Study on incipient channelization due to seepage erosion

Abstract

Overland and seepage flows have been hypothesized as dominant factors in the initiation and evolution of gullies and channels. Many studies on channelization due to erosion by overland flow have been carried out whereas only few studies have focused on seepage erosion. This study aims to acquire fundamental knowledge on the seepage erosion from experiments and a linear stability analysis. We use the Dupuit-Forchheimer equation and a description of the retreat of the seepage front. The retreat speed consists of two terms: the first is assumed to be a power law function of a specific discharge at the front exceeding a critical discharge, and the second is assumed to be a diffusion-like function of front shapes, in which the retreat speed is enhanced and retarted by the convexity and concavity of fronts, respectively. We find that the characteristic channel spacing becomes infinitely small when the effect of the front shapes is excluded. We conduct a series of experiments with various depth of sediment layers and chamber slopes in order to estimate the diffusion-like coefficient. We find that the width of gullies increases with increasing grain size and bed gradient, while channel spacing decreases. We propose a relationship between the diffusion-like coefficient and the combination of the two functions, water level at the seepage front and weight of a failure block.