Author
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Wilson, Glenn |
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Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/6/2008 Publication Date: 11/30/2009 Citation: Wilson, G.V. 2009. Mechanisms of ephemeral gully erosion caused by constant flow through a continuous soil-pipe. Earth Surface Processes and Landforms. 34:1858-1866. Interpretive Summary: Ephemeral gully erosion, gullies that can be filled in by tillage, contributes about one third of the total erosion in the US. Surface runoff is generally thought to be the only cause of ephemeral gully erosion, but flow through large pores, called soil pipes, may also contribute. The conditions under which flow through soil pipes cause ephemeral gully erosion were tested. A 1 cm diameter soil-pipe was formed in a 10 cm soil bed immediately above a compacted layer that prevents deeper percolation. Flow into the soil-pipe was set at constant rates equivalent to 15 cm and 30 cm water pressures. Experiments were performed for pipe flow alone and pipe flow along with rainfall. Despite a constant flow rate into soil pipes, pipe flow was highly variable due to erosion within the soil pipe that clogged the pipe until water pressure increases forced the clog out of the pipe. Short (10-20 second) periods of no flow were followed by surges in flow containing high sediment concentrations that included soil clods. Increases in soil water pressures inside soil pipes during these surges in flow were observed but were likely not representative of the water pressures measured in the soil around the pipe. The lack of equilibrium between the soil and the pipe resulted in total pressure gradients in the opposite direction of pipe flow during early stages. Pipe flow rates and sediment concentrations during periods of clogging and surging were likely more extreme than observed due to the sample collection period smoothing out the flow rate. A modification of the Slot Erosion Test provided estimates of shear stress between 2.2 to 5.3 kg m-1 s-2. These results will guide development of approaches for predicting and controlling ephemeral gully erosion for conditions of shallow soil over a compacted layer where soil pipes are likely to contribute. Technical Abstract: Ephemeral gully erosion is considered to driven be convergent surface flow while the role of subsurface flow through soil pipes is often overlooked. This study sought to characterize the processes and quantify the soil erosion properties associated with ephemeral gully erosion by pipe flow. A 1 cm diameter soil-pipe was formed in a 10 cm soil bed immediately above a water restricting layer. Constant flow into the soil-pipe was established at rates equivalent to15 cm and 30 cm pressure heads. Experiments were performed for pipe flow alone and with rainfall. Despite a constant flow rate into soil pipes, pipe flow was highly unstable due to internal mass wasting clogging soil pipes until pressure increases forced the debris plug out of the pipe. Short (10-20 second) periods of negligible flow were followed by surges in flow with high sediment concentrations that included high proportions of aggregates. Increases in soil water pressures associated with these debris flows were observed but were likely not representative of the pressures inside the soil pipes due to hydraulic nonequilibrium between the soil pipe and soil matrix. Hydraulic nonequilibrium resulted in hydraulic gradients in the opposite direction of flow through the soil pipe during early stages of pipe flow. Pipe flow rates and sediment concentrations during debris flow periods were likely more extreme than observed due to integration over the 3 minute sample collection interval. Application of the Slot Erosion Test to conditions of constant flow rate through a pipe observed to enlarge at the front face provided estimates of shear stress between 2.2 to 5.3 kg m-1 s-2. However, the technique did not prove effective for estimation of the soil erodibility coefficient. |
