Irrigation Engineer Lesson Regulating Structures Canal for Flows Version 2 CE IIT, Kharagpur Instructional objectives On completion of this lesson, the student shall be able to learn: The necessity of providing regulating structures in canals. The basics of canal drops and falls. The importance of canal regulators. The need for Groyne Walls, Curved Wings and Skimming Platforms. The functions of escapes in a canal. S":".°°!\." 3.9.0 Introduction A canal obtains its share of water from the pool behind a barrage through a structure called the canal head regulator. Though this is also a regulation structure for controlling the amount of water passing into the canal (with the help of adjustable gates), it shall be discussed under diversion works (Module 4). In this lesson, attention is focussed on structures that regulate the discharge and maintain the water levels within a canal network (Figure 1). DlSTRlBUTARY HEAD REGU LATGR ESEAPE .»>/J FIGURE1. Canal structures for flow regulation and control These structures may be described as follows: 1. Drops and falls to lower the water level of the canal 2. Cross regulators to head up water in the parent channel to divert some of it through an off take channel, like a distributary. 3. Distributary head regulator to control the amount of water flowing in to off take channeL 4. Escapes, to allow release of excess water from the canal system. These structures are described in detail in this lesson. 3.9.1 Canal drops and falls A canal has a designed longitudinal slope but has to pass through an undulating terrain. When a canal crosses an area that has a larger natural surface slope, a canal drop, also called fall in India, has to be provided suitably at certain intervals (Figure 2) CANALWATERLEVEL ) NATURALIERGUNBSURFACELEVEL ,.../ ALEJNGCANALALEGNMENT l LCiCATlQflxf. (HF CANAL DRCIWFALL E: {§AN.f5tL EN EX£3AVATl£I}N ; i l§-»0C*.TlC}i*~i% 01: CANAL ERQPEFALL F : CANAL W FELUNG Frc;auRE 2 . Typical location for providing canal irlrop or fall The location of a fall has to be judiciously worked out such that there should be a balance between the quantities of excavation and filling. Further the height of the fall has to be decided, since it is possible to provide larger falls at longer intervals or smaller falls at shorter intervals. It may be observed that the portion of the canal which is Version 2 CE IIT, Kharagpur running in filling (Figure2) may be able to serve the surrounding area by releasing water by gravity. For the portion of the canal that is running in excavation, if surrounding areas have to be irrigated, it has to be done through pumping. There are various types of fall structures, some of which are no more provided these days. However, there are many irrigation projects in India which have these structures in the canal network, as they were designed many years ago. Many of these structures used boulder masonry as their construction material, whereas now brick masonry or, more commonly, mass concrete is being used commonly in modern irrigation projects. 3.9.2 Falls of antiquity The Ogee type of fall has been one of the first to be tried in the Indian canal irrigation system, probably since more than a century back (Figure 3a). However, according to the earliest structures provided, the crest of the fall was in the same elevation as that of the upstream section of the canal. This caused a sharp draw-down of the water surface on the upstream side. On the downstream, the drop in elevation added energy to the falling water which exited the falls as a shooting flow, causing erosion of the canal bed immediately downstream. These difficulties were later removed by raising the crest level of the fall above the upstream canal bed level and providing suitable stilling basin with end sill at the downstream end of the fall which kills most of the excess energy of the leaving water by helping to form a hydraulic jump (Figure 3b). , 1: EFEES? DPFF-ELL F?A|.SE.'Q +~.BCI\;'E EAf-EEIQL EEZE l £\!E§ *~mr wen fE}m.a. 52:.t Ff r LWE 5 ,-:g,z«.»;g,ai-; r-mean;é,«tS.?:Cil~§RY ttcmzitets (E3 FFHSURE 3, fa) K7.3ge¬e~«ty;3e fall rnatdeof rubblemasonry (by:Same typeoffafi, but made of concrete and equipped with at stilling basin!for energy dissipation Version 2 CE IIT, Kharagpur The rapid-fall was tried in some of the north-Indian canals which were constructed with boulders cemented together by lime concrete (Figure 4). These were quite effective but, the cost being prohibitive, was gradually phased out. ~ BED OF RAPID NV WATER SUHFAGE FALL F EGURE 4. Rapid fialls The trapezoidal-notch fall consists of one or more notches in a high crested wall across the channel with a smooth entrance and a flat circular lip projecting downstream from each notch to disperse water (Figure 5). This type of fall was started around the late nineteenth century and continued to be constructed due to its property of being able to maintain a constant depthdischarge relationship, until simpler and economical alternatives were designed. FIGURE5. The notchfal% Version 2 CE IIT, Kharagpur 3.9.3 Modern falls Some falls have been commonly used in the recent times in the canal systems of India. These are described in the following sections. Detailed references may be had from the following two publications of the Food and Agriculture Organisation (FAO): 1. FAO Irrigation and Drainage paper 26/1: Small Hydraulic Structures, Volume 1 (1982) 2. FAO Irrigation and Drainage paper 26/2: Small Hydraulic Structures, Volume 2 (1982) These books are also available from the web-site of FAO under the title Irrigation and Drainage Papers at http://wvvw.fao.org/ag/agL/pub|ic.stm#ag|wbu. 3.9.3.1 Falls with vertical drop These are falls with impact type energy dissipators. The vertical-drop fall (Figure 6) uses a raised crest to head up water on the upstream of the canal section and allows it to fall with an impact in a pool of water on a depressed floor which acts like a cushion to dissipate the excess energy of the fall. This type of fall was tried in the Sarda canal of Uttar Pradesh, which came to be commonly called as the Sarda-type fall. WGLIFEEE . Vertical drop fall Version 2 CE IIT, Kharagpur Typical plan and section of a Sarda-type fall is shown in Figure 7. Usually, two different crests for the fall are adopted, as shown in Figure 8. For canals conveying discharges less that 14m3/s,crest with rectangularcross sectionis adopted,and for discharges more than that, trapezoidal crest with sloping upstream and downstream faces is chosen. Hn!es5t0.15 x0.15; l;...... ..nuwnsmmm F.5.L Upstream F.5..l.I SuctionX - Y - 2 ';t'*M¢~a.;,_;_;.u; 4ii,l.,.lilf Amlgkhgtrg--aAViA_4§% Plan FIGURE 7. SARDA TYPE FALL Version 2 CE IIT, Kharagpur q E § § in E 5 i mono: « FIGURE 8 Typesof cressnsectiors forsaradaFall {a}Rectangular Crest(Urowned Film} (bl TrapezulaiCresl {Freeflow} For smaller discharges, the following a may be provided. o Well drop fall (Figure 9) o Pipe drop fall (Figure 10) o Baffled apron drop (Figure 11) § 4'3: cuménlxcnnemln Haifplancf sopandhaifman:2!boitom FIGURE 9 . WELL DROP All dimensions inem,unless otherwise stated Ear:rim E-mud Laneiiuhw FIGURE 10. Pipe drep epillway Version 2 CE IIT, Kharagpur ENTRANCE SE«CTlC)N SHJE WAL . WENG WALL BAFFLE WALL SIDE WALL ENTRANCE SiLL(ClPTlONAl_} SlDE WALL BACKFILL CHUTE WIDTH L~SEGTlON FlGURE.ll PLAN AND SECTION OF BAFFLED APRON DROP 3.9.3.2 Falls with drop along inclined glacis These are falls with and inclined glacis along which the water glides down and the energy is dissipated by the action of a hydraulic jump at the toe of the structure. Inclined drops are often designed to function as flume measuring devices. These may be with and without baffles as shown in Figures 12 and 13 respectively and supplemented by friction blocks and other energy dissipating devices (Figure 14). Version 2 CE IIT, Kharagpur UfS WING WALL K DISWW3WALL ,TOP OF PETCHING _______________ __ 9_S_!.F.: - _ _ __ __ smvs PiTc2H1G EH5 HFL _ . . . ---..-........ A,» -.....-«.....V....._.. A UisCURTAFN WALL [05 w;;g_;_ DEFLECTDR WALE. OR DISCURTAIN WALL FIGURE12.SECWONAL VEEWTHRGUGH GLACISFALLSWFFHOUT BAFFLES Itf/I/If/l7Ii FIGURE 13. PLAN AND SECTION OH STANDING H...- WAVE FLUME- FALL FHEURE M Flumed @3035 Wilhmetered rat Similar type of fall was also developed in Punjab which was called the CD0 type fall, as shown in Figure 15 (for hydraulic drop up to 1m) and Figure 16 (for hydraulic drop above 1m). Version 2 CE IIT, Kharagpur LEuEL L, 3 (PDRTIGN Lnuslmnawmsscracanq FSGUREES; EULCI! PUNJAB TVPEFAi.E. U11"-T0 EmDRQF Version 2 CE IIT, Kharagpur 7W 3 W-."r-4§ 3% '3-s"*-F-V? $ in L,E)h|t'5il?TUE}l§%JAL 5§%EC7§"l{3£sl f%{3ll}REt1 E.GEEPi!-llJAEl T'fPE:FALL FDRGREATER TQANL tm DREIP The glacis type falls may be modified in the following ways: (a) Flumed or un-flumed, depending upon the crest width being smaller or equal to the bed width of the canal (Figure17). (b) Meter or non-meter fall depending upon whether the canal fall may be used to measure the discharge as well. Details of a meter-fall is described in Lesson 3.10 Version 2 CE IIT, Kharagpur Be Br (A) , 3 At . . (3) Figure17 (A)A umed glacisfallwithfallwidth(B;) beinglessthan canalwidth(Bc). (B)Anan- umedglacis fall,where(BF)is thesameas (Be). Version 2 CE IIT, Kharagpur The following appurtenant structures should be considered while providing a verticaldrop or a glacis-type fall: - The floor of the falls should be able to resist the uplift pressure under the condition of dry canal and a high ground water table. 0 Cut-off walls or curtain walls either of masonry or concrete should be provided at the upstream and downstream ends of the floors of the falls. o Bed protection with dry brick pitching should be provided in the canal just upstream and downstream of the fall. 0 Side protection should be provided at the upstream and downstream splays with brick pitching. Since falls are structures across a canal, it is usual for providing a bridge along with the fall structure for crossing the canal. 3.9.4 Canal regulators These include the cross regulator and the distributary head regulator structures for controlling the flow through a parent canal and its off-taking distributary as shown in Figure 1. They also help to maintain the water level in the canal on the upstream of the regulator. Canal regulators, which are gated structures, may be combined with bridges and falls for economic and other considerations, like topography, etc. A typical view of a distributary head regulator and a cross regulator (shown partly in section) is illustrated in Figure 18. Version 2 CE IIT, Kharagpur Figure 153WDistributary head regulator and parent canal cross regulator showing combination with glacis falt and bridge Gates and gate hoisting arrangements have not been shown for clarity In the figure, the gates and gate hoisting arrangements have not been shown, for clarity. Further, the floor of the regulators would be protected on the upstream and downstream with concrete blocks and boulder apron. A typical sectional drawing through a regulator is shown in Figure 19. TOP OFD0tNl.A3'l3.O urnF.L 311.80 } W5FlS.i..3T0.85 2RQWS DFEDUXEODXHH 3L9gg_3 gsgggxsuo QC ELGCKS OVER BGDGRADED HLTEH ' __DIS BED E08..?5 FIGURE 19. Section through a typical regulator Version 2 CE IIT, Kharagpur The angle at which a distributary canal off-takes from the parent canal has to be decided carefully. The best angle is when the distributary takes off smoothly, as shown in Figure 20(a). Another alternative is to provide both channels (off-taking and parent) at an angle to the original direction of the parent canal (Figure 20b). When it becomes necessary for the parent canal to follow a straight alignment, the edge of the canal rather than the centre line should be considered in deciding the angle of off-take (Figure 20c). GFFT._a.KtNG ,, it CANAL DFF tnisiamve giigfmg T 1 1»-My 3 ;-K? I. J.s Nn LEFTBANK 'A§¥FP3'ET i LEeE;«lu "M . Pmerxrr {LR ;CR_07SREGUl_;3.TCiR , , HR;:HEAg RESLJLATGR SANAL {3} .w.+ PARENT CAMXL f v\, L PARETNVT EANAL 1 lb? iii} Fl.:«"._-=LlRE 20. Alignmertttypes for off taking canal from asparenticanal. la) Smoothoff take {E1}Both inclined to originaliiflow; {Tc}Parent canal f1dw5.s.tr.aightwiith r.er;iuc»ed width. To prevent excessive entry of silt deposition at the mouth of the off-take, the entry angle shouldbe keptto between60° and 80°. For the hydraulicdesignsof crossregulators, one may refer to the Bureau of Indian Standard code IS: 7114-1973 Criteria for hydraulic design of cross regulators for canals. The water entering in to the off-taking distributary canal from the parent canal may also draw suspended sediment load. The distributary should preferably be designed to draw sediment proportional to its flow, for maintaining nonsi|tation of either the parent canal or itself. For achieving this, three types of structures have been suggested as discussed below along with the relevant Bureau of Indian standard codes. Version 2 CE IIT, Kharagpur 3.9.5 Silt vanes (Please refer to IS: 6522-1972 Criteria for design of silt vanes for sediment control in offtaking canals for more details) Silt vanes, or Kings vanes, are thin, vertical, curved parallel walled structures constructed of plain or reinforced concrete on the floor of the parent canal, just upstreamof the offtaking canal. The height of the vanes may be about one-fourthto onethird of the depth of flow in the parent canal. The thicknessof the vanes shouldbe as small as possibleand the spacingof the vanes may be kept about 1.5 times the vane height. To minimize silting tendency, the pitched floor on which the vanes are built should be about 0.15 m above the normal bed of the parent channel. A general three dimensionalview of the vanes is shown in Figure 21 and a typical plan and sectional view in Figure22. OF-"FTAl(lNG CANAL PARENT CANAL '\ \ FJGURE21. View of siltvariesfordivertingsedimentbed loadof parentcanalawayfromofftake Version 2 CE IIT, Kharagpur _;;-*-*0.35!!t thickwewait J; 51 3.35mtixicle toewall T x" 8.35m center not warm to backme g... cmwater , f Plan at upstream and of vane ,.;_L.....-.....,. mmumm: hf» 0.2m thick $4.. masonry aw Sec A B CD FIGURE22.Sll.TVANES 3.9.6 Groyne walls or curved wings (Please refer to IS: 7871-1975 Criteria for hydraulic design of groyne wall (curved wing) for sediment distribution at offtake points in a canal for more details) These are curved vertical walls, also called Gibbs groyne walls, which project out in to the parent canal from the downstream abutment of the off-taking canal. The groyne wall is provided in such a way that it divides the discharge of the parent canal in proportion of the discharge requirement of the off-taking canal with respect to the flow in the downstream parent canal. The groyne wall extends upstream in to the parent canal to cover 3Ato full width of the offtake. The proportional distribution of flow in to the offtaking canal is expected to divert proportional amount of sediment, too. A general view of a groyne wall is shown in Figure 23. Version 2 CE IIT, Kharagpur (j$;:l.'r,:i,;ac;i5.igg,t PAREN? CANAL CANAL. eowustee QARENT CANAL FIGURE23. Viewat girdyne wall{curvedwing) Bgtprejected lengthofgroynefwell} ehould veryfrernlGT5to i.iZ1U BC. whereBCis the bed width of the efftai