10. Optimal Design of Transmission Canal

A transmission canal conveys water from the source to a distribution canal. Many times, the area to be irrigated lies very far from the source, requiring long transmission canals. Though there is no withdrawal from a transmission canal, it loses water on account of seepage and evaporation. Hence, it is not economical to continue the same section throughout the length of a long transmission canal. Instead, a transmission canal should be divided into subsections or reaches, and the cross section for each of the subsections must be designed separately. This would result in reduced cross sections in the subsequent reaches. The reduced cross section not only results in cost saving for earthwork, lining, and water lost but also requires less cost in land acquisition, construction of bridges, and cross-drainage works. This chapter addresses the problem of design of transmission canal. Using the least-cost section equations presented in Chap. 8 and applying grid search method, equations for computation of the optimal subsection length and corresponding cost of a transmission canal have been obtained. The optimal subsection length of the transmission canal is independent of the length of the transmission canal. The optimal design equations along with the tabulated section shape coefficients provide a convenient method for the optimal design of a transmission canal. The present method can be extended in developing equations for the optimal design of a transmission canal having unequal cost of transitions and unequal length of subsections. The suggested equations are applicable for all the regular canal shapes. The section shape coefficients to be used in designing a transmission canal have been obtained for triangular, rectangular, and trapezoidal canals. The method can be extended to find the coefficients in the optimal design equations for other shapes such as the circular section, parabolic section, rounded corner trapezoidal section, etc., if the corresponding seepage functions are developed. Direct optimization procedures may be adopted for the optimal design of irrigation canal sections and for the transmission canal, but they are of limited use and require considerable amount of programming and computation. On the other hand, using the optimal design equations along with the tabulated section shape coefficients, the optimal design variables of a canal can be obtained in single-step computations.