Design of Irrigation Canals (Part - 1) - Civil Engineering (CE) PDF Download

Introduction

The entire water conveyance system for irrigation, comprising of the main canal, branch canals, major and minor distributaries, field channels and water courses have to be properly designed. The design process comprises of finding out the longitudinal slope of the channels and fixing the cross sections. The channels themselves may be made up of different construction materials. For example, the main and branch canals may be lined and the smaller ones unlined. Even for the unlined canals, there could be some passing through soils which are erodible due to high water velocity, while some others may pass through stiff soils or rock, which may be relatively less prone to erosion. Further, the bank slopes of canals would be different for canals passing through loose or stiff soils or rock. In this lesson, we discuss the general procedures for designing canal sections, based on different practical considerations. 

Design of lined channels

The Bureau of Indian Standards code IS: 10430 -1982 “Criteria for design of lined canals and guidelines for selection of type of lining” (Reaffirmed in 1991) recommend trapezoidal sections with rounded corners for all channels-small or large. However, in India, the earlier practice had been to provide triangular channel sections with rounded bottom for smaller discharges. The geometric elements of these two types of channels are given below

Triangular section  

For triangular section, the following expressions may be derived 

A = D² (cot )               (1)

 P = 2 D (cot )             (2)

 R = D / 2                     (3) 

The above expressions for cross sectional area (A), wetted perimeter (P) and hydraulic radius (R) for a triangular section may be verified by the reader.

Trapezoidal section 

For the Trapezoidal channel section, the corresponding expressions are: 

 A = B D + D² (cot )             (4)

  P = B +2D(cot )                (5)

 R = A / P 

The expressions for A and P may, again, be verified by the reader. In all the above expressions, the value of is in radians. 

The steps to be followed for selecting appropriate design parameters of a lined irrigation channel, according to IS: 10430 may be summarized as follows: 

1. Select a suitable slope for the channel banks. These should be nearly equal to the angle of repose of the natural soil in the subgrade so that no earth pressure is exerted from behind on the lining. For example, for canals passing through sandy soil, the slope may be kept as 2H: 1V whereas canals in firm clay may have bank slopes as 1.5H: 1V canals cut in rock may have almost vertical slopes, but slopes like 0.25 to 0.75H: 1V is preferred from practical considerations. 

2. Decide on the freeboard, which is the depth allowance by which the banks are raised above the full supply level (FSL) of a canal. For channels of different discharge carrying capacities, the values recommended for freeboard are given in the following table: 

 3. Berms or horizontal strips of land provided at canal banks in deep cutting, have to be incorporated in the section, as shown in Figure 3. 

The berms serve as a road for inspection vehicles and also help to absorb any soil or rock that may drop from the cut-face of soil or rock of the excavations. Berm width may be kept at least 2m. If vehicles are required to move, then a width of at least 5m may be provided. 

4. For canal sections in filling, banks on either side have to be provided with sufficient top width for movement of men or vehicles, as shown in Figure 4. 

The general recommendations for bank top width are as follows: 

Next, the cross section is to be determined for the channel section. 

5. Assume a safe limiting velocity of flow, depending on the type of lining, as given below:  

• Cement concrete lining: 2.7 m/s
• Brick tile lining or burnt tile lining: 1.8 m/s
• Boulder lining: 1.5 m/s 

6. Assume the appropriate values of flow friction coefficients. Since Manning’s equation would usually be used for calculating the discharge in canals, values of Manning’s roughness coefficient, n, from the following table may be considered for the corresponding type of canal lining.  

7. The longitudinal slope (S) of the canal may vary from reach to reach, depending upon the alignment. The slope of each reach has to be evaluated from the alignment of the canal drawn on the map of the region. 

8. For the given discharge Q, permissible velocity V, longitudinal slope S, given side slope , and Manning’ roughness coefficient, n, for the given canal section, find out the cross section parameters of the canal, that is, bed width (B) and depth of flow (D). 

Since two unknowns are to be found, two equations may be used, which are: 

• Continuity equation: Q = A * V                         (6)
• Dynamic equation: V = 1/n (AR^2/3 S^1/2)          (7)

In the above equations, all variables stand for their usual notation as mentioned earlier, A and R is cross sectional area and hydraulic radius, respectively.