Pipeflow and Internal Erosion Observations and Modeling.

Many embankment failures, landslides, and gullies have been attributed to erosion by piping but observations suggest varying flow and erosion processes being involved. One such piping process is flow through a discrete macropore or soil pipe. Preferential flow through soil pipes can result in internal erosion such that gullies form when the soil above collapses. Soil pipes have also been attributed to stabilizing hillslopes by acting as drains thereby increasing the soil strength or destabilizing hillslopes when water pressures inside the soil pipe buildup as a results of pipe clogging.  Laboratory studies have shown that clogging of soil pipes can result in surges in flow and potential pressure buildups but measurements within soil pipes is limited. Recent field studies have shown pressure buildups within soil pipes but under limited conditions. Laboratory studies have shown that internal erosion can result in pipe collapse and field observations of collapses have suggested that their locations are associated with water restrictive layers that can foster lateral preferential flow. Richards' equation, with the soil pipe treated as a highly conductive porous media instead of a void, has been used to model pipeflow. This approach does not fully capture the dynamics of the hydraulic nonequilibrium between the soil and pipe. Incorporating internal erosion into Richards' equation based flow models has several limitations including turbulent flow, pipe enlargement, and representation of the pipe properties. This paper will review piping to highlight our current understanding of the processes involved and will identify knowledge gaps in our ability to measure and model pipeflow and internal erosion.