Looking through the window of disturbance at post-wildfire debris-flow hazards

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Abstract

The extreme heat from wildfire alters soil properties and incinerates vegetation, leading to changes in infiltration capacity, ground cover, soil erodibility, and rainfall interception. These changes promote increases in runoff and sediment transport that increase the likelihood of runoff-generated debris flows. Over a period of several years, referred to as the window of disturbance, the landscape recovers and wildfire-induced changes become less accentuated. Debris flows are most common in the year immediately following wildfire, but changes in the likelihood and magnitude of debris flows throughout the window of disturbance are not well constrained. Assessing debris-flow hazards throughout the post-wildfire recovery period is complicated, in part, by the myriad of wildfire-induced changes and their nonlinear relationships with sediment transport and runoff generation processes. In this study, we combine measurements of soil hydraulic properties with vegetation survey data and numerical modeling to understand how debris-flow threats are likely to change in steep, burned basins during the first two years of recovery. We focus on documenting recovery following the 2016 Fish Fire in the San Gabriel Mountains, CA, USA and demonstrate how a numerical model can be used to predict temporal changes in debris-flow properties and initiation thresholds within that region. Substantial increases in sorptivity, which represents the capillarity contribution to infiltration, and reductions in the percentage of bare soil occurred during the first 18 months following the Fish Fire. Numerical modeling suggests that these changes lead to a roughly 40% increase in the 15-minute rainfall intensity-duration threshold associated with debris-flow initiation as well as more than a three-fold decrease in debris-flow volume from post-fire year 1 to post-fire year 2. These results provide valuable constraints on changes in debris-flow thresholds within the San Gabriel Mountains as well as a general framework for exploring the impact of changing vegetation and soil hydraulic properties on debris-flow magnitude and susceptibility.

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Additional publication details

Publication type Conference Paper
Publication Subtype Conference Paper
Title Looking through the window of disturbance at post-wildfire debris-flow hazards
Volume 7
Year Published 2019
Language English
Publisher Mountain Scholar
Contributing office(s) Geologic Hazards Science Center
Description 8 p.
Larger Work Type Book
Larger Work Subtype Conference publication
Larger Work Title Debris-flow hazards mitigation : mechanics, monitoring, modeling, and assessment ; proceedings of the Seventh International Conference on Debris-Flow Hazards Mitigation
First page 516
Last page 523
Conference Title Seventh International Conference on Debris-Flow Hazards Mitigation
Conference Location Golden, CO
Conference Date June 10-13, 2019
Country United States
State California
Other Geospatial San Gabriel Mountains