Soil Pipe Flow and Internal Erosion: The Integration of Soil Pedology and Hydrology.

Many embankment failures, landslides, and gullies have been attributed to erosion by piping. One such piping process is flow through a discrete macropore or soil pipe. Soil pedology and hydrology interact to determine the location of soil pipes, flow rates through soil pipes and rates of internal erosion. Soil pipes tend to develop in duplex soil in that water restricting horizons cause a proliferation of biopores at the interface and foster lateral subsurface flow by perching water. Internal erosion can enlarge these preferential flow paths to the extent that pipe's collapse, thereby forming flute holes, sinkholes and ephemeral gullies at the surface. The soil hydropedologic properties determine the erodibility of the pipe surfaces and shear forces acting on pipe walls.  Laboratory studies of these processes exist to a limited degree but field characterization of soil pipeflow and internal erosion are seriously lacking. This paper will review the connections between hydrologic and pedologic soil properties with soil pipeflow processes using observations of soil pipes in Goodwin Creek Watershed. A 6.5 ha catchment contained 72 pipe collapse openings, 16 sinkhole depressions, and seven gullies that appeared to be formed by tunnel collapse. Soil pipes and gullies in the catchment were instrumented with pressure transducers and flow velocity meters for monitoring subsurface flow at multiple locations and surface flow at a downslope location. Soils in the catchment contain a fragipan and evidence of past erosion-sediment deposition features that control the location of soil pipes and hydrologic response of the catchment. Soil pipes quickly responded to storm events with some serving as sources of surface flow (artesian seeps) and others, occasionally positioned immediately downslope, serving as sources of subsurface flow by routing surface runoff into soil pipes. This paper will use field observations to highlight gaps in our understanding of the hydropedologic processes and ability to map their location and measure pipe flow and internal erosion rates in situ.