Modelling post-wildfire debris-flow inundation in the Southwestern United States

Abstract:

The impacts of post-wildfire debris flows are becoming more prevalent due to increases in fire frequency and the expansion of the wildland-urban interface across the western United States. There are established methods used to rapidly assess post-wildfire debris-flow likelihood and volume, but there is a need to develop similar methods to assess the downstream effects of these flows. While many process-based debris flow models already exist, they may require detailed information about flow constituents and are computationally expensive, making them impractical for the purpose of rapid hazard assessments over large areas. Here we present a computationally efficient model, which is composed of a series of empirical equations coupled with a flow-based routing algorithm, that can be used to provide estimates of debris flow inundation extent downstream of multiple watersheds (1-10 km²) in several minutes. Inputs to the model include a digital elevation model, debris flow volume, and an initiation point (which can be located at any point upstream of areas of deposition). Flow mobility and lateral spreading are controlled by topography, volume, and two user-specified parameters for flow viscosity and yield strength. We tested the model by comparing simulated inundation limits with the extent of observed debris-flow deposits at three sites: the 2010 Schultz Fire (AZ), 2016 Fish Fire (CA), and 2017 Thomas Fire (CA). Model performance was assessed with a similarity metric that quantifies the overlap between modeled and observed deposits. The similarity metric varies from -1 (complete mismatch) to 1 (complete agreement). Results suggest the model can reproduce the observed inundation extent at all three sites (similarity metric between 0.3-0.5), which is particularly encouraging given differences in climate and lithology among the three study areas. A better understanding of post-wildfire debris-flow inundation will help mitigate the negative impacts of future fires.