16-8 Improving estimates of postfire debris-flow hazard across the western United States

Session: Wildland Fire: An Agent of Geomorphic, Ecologic, and Societal Change

Presenting Author:

Andrew Graber

Authors:

Graber, Andrew P¹, Selander, Brittany D², Barnhart, Katherine R³, Gorr, Alexander N⁴, Kean, Jason W⁵, King, Jonathan M⁶, Kostelnik, Jaime⁷, Rengers, Francis K⁸, Thomas, Matthew A⁹

Abstract:

The United States Geological Survey (USGS) uses a suite of empirical models to rapidly assess hazards from debris flows after wildfire in the western United States. One of these models (the “M1” model) is a logistic regression equation that uses topographic, burn severity, and soil information to predict debris-flow likelihood and identify quantitative rainfall thresholds that can be used as guidance for warning and decision support. The M1 model was calibrated to a debris-flow inventory from southern California (USA) and has been applied throughout the western United States. But, limited spatial coverage in the calibration dataset motivates evaluation of M1 model accuracy when applied outside of the calibration region (e.g., the Sierra Nevada or the eastern Cascade Range, USA). Previous test cases showed that M1 overpredicts debris-flow likelihood and underpredicts rainfall thresholds for some locations (e.g., Arizona, Colorado, USA). We sought to improve the regional applicability of a debris-flow likelihood model by expanding the debris-flow inventory used for calibration, testing multiple versions of M1-like models, and generating a revised model. The new inventory includes debris-flow data from more locations paired with a rainfall metric that normalizes recorded rainfall intensity for each debris-flow observation by the relevant 1-year recurrence interval storm to account for regional variability in the rainfall intensities associated with debris-flow triggering. The revised model predicts debris-flow likelihood and estimates rainfall thresholds with better performance than the original M1 model, based on comparisons of model predictions to observed rainfall and debris-flow activity. These predictions result in reduced false positives relative to M1 and rainfall thresholds that are better aligned with relative differences in regional climatology and debris-flow activity.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6538

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