Subsurface Drainage in Soils with High Hydraulic Conductivity Layers

Abstract

ABSTRACT THE Richards equation for two-dimensional, saturated-unsaturated flow during drainage was solved for layered soils using finite difference meth-ods. Solutions were obtained for soils in which the deeper layer has a higher hydraulic conductivity than the surface layer. Solutions were presented to show the distribution of equipotential lines and position of the water table during drainage processes. When the drain depth is increased so that it is closer to the high conductivity layer, head loss due to convergence near the drain is reduced. Thus, deeper drains may significantly increase drainage rates even if the hy-draulic head at the outlet remains unchanged. This effect is larger for deep profiles and narrow drain spacings than for shallow profiles and wide spacings. The effect of increasing the drain depth also increases with the hydraulic conductivity of the bottom lay-er. When outlet conditions are limiting, the most efficient drain depth is that of the layer interface. Solutions for various cases showed that further in-creasing the depth had only a slight effect on water table drawdown.