Debris-flow response of southern California watersheds burned by wildfire

Evaluation of the erosional response of 86 recently burned basins in southern California to the heavy winter rainfall of 1997-98 shows that debris flows were produced from only 30 of 86 basins examined. The remaining basins produced sediment-laden streamflow, hyperconcentrated flow, or no discernable response. Basins underlain by sedimentary rocks were most likely to produce debris flows dominated by large materials, and sand- and gravel-dominated debris flows were generated primarily from decomposed granite terrains. For a given basin area, there is a threshold value of slope above which debris flows are produced, and below which other flow processes dominate. The presence or absence of a water-repellent layer in the burned soil and an extensive burn mosaic had little bearing on the generation of large grain size debris flows. The presence of water-repellent soils appears to have affected the generation of sand- and gravel-dominated debris flows.