Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

Wildfire-induced changes to soil and vegetation promote runoff-generated debris flows in steep watersheds. Postfire debris flows are most commonly observed in steep watersheds during the first wet season following a wildfire, but it is unclear how long the elevated threat of debris flow persists and why debris-flow potential changes in recovering burned areas. This work quantifies how rainfall intensity-duration (ID) thresholds for debris-flow initiation change with time since burning and provides a mechanistic explanation for these changes. We constrained a hydrologic model using field and remotely sensed measurements of soil-infiltration capacity, vegetation cover, runoff, and debris-flow activity. We applied this model to estimate rainfall ID thresholds for debris-flow initiation within three burned areas in the southwestern United States over a postfire recovery period of three to four years. Modeling suggests ID thresholds are lowest immediately following the fire (below a one-year recurrence interval [RI] storm) and increase with time, such that a 10- to 25-year RI storm would be required to generate a debris flow after three years of recovery. Modeled changes in rainfall ID thresholds result from increases in soil infiltration capacity, canopy interception, hydraulic roughness, and median grain size of sediment entrained in an incipient debris flow. The relative importance of each of these factors varied among our three sites. Results improve our ability to assess temporal changes in postfire debris-flow potential, highlight how site-specific factors may alter the persistence of postfire debris-flow hazards, and provide additional constraints on the timescale of recovery following wildfire. © 2021. The Authors.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Movement of Sediment Through a Burned Landscape: Sediment Volume Observations and Model Comparisons in the San Gabriel Mountains, California, USA

Post-wildfire changes to hydrologic and geomorphic systems can lead to widespread sediment redistribution. Understanding how sediment moves through a watershed is crucial for assessing hazards, developing debris flow inundation models, engineering sediment retention solutions, and quantifying the role that disturbances play in landscape evolution. In this study, we used terrestrial and airborne lidar to measure sediment redistribution in the 2016 Fish Fire, in the San Gabriel Mountains in southern California, USA. The lidar areas are in two adjacent watersheds, at spatial scales of 900 m2 to 4 km2, respectively. Terrestrial lidar data were acquired prior to rainfall, and two subsequent surveys show erosional change after rainstorms. Two airborne lidar flights occurred (1) 7 months before, and (2) 14 months after the fire ignition, capturing the erosional effects after rainfall. We found hillslope erosion dominated the overall sediment budget in the first rainy season after wildfire. Only 7% of the total erosion came from the active channel bed and channel banks, and the remaining 93% of eroded sediment was derived from hillslopes. Within the channelized portion of the watershed erosion/deposition could be generally described with topographic metrics used in a stream power equation. Observed sediment volumes were compared with four empirical models and one process-based model. We found that the best predictions of sediment volume were obtained from an empirical model developed in the same physiographic region. Moreover, this study showed that post-wildfire erosion rates in the San Gabriel Mountains attain the same magnitude as millennial time scale bedrock erosion rates. © 2021. The Authors. This article has been contributed to by US Government employees and their work is in the public domain in the USA.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Postwildfire Soil-Hydraulic Recovery and the Persistence of Debris Flow Hazards

Deadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. We develop a simulation-based framework to quantify changes in the hydrologic triggering conditions for debris flows as postwildfire infiltration properties evolve through time. Our approach produces time-varying rainfall intensity-duration thresholds for runoff- and infiltration-generated debris flows with physics-based hydrologic simulations that are parameterized with widely available hydroclimatic, vegetation reflectance, and soil texture data. When we apply our thresholding protocol to a test case in the San Gabriel Mountains (California, USA), the results are consistent with existing regional empirical thresholds and rainstorms that caused runoff- and infiltration-generated debris flows soon after and three years following a wildfire, respectively. We find that the hydrologic triggering mechanisms for the two observed debris flow types are coupled with the effects of fire on the soil saturated hydraulic conductivity. Specifically, the rainfall intensity needed to generate debris flows via runoff increases with time following wildfire while the rainfall duration needed to produce debris flows via subsurface pore-water pressures decreases. We also find that variations in soil moisture, rainfall climatology, median grain size, and root reinforcement could impact the median annual probability of postwildfire debris flows. We conclude that a simulation-based method for calculating rainfall thresholds is a tractable approach to improve situational awareness of debris flow hazard in the years following wildfire. Further development of our framework will be important to quantify postwildfire hazard levels in variable climates, vegetation types, and fire regimes. © 2021. The Authors. Journal of Geophysical Research: Earth Surface published by Wiley Periodicals LLC on behalf of American Geophysical Union. This article has been contributed to by US Government employees and their work is in the public domain in the USA.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Extreme Precipitation Across Adjacent Burned and Unburned Watersheds Reveals Impacts of Low Severity Wildfire on Debris-Flow Processes

In steep landscapes, wildfire-induced changes to soil and vegetation can lead to extreme and hazardous geomorphic responses, including debris flows. The wildfire-induced mechanisms that lead to heightened geomorphic responses, however, depend on many site-specific factors including regional climate, vegetation, soil texture, and soil burn severity. As climate and land use change drive changes in fire regime, there is an increasing need to understand how fire alters geomorphic responses, particularly in areas where fire has been historically infrequent. Here, we examine differences in the initiation, magnitude, and particle-size distribution of debris flows that initiated within the area burned by the 2019 Woodbury Fire in central Arizona, USA, and those that initiated in a nearby unburned area. Despite similar rainfall intensities, unburned watersheds were less likely to produce debris flows. Debris flows in unburned areas initiated from both runoff and shallow landslides, while debris flows only initiated from runoff-related processes in the burned area. The grain-size distribution making up the matrix of debris-flow deposits within the burned area generally had a lower ratio of sand to silt relative to debris flows that initiated in the unburned area, though there were no systematic differences in the coarse fraction of debris-flow sediment between burned and unburned areas. Results help expand our ability to predict postwildfire debris-flow activity in a wider range of settings, specifically the Sonoran Desert ecoregion, and provide general insight into the impact of wildfire on geomorphic processes in steep terrain. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 10 March 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]