OI') 00..:..
~~Ol&J w_1--a: ~6~I-l&J d 6
o ~, ~W 0"'ci,~ ~o::-
:z: ~lL.0:I- WwOQ O::~1--
I'l~(/)o::u" ~Lt':"0 >-Iw WUI--(Xl U(/)-l:SO(/)N
~ 0~ z
w~ I--
~-l
6 a..
lO6
~0
o6>U)" /'0~ 8:1·z 8.- 0.5~ 0- 03Uo...J a.~ e
1'6('3,·z/'0
0.90.70.80.6Ot<:.~ ~ ~ g g 8
0
-..••
o 0 oOQ
LACEY REGIME FORMULA
z'!4V=(QfJ6i40
I PLATE NO. J 3: LACEY'S REGI ME .DISCHARG~ AND VELOCITYDIAGRAM
-- -- ._ .._,
1
----_." _.~._-_. -.-..- '-r- vv./'"
._. ---1------ .- ~- ... .- . - . -- _._- _ . .._- .- _._.- . _. .--- . ----: .... _- ."._---
. _. ----_ . .... __ . .-- -- -~ _. - --' .."--_. ---- --- ..---------~- _.'-- ..- -- _. --
~ ~- f- .._. .... -. -- .•. _ ...... _._- ..- .._ ... "._--- _._-- ----
--
-
-
I 0.2 0.3 0.4C .:> O. I 20 30 .•0 : D 70 KIO 200 00
PLATE NO. 14: GRAPH FOR BED WIDTH AND DEPTH .RATIOFOR UNLINED CHANNELS
>-a:i::>oID
BANK MAX.60 !1BElt..•• 5001+" 3 SPOIL BANK a.ROAD ~ N.S._, 1 _
z D~ +--r; M..r-I50 _ --i 5001'') ~ZV~50 150 Z
300 WIDE GAPS TO BE l.EFT AT~EVERY75 MIN. SPOIL BANKS'ONBOTH SIDES.IF D>3M LEDGES ARE PROVIDEDAT 3M INTERVALS.
/MIN.300--l I-
{MIN.,300+I I-<-
N.S. BETWEEN BED AND F.S.L.DIGGING MORE THAN BALANCING DEPTH
I+~~ ~(5SERIAL NO. I TO 8 INDICATE ORDEROF DISPOSAL OF SURPLUS EARTH.
N.S. BETWEEN BED AND F.S.L.DIGGING l.ESS T"HAN BALANCING DfPT'H
I
8~IDO
ZOO) 300 a IDOWEL t-t--I- f+ 300 z
BERM . =-=-=~-:::-.,:t t ~. ~ N.S.
l-50gp .J<tZ~o
>-a:0,
,~
,g INsPECTIONROAD
Design of Lined Canals
It has been est imated that seepage losses inirrigation channels constitute 35 to 50 per cent of theavailable water at the canal head. The necessity of liningirrigation canals with a view to save these lossescannot be over-emphasised. As more and morewater resources are utilised and water demandin creases the canal lining will ha\'e to play anirnp~rtant role in conserving losses and th'?l"Cby extendand improve irrigation facilities.
Lining of an irrigation channel is resorted toachieve all or some of the following objectiveskeeping in vIew the overall economy of theproject.
(a) Reduction of seepage losses resulting in asaving of water which can be utilised for addi-tional irrigation.
(b) Pr~vention of water-logging by reducingseepage to watcr table.
(c) Reduction in area of cross-ser:tions (andthereby, saving ia land) due to increase inpermissible velocity by reduction ill the valueof rugosity and availing of steep:r slop(;, whereavailable.
(d) Improven,ent of discharging capacity ofexisting ,:hannel~.
(e) Improvement of operational efficiency.
(j) Prevention of weed growth.
(g) Reduction of maintenance cost.
Reduction in evaporation and transporta-tion losses due to reduced exposedarea.
(i) Elimination of silting problems, inherent inunlined canals, due to higher veloci ties.
(j) Increase in available head for power genera-tion as a flatter gradient can be provided inpower channels due to reduced value of'N'.
(k) Reduction in erosion which occurs in unlinedchappels constru((t~d in ~t~ep lands.
In order to establish necessity of lining in acertain area, it is necessary to quantify the contribu-tion of each of the factors mentioned above forassessing the overall economy achieved bylining.
4.2 Types of Lining
The various types of lining can be grouped into twocategories: (i) Exposed and hard surface linings and(ii) Buried membrane linings. The advantages anddisadvantages of various types of lini ngs which needbe kept in view while selecting the type of linil"'gto be adopted are indicated below:
4.2.1 Exposed and Hard Surface Linings
Exposed linings include all linings exposed toabrasion erosion and deterioration effect of theflowing water, operation and maintenance equipmentand other hazards. Such linings are constructed ofcement concrete, asphaltic materials bricks, andboulders/stones. Although, the initial cost of theselinings is generally high, the reinforced cementconcrete linings being the costliest, they are usuallyrecommended for use only where structural safety isthe prime consideration.
(a) Cement Concrete
Cement concrete in-situ lining is one of the mostconventional type of lining which has successfullybeen used in India and other parts of the world. Asuitably designed and properly constructed Cemei1tconcrete lined channel can withstand high velocitieswhich are considered appropriate for a canal. Cementconcrete lining is more preferable than any other liningwhere channel is to carry high velocity water becauseof its greater resistance to erosicn. Velocities upto2.5 m/sec. are generally considered permissible withadequate water depth although higher \:elocities up to 5m/sec in case of Kosi Feeder canal in U.P. have beenprovided. Cement concrete linil1g eliminates wcedgrowth and thereby, improves flow characteristicsBurrowing animals cannot penetrate concrete andprovision of concrete lining reduces maintenancecharges to a minimum.
A distinct disadvantage of concrete lining is itslack of (ixtensipility. which reS\llts in frequent crack§
due to contraction taking place on account oftemperature changes drying, shrinkage and settlementof subgraue. It is also likely to be damaged byalkaline water. Cement concrete lining withoutreinforcement may be damaged due to excessiveexternal water pressure. However, reinforced concretelining can withstand the external water pressur~ butat a very high cost. Where unexpected waterpressures are encountered, unreinforced lining willcrack more easily than the reinforced lining and willrelieve the pressure thereby reducing the area ofdamage.
In this typ<: of lining cement mortar is applied bypneumatic pressure. Shotcrete lining can be easilyplaced over rough subgrade and is therefore, bettersuited for use on existing cuts where finishing to exactshape and slope would be expensive. The liningmay be constructed with or without reinforcement(in the form of mesh or expanded metal), althoughreinforcement increase its useful life, especially, whenlaid over earth subgrade.
Since the thickness of lining is limited, 5.0 emmostly, such linings are applied on smaller channelsor where operational requirements are J10t severe. Thelining is also likely to be damaged- by external waterpressure easily due to .small thic~n.ess. I~ is verydifficult to control the thIckness of IIDlDg, whIch mayleave areas where thickness is less than the minimumrequired. Such patches constitute the areas of weaknesswhere lining may give way.
This type of lining is made of a mixture of cementand natural sandy soil. This type of lining maysometimes result in considerable saving as comparedto cement concrete lining. It may be adopted oneconomic considerations both for small and largechannels. The use of soil cement lining is, sometimes,handicapped by the non-availability of sui(able soil.It is not weather resistant.
Asphaltic concrete lining may be substituted forcement concrete lining where it ""orks out to becheaper and aggregate is not of sufficiently geodquality for cement concrete but is sljita~le for asphalticconcrete. Asphaltic concrete has greater ability towithstand changes in the subgrade. Asphaltic concretelining can b~ used for repairing cement concrete liningby placing a resurfacing layer of asphaltic concrete.Disadvantages of the ,asphaltic concrete lining are:velocities in the lining are limited to 1.5 mjsec, dangerof weed growth resulting in plolDcturing of linings,insufficient resistance to external hydrost~tj0 pressure,and danger of ~Iidil1g dming hot season.. '
(e) Brick Lining
This type of lining has been extensively uscd inIndia and elsewhere. The brick linings, are not onlyequally impermeable as C. C linings, but have theadditional advantage of low initial and maintenancecost, quicker construction and natural safeguardagainst cracking due to closely spaced joints. Thistype of lining is economical where aggregates forconcrete lining are nN available. It does not requireskilled labour as needed for concrete lining.
(f) Exposed Mernbrane Lining
The various types of exposed membrane liningsare: sprayed in place asphalt cements, prefabricatedsheets of asphaltic materials, fibre glass and films ofplastic and synthetic rubber. Experiments have shownthat where as the exposed membranes have low resis-tance to puncturing and disintegrate rapidly, thickersheets with greater resistance are expensive.
(g) Earth Linings
Thick compacted earth lining may prove to beeconomical where suitable material for construction isavailable at site The lining is durable and can with-stand considerable external hydrostatic pressure andcan be provided over expansive clays also .. Bentonitehas shown considerable promise for use as a goodlining material. Bentonite containing large percentageof montmorillonite, is characterised by hi gh waterabsorption accompanied by swelling and impervious-ness. It can be used as 5.0 em thick membranecovered by protective blanket or as a mixed in placelayer of soil bentonite well compacted.
(h) Boulder Lining
Boulder lining may prove to be cheaper thancement concrete where boulders obtained duringexcavation are readily available. Boulder lining hasvery little flexural resistance or flexibility and theslightest settlement of the sub-grndc may cause dist-ress. Such lining is not satisfactory for canal infilling.
(i) Pre-cast Concrete Lining
Concrete tiles are more compact and impervious.The usual size of concrete tiles is 500 x 250 x 50 mm or250 X 250 X 50 mm. They are laid in 1:3 cement mortarin single or double layer directly on the compact sub-grade. A spacing of 3.5 to 5 m is usually providedfor siting of expansion joints on sleepers. The jointsare sealed with bituminous mastic.
T~e pr~-cast tiles are manufac~ured by hand castingon vlbratmg table or/are machme made dependingupon requirement and availabili.ty of equipment.
4.2.2 Buried Membrane Linings
Hot applied asphaltic, prefabrica1cd asrh?lt m~1te-rials, plastic fill11 pi',1 a Ja)Cf of ter,!cJ;itc (r eth(:\
types of clays protected by earth or gravel cover arecheap linings. These linings can be provided imme-diately after completion of excavation or even later.Memhrane linings are susceptible to damage by weedroat and p~rmissible water velocity is limited to avoiderosion. The life of the lining is uncertain.
(a) Sprayed-in place asphaltic membrane lining; Itis composed of a special high softening point asphaltand sprayed in-situ at a high temperature and is laid onthe prepared sub-grade to form approximately 6 mmthick water proof barrier. It is protected from damageand weathering usually by a protective layer of earthand gravel. It provides an effective and cheap meansof seepage control and can be satisfactorily laid in acold and wet weather.
(b) Pre-fabricated asphaltic membrane linings:These ready made membranes are used in smallerchannels or in relatively short reaches of large canals,where the use o'f sprayed-in-place linings is costlier onaccount of the requirement of special equipment andskilled personnel. Such linings are fabricated fromlow cost material of adequate water tightness anddurability. They are relatively thin and light and canbe transported to long distances, stored in hot weatherand placed at low temperatures.
(c) Plastic film and synthetic rubber membranelining: These linings using polyvinyl and polytheleneplastic and butyl coated fabrics have been tried ex-perimentally on a limited scale. Out of the varioustypes tested so far, polyvinyl and polythdene appearto be the best. A protective cover of earth or earth andgravel- is provided. Plastic film can be manufacturedin any convenient size. More over being light it canbe transported and handled easily. Synthetic rubberalso h3s found use in canal lining although it is highlydurable, it undergoes mechanical damage whenexpo~ed.
(d) Bentonile and clay membrane linings: Bentonitehas got the peculiar characteri.stics of b.ecomi.ng im-pervious on wetting due to swellll1g and ImbIbIng ofwater. As such, it is a very useful material, forcontrolling seepage from canals, if available locallyat low cost. It h~ISbeen used by spreading as a mem-brane 25 to 50 mm Or more in thickness over the canalsub-grade and covering with a 15 to 30 em protectiveblanket of SUitable earth or grawl.
For further details of linings, standard manualsand reports such as the CBl? Technical Report No. 14may be referred.
4.3 Considerations for SelectionKeeping in view the adv~ntages and disa?va.ntages
of various types of linings dIscussed above, It IS seenthat 110 single type of lining can be recommended forall the conditions encountered_ Type of subgrade,positions of water-table, climatic c~nditions.' availa-bility of materials, size of canal, servIce requJrements
and experience are the major factors affecting theeconomy and selection of suitable lining material.Adoption of a particular type of lining material willrequire consideration of all these factors and hence itis not possible to recommend anyone type ofliningsuitable for all conditions. However, a broad indica-tion can be given for the type of lining that may beconsidered for various sizes of canals. Followingclassifications of channels may be made for decidingthe type of lining for a particular channel.
(a) Channels with Bed Width upto 3.0 m
U) Single burnt clay tile or brick lining shOUld bepreferred where seepage considerations are ofover-weighing importance.
(ii) Soil cement lining may be adopted at reason-able cost when burnt clay tiles are notavailable and the initial cost is the foremostconsideration.
(Ui) Polythelene plastic or alkathene lining withadequate earth/tile cover may be adoped forstable channels.
(iv) 50 to 80 mm thick asphaltic. concrete liningmay be adopted for contmuous runningchannels where there is no likelihood of weedgrowth.
(v) 25 to 50 mm thick bentonite membrane liningwith earth cover may be considered wherevereconomical. '
(b) Channels with Bed Widths between 3.0 and8.0 m
(i) Lining of single burnt clay tile should bepreferred.
(ii) Precast concrete lining may be adopted if coarseaggregates for cOllcrete are available withineconomical leads and burnt clay tile liningproves to be uneconomical. However theconcrete lining is more impervious than burntclay tile lining.
(iii) Composite lining (membrane in bed and bricktile or concrete lining on sides) may be adoptedfor existing channels which have become stableand no danger of scour is expected.
(iv) Bentonite membrane lining protected by earthcover.
(c) Channel with Bed Width Greater than 80m
(i) l;D-.situ concrete lining or precast concretelImng may be preferred for reducing the see-page losses.
(ii) Burnt clay tile lining (double on sides andsingle on bed) may be adopted where aggre-gates for manufacture of concrete are notavailable within economical leads.
(iii) Thick COffiPJ.Clct.l carth lining may be providedwhere suitable and sufficient materials areavailable and the job is large enough to warrantuse of heavy earth moving equipment.
4.4 Flow Formula and Velocities
The same formulae as for unlined channels willapply in gener31 to lined canals. However, Manning's
formula, i.e., V = J R2/3 S IJ2 is widely used for linedncanals. Experiments in various research institutes havebeen conducted by taking actual observations forfinding the value of rugosity coefficient on lined canals.Some values of' n' for various surfaces as recommen-ded by ISI are given in Table VI.
TABLE VIValue of Coefficient of Rugosity Coefficient 'n' for Lined Channels
Concrete with surface as indicated below
Formed, no finish/PCC tilesor slabsTrovel/Float finishGunited finish
Concrete bottom float finished andsides as indicated belowHammer dressed stone masonryCoursed rubble masonryRandom rubble masonryMasonry plasteredDry boulder lining
Brick tile lining
(b)(c)
II
0.018-0.0200.015-0.0170.018-0.022
(a)(b)(c)(d)(e)
III
0.019-0.0210.018-0.0200.020-0.0250.015-0.0170.020-0.030
0.018-0.020
Note: With channels of an alignment other than straight, loss ofhead ctue to resistance forces will be increased. A smaUincrease in the value of 11 may be made to allow for addi-tional loss of energy. There are no rigid velocity limitsfor lined canals. If the water is clear from silt and sand,considerable velocities can be allowed in lined section.This is. however, not possible in actual practice. Incanals taking of from rivers, the slope is further limitedfrom considerations of command. In view of this velocityis usually limited to 2 m/sec and the critical velocity ratioshould be aimed at higher than unity though criticalvelocity ratios are not applicable to lined canals but thepossibility of silting cannot be neglected.
4.5 Design of Lined SectionsIt has been seen that the most economical section,
i.e., having maximum area with minimum perimeter,is the one with a curved bed with its centre at theF.S ..L. and the sides tangential to the curve. This typeof section, however, does not give any horizontal por-tion in the bed and is suitable for small channels onlycarrying discharges upto 60 cumecs. For dischargesexceeding 60 cumecs, it becomes necessary to providehorizontal bed to ke2p depth within reasonable limits.
The bed is however, joined to the side slopes in acurve of radius equal to F.S.D.
The water surface slope, side slopes and the valueof 'n' having been decided upon, the section may bewOlked out from Manning's formula as given below:
i.For Small Channels
(i) Side Slopes 1:1(ii) Angle IjJ 45°(iii) Sectional Area (A) 1.785 dZ
(iv) Wetted Perimeter (P) 3.57 d(v) Hydraulic Mean Depth (R) 0.5 d
(vi) Velocity V=_l R2j3 S 1/Sn
(vii) Discharge Q=A X V
2. For Large Channels
(i) Side Slopes 1:1 1.25:1(ii) Angle 0 45° 38° 4Q'(iii) Length of Tangent (t) 0.4142 d 0.35d(iv) Sectional Area (A) bd+ 1.785d2 bd+ 1.925d2
(v) Wetted Perimeter (P) b +3.57 d b+3.85 d
(vi) HydraUlic mean depth R=:(vii) Velocity V=I/n R~/3 SIj2(viii) Discharge Q=A x V
Typical examples have been given in Appendix II.
1.25:138° 40'1.925 d2
3.85 d0.5 d
4.6 Side Slopes
In order to keep down the thickness of lining onsides it is usually made to rest on slopes correspond-ing to the angle of repose of the natural soil so thatthe slopes shall be snch that no earth pressure is ex-erted on the back of lining. The slopes steeper thanthese have to be designed as gravity sections and re-quire greater thickness and are consequently unecono-mical. The side slopes 1:1 to 1.25:1 the .flatter beingfor deeper canals have been found convenient to work.Generally, a slope 1:1 for depth less than 366 m and1.25:1 slope for more than 3.66 m is adopted. In fillingcompaction at optimum moisture content is done andstable slopes for such comp:lcted banks are adopted.
4.7 Full Supply Depth
Full supply depth is more a matter of assumption.It is however, guided by the following considerations:
(i) Having fixed the alignments and F.S.L. fromthe command point of view~,the depth may besuch as to give balancing depth as far aspossible.
(ii) Velocity may not exceed 2 m/sec.
(iii) The bed of the channel may be higher than the
sub·soil water level as far as possible, so as toavoid pumping during constructions anduplift pressures after closure of channel.
In case of main and branch canals the depthshould be such that it gives bed width enoughto provide convenient working space within it,consistent with maximum economy in the costof excavations, land and masonry works.
4.8 Bank Width and Free Board
The following bank widths shall be provided:(i) 8 m for cutting and filling reaches for main
canals.(ii) 6.5 m for cutting reaches of branch canals.
(iii) 6.5 ill on left side and 5.0 m on right side forfilling reaches of branch canals.
A free bO'lrd of 0.75 m shall be provided for canalscarrying a discharge of more than 10 cumecs. Forcanals carrying a discharge less than 10 cumecs a freeboard of not less than 0.60 m shall be adopted. Freeboard shall be measured from the full supply level tothe top of lining.
4.9 Berms and Dowels
A typical cross-section is .shown in plate No. 16.No berms are provided for lined canals to reducechances of water finding its way behind lining. Adowel is also provided for this purpose, i.e., to preventrain water on bank flowing into the channel. Boththe banks and dowels should be sloped outwards todrain off the water quickly into the catch water drainprovided at the outer edge of bank. Other detailsprovided are in the same manner as for unlined chl'\n-nels.
4.10 Drainage and Pressure Release Arrangements
Failures of canal lining in most cases cccur due toexcess hydrostatic pressure behind the lining resultingfrom either high water-table condition or pressurebuild up clue to time lag in drainage of the sub-gradefollowing drawdown of the canal. In oreler, the~efore,to improve stability of canal under such conditIons,it is essential to eliminate the excess pressure by pro-vision of adequate drainage arrangement.
4.10.1 Various Condit ions of Water Table
The drainage arrangement to be provided woulddepend upon the site conditions, viz. Subsoil wat~rlevel and permeability of subgrade. The hydros.taycpressure at the soil-lining interface is the dete[mJl1lDgfactor for the stability of lhe lining. Even when thepressure build·up behind the l!ning is controlledthrough provision of adequate dramag~, pressure lagin a poorly draining subgrade may be Important. fromthe point of view of the stability ~f the . b.a~k Its~lf.Any inadequacy of drainage at the tIme of InItIal deSign
or due to subsequent malfunctioning will lead togradual accumulation of water wi thin the contactspace and cause failure of lining. Drainage arrange-ments have, therefore, to be designed such that excesshydrostatic pressures are eliminated. The necessityfor provision of filter behind the lining is also stressedfrom the considerations of inhibiting the movement offine material from the subgrade following drawdownwhich, if not guarded against, may lead to progressiveformation of ca vities resulting in subsidence of thelining. The various considerations governing the designof drainage arrangements and pressure release valvesfor different water-table conditions are given below:
(a) Wafer-Table below Canal Bed: If the water-table is below canal bed and the bank-fill isfree-draining, there will be no time lag in thedissipation of drawdown pore pressures in thebackfill as such no drainage arrangement isnecessary in such a condition.
When the water-table is below canal bed andthe bank-fill is of low permeability, the back-fill will get saturated in course of time due toseepage of water through joints and cracks,and in case of drawdown the water will not bedrained out as quickly as the drawdownoccurs. Hence, pressure will be built upbehind the lining necessitating a well designeddrainage arrangement.
(b) Water-Table between Canal Bed and F.S.L.:Whether the bankfill is composed offree·drain-ing or clayey material, the soil behind thelining will remain submerged to the elevationof the water-table. In case of lowering oremptying of the canal, lining will be subjectedto hydrostatic pressu're due to water-table andalso drawdown pressures arising from drain-.age of the backfill above the elevation ofwater-table. Drainage must, therefore, beprovided to help in reducing such pressures tosafe limits.
(c) Water-Table or H.F.L. above Canal F.S.L.: Ifthe water-table outside the canal is above theF.S.L. of the canal, the canal linina wouldcontinuously be subject to uplift pressuresduring norma} running of the canal. In 0rderto release these pressures, extensive drainagearrangements consisting of longitudinal drainsin the bed, drains or continuous invertedfilter behind the side slope and pressure re-lease valves to discharge seepage water intothe canal will have to be provided.
4.10.2 Selection of Drainage Arrangements
The drainage arrangements provided to reduce orelimin~te hyd.rost.atic pressure behind lining usuallycompnse 10ngJtudmai drams and cross-drains in thebed and pressure release valves or continuous filter
below the lining on th3 sides, The adequacy of v.ui-ous drainage arrangem~nts can be determined on themodel.
In case of high ,vater-table, the lining will be su-bjected to uplift pressure corresponding to head differ-ence irrespective of the type of subgrade. In case ofthe lining being subjected to drawdown, however, theexcess pressure will depend upon the rate of drawdownand the drainage characteristics of the sub grade. Inmost cases it should be possible to adjust the draw-down by suitably restricting the rate of drawdownto an extent that there is little pressure lag. Insuch cases, drainage may be provided as a precaution-ary measure. A broad categorisation based on Cass-agrande's classification in?i~atiJlg the ran,ge ?f freedraining and poorly dra1l11l1g subgrade IS gIven asbelow:
Th~ first row should b~ provided at the junction of thecurve and the sides. The number of rows on the bedof canals shall be such that for every 10 m bed widtha row is provided. The spacing of the pressure releasevalves in a row should be decided on the basis ofexperiments carried out on models simulating siteconditions, Valves in adjacent rows should be stagger-ed, The longitudinal drains consisting of open jointedpipes encased in graded filter, should have outlets in atrench of width 45 cm and depth 50 cm. The trenchshould be filled with graded filter and have outlet intothe canal through pressure release valves. The outletmay be provided through precast concrete boxes colh:c-ting water from drains with pressure release valves onthe top of boxes.
The performance of metallic pressure release valvesrecommended by ISI has not been satisfactory. The
(i) Free draining(ii) Poorly draining
Clear gravel on clear sandVelY flnes:lnd, mixtures ofsand, silt clnd clay, clays.Homogeneous clays belowzone of weathering
Permeability greater than 1O-4cm/sfcPermeability less than 10-4 cm/secand greater than 10-6 cm/secPermeability less than 10-6 cm/sec
The criteria may be used to broadly decide thenecessity or otherwise and type of drainage measuresto be provided below the side lining. In case permea-bility is less than 10-6 cm/sec. no drainage may berequired. In case of permeability higher than10.4 cm/sec, the drainage of the backfill material islikely to closely follow the drawdo'vvn in the canal, andno continuous fllter may be required and only pressurerelease valves may be provided to drain out the see-page water as a precautionary measure. In case ofpoorly draining soils (K< 1O-4cm!sec), continuousfilter should usually be necessary in case of fastdrawdown dL;e to lag in drainage unless the rate ofdrawdown can be controlled by cross drains or pres-sure release valves. In case of high water-table condi-tions, contJI1UOUS tllter with non-return valves todischarge seepage waters into the canal would berequired to elimInate the excess hydrostatic pressurebehind the lining. Details of filter material aroundpressure release valve is given in Plates No. 19 and 20.
4.10,3 Pressure Release Valves
Plessure release valves which open into the canalare provided to relieve excessive hydrostatic pressurebehind lining. The v3.1ves should be provided in poc-kets filled with graded filter underneath the lining.The pockets may be cylindrical with depth and diame-ter as 85 cm or cubical with sides as 85 cm. Hori-zontal and vertical pressure release valves are installed.50 mm diameter valves are normally used on theside slopes and 150 mm diameter valves are used inthe bed. 1he 11umber of rows of pressure releasevalves in the slnpes of canal should be such that foreach 4 m widt!l along the slope a row is provided.
flaps are easily damaged and leakage of water is sub-stantial. There is a need for evo]vlDg a better designand should be made of plastics or fibre glass whichare strong but have no resale value. This wouldd:scourage pilferage which is common with metallicvalves. Suitable non-metallic pressure release valvesfor use in bed and sides of lined canals were developedfor Shard a Sahayak Project. These v(llves were testedby 1.R.I., Roorkee and their hydraulic performancewas found to be very satisfactory.
Details of one type of pressure release valves aregiven in Plate No. 17 and details of filter materialaround pressure release valve is given in Plates No.19 and 20.
4.10.4 Provision of Dwarf Regulator
Dwarf regulators are regulators of part height con-structed across lined canals at suitable intervals forpondipg lip water to counteract excess uplift pressurebelow the lining. These are provided in areas wherethe water-table remains continuously high and theconventional pressure release arrangements are notlikely to prove adequate. The height and spacing ofthe dwarf regulators will depend upon the minimumdepth of water required to be maintained inside thecanal for counter balancing residual pressures for safetyof the canal. The regulators also help in maintainingminimum water depth inside the canal even when thecanal is operated at part capacity with lower normaldepths. Their disadvantage is that the canal madestable with dwarf regulator cannot be emptied forrepairs ((ill the water-table goes down to bed level)and these may cause additional head loss in the canal.
The under drainage arrangements should be as per therecommendations of I.S. 4558-1968.
4.11 Lining of Expansive Soils
Canals excavated in expansive soils pose severalproblems of stability. To have economical sections~LDdprevent erosion due to given velocities, it is neces-sary to line the canal bed and slopes. Pre-cast cementconcrete slabs for side slopes and in-situ concrete forbed are common types of lining adopted for canals.However, it is often experienced that the lining male-rial directly placed against the soil undergoes defor-mations, disturbing the lining and throwing the canalout of commission. This deformation is traceable tothe unduly high swelling pressures developed by expan-sive soils when they absorb water. The heaving ofsoil mass is often attempted to be contained by prote-cting the soil with a thin layer of murum or gravel.From experiments in the laboratory and field it is con-cluded that the deformations can be correlated to thethickness of the murum layer and swelling pressurein the soil.
To connter the swelling pressures and preventdeformation of the rigid lining material a CohesiveNon-Swelling layer (CNS) of suitable thickness depen-ding on the swelling pressure of the expansive soil issand-witched between the soil and rigid liningmaterial.
The CNS material may contain gravel or murumor a combination. of sand and mUrtllTI or gravel.Typical gradation based on the concept of stresstransfer from micro-particles to the soil may be indiocated as :
(a) Clay 15·20%(b) Silt 15-20% or more(c) Gravel and sand 60-70%
The minimum gradation shall be ensured to avoidswelling of clay particles in the voids and to restrictthe pore size.
The shear parameters m:1Y range from 10 to 30KN/m2 for cohesion and angle of internal friction12° to 25°.
Q = 300 eumecsN = 0.018
1S = 10000'Side Slope = 1:1Assume section 26.0 m X 5.7 m (d = 5.7m)Section Area = bd + 1.785 d2
= 26.0 ~< 57 + 1.78x5 5.72
= 148.2 + 5'7.99 = 206.19 m'~= b + 3.57 d= 260+ 3.57 x 5.7 =46.35 m
A 206.19 4.45 m- --Pw 46.35-- =
1 X 4.452/3 ( 1 y/2X 100000.018
1 1--~--- X 2.71 X-ioo--0018
00271= 1.51 m/see
.0180
Q 206.19 X 1.51 X 311 cumecsHence O.K.
Example IIDesign of Distributary with the following data:-Q = 20 cumccs
1S = 10 000
n = 0.014Side slope = 1.25 : 1
Let Depth of Channel = dArea = 1.925 d2
R = d/2
V _1_ R2f3 Sl/2n
1 1 l~R2f3 ----
0.014 (10,000)0.715 R2/3 .. (1)
Also V = _Q_ = _2 __0 _R 1.925 x d'
Equating (1) and (2)
0.'715 R2/3 = 20 As R = d/21925 x d2
0.715 (-f- }2/3 X 1.925 d2=20
From aboved = 3.25 m
Design a lined ean~11with the following data withthe help of the curves in plate No 18
Q = 300 cumecsN = 0.018
Side slope = J .25 : 1Longitudinal slope = 1: 10000From the curvesAssuming a bed with of 30 mDepth required is 5.20 mVelocity of flow would be around 1.44 m/sec.
NATURAL GROUND IS ABOVE TOP OF LINING
PLATE NO· 113:TYPICAL CROSS S'OCTIONS .FOR LINED CANALS
(A) FOR OTHER VALUES OF Ni:TODETEiiMlNE RREAD B£D WIDTH AND DEPTH AGAINST DISCHARGE MULTIPLIED BYN/N INeO'016'N," 0.015 0.016 '0.020 0.022 '
;i=0.833 0.889 1.111 1.222
2. TO DETER MINE "f' 2READ FOR THE DISCHARGE CALCULATED ABOVE & MULTIPLY IT. IN/NI'N1= 0.~15 0.016 0.020 0.022(N/N,) = '.44 1.266 0.810 0.6693. TO DETERMINE V.READ FOR DISCHARGE CALCULATE ABOVE & MULTIPLY IT. NINI IN:OO.OIBINI =0.015 0.016 0.020 0.022
tN/NI )=1.200. 1.125 0.900 0.818
(8) FOR OTHER VALUE OF SLOPES. 1/2I. READ BED WIDTH & DEPTH AGAINST DlsOIARGE DIVIDeD BY 15(15) 1/2WHERE sl'S·THE NEW SLOPE.2. TO DETERMINE f 1/2READ FOR DISCHARGE CALCULATED ABOVE & MULTIPLY BY (s,/SI 112
3. TO DETERMINE V. 1/2READ FOR THE a CALCULATE.D ABOVE & MULTIPLY BY IS/51 1/2
PLATE NO· 18: CURVES.FOR DESIGN OF LINEDCHANNEL SIDE SLOPE 1·25: IN= 0·0 18 S" O·I / 1000
~ It OF PRESSUREI RELIEF VALVE
"
.. ..• p. • ..• ~ A.. .. A
\.. • /A
• A
•A !> '.
" ".. I>5CM.'7" 15,~M" 85cM.,;
A I> "~to
AI>/A .. .. ..•.. ... ..
..; :',A:-':.... "'.'.'~;'l' :
, " ", ,J.\'
GRAVEL 015 = 7.5",",
COARSE SAND 015 = 0.6m .••.
FINE SAND 015 "" 0.09 mm.
GRAVEL 0/5'" 7.5mm.
COARSE SAND 015.= 0.6 Mm.-
FINE SAND 015 ='0.09mm.
NOTE :---- THE GRADED FILTER SHALL BE DESIGNED IN SUCH A WAY THAT THERE IS NO LOSS OF SOIL PARTICLES. THE GRADATION
CURVE OF BED MATERIAL SHOULD BE OBTAINED FROM THE SIEVE ANALYSIS. THE 151. SIZE IDI5} OF THE LAYER (A)SHOULD BE AT LEAST FOUR TIMES AS LARGE AS 15~. SIZE OF THE SOIL AND LESS THAN FOUR TIMES 85 PERCENTSIZE 085 OF THE SOIL. DESGIN OF THE OTHER LAYER SHOULD BE DESGINED IN A SIMILAR WAY TILL THE Rl::aUIREMENTOF THE FILTER OPENI NG IS MET.THE 151. SIZE OF VARIOUS LAYERS SHOWN ARE ONLY TYPICAL.
~ t OF PRESSUREI RELIEF VALVE
A"
• ..• ". • ".. " A
~ .. A
• P A
• j
".. "
A b " "
'" t>IS,CM .. 85cM.6
A .. " A" "A .. " A
" " "
GRAVEL 015 = 7.5m",
COARSE SAND DI5 = 0,8m .•• ,
fiNE SAND DI5 -== 0.09 mm.
GRAVEL DI50: 7.5mm.
COARSE SAND D15!= 0,8 Mm.
FINE SAND DI5 =0,09mm.
NOTE :---- THE GRADED FILTER SHALL BE DESIGNED IN SUCH A WAY. THAT THERE IS NO LOSS OF SOIL PARTICLES, THE GRADATION
CURVE OF BED MATERIAL SHOULD BE OBTAINED FROM THE SIEVE ANALYSIS. THE I SI. SIZE 10151 OF THE LAYER I A)SHOULD 8E AT LEAST FOUR TIMES AS LARGe AS 15 I. SIZE OF THE SOIL AND LESS THAN FOUR TIMES 85 PERCENTSIZE D85 OF THE SOIL. DESGIN OF THE OTHER LAYER SHOULD BE DESGINED tI A SIMILAR WAY TILL THE REQUIREMENTOF THE FILTER OPENI NG IS MET.THE 151. SIZE OF VARIOUS LAYERS SHOWN ARE ONLY TYPICAL,
~~:-~',~.
8.. .VC~~<..,~.
GRAVEL D13" l(S ••.
COARse:. SAND DIS ·:0.•••.••INE SAND 015 = o.oe •••
NOTE:----THE GRADED ••ILTER SHALL 8f DESIGNED IN SUCH A WAY THAT THERE IS NO LOSS OF SOIL PARTICLES. THE
GRADATION CUR\/! OF SED MATERIAL SHOULD 8E OBTAINED FROM THE SIEVE ANALYSIS. THE 151. SIZE lD15'01' THE L.AY!R IAI SHOULD BE AT LEAST fOUR TIMES AS LARGE AS 15 J( SIZE OF THE SOIL AND LESS THANfOUR T1MES 85 PERCENT SIll!; DS5 OF' THE SOIL. DESIGN OF THE OTHER LAYER SHOULD BE DESIGNED IN ASIMILAR WAY TILL THE REOUIR!MENT OF THE. FILTER OPENING IS MET. .THE 15:.1SIZE OF VARIOUS LAYER SHOWN ARE ONLY rYPICAL.
Power Channels
A power channel constitutes an important compo-nent of a hydro electric project. Its proper planning anddesign and maintenance assumes great significancein the efficient functioning of the power project. Inaddition to the requirements for planning and layoutof irrigation canals, power channels need conside-rations of their own, like water-power studies, surgesetc,.
5.2 Planning
A prerequisite in the planning of a power channelis to fix the discharge capacity which is based on thewater-power studies to be made for arriving at theinstalled capacity of the power plant. These studiesinclude the flow duration curves and mass curves foravailable discharge or storage capacity of reserv0ir andextent of balancing storage/pondage requirements to beprovided at suitable points of water conductor systemto suit the load demand and type of operation ofpower stations. In case of stage development of theproject the capacity of the channel may have to befixed for the ultimate stage of power development asindicated by economic studies.
5.3 Design
Power channel design is determined by the type ofsystem adopted. The following three types of opera-tion are normally ad0pted:
Constant discharge in channel with constantwater levels upstream to downstream withbypass arrangement from upstream to down-stream;Channels with balancing reservoir to take careof fluctuations. This will in many cases, resultin reduced capacity requirements of the channelupstream of the reservoir; and
Lock operation of channel, in which the channelis used similar to a lock and the discharges inchannel fluctuate with the load.
5.4 Cross-SectionThe cross-section of the power chanr.el, bed slope
~tc., ar~ designed On tbe basis of economic .studies
considering the optimum cost of construction and costof energy lost due to head loss in friction. Also, theside slope of the channel section is designed to suit thedrawdown conditions in the power channel.
5.5 For power channels carrying Water to the powerhouse the phenomenon of surges due to variation ofdischarge on account of load demand or rejectionshould be analysed fully. Sufficient free board shallbe provided to avoid overtopping of water on eha11l~elsides which may endanger the channel section.Depending on the topography, provision of sLiitatl~spillover or balancing reservoir of sufficient capacity10 act as an open surge basin at forebay may be consi-dered. Factors involved in the analysis of surgephenomenon are as follows:
(a) Hydraulic section, slope of the channel andvelocity of flow in the channel;
(b) Amount of load rejection or load acceptance;
(c) Rate of closure of units or acceptance of load;and
(d) Size of forebay or surge basin on the channel.
Criteria for analysis of the maximum and minimumsurges in power channel shall be the same as for thesurges in head race tunnels. Maximum surge heightin a power channt! due to load rejection may be calcu-lated from the empirical formulae given below:
(a) For abrupt closure hma", = v'i<.'i+2Kh(b) For gradual closure within the period required
for the first wave to travel twice the length ofthe channel.
~+V";h2 g
where,
hm." = maximum surge wave heightV2
K - = velocity head2g
V mean velocity of flow, andh '? effective pepth = _ area of cross-section. , .. .. , top width
5.6 Lining of Power ChannelPower channels should preferably be lined since:(a) It is hydraulically more efficient thus ensuring
smaller cross-~ection, relatively flatter slope forthe same discharge capacity resulting ineconomy.
(b) Loss of water due to seepage or leakage isminimised.
(c) Closure of power channel for repairs, if any,are remote and also they would be of shortdurati on only thus interrupting power gene-ration for brief periods only.
(d) Cost of operation and maintenance is lower,and
(e) Weed growth is mioimised.
5.7 Drawdown
Depending upon the pattern of load demand andextent of stor<lge provided, fluctuations occur in waterlevel of the power channel due to utilisation of balanc-ing storage. This effect of drawdown and the rate atwhich it occurs govern the stability of the side slopesand lining of the channel. In cases of excessive andrapid draw down suitable automatic control structuresmay have to be provided to regulate the rate. of draw-down. For instant a crest may be required at theoutlet of the channel into a balancing reservoir tocontrol the drawdown.
5.8 Sediment ControlNecessary desilting arrangements i.e., silt ejectors
shall be provided to remove sediment content to adegree safe for operation of generatin.g units as .givenin Table VII. The quantity ;1l1d size of sedimentthat can be permitted depends on the type of turbine,its head, the size and mineralogical content of thesediment.
The exact requirement is fixed in consultation withmanufacturers. Upstream of desilting arrangements, the
channel is provided with extra capacity to allow fordischarge required to flush out the sediment.
Generally a provision of 15 to 20 percent extradischarge for flushing purposes is considered to bequite adequate. Prototype studies have shown thatthere is no appreciable increase in the efficiency ofthe ejectors with further increases in the escapagedischarge.
In channels carrying considerable sediment, it maybe desirable to design the channel throughout for fullcapacity even though balancing reservoir is provided,due to necessity for by-passing the balancing reservoirand reducing or avoiding sediment deposition in it.Balancing reservoir and canal system are so designedand operated as to mini mise sediment deposition inthe reservoir and reduce the necessity of dredgingwhich add to difficulties in operation and maintenance.
TABLE VIISafe Particle Size for generating units.
Head(m)
Below 100100 to 300Above 300
5.9 Trash Rack
Largest Particle Size(mm)
0.50.20.1
Suitable trash racks are provided at the exit end ofpower channel that is at the forebay portion where-from the penstocks offtake to avoid trash entering thepenstocks which would otherwise damage parts of thegenerating unit.
Suitable provisions are also made for surplusingarrangements in the balancing reservoir in lhe event ofsudden load rej~ction. Also in case of very longpower channels provisions may be made f..)I" escape,regulations in power channels for emptying it in caseof emergency/periodical closure, if any.
Construction, Operation and Maintenance
Earth channels for irrigation or power projectsshould be constructed after thorough ~xaminatjon ofthe materials met with along the alignment. Thesamples of the soil should be examined ill the labora-tory and the results of the test sh?uld be utilised formaking of the canal banks, ensunn~ that losses ofwater by seepage and other undeslfable factors areavoided. The earthwork should be clone as per I.S.4701 (Latest).
6.1.1 Service Roads
In addition to an inspection road provided on thetop of the bank in the case of main and bran~h canalsand at the toe of the banks in the ca?e of dlstflbuta-ries a service road is usually provIded on the otherside of the toe of bank. This road is used for trans-porting materials during construction and as a publicroad after construction. In the case. of main. andbranch canals service roads are sometImes prOVIded,on both the ba'nks particularly in the re.aches of th.eGovernment waste land and in propnetory areas Ifthe cost ofland is not prohibitive. A width of usual~y6 m including 2 m for the boundary trench, ISprovided for this purpose.. The boundary trenchwhich is usually 0.30 m deep IS excavated as the nameimplies to mark the boundary of canal land.
6.1.2 BorrolV Pits
If the earthwork from digging is less than thatrequired for making the banks, borrow pits have tobe dug out. Borrow pits, should ftrst be dug fr?minside the channel section from beds o! berms whichif properly taken, silt up after running a.nd do notspoil the section. In the b.ed, the borrow ~ltS may betaken in the Central half wld~h of the sectton. Alongthe direction of flow a portIOn not less than half ~helength of borrow 1?its ~hould be lef~ bet~~en successIveborrow pits. ThIS wIll ensure qUick Slltlllg up of thepits Where berms are wide, similar discantinuouspockets may be taken from the berms. If the internalborrow pits are inadequate to meet the demand, .theremaining earth has to be borrowed fr~m outSide.The borrow pits should not start at a distance lessthan 5 m from toe of embankment in small channelsand 10 m in the case of large channels. They should
not be deeper than 1 ill and should be connected by adrain so that they do not hold stagnantwater.
6 1.3 Spoil Banks
Earlh from excavation of a channel surplus to therequirements of banks is deposited as spoil banks.The height of the spoil banks is limited to 6 m. A 3 ill
space is left between the outer edge of banks and thetoe of the spoil for the drain. 3 m wide gaps sh0uld belefl in the spoil banks after every 100 m length, to dis-pose off the rain water coming in the drains.
6.1.4 Boundary Stones
Boundary stones are fixed at the land boundaryevery 250 m apart and at every change of land width.These are considered very necessary to avoid anyencroachment in Government land. On main canalsand branches, distance marks are often provided onboth banks, fixed in dowels with top of concrete blockflush with its top. On small channels these are ftxedon one side only along the service or inspectionroad.
6.2 Maintenance of Channels
The importance of proper maintenance of canalsystem is well understood by the field engineers andneeds hardly any emphasis. For proper maintenanceit is necessary that regular hydraulic surveyscomprising longitudinal sections and cross-sections,measurement of discharge and corresponding waterlevels attained in different reaches etc., be carriedout. Stress should also be laid on the importance oferadication of weeds, jungle clearance, berm, cutlingetc. It should be obligatory for the fteld engineersthat the longitudinal sections and cross-sections atsuitable intervals of each channel may be examinedonce in five years and revised, if necessary.Inspection roads along the canals shotlld be kept infit condition and canal banks to be maintainedproperly for the removal of rain water away fromthe canals.
6.2.1 Maintenance of Unlined Channels
The unliped channels suffer from the problem ofsilt entry. The position is further aggravated when
their full supply discharge is increased. The mainproblem confronting the engineers charged with them::tintenance and operation of such channels, involvescomplex relationships governing the flow of water andthe sediment which it transports.
The following aspects, therefore, should beconsidered to keep these channels in regime
(i) Hydraulic functioning of channel.
(ii) Equitable di,tribution of irrigation supplies.
(iii) Judicious silt clearance, weed clearance bermtrimming and jungle clearance.
(iv) Proper maintenance of the inspection road forany emergency.
All these aspects are briefly discussed below:
6.2.1.1 Hydraulic Functioning of a Channel: Theperfectly developed "regime theory" for the designand proper maintenance of unlined. canals haswon worldwide recognition. Canals which have runfor long periods may have alrea~y attai~ed finalregime and there J?ay ~e n? dl.fficulty In theiroperation. But th1s regll1:-e IS ~I~turbed when achannel is remodelled. ThIS pOSitIOn IS furtheraggravated when their full supply discharge isincreased.
In the case of channels where regime is yet to beattained, regular watch need be kept by conducti~ghydraulic surveys at regular llltervais. ~he hydrauhcsurvey of each channel s~ould be carne~ out everyalternate year and should melude observatIOns of bedlevels, berm levels, full supply levels, natural surfacelevels crest ievels, and working heads of flumes, fallsreguI~tors and sizes of outlets. If a channel showsrepeated tendency to silt up or scour then .suitableremedial measures be undertaken to bnng thechannel back to the regime conditions.
The rules adopted for the rcgulation of riversupplies at head works if not judicious, also affect thehydraulic functioning of barrage ponds and t~e .can.alsoff taking therefrom. The keen demand of IrngatJOnwater compeils field engineers to overlook the main-tenance aspect and the flushing of sediment de\?osits inthe undersluice pockets/ponds which IS vital forefficient operation of can~ls, i.s seldo.m resorted to asthe water required for flushing IS conSidered a waste.Thus, review of regulation rules is very essential forproper maintenance of canals.
Another aspect in this regard is inadequate dissi-pation of energy downstream of falls ~n unlined canalswhich causes bed scour and bank erosIOn.
6.2.1.2 Equitable Distribution of Irrigation Sup-plies: The irrigation system i~ requi.red to be properlyoperated and maintained w1th a view to iecure opt!-
mum crop yield at minimum cost. The primary aimis to see that water is conveyed to the cultivated landand utilised there in an efficient manner to achieveoptimum use of the capital invested in the irrigationsystem. For this purpose the distribution n~tworkshould be maintained in good shape and the flow ofwater closely controlled. Another significant factor isthe distribution of supplies from the channel into thefield, which contributes a great deal to their economicutilisation. With proper distribution, more area isbrought under irrigation with the same supply andthere is less wastage. Besides, the water also reachesthe tail-end of the system instead of benefiting a fewin the head reaches.
6.2.1.3 Judicious Silt and Weed Clearance:(a) Silt Clearance: As regards silt clearance,the prevalent instructions provide that siltclearance u pto theoretical bed level and sidesas indicated by bed bars is an important aspectof maintenance of the canal system. It, however,provides only temporary relief towards restoringregime of an irrigation channel. Since irrigationchannels are required to be non-scouri ng and non-silting, the cause of silting has to be investigated and.remedial measures undertaken.
(b) Weed Growth and its Removal: Weed growthreduces the carrying capacity of irrigation channeland deteriorates its regime and hydraulic function.The decrease in the carrying capacity ismainly due to two accounts, decrease in thecross-sectional area, and increase in the frictionalresistance to flow.
Some of the important factors which encourage thegrowth of weeds in a canal system are: temperatureof water and its chemical compositions, velocities offlow, deposition of silt 011 the bed, depth of water a'lldreduction of sunlight.
The types of control may be divided into:
(i) Regulation measures, alternate wetting anddrying.
(ii) Chemical measures.
(iii) Mechanical
(iv) Increase in the velocity of flow by regradingthe slope of the channel. This remedy wasadopted on the Bhakra Canal. The channelswere regraded so thRt the minimum 'velocitywas not lower than 0.61 m/sec.
62.1.4 Maintenance of Banks and lnspection Roads:A dry road surface disintegrates rapidly and it is,therefore, necessary that road should be wateredregularly. On the main canals and branches, afterwatering the surface should be worked with rings to
give a good even surface. When, however, the soil isp:::Jor for this treatment, the road surface should beproperly levelled after watering and subsequentlyscraIJed to make it smooth. On the distributarit:s, thewatering of road surface should be resorted as fre-quently as possible. Ruts should be repaired as earlyas possible. Regarding boundary road, it is usual toconstruct a small dowel, along the outer edge of theroad. The boundry road is flooded of watered andafter such watering the boundary road should be pro-perly scraped by means of a scraper. Inspections roadswhich are not metalled, should be closed to trafficfollowing rain. This is to be done because not onlythe surface gets badly damaged by the traffic, imme-diately after rainfall but it may not be safe for themoving traffic also on account of the road bt:ing softand slippery. As soon as the road is firm enoughafter rain, but before it is dry, it should be scrapedand then rolled over with a fairly heavy roller.
It should be realised that both banks of a channelneed be given equal attention, because, otherwise, thesafety of the channel will be jeopardized and leakage,breaches and over-flow etc. may occur at the non-inspection bank.
62.1.5 Generally it is observed that tail reaches aredeprived of their due share of water. The reasons forthis could be due to low discharges or more trans-mission losses in canals. In view of this, dischargeobservations should be carried out in various reachesof canals to assess the transmission losses. In caseseepage loss/transmission loss are observed to be morethan those planned, this should be studied and reme-dial measures adopted in those reaches to conserve thewater.
6.2.1.6 As per regulation rules minors and distribu-taries should be fed at full supply level, in case of lowdischarges in main/branch canals to feed the outlets.Observations should be made whether minors/distri-butaries are fed at FSL or not. This may be p'ossiblewith and without cross regulator depending upon FSLof paraet and off-take channels and other designassumptions. In case the system is not able to meetthis requirement, feasibility of providing cross regula-tor at suitable locations may have to be examined toassure full supply to minor/distributary. As pJintedout earlier gauge discharge observations should becarried out. Gauges on the "pstream and downstreamof the structures should be installed to assist in takingthe gauge readings and discharges.
6.22 1I1aintenance of Lined Chann~ls
Lining of irrigation channels and their subsequentmaintenance is of great importance. Lining affectsdirectly the design as well as the operation of irriga-tion system, the lining not only facilitates more pro-per operation but also reduces maintenance cost. Thereduction in maintenance cost would be substantial
only if the lining is sufficiently strong to resist thehydrostatic pressur~ behind it and the backfill hasbeen properly compacted so as to avoi d any s'.:ttle-ment. Lining also ensures the stability of sectionwhich in the case of distributaries, reduces remodel-ling and alteration of outlets. The following lTIrasuresshould be taken for maintaining the channel in per-fect condition.
6.2.2.1 Proper Drainage of Banks : The catchwater drains alongwith their outfalls shouldbe thoroughly cleaned before the rainy seasonand again from time to time as required. Propercross slopes of the banks should be maintained so thatrain water is drained off quickly through the catchwater drain, thereby reducing the chances of pressurebeing exerted on lining due to percolation. Tb facili-tate drainage of the bank and to avoid flow on thebank along the canal, passage for water at suitableintervals should be provided. The slupe at the top ofdowel be maintained properly so that rain walershould drain towards the bank and not towards thelining.
6.2.2.2 Removal of Weeds: The growth of weedson the linin.g. is removed with a sharp edge andcracks/open JOInts are raked and filled with mortarand bitumen.
62.2.3 .Mainfenance of inner slopes in high cuttingreaches: For preventing erosion of soil on innerslopes in cutting reaches, the grass grown there
. should nOI be allowed to be cut from roots.
6.2.2.4 Repairs to .cra~ks in .lining and joints:Whenev~r any opportunIty IS avaIlable for loweringor stoppIng the suppbes for any reason, efforts shouldbe mad~ to repaIr the cracks and joints in lining atthe earbest. These should be properly raked and thenfilled with cement mortar and bitumen as per siteconditions.
6.2.2.5 Sand Grouting behind lining: Fromperusa.l .of causes of damages to lining it is clearthat b.llll1g s~abs generally fail due to formationof .cavlty behInd the slabs. The cavity is filled byfillu:g sand behi~d. lining. A hole of about 150 111mdla IS C~lt III the Iming slab ~nd water is poured in thehole WIth drums. If suffiCIent quantity of water istak~n by the .h?le, th~n it is established that a cavitybehlDd the lInIng eXIsts. In that case coarse sand ispoured III .the hole alongwith water so that sand flowsIII the cavlly WIth the flow of water. The process iscontin~ed till the inflow of sand ceaSes and wholecavIty IS filled. In some caSes intake of the sand isabnorm.al :v~ich is attributed to existence of widercracks 1n IIn Ing at greater depths, which does notallow the sand to be retained in the cavity. In that
case, in order to provide a check to the flow of sandthrough the crack into the channel, sand mixed withshingle is poured along with water till the cavity istilled. Sand grouting behind lining is very importantfor the life of lining and safety of the canal. It is asort of continuous process which should be consideredas an essential part of general maintenance of alined canal system.
6.2.2.6 Membrane lining: In cases of membranelining buried under earth there is everychance of rupture due to animal hooves orweed growth. Location of rupture point shouldbe ascertained and earth burden should be removedand cleaned. Patch of membrane should be addedso as to make the joint water tight. Overlaping ofISO mm on all sides is suggested to provide properwater seal joint. Animal/cart crossings could also beprovided.
6.2.2.7 In case of canals where the sub-soil waterlevel is high above the bed level, closure of the canalshould not be sudden ,is otherwise the lining is likelyto be damaged due to excessive hydrostatic pressuresbehind the lining Experiments have been conductedin the Irrigati()n and P,_,wer Research Institute,Amritsar, regarding the safe limits of hydroslatic pres-sure which single brick tile and double brick tile linings
can withstand without getting damaged. It has beenfound that single tile lining can withstand differentialpressure up to 15 cm whereas double tile lining cantolerate pressures up to 24 eill.
In the particular case tested in the Research Insti-tute, it has been found that drawdown rate of 30 emper hour is safe in case of single tile lining whereasdouble tile lining remains safe up to drawdown rateof 45 em/hour. However, each case of a lined canalshould be stu died separately to assess the safe draw-down rate and the same should be kept in view by theoperation and maintenance staff.
6.2.2.8 As indicated in Para 2.10 the silt charge in thewater mtering into canal at diversion works isconsiderable inspite of providing silt excluders in thehead works. It is observed that the canals are runin the initial stages of the project at very low dischargeresulting in low velocities and high silting whichred lIces the capacity of canal. Taking into considera-tion these factors it is desirable that the main canalstaking off from the diversion structures should as faras possible not be run at less than 0.5 to 0.7 ofdesigned discharge particularly during high flowperiods. Portions of discharge in excess of irrigationdemand can be escaped through the escapes providedin the system.
(1) Hand book of Applied Hydraulics by C.V. Davis(2) Stable Channels in Alluvium by Lacey(3) Uniform Flow in Alluvial Rivers and Canals by Lacey(4) Hydraulics by Lea(5) Irrigation Engineering by K.R. Sharma(6)· Irrigation (Design and Practice) by K.B. Khushalani(7) Irrigation Pratice and Engineering by Etcheverry.(8) Principles of Irrigation, Roads, Buildings and Water Supply of Towns by Strange.(9) Irrigation Manual by W.H. Ellis
(10) Irrigation Engineering by Wilson(l1) Hand book of Hydraulics by Harace Williams King(12) Irrigation Engineering Vol.1I by Ivan E. Halk(13) The New Irrigation Era YoU-No. 3 Journal of the Irrigation Branch, Madras PWD.(13) Drainage and Flood Control Engineering by George W. Pickels(15) Canals and Related Structures (Design Supplement No.3) U.S.B.R.(16) Canal Linings-U.S.B.R.(17) Design of Channels by Shri Kanwar Sain I.S.E.(I 8) Irrigation Pocket Book by Buckley(19) Irrigation Engineering by Bharat Singh(20) C.B.I.P. Manual on Current Practice in Canal Design in India.(21) Irrigation Power Engineering by Punmia(22) Irrigation Engineering by Varshney(23) Mnnual on Cnnal Lining CBIP Technical Report No. 14 Sept. 1975(24) 1.S. 4745-1968 Code of Pratice for Design of Cross-Section for Lined Canals.(25) 1.S. 4839 Code of Practice for Maintenance of Canals
(Part I) 1979 Unlined Canals(Pa rt IT) 1979 Lined Canals
(26) I.S. 5968· 1970 Guide for Planning and Layout of Canal System for Irrigation.(27) 1.S. 7112-1973 Criteria for Design of Cross-section for Unlined Canals in Alluvial Soil.(28) 1.S. 7916-1975 Code of Practice for Open Power Channels.(29) 1.S. 4558-1968 Code of Practice for Under Drainage of Lined Canals(30) 1.S. 5331-1969 Guidefor Selection of Type of Lining for Canals.(31) 1,S. 9451-1980 Guidelines for Lining of Canals in Expansive Soils.(32) C.B.J.P. Publication No. 144: Syposium on Operation and Maintenance of Canal Systems.
