CRANK IT UP: AMPLIFICATION OF POST-WILDFIRE PEAK RUNOFF BY DEBRIS

In burned steeplands, the peak discharge of post-wildfire water runoff can substantially increase from the addition of debris. Yet, methods to estimate this increase are lacking. We quantify the potential amplification of peak stage and discharge using new video observations of post-wildfire runoff, compiled data on post-wildfire peak flow (Q_p), and a physically based model. Comparison of flood and debris-flow data with similar distributions in drainage area (A) and rainfall intensity (I) show that the median runoff coefficient (C = Q_p/A I) of debris flows is 35 times greater than that of floods. The striking increase in Q_p can be explained using a fully predictive model, which is adapted from Hungr [Earth Surf. Process. Landforms, 2000]. This model describes the additional flow resistance caused by the emergence of coarse-grained surge fronts present in our video observations. The high friction within the granular front creates a moving dam that increases the flow depth of water and sediment supplied from upstream. The model, or a complementary empirical equation, can be used to make quantitative predictions of peak discharge needed for assessing post-wildfire hazards and planning mitigation.