Understanding soil-pipe flow and its role in ephemeral gully erosion

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 10 mm 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 min. Either the tunnel collapsed 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, and 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. Copyright (C) 2011 John Wiley & Sons, Ltd.