POST-FIRE RUNOFF RESPONSE ACROSS MULTIPLE HYDROGEOMORPHIC PROVINCES IN CALIFORNIA: A LOOK AT USING PEAK BULK-FLOW MULTIPLIERS TO ESTIMATE FLOW TYPE

Wildfire increases the likelihood of flash floods and debris flows in mountainous regions by promoting increases in runoff and surface water velocity which increase the entrainment of sediment and debris. The likelihood of post-fire floods and debris flows is often greatest within the first two years following wildfire and can lead to loss of life and substantial damage to property and infrastructure. The current state of practice used by emergency response agencies to estimate post-fire flood and debris-flow hazards is to adjust pre-fire clearwater flow estimates using a bulked-flow multiplier to account for increases in sediment and debris entrainment. To evaluate this approach and improve our understanding of post-fire flooding and runoff-induced debris flows, we initiated a study to quantify the rainfall and runoff response in watersheds across a range of hydrogeomorphic provinces throughout California that burned at moderate to high soil burn severity using a network of rain gages, stream gages, and ground-based reconnaissance. Rainfall and stream gage data from runoff events in the first year following fire were compiled and compared to the flow type (e.g., flood flow, hyperconcentrated flow, and debris flow) that occurred. Results show that for mostly one- to two-year recurrence interval storms, the bulked-flow multipliers, which are calculated as the ratio of the measured peak discharge to the theoretical maximum clearwater flow based on the Rational Method, are typically less than one for clearwater flows, between one and two for hyperconcentrated flows, and greater than two for debris flows. Moreover, bulked-flow multipliers calculated as the ratio of the measured peak event discharge to the pre-fire discharge using regional regression equations with a two-year return interval generally showed an order of magnitude increase, with ratios less than 10 associated with clearwater flows, ratios between 10 and 20 associated with hyperconcentrated flows, and ratios greater than 20 associated with debris flows. We conclude that the bulked-flow multiplier approach provides a reasonable estimate of flow type for our initial test cases based on comparisons with field observations, but additional test cases are needed to evaluate the statistical consistency of the ratios we present here.