Numerical calculation of soil pipe flow and its effect on water dynamics in a slope

Abstract

AbstractA chain of connected macropores, which develops nearly parallel to the impermeable subsurface layer, is commonly found in hillslopes and referred to as a soil pipe. Field observations have revealed that water flow in such a pipe (pipe flow) has large hydrogeomorphic effects, since it contributes to rapid transfer of storm water and subsurface soil erosion. This study proposed a simple simulation method to describe water dynamics in a piped hillslope. The applicability of the method was examined using data obtained from a bench‐scale experiment using pseudo soil pipes. Results of the experiment showed that (1) the flume‐bed pore water pressure became small around the upper ends of the pipes; (2) when pipe outlets were connected to a stream channel (the open pipe condition), the flume‐bed pore water pressures in the whole slope were much lower than those under the ‘no pipe’ condition; (3) when pipe outlets were not connected to a stream channel (the closed pipe condition), the flume‐bed pore water pressure at the pipe outlets was higher than that under the ‘no pipe’ condition. These results were reproduced by solving the two‐dimensional Richards equation assuming an equivalent soil matrix layer for the original ‘pipes plus soil matrix’ part, which had a larger saturated hydraulic conductivity than the surrounding soil matrix. This simple simulation method has an advantage over previous pipe flow models: both the pipe flow and the interaction between the soil pipes and the surrounding soil matrix can be computed by solving the Richards equation without using any further assumptions for the boundary conditions between the soil pipes and the soil matrix. Results of numerical experiments using the simple method showed that (1) under the closed pipe condition, the flume‐bed pore water pressure at the pipe outlets increased as the pipe length increased, and (2) once the open pipes clogged, flume‐bed pore water pressures in the whole slope increased, and the greatest increase occurred at the boundary between the clogging and non‐clogging parts. Copyright © 2004 John Wiley & Sons, Ltd.