Preferential Flow through Soil-Pipes Results in Internal Erosion: Role in Ephemeral Gully Erosion.

Sheet and rill erosion have been historically considered the main sources of stream sediment, however, in many cases the dominant source is gully erosion and hillslope failure. Estimates suggest that ephemeral gully erosion ranges from 18 to 73% of the total erosion and bank erosion can contribute 10 to 90% of stream sediment. Subsurface flow can contribute to gully erosion and slope failure by water pressure impacts on the soil properties, seepage gradient forces, and preferential flow through soil-pipes. Preferential flow through small biological (root channels and animal burrows) and soil-structural features can result in formation of soil pipes due to internal erosion of the flow paths. Pipe flow is commonly found in duplex soils in which a water restricting horizon facilitates lateral flow. Landslides and embankment failures have been attributed to pressure build ups when soil-pipes clog. It has been postulated that pipe erosion can occur with no visible evidence until tunnel collapse results in a fully developed ephemeral gully. Questions exist as to the conditions under which preferential flow through biological and structural features can result in pipe flow and the role of pipe flow on gully formation and slope failure. Experimental studies will be presented on flow through continuous soil-pipes of various sizes under different hydrologic conditions. Soil pipes smaller than 6 mm diameter did not exhibit pipe flow under constant heads of 15 cm. For preferential flow through pipes initially > 6mm diameter under constant head conditions, flow rates increased with time due to internal erosion enlarging the soil pipes and occasionally resulted in tunnel collapse. For conditions of constant flow into soil pipes, internal erosion resulted in unsteady flow through the pipes as eroded material plugged the pipe until pressure increases forced the material out of the pipe. However, such conditions did not exhibit sudden development of mature ephemeral gullies by tunnel collapse. These results suggest that macropores developed from small (<6 mm) roots, may not result in ephemeral gullies by piping but if initiated, pipe flow under constant head conditions can result in tunnel collapse.