Interflow of Water through a Sloping Soil with Seepage Face

Abstract

AbstractA theoretical analysis of interflow pathways, source areas, and rates is presented for saturated, steady, and homogeneous conditions. Investigated were the effects of slope length to soil depth ratio, soil slope, slope‐of the seepage face, boundary condition at the upper end of the slope, leakiness of the base, and of ditch water covering the seepage face. Two regions of interflow could be distinguished: (i) flow through the main body of the soil (region 1), and (ii) flow from infiltration near the seepage face (region 2). For soil on an impermeable base, all the interflow water in region 1 entered the soil through a small area near the upslope end and flowed parallel to the slope before seeping out of the face near the base. For a slope length to soil depth ratio of 10 or larger, this part of interflow could be approximated as one‐dimensional flow with hydraulic gradient equal to the sine of the soil slope angle. Presence of a seepage face at the lower end of the slope caused considerable additional infiltration to contribute to interflow, entering from a distance near the face of about 2.5 times the depth of soil (region 2). This flow increased as the face slope (with respect to the base) increased, but was not influenced by slope length and was influenced in a minor way by soil slope. When the ditch water covered the seepage face completely, the infiltration in region 2 was prevented. For soil on a leaky base with leakage rate as low as 1/100th the hydraulic conductivity of the soil, all the water entering upslope of a short distance from the seepage face flowed downward into the leaky base. The slope length and soil volume traversed by interflow were reduced drastically. The boundary between the interflow and leakage flow regions could be approximated by a straight line of slope v/K tan ω passing through the origin, where v is the leakage rate, K is the hydraulic conductivity of soil, and ω is the soil slope angle.