Post‐fire thin debris flows: sediment transport and numerical modelling

The creation of a hydrophobic layer in the soil during fires in semi‐arid environments inhibits the infiltration of rainfall. This leads to increased rates of runoff and associated sediment transport. When the hydrophobic layer is deposited beneath the soil surface, a perched water table develops which may cause thin (1–2 cm) hillslope failures that are distinguishable from features caused by rilling and sheetflow. Evidence for these failures was observed after a fire near Santa Barbara, California. The amount of sediment eroded from some hillslopes was substantial, with 290 kg of sediment per metre width of hillslope delivered to the valley floor. The mechanics of these failures are examined with a numerical model that incorporates a stability analysis with subsurface flow routing along a typical hillslope profile. The model correctly predicts the location of the failures as well as the rainfall amount necessary to trigger them.