Design of Irrigation Canals (Part - 4) - Civil Engineering (CE) PDF Download

Unlined alluvial channels in sediment laden water

It is natural for channel carrying sediment particles along with its flow to deposit them if the velocity is slower than a certain value. Velocity in excess of another limit may start scouring the bed and banks. Hence, for channels carrying a certain amount of sediment may neither deposit, nor scour for a particular velocity. Observations by the irrigation engineers of pre-independence India of the characteristics of certain canals in north India that had shown any deposition or erosion for several years, led to the theory of regime channels, as explained in Lesson 2.10. These channels generally carry a sediment load smaller than 500ppm. The first regime equation was proposed by Kennedy in the year 1895, who was an engineer in the Punjab PWD. Lindley, another engineer in the Punjab proposed certain regime relations in 1919. Later these equations were modified by Lacey, who was at one time the Chief Engineer of the UP Irrigation Department. In 1929 he published a paper describing his findings, which have been quite popularly used in India. These have even been adopted by the Bureau of Indian Standards code IS: 7112-1973 ‘Criteria for design of cross section for unlined canals in alluvial soils” (Reaffirmed in 1990), which prescribes that the following equations have to be used: 

       (13)

P = 4.75            (14)

R = 0.47 (Q/f) ^1/3          (15)

Where the variables are as explained below: 

• S: Bed slope of the channel
• Q: the discharge in m3/s
• P: wetted perimeter of the channel, in m
• R: Hydraulic mean radius, in m
• f: The silt factor for the bed particles, which may be found out by the following formula, in which d50 is the mean particle size in mm. 

                             ₍₁₆₎ 

The Indian Standard code IS: 7112-1973 has also recommended simplified equations for canals in certain parts of India by fitting different equations to data obtained from different states and assuming similar average boundary conditions throughout the region. These are listed in the following table.

It may be noted that the regime equations proposed by Lacey are actually meant for channels with sediment of approximately 500ppm. Hence, for canals with other sediment loads, the formula may not yield correct results, as has been pointed out by Lane (1937), Blench and King (1941), Simons and Alberts on (1963), etc. however, the regime equations proposed by Lacey are used widely in India, though it is advised that the validity of the equations for a particular region may be checked before applying the same. For example, Lacey’s equations have been derived for non-cohesive alluvial channels and hence very satisfactory results may not be expected from lower reaches of river systems where silty or silty-clay type of bed materials are encountered, which are cohesive in nature. 

Application of Lacey’s regime equations generally involves problems where the discharge (Q), silt factor (f) and canal side slopes (Z) are given and parameters like water depth (D), canal bed width (B) or canal longitudinal slope (S) have to be determined or Conversely, if S is known for a given f and Z, it may be required to find out B, D and Q. 

Longitudinal section of canals: 

The cross section of an irrigation canal for both lined and unlined cases was discussed in the previous sections. 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. Further in Lesson 3.6, the alignment of a canal system was shown to be dependent 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 at 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” (Reaffirmed 1992). At each point in plan, the chainages and bed elevations marked clearly, as shown in Figure 13. The canal bed elevations and the FSLs at key locations (like bends, divisions, etc) are marked on the plan. It must be noted that the stretches AB and BC of the canal (in Figure 13) shall be designed that different discharges due to the offtaking major distributary. Hence, the canal bed slope could be different in the different stretches. 

figure 13. Typical layout of a canal showing bed and canal full supply levels

The determination of the FSL starts by calculating from the canal intake, where the FSL is about 1m 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 offtake between two successive key locations and no change in longitudinal slope is provided, then the crosssection would not be changed, generally, and accordingly these are marked by the canal layout. 

At the offtakes, 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 offtake, 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. 

Figure 14. Longitudinal section of a canal assuming no withdrawals in this stretch

A typical canal section is shown in Figure 14, for a canal stretch passing through varying terrain profile. Here, no withdrawals 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 0.5m be provided for canals carrying discharges less than 10m3/s and 0.75m for canals with higher discharges. 

Important terms

Free Board: A depth corresponding to the margin of safety against overtopping of the banks due to sudden rise in the water level of a channel on account of accidental or improper opening or closing of gates at a regulator on the downstream.

Borrow pits: Specific site within a borrow area from which material is excavated for use is called a borrow pit.

Spoil Banks: Piles of soil that result from the creation of a canal, deepened channel, borrow pit, or some similar structure.