PIPE FLOW IMPACTS ON EPHEMERAL GULLY EROSION

Authors: Wilson, Glenn; Cullum, Robert; Romkens, Mathias

Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/5/2006
Publication Date: 4/2/2006
Citation: Wilson, G.V., Cullum, R.F., Romkens, M.J. 2006. Pipe flow impacts on ephemeral gully erosion. Federal Interagency Sedimentation Conference Proceedings, April 2-6, 2006, Reno, Nevada. CDROM.

Interpretive Summary: Soil loss from shallow gullies can be a major source of sediment yet the processes that causes these shallow gullies to form is not well understood. When infiltrating water reaches a less permeable layer it can temporarily perch and move laterally over the water-restricting layer. Lateral flow through large soil pores, called soil pipes, has been proposed to cause the initial formation and continued growth of shallow gullies. The objective of this study was to determine the effect of lateral flow through soil-pipe with and without rainfall on gully formation. For test with rainfall, it was applied at 65 mm/h for 1 hour under dry conditions, followed 30 minutes later by a 30 minute rainfall under wet conditions, then 30 minutes later a final 30 minute rainfall under very wet conditions. The soil was at a 5% slope and consisted of 30 cm of Providence silt loam soil packed to a bulk density of 1.35 g cm-3 over a 5 cm thick clay loam layer packed to 1.57 g cm-3. Pipe flow was simulated using a 2 cm diameter porous pipe that extended 50 cm into the soil bed from the upper end. Tensiometers were inserted into the soil bed at 12 positions to monitor soil water pressures during flow events. Rainfall without pipe flow and pipe flow without rainfall did not result in mass wasting. However, when rainfall and pipe-flow were combined they did produce pop-out failures that form shallow gullies. The total soil losses by sheet erosion from runoff were 2-3 times higher with rainfall and pipe flow combined than by rainfall alone. The total soil loss by gully erosion was 5 times higher than sheet erosion as a result of pipe flow combined with rainfall.

Technical Abstract: Rills and ephemeral gullies are major sources of sediment yet their development is not well understood. Lateral flow through soil pipes over water-restricting horizons has been postulated to initiate the development and head cut migration of ephemeral gullies. The objective was to determine the effect of subsurface pipe flow above a water-restricting horizon on ephemeral gully formation during rainfall events and specifically to quantify the effects of hydraulic head on ephemeral gully erosion. A rainfall simulator applied rainfall at 65 mm/h to a 1.5 m long by 1 m wide soil bed at a 5% slope. Rainfall was applied for 1 hour under dry antecedent conditions, followed 30 minutes later by a 30 minute rainfall under wet conditions, then 30 minutes later a final 30 minute rainfall under very wet conditions. The soil profile consisted of 30 cm of Providence silt loam soil packed to a bulk density of 1.35 g cm-3 over a 5 cm thick water restricting layer packed to 1.57 g cm-3. Pipe flow was simulated using a 2 cm diameter porous pipe that extended 50 cm into the soil bed from the upper end. Pipe flow was controlled under a constant head of 0 (no pipe flow) and 30 cm. Tensiometers with pressure transducers were inserted into the soil bed at 12 positions to monitor soil water pressure dynamics during flow events. Rainfall and pipe flow individually did not result in mass wasting, however, their combination did produce pop-out failures. The total soil losses by sheet erosion were 2-3 times higher with rainfall and pipe flow combined than by rainfall alone. The total soil loss by ephemeral gully erosion was 5 times higher than sheet erosion as a result of pipe flow combined with rainfall.