THE SPATIAL DISTRIBUTION OF POST-FIRE DEBRIS FLOWS IN RELATION TO OBSERVED RAINFALL ANOMALIES

A range of hydrologic responses can be observed in steep, recently-burned terrain, but predicting the spatial distribution of large debris flows remains challenging. Studies from rainfall-induced landslides in unburned areas show evidence of hydroclimatic tuning of landslide triggering, where the spatial distribution of events is better predicted by the observed rainfall anomaly relative to climatic norms than by absolute rainfall. In this study, we test whether the spatial distribution of debris flows in response to rainfall can be similarly predicted by rainfall anomaly. The Dolan Fire burn scar in Monterey County, California, spans a sharp hydroclimatic gradient and experienced a widespread debris-flow triggering storm in January 2021, providing an ideal natural experiment in which to test the predictive power of rainfall anomaly versus absolute rainfall. We use remote and field methods to map debris-flow response and examine its spatial heterogeneity across the study area. Together with rainfall data, our mapping reveals that the observed anomalies in peak 15-minute rainfall intensity relative to the intensity of the 1-year return interval storm predict debris-flow occurrence more effectively than the absolute peak I₁₅. While such a pattern is particularly noticeable at the Dolan Fire due to the combination of a sharp hydroclimate gradient and widespread rainfall, we also present similar evidence from several other burned areas in Northern California. Our findings suggest that debris-flow processes and threshold rainfall required for debris-flow initiation may be tuned to local hydroclimate, with implications for rapid post-fire debris-flow prediction and reconnaissance, even in the face of limited burn severity data.