CHARGING THE SEDIMENT CAPACITOR: EXPLORING A MODEL FOR THE TRANSITION OF WATER RUNOFF INTO DEBRIS FLOW

Sparsely vegetated alpine areas and recently burned steeplands are susceptible to debris flows following intense rainfall. These areas commonly have abundant loose channel or hillslope sediment situated downslope of low-permeability surfaces, such as bedrock in alpine areas or hydrophobic soils in burned areas. Debris flows in these settings are often triggered by runoff from the low-permeability surfaces, which at a critical discharge mobilizes loose sediment downslope into a debris flow. While a variety of conceptual and physically based models have been proposed for this initiation process, there is a lack of consensus regarding the mechanics by which water runoff can transform loose sediment into flowing debris. Here, we present a model to explain recent observations of this process at two field sites: a bedrock-dominated basin in central Colorado (Chalk Cliffs) and a recently burned watershed in southern California (Arroyo Seco). The mathematical model is based on the hypothesis that low-gradient sections of channel topography act as “sediment capacitors.” The sediment capacitors temporarily store incoming bedload transported by water flow and periodically release the accumulated sediment as a debris-flow surge. The model consists of a system of coupled 1-D equations for water flow, bedload transport, slope stability, and mass flow. Comparison with video of debris-flow initiation shows that the model reproduces the observed dynamics of the process. The model also generates similar patterns of surge magnitude and frequency to those observed at the two field sites. We systematically vary key parameters in the sediment capacitor model (such as rainfall intensity, slope, grain size, and sediment supply) to explore potential controls on debris-flow initiation and surge magnitude and frequency. These calculations provide theoretical guidance for explaining regional variations in empirical rainfall intensity thresholds developed for burned steeplands and sparsely vegetated alpine areas.