Assessment of Landslide and Debris-Flow Impacts from California Wildfires
Wildfire increases the susceptibility of steep slopes to landslides and debris flows. A tragic recent example occurred in Montecito, California, where heavy rainfall on the Thomas fire triggered debris flows that killed 23 people and damaged or destroyed over 400 homes.
Wildfire-affected landslide assessment: tasks and benefits 2019
Debris-flow hazard following major wildfires in California
Steep drainage basins recently burned by wildfire can generate deadly and destructive debris flows. To help reduce losses from this type of landslide, the USGS delivers debris-flow hazard assessments following major wildfires in the western United States. The assessments are used by federal and state land and emergency management teams who evaluate and communicate immediate post-fire risk. The assessments are also used by the National Weather Service (NWS) to guide debris-flow alerts delivered as part of their Flash Flood Warnings. The 2019 Disaster Supplemental Appropriation (P.L. 116-20) has enabled progress to close two key gaps identified by stakeholders in assessing the post-fire debris-flow hazard: (1) mapping where debris flows travel and end up, and (2) defining how long the debris-flow hazard will persist. Targeted data collection and studies are leading to new operational capabilities that will improve post-fire risk assessment and early warning by the USGS and its partners, the need for which is expected to increase (Kean and Staley, 2021).
Most landslide hazard maps only portray areas of the landscape where landslide begin. However, the paths they travel and where they end up is often well downslope or down channel. These areas are also often the locations of people, building, and lifelines; hazard assessments describing landslide travel and deposition are needed to assess risk. Detailed mapping following the Thomas Fire and the 2018 Montecito, California, debris-flow disaster provided the means to rigorously test available debris-flow inundation models in a high-performance computing environment. Findings show that reducing uncertainty in the debris-flow volume is critical for improved estimates of hazard guiding future data collection priorities (Barnhart and others, 2021a). The increased capacity and experience gained with debris-flow inundation models was also used to evaluate the tsunami threat from a very large coastal landslide newly identified in Prince William Sound, Alaska (Barnhart and others, 2021b). Congressionally directed work in coastal Alaska along with a budget increase have allowed the USGS Landslide Hazards Program to sustain some of the capacity gained in this area.
Recovery of vegetation after wildfire reduces runoff and debris-flow generation; however, the duration and evolution of the hazard varies with a range of physical, ecological, and climate factors. Partners at the University of Arizona have assessed how the return period of debris-flow initiating rainfall increases from about one-year immediately after the fire to more than 10 years following four seasons of vegetation growth (Hoch and others, 2021). These findings, combined with modeling studies exploring the changes in soil hydraulic properties with time after wildfire, provide improved rainfall thresholds used by the National Weather Service to issue debris-flow watches and warnings for burn areas (McGuire and others, 2021; Thomas and others, 2021). Growth in the capacity to model debris-flow inundation, combined with new data and methods for updating debris-flow hazard maps and rainfall thresholds, allow the USGS to better address the growing need to evaluate and address post-fire impacts and risk by a wide range of stakeholders including federal, tribal, state, and local land and emergency managers.
Return to 2019 Supplemental Appropriations Activities.
Additional Resources:
Lancaster, J.T., Swanson, B.J., Lukashov, S.G., Oakley, N.S., Lee, J.B., Spangler, E.R., Hernandez, J.L., Olson, B.P.E., DeFrisco, M.J., Lindsay, D.N., Schwartz, Y.J., McCrea, S.E., Roffers, P.D., and Tran, C.M., 2021, Observations and Analyses of the 9 January 2018 Debris-Flow Disaster, Santa Barbara County, California: Environmental and Engineering Geoscience, v. 27, no. 1, p. 3-27.
Sidder, A., 2021, The wildfire one-two: First the burn, then the landslides, Eos, v. 102, 22 June 2021.