In this article we will discuss about the longitudinal section of canals, with the help of suitable diagrams.
The longitudinal slope of a canal therefore is also known or is adopted with reference to the available country slope. However, the slope of canal bed would generally be constant along certain distances, whereas the local ground slope may not be the same. The alignment of a canal system depends on the topography of the land and other factors.
The next step is to decide on the elevation of the bed levels of the canal certain intervals along its route, which would allow the field engineers to start canal construction at the exact locations. Also, the full supply level (FSL) of the canal has to be fixed along its length, which would allow the determination of the bank levels.
The exercise is started by plotting the plan of the alignment of the canal on a ground contour map of the area plotted to a scale of 1 in 15,000, as recommended by Bureau of Indian standards code.IS: 5968 – 1987 “Guidelines for planning and layout of canal system for irrigation”.
At each point in plan, the chain-ages and bed elevations marked clearly, as shown in Figure 3.22. The canal bed elevations and the FSLs at key locations (like bends, ivisions, etc.) are marked on the plan. It must be noted that the stretches AB and BC of the canal (in Figure 3.22) shall be designed that different discharges due to the off taking major distributary. Hence, the canal bed slope could be different in the different stretches.
The determination of the FSL starts by calculating from the canal intake, where the FSL is about 1 m below the pond level on the upstream of the canal head works. This is generally done to provide for the head loss at the regulator as the water passes below the gate. It is also kept to maintain the flow at almost at full supply level even if the bed is silted up to some extent in its head reaches.
On knowing the FSL and the water supply depth, the canal bed level elevation is fixed at chainage 0.00kM, since this is the starting point of the canal. At every key location, the canal bed level is determined from the longitudinal slope of the canal, and is marked on the map. If there is no off take between two successive key locations and no change in longitudinal slope is provided, then the cross-section would not be changed, generally, and accordingly these are marked by the canal layout.
At the off takes, where a major or minor distributary branches off from the main canal, there would usually be two regulators, One of these, called the cross regulator and located on the main canal heads up the water to the desired level such that a regulated quantity of water may be passed through the other, the head regulator of the distributary by controlling the gate opening. Changing of the cross regulator gate opening has to be done simultaneously with the adjustment of the head regulator gates to allow the desired quantity of water to flow through the distributary and the remaining is passed down the main canal.
The locus of the full supply levels may be termed as the full supply line and this should generally kept above the natural ground surface line for most of its length such that most of the commanded area may be irrigated by gravity flow. When a canal along a watershed, the ground level on its either side would be sloping downward, and hence, the full supply line may not be much above the ground in that case.
In stretches of canals where there is no off take, the canal may run through a cutting within an elevated ground, and in such a case, the full supply line would be lower than the average surrounding ground level. In case irrigation is proposed for certain reaches of the canal where the adjacent ground level is higher than the supply level of the canal, lift irrigation by pumping may be adapted locally for the region.
Similarly, for certain stretches of the canal, it may run through locally low terrain. Here, the canal should be made on filling with appropriate drainage arrangement to allow the natural drainage water to flow below the canal. The canal would be passing over a water-carrying bridge, called aqueducts, in such a case.
As far as possible, the channel should be kept in balanced depth of cutting and filling for greatest economy and minimum necessity of borrow pits and spoil banks.
The desired canal slope may, at times, is found to be much less than the local terrain slope. In such a case, if the canal proceeds for a long distance, an enormous amount of filling would be required. Hence, in such a case, canal falls are provided where a change in bed elevation is effected by providing a drop structure usually an energy dissipater like hydraulic jump basin is provided to kill the excess energy gained by the fall in water elevation. At times, the drop in head is utilized to generate electricity through suitable arrangement like a bye-pass channel installed with a bulb-turbine.
A typical canal section is shown in Fig. 3.23 a canal stretch passing through varying terrain profile. Here, no with drawafs have been assumed and hence, the discharge in the entire stretch of the canal is assumed to remain same. Hence, the canal bed slope and water depth are also not shown varying. It is natural that if the canal has outlets in between, the change in discharge would result in corresponding changes in the full supply line.
The elevation of the banks of the canal is found out by adding the freeboard depth. Though the free board depth depends upon many factors, the Bureau of Indian standards code IS: 7112 – 1973 “Criteria for design of cross sections for unlined canals in alluvial soils” recommends that a minimum free board of 0. 5 m be provided for canals carrying discharges less than 10 m3/s and 0.75 m for canals with higher discharges.