Insights from a 25-year database of post-fire debris flows in California

Background Post-wildfire debris flows (PFDFs) frequently threaten life, property and infrastructure in California. To date, there is no comprehensive assessment of their spatial distribution, seasonality, atmospheric drivers and interannual variability across the state. Aims We develop a database of PFDF events in California for the period 2000–2024 and analyze the database to describe spatial and temporal variability and impacts of PFDFs. Methods We use peer-reviewed literature, media and agency reports to compile the PFDF event database and various meteorological sources to classify events by storm type. Key results and conclusions We identify 97 PFDF events producing 595 individual PFDFs; events occur predominantly in the Transverse Ranges and the Sierra Nevada. There is high interannual variability in PFDF events. Event frequency tends to be greatest following years with well above-average area burned. PFDF events occur predominantly in the cool season (October–May) and 55% are associated with atmospheric rivers. Approximately 31% of PFDF events occur in the warm season (June–September) associated with the North American Monsoon, tropical cyclones and other thunderstorms. Implications Improved understanding of where, when and why PFDFs occur supports hazard planning and mitigation efforts and allows us to track changes in a warming climate.