The Rainfall Intensity‐Duration Control of Debris Flows After Wildfire

• Published in: Geophysical research letters Vol. 50; no. 10
• Main Authors: Thomas, Matthew A., Lindsay, Donald N., Cavagnaro, David B., Kean, Jason W., McCoy, Scott W., Graber, Andrew P.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-05-2023; Wiley
• Subjects: Accumulation; Debris flow; Detritus; Duration; Fires; flooding; Hazard assessment; hillslope hydrology; postfire hazards; Precipitation; Rain; Rainfall; Rainfall intensity; Rainfall measurement; Rainfall rate; Rainstorms; Runoff; Slurries; Storm rainfall; Storms; Thunderstorms; Wildfires; United States > US; California
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL103645

Increased wildfire activity in the western United States has exposed regional gaps in our understanding of postfire debris‐flow generation. To address this problem, we characterized flows in an unstudied area to test the rainfall intensity‐duration control of the hazard. Our rainfall measurements and field observations from the northern Sierra Nevada (California, USA) show that debris flows resulted from a short burst rainfall during a low‐accumulation storm. In contrast, a much higher accumulation storm (∼10 times more rainfall) with lower short‐duration rainfall rates only produced low‐hazard flooding. We conclude that total storm rainfall is not an ideal metric for identifying the rainfall conditions that initiate runoff‐generated debris flows in the first year after wildfire. Rather, a focus on short‐duration (<1 hr), high‐intensity rainfall that can occur during localized thunderstorms, or bands of intense rainfall during prolonged rainstorms, is more beneficial for the purposes of hazard assessment and warning.
Plain Language Summary
The rainfall conditions that trigger fast‐moving slurries of water and debris (or “debris flows”) after wildfire have been studied extensively in southern California. However, the transferability of these concepts to areas further north along the western margin of the United States in regions that are being affected by increased fire activity is unclear. We present rainfall measurements from contrasting storm types that affected an area burned by the 2021 Dixie Fire in northern California and describe the resultant postfire flows, which ranged from nuisance water‐rich flows to large and dangerous debris flows. Short bursts of rainfall, as opposed to prolonged (multiday) periods of rainfall, initiated the most hazardous flows. We conclude that rainfall intensity measured over short durations, not storm rainfall total, is the most useful measure of rainfall for identifying runoff‐induced debris‐flow hazard potential in the first year after fire.
Key Points
Postfire debris flows described in region where they were previously undocumented
Flow type and sediment yield controlled by differences in short‐duration rainfall rates
Debris‐flow initiation not sensitive to storm rainfall totals in first year after fire