No abstract

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov/ or call 1-888-ASK-USGS (1-888-275-8747).For an overview of USGS information products, including maps, imagery, and publications, visit http://store.usgs.gov/.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. AbstractWildfire can significantly alter the hydrologic response of a watershed to the extent that even modest rainstorms can generate dangerous flash floods and debris flows. To reduce public exposure to hazard, the U.S. Geological Survey produces post-fire debris-flow hazard assessments for select fires in the Western United States. We use publicly available geospatial data describing basin morphology, burn severity, soil properties, and rainfall characteristics to estimate the statistical likelihood that debris flows will occur in response to a storm of a given rainfall intensity. Using an empirical database and refined geospatial analysis methods, we defined new equations for the prediction of debris-flow likelihood using logistic regression methods. We showed that the new logistic regression model outperformed previous models used to predict debris-flow likelihood.

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff‐generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity‐duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post‐wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris‐flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post‐wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post‐wildfire debris‐flow studies in Arizona only constrain the peak rainfall intensity within debris‐flow‐producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h−1 lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15‐min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris‐flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd.

Wildfire alters the hydrologic and geomorphic responses of burned areas relative to nearby unburned areas, making them more prone to runoff, erosion, and debris flow. In post-wildfire settings, debris flows often initiate when runoff concentrates on steep slopes and rapidly mobilizes sediment. Rainfall intensityduration (ID) thresholds have been proven useful for assessing post-fire debris-flow potential but can vary substantially from one location to another as a result of hydrologic factors that control rainfall-runoff partitioning. Debris-flow initiation thresholds based on a slope-dependent dimensionless discharge criterion, which have the theoretical benefit of being consistent from site to site, have also been proposed but not extensively tested. We monitored debris-flow activity in 12 small (< 1 km 2 ) watersheds burned by the 2018 Buzzard Fire in New Mexico, USA, documenting 24 debris flows during the first several months following the wildfire. We use a recently proposed dimensionlessdischarge threshold in combination with rainfall-runoff modeling to estimate basin-specific rainfall ID thresholds for debris-flow initiation. These model-derived thresholds compare well with observations. Areas burned at low severity are characterized by higher infiltration capacity, rainfall interception, and hydraulic roughness relative to areas burned at moderate or high severity, but differences in rainfall ID thresholds between these two areas can be predominantly attributed to wildfire-induced changes in hydraulic roughness. Results highlight the utility of thresholds based on dimensionless discharge relative to those based on rainfall intensity and also provide additional data that will help constrain general models for the prediction of rainfall ID thresholds.