Author
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Wilson, Glenn |
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Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/7/2011 Publication Date: 7/15/2011 Citation: Wilson, G.V. 2011. Understanding soil-pipeflow and its role in ephemeral gully erosion. Hydrological Processes. 25(15):2354-2364. Interpretive Summary: The role of flow through large pores, called soil pipes, in causing small, termed ephemeral, gullies to develop is not well understood. Experiments were carried out on soil pipes to better understand the processes of erosion inside soil pipes and its role in ephemeral gully development. Soil beds of 140 cm length, 100 cm width and 20 cm depth had a single soil pipe of initial sizes 2, 4, 6, 8, and 100 mm outside diameter. The soil pipes extended from a water reservoir to the end of the soil bed. Experiments were run on Providence silt loam and Smithdale loam soils under a constant water level in the reservoir of 15 cm above the soil pipe opening. This level was maintained for 30 minutes or until either the soil above the pipe collapsed or the water level could not be maintained. Soil pipes that were initially 2 and 4 mm diameter clogged immediately at their opening without flowing, whereas, pipes that were the 6, 8, and 10 mm diameters at the start enlarged by 268, 397, 699%, respectively. The rapid internal erosion resulted in large, rapid changes in flow rates and sediment concentrations with periods of no flow when pipes clogged followed by surges of high flow and high sediment concentrations when they reopened. Tensiometers n the soil near the soil pipes did not exhibit pressure changes typically seen when pipes clog. Flow through 10 mm diameter soil pipes eroded so much inside that the soil above them collapsed. Tunnel collapse typically occurred within minutes of the start of flow suggesting that ephemeral gullies caused by erosion inside soil pipes could be mistaken as being caused by surface runoff if observations were made after the runoff event instead of during the event when flow is occurring in soil pipes. Technical Abstract: The role of soil pipeflow in ephemeral gully erosion is not well understood. Experiments were conducted on continuous soil pipes to better understand the role of internal erosion of soil pipes and its relation to ephemeral gully development. Soil beds of 140 cm length, 100 cm width and 20 cm depth had a single soil pipe of different initial sizes (2, 4, 6, 8, and 100 mm outside diameter) extend from a water reservoir to the outlet. Experiments were run on Providence silt loam and Smithdale loam soils under a constant head of 15 cm established for 30 minutes or until either tunnel collapse or the head could not be maintained. Soil pipes that were initially 2 and 4 mm clogged instantaneously at their mouth and did not exhibit flow, whereas, pipes initially > 6 mm enlarged by 268, 397, 699% on average for the 6, 8, and 10 mm diameters, respectively. Critical shear stress values were found to be essentially zero and erodibility values gave erosion indexes that were extremely high. The rapid internal erosion resulted in erratic flow and sediment concentrations with periods of no flow as pipes were temporarily clogged followed by surges of high flow and high sediment concentrations. Tensiometers within 6 cm of the soil pipes did not exhibit pressure increases typically associated with pipe clogging. Flow through 10 mm diameter soil pipes exhibited tunnel collapse for both soils tested. Tunnel collapse typically occurred within minutes of flow establishment suggesting that ephemeral gullies could be misinterpreted as being caused by convergent surface flow if observations were made after the runoff event instead of when flow is first established through soil pipes. |
