WATER AVAILABILITY AND DEBRIS-FLOW GENERATION IN BURNED AREAS

This work demonstrates the hypothesis that, during the first one to two years after a fire, debris flows are rainstorm-intensity limited and not rainstorm-volume limited. That is, it is common to generate more than enough water, but to produce a debris flow the water must be delivered in sufficiently large doses. The volume-independence is shown by a dataset of 46 debris flows from eight burned areas in California, Colorado, and Utah. Assuming that a debris flow is composed of 40% water and 60% solids, these events were generated during rainstorms that produced an average of 14 times as much water as necessary to develop a debris flow. Furthermore, when infiltration is accounted for, the rainstorms still generated an overabundance of water. Intensity-dependence is shown by a number of cases where the exact timing of debris flows can be pinpointed and are contemporaneous with high intensity bursts of rainfall. This dependence has been previously shown for certain types of non-fire-related debris flows, but the vulnerability of runoff-initiated flows in burned terrain is even stronger. Finally, a number of cases are shown where high volume storms without high intensity bursts do not generate debris flows.