Internal erosion during soil pipe flow: Role in gully erosion and hillslope instability

Many field observations have led to speculation on the role of piping in embankment failures, landslides, and gully erosion. However, there has not been a consensus on the subsurface flow and erosion processes involved and inconsistent use of terms have exasperated the problem. One such piping process that has experienced a lot of field observations but very limited mechanistic experimental work involves flow through a discrete macropore or soil pipe. Questions exist as to the conditions under which preferential flow through soil pipes: result in internal erosion, stabilize hillslopes by acting as drains, result in hillslope instability by causing pressure buildups, result in ephemeral gully formation or reformation of filled-in gullies. The objective of this paper was to review discrepancies in terminology to better explain the piping processes and highlight the experimental work done to date on the specific processes of soil pipeflow and internal erosion. The studies reviewed include those that examined the process of slope stability as affected by the clogging of soil pipes, the process of gullies reforming due to mass failures caused by flow into discontinuous soil pipes, and the process of gully initiation by tunnel collapse due to pipes enlarging by internal erosion. In some of these studies the soil pipes were simulated with perforated tubes placed in the soil, while in other studies the soil pipes were formed out of the soil itself. Analytical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes in order to calculate critical shear stress and erodibility properties of soils. Numerical models have been applied to describe flow through soil pipes but incorporation of internal erosion into such models has proven complicated due to enlargement of the pipe with time as well as temporary clogging of soil pipes. These studies and modeling approaches will be described and a discussion will ensue that considers the gaps in our understanding of pipe flow and internal erosion processes and our ability to model these processes.