Theoretical Aspects of Seepage from Open Channels

Seepage from open channels was evaluated by analog in relation to depth and shape of the channel, position of the water table, and the saturated and unsaturated permeability of the soil. Subsoil conditions included an impermeable layer and a permeable layer at varying depth. The case of clogged bottoms and banks was treated analytically. For trapezoidal canals with 1:1 side slopes, a bottom width W_b, and a surface width W_s, soil conditions at distances more than 5 W_b below the bottom have little effect on seepage. Lowering the water table increases the seepage, but when the water table at a horizontal distance of 10 W_b has reached a depth of 2.5 Ws below the water level in the channel, the seepage is already near that at infinite depth of the water table. Seepage increases with increasing water depth, but at a slower rate than the discharge for uniform flow. Therefore, canals convey water more efficiently when deep than when shallow. Other analyses refer to the effect of bottom treatment for seepage reduction, and to the effect of unsaturated flow on seepage.