High-resolution modeling of overland flow and sediment transport following wildfire: Insights into initiation mechanisms and sediment sources for runoff-generated debris flows

Abstract:

Sediment transport in steep landscapes may be facilitated by both water-dominated flows and debris-flow processes. Given the differences in erosion potential and mobility possessed by runoff and debris flows, knowledge of the conditions that determine the runoff-to-debris-flow transition has important implications for hazard assessment as well as our understanding of the long-term evolution of steep bedrock channels. Debris flows in alpine areas and burned steeplands are frequently triggered by runoff following high-intensity rainfall, but the mechanics by which runoff generates a debris flow are not well understood. To examine the connections between runoff and debris flow initiation, we developed a numerical model that couples overland flow with sediment transport and debris-flow processes. We applied the model to study erosion and debris-flow initiation that occurred during a rainfall event that produced numerous debris flows within a burned drainage basin in the San Gabriel Mountains, CA, USA. Input data for the numerical model was constrained by rain gauges, stage measurements at the basin outlet, soil-moisture sensors, and high-resolution topographic data obtained using a terrestrial laser scanner (TLS). Numerical model predictions, which compare well with TLS-derived measurements of topographic change, indicate that hillslope erosion at our study site was primarily the result of raindrop-induced sediment transport. Further, results indicate that the majority of sediment eroded from the hillslopes was deposited within the channel system during the storm with only minor amounts being transported out of the basin in suspension. Based on model results, we hypothesize that numerous debris flows were generated from the mass failure of sediment dams that built up within the channel system throughout the storm. This study adds to our understanding of sediment transport in steep landscapes and provides insight into the topographic and hydrologic factors that transform runoff into debris flow.