Simulated riverbed response to post-wildfire debris flow input in river networks

Post-wildfire erosion presents threats to watersheds beyond direct risks from fires, including increased erosion and delivery of sediment to stream channels. Excessive sediment loading can increase flood risk, alter aquatic habitat, and/or be transported downstream to impact water supply reservoirs. Murphy et al. (2019) developed a novel framework to predict post-wildfire sediment generation and sediment impacts downstream from burned areas by linking debris flow generation and network-scale sediment routing models. We advance and apply the coupled framework to evaluate critical water supply reservoirs in the Wasatch Range above Salt Lake City, Utah. Post-wildfire debris flow generation is predicted using a probabilistic model for the western U.S. (Staley et al., 2017), coupled with an empirical model to predict debris flow volumes (Gartner et al., 2014). Debris flow sediment delivery to the stream network is estimated using a geometric model (Murphy et al., 2019). Grain size distributions (GSDs) of post-fire debris flows are based on data compiled from the literature and recent field surveys conducted across Utah (Wall et al., 2021). Finally, the primary focus of this work is to describe the preliminary results from a network-based Lagrangian sediment transport model used to route debris flow sediment through the river networks (Czuba, 2018). Model outputs provide transport time of sediment, accumulated sediment volume and GSD within the network, and reservoir sedimentation at each timestep. Comparison with estimates of reservoir sedimentation rates from recent bathymetric surveys enables field verification of our pre-fire model predictions. In each watershed, we then simulated fluvial sediment routing for 10-years after modeled wildfire and debris flow generation, and observe a considerable time lag between the occurrence of a wildfire far upstream of a reservoir and the subsequent impact to reservoir storage downstream. This ongoing work reveals that due to coarse sediment transport time lags, the timing of reservoir impacts may exceed typical post-wildfire monitoring timescales (1-2 years). This modeling study will improve the current understanding of downstream sediment impacts due to post-wildfire erosion, enhance hazard assessments, and inform forest, fire, and water management.