Mechanisms of ephemeral gully erosion caused by constant flow through a continuous soil-pipe

Ephemeral gully erosion is considered to be driven by convergent surface flow while the role of subsurface flow is often overlooked. This study sought to characterize and quantify the soil erosion mechanisms associated with ephemeral gully erosion by pipe flow. A soil pipe (1 cm o.d.) was formed in a 10 cm soil bed immediately above a 5 cm water restricting layer. Flow into the soil pipe was established at steady‐state rates of 190 and 284 l h−1. 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 Seconds) periods of negligible flow were followed by surges in flow with high sediment concentrations that included a high proportion 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 non‐equilibrium between the soil pipe and soil matrix. Hydraulic non‐equilibrium 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 three minute sample collection interval. The Slot Erosion Test (SET) was extended to conditions of constant flow rate through an internal soil pipe that was observed at the front face of a soil bed. The modified SET 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. Published in 2009 by John Wiley & Sons, Ltd.