SEEPAGE FROM CANALS FOUNDED ON FINITE PERVIOUS SOIL WITH ASYMMETRIC DRAINAGES

Abstract

Cana seepage estimation is of considerable importance for economic justification of canal lining. Determination of phreatic surface due to seepage from canal helps in estimating the area likely to be water—logged. Seepage from canals is substantial component oflhe total recharge.to the subsurface reservoir. Accurate estimation of canal seepage is necessary for evaluating sustained yield of subsurface reservoir. Seepage losses from unlined canals depend on the shape and size of the canal cross—section, location and levels of drainage on either side of the canal and coefficient of permeability and depth of the subsoil. Seepage losses also depend upon the rate of infiltration/evaporation from the free surface. In this solutions for the following problems have been obtained. 1. Seepage from a canal to a homogeneous, isotropic medium extending to finite depth with asymmetrically placed drainages. 2. Seepage from a canal to a homogeneous, isotropic medium extending to infinite depth with symmetriCally placed drainages and infiltration/evaporation from the free surface zone. Solution of the first problem has been obtained in two stages. In the first stage shape of the canal is neglected (water depth is considered small as compared to its width). In the next stage effect of the channel shape on the seepage and phreatic surface is determined. Analytical solutions of these problems have been obtained using Zhukovsky's function and conformal mapping. The values of resulting integrals have been obtained numerically. The values of seepage losses from canal and free surface profile are obtained by solving the equations for various values of Li/hi, L2/h1, T/h1, B/hi, H/hi and h2/h1. Results are presented in the form of nomographs, which can be used to quickly obtain canal seepage discharge for the known values of physical parameters. It is observed that the seepage from canal increases with increase in the depth of the im ervious layer. However the effect of increase in the depth of impervious layer on canal seepage is negligible for very low values of T/hi and for very large values of T/hi. The seepage from canal increases with increase in the bed width, water depth and side slope of the canal and decrease in the drainage's distance. However increase in the canal seepage due to increase in canal water depth and canal side slope is very small. Seepage to the drainage near to the canal and at lower level is more than that to the drainage at larger distance and at higher level. If level of one of the drainage is raised, the seepage from canal to this drainage decreases and that to the other drainage increases. With further rise in the level of this drainage, the seepage discharge to the raised drainage keeps on decreasing. At a certain depth of drainage below canal level this drainage may not receive any seepage discharge. This depth is taken as critical depth he. It is seen that the value of critical depth he/hi decreases, when is increased and also if L2/h1 is decreased. Variation in he/hi is more pronounced when Li/hi and L1/h1 are of similar magnitude. If T/hi or B/h1 are increased, he/hi also increases. The free surface rises with increase in bed width, increase in drainage distance and-decrease in the.depth of the impervious layer. In Second problem canal water depth is assumed to be very small, drainages are considered symmetrically placed and pervious medium extending upto large depth. However the infiltration to or evaporation from the free surface zone is considered. Solution for this problem is obtained by replacing complex seepage potential by an analytical function SI = —iw/k + s/k z, where e denotes rate of infiltration (c <0 for evaporation). The 0—plane and the 11—plane are mapped on to semi infinite t—plane. Effect of infiltration/evaporation and drainage bed width on seepage discharge and five surface profile are determined. It is observed that 8 infiltration rate increases the canal seepage decreases. The free surface profile rises with increase in the value of infiltration rate. As expected, with decrease in drainage bed width the free surface rises and the seepage from the canal decreases.