THE WILDFIRE - DEBRIS FLOW CONNECTION IN THE U.S. INTERMOUNTAIN WEST

Wildfire can have profound effects on the hydrologic response of a watershed, and debris flows are among the most hazardous consequences of rainfall on burned hillslopes. The debris-flow response of burned terrains differs significantly from that of undisturbed landscapes in several ways. Data from 210 recently burned debris-flow producing basins located throughout the U.S. Intermountain West show that the great majority of debris-flow events occur within the two years immediately following wildfires and in response to low-recurrence (<2 to 10 year), short-duration (<1 hour) storms. Most debris flows triggered in response to these storms initiate through a process of progressive bulking of storm runoff with sediment eroded from hillslopes and primarily, channels. Although some landslide-initiated debris flows can occur in burned basins, these most frequently occur in response to long-recurrence rainfall events, and can contribute just a small proportion to the total volume of material transported from the basin. We developed a GIS-based approach for evaluating potential debris-flow hazards from basins burned by wildfire in the U.S. Intermountain West. The approach relies on data that is readily available immediately after a fire, is used to identify those basins most likely to produce debris flows, and to estimate the magnitude of the possible debris-flow response at the outlets of the basins. A logistic multiple-regression model describes the probability of debris-flow occurrence from individual basins as a function of the percentage of the basin area burned at high and moderate severities, the abundance of clay in the soils, the soil liquid limit, the percentage of the basin with slopes greater than or equal to 30%, the basin ruggedness, and the average storm-rainfall intensity. A separate multiple-regression model estimates debris-flow peak discharge at basin outlets as a function of the area of the basin with slopes greater than or equal to 30%, the area of the basin burned, and average storm-rainfall intensity. We evaluated the effectiveness of the approach using information from the 2003 Hot Creek Fire in Central Idaho and its response to a storm on August 3, 2003. This approach can be used to focus mitigation efforts and aid in decisions about emergency warnings and evacuations.