The Central California coast, a region characterized by active tectonics and steep terrain, is well known for producing destructive and hazardous debris flows during rainstorms. A new study led by the U.S. Geological Survey sheds light on how wildfire influences debris-flow susceptibility in the region.

A team of researchers from the USGS and partnering institutions conducted the study, “Postfire hydrologic response along the Central California (USA) coast: insights for the emergency assessment of postfire debris-flow hazards”, in the aftermath of atmospheric-river storms that brought intense rainfall across multiple burn areas in Central California in early 2021. The storms provided a unique opportunity to document the occurrence of postfire debris flows, offering valuable insights into the impact of wildfires on the region's hydrologic response. 

A debris flow is a fast-moving mixture of water, rock, soil, and vegetation that moves downhill. Wildfires can increase the likelihood of debris flows during heavy rainfall events by removing vegetation, killing roots, and exposing soil, which can destabilize sediment on hillslopes and in ravines. 

The team evaluated the predictive performance of the U.S. Geological Survey M1 likelihood model, a tool currently used for emergency assessment of post-fire debris-flow hazards in the western United States. By comparing the model's predictions with real-world data from the 2021 storms, they sought to determine its effectiveness in accurately assessing post-fire debris flows.

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The study's findings indicate that the M1 model shows potential for predicting the likelihood of post-fire debris-flow hazards along the central California coast. However, additional field-verified inventories of post-fire hydrologic responses will be essential to refine the model further, particularly in understudied, fire-altered regions of northern California and the Pacific Northwest. 

With wildfire activity on the rise throughout mountainous areas of the USA, understanding the relationship between wildfires and debris-flow susceptibility has become increasingly vital. Studying post-fire debris-flows on the steep, tectonically active central California coast can yield insights into how these events may unfold elsewhere. 

"As wildfires continue to impact the landscapes of central California and beyond, we need accurate tools to assess and predict the potential hazards posed by post-fire debris flows, which can occur within minutes of heavy rainfall,” said Matthew Thomas, USGS Research Hydrologist and the lead author of the study. “Characterizing the link between wildfires, meteorological events, and debris flows can inform regional hazard assessments.”