A MODEL FOR SEDIMENT ACCUMULATION AND DEBRIS FLOW PRODUCTION IN BURNED AREAS

Sediment movement by debris flow can be conceptualized in a similar manner as sediment movement by running water, with erosion, transport, and depositional processes interacting and dominating different parts of the flow. In burned areas, debris flow initiation often results from widespread sheetwash, rilling, and gullying concentrated in the upper part of the drainage basin. During transport of the flow in the central and lower part of the basin, sediment bulking due to channel scour dramatically increases the volume of the flow and peak discharge levels. Deposition of levees in the central part of the debris channel can absorb a large portion of the moving debris, and has been measured at some locations to account for over 30% of the total eroded and scoured debris volume. Volumes of fans at the mouths of the drainage basins therefore represent the accumulation of eroded and scoured sediment, less the volume of levees. Our observations of recent debris flows in California, Colorado, and Utah, coupled with measurement of erosion, channel scour, and levee volumes for 36 different flows have shown that channel scour rates accelerate once the debris flow has reached a threshold volume in the range of 200 to 500 cubic yards. Thresholds are also hypothesized for sediment accumulation rates following the fire, rainfall levels, and the effects of time. Although conceptual, the developed model has strong implications for the design and effectiveness of various mitigation measures.