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Post-Wildfire Debris Flow Risk Assessment of Colorado and Utah Watersheds

As part of the Strategic Alliance for Risk Reduction II (STARR II), we sought to understand how risks to life and economic costs change under post-fire conditions and during hydrologic recovery. Focusing on three recently burned watersheds in Utah and Colorado¡ªGrizzly Creek, Cameron Peak, and Jacob City¡ªa study was initiated assessing approximately 621 square miles (1,000 square kilometres) for risks and conditions one, three, and five years after the disaster.

Previous studies had indicated that debris flow hazards are dramatically higher during the first year following a wildfire, with rapid watershed recovery thereafter. Our study corroborated this view, highlighting the heightened risks immediately after a fire, and we developed annual hazard debris flow and area-based probability-intensity curves.

Our team modeled hundreds of thousands of debris flows across multiple scenarios and cataloged risk elements such as roads and buildings to assess quantitative risk based on probability and impacts. Each site received both a quantitative and a classified risk result. To address post-fire debris flow risk across large study areas, the team implemented new methods including the validation of hazard assessments against subsequent debris flow events.

STARR II personnel have been invited to present these findings to professional groups, write results for publication, and hold workshops for technical team members.