Steady State Flow to Drains in Terraced Multilayered Anisotropic Soils

Abstract

ABSTRACT Results for steady-state water flow in layered soils with subsurface drains are presented. The soil was assumed to consist of one or more layers bounded below by an impervious barrier at an infinite depth and above by a bench terraced soil surface. Each soil layer was considered water-saturated and uniformly isotropic or anisotropic within the layer. Anisotropy was defined such that I 9^ x: where | and K are the horizontal and vertical hydraulic conductivities, respectively. The finite element method was used in the numerical solution for steady and saturated flow. Although the numerical techniques presented can handle any number of layers, results are only presented for one- and two-layered profiles. The effect of the degree of anisotropy (R) and equivalent hydraulic conductivity (K) for individual layers (where R = [|/x:]^^2 and K = [| xY^^) on drain outflow, water seepage at the soil surface and the flow within each layer was investigated. The presence of subsurface drains increased the total flow into the terraced soil surface and decreased (often eliminated) water seepage outflow from the bottom of the terraced slope. In a two-layered system, the simple effects of drain depth, degree of anisotropy, and equivalent hydraulic conductivity of the individual layers were masked by interactions of their effects. At the shallow drain depth, when the equivalent conductivities of the two layers were the same, flow increased as degree of anisotropy increased if the lower layer was anisotropic. Flow decreased as the degree of anisotropy increased if the upper layer was anisotropic. At deeper drain depths, flow tended to increase as degree of anisotropy decreased. When the lower layer was more conductive than the upper layer, flow increased significantly as the degree of anisotropy in the upper layer decreased and as drain depth increased. When the lower layer was anisotropic, flow increased as degree of anisotropy increased at shallow depths but gradually changed to decreased flow with increasing degree of anisotropy for deep drains.