EVALUATION OF BULKED-FLOW MULTIPLIERS USED DURING EMERGENCY RESPONSE EFFORTS TO ESTIMATE POST-FIRE PEAK DISCHARGE ACROSS A RANGE OF HYDROGEOMORPHIC CONDITIONS, CALIFORNIA (Invited Presentation)

Wildfire can radically alter watershed processes resulting in increased risks to life, property and infrastructure from flash floods and debris flows, particularly within the first two years following wildfire. However, the magnitude of post-fire (PF) flows is difficult to predict because necessary data on soil properties, surface roughness, and canopy cover, are often lacking. Consequently, the current state of practice used by emergency response agencies to estimate PF flood and debris-flow hazards is to adjust pre-fire flow estimates using a simple bulked-flow multiplier (BFM) to account for increases in PF runoff plus sediment and debris entrainment. Current BFMs, defined as the ratio of PF to pre-fire peak runoff, are a function of several factors including watershed size, burn severity, sediment and debris availability, and the storm recurrence interval (RI) of interest. BFMs applied by both federal and state response teams in California typically do not exceed 3 for PF floods and 10 for PF debris flows in the first two years following fire. To test the accuracy of current methods used to predict peak PF flows, we quantified the rainfall and runoff response in watersheds burned at moderate to high severity across a range of hydrogeomorphic provinces throughout California using a combination of rain gages, stream gages, and ground-based reconnaissance. Rainfall and runoff data from events in the first two years following fire were compiled and peak BFMs were calculated using pre-fire flow estimates developed using regional regression equations roughly matched to the rainfall RI. Preliminary measured BFMs for PF flood flows were comparable to values obtained with existing methods. However, BFMs for debris flows were much higher with an interquartile range between 10 and 50, with the highest values concentrated in southern California. Although additional test cases are needed to evaluate the statistical consistency of the BFMs we present, the results illustrate the need to advance PF research across geomorphic provinces with diverse hydrogeomorphic conditions. Information obtained from such studies will improve our knowledge on PF runoff response to better assess downstream impacts to life, property, and natural resources, and inform the development of best management mitigation practices and techniques.