Post-wildfire debris-flow runout mapping using geospatial analysis

Post-wildfire debris flows are a significant natural hazard that is becoming more prevalent because of climate change and increasing populations in the wildland-urban interface. In the United States, the US Geological Survey (USGS) conducts regular assessments of debris-flow probability and volume following major wildfires, but these assessments do not include estimates of potential runout length, which is needed for identifying infrastructure potentially at risk from post-wildfire debris flows. Runout can be simulated by physically based dynamic models, but these models require detailed local site information that necessitates field measurements. As an alternative, topographically based runout models, such as the Average Channel Slope (ACS) model, can be used to estimate the likely maximum runout length of debris flows using widely available existing geospatial datasets. Here, a geographic information systems (GIS)-based modeling workflow was developed to run the ACS model for rapid assessment of post-wildfire debris-flow runout. Modeled runout length was tested against 34 observed debris flows across six US regions, with regionally averaged modeled runout ranging from 86 to 107% of observed runout. The model was then used to estimate potential runout length for 16 recent wildfires in the western US and the results were displayed in an interactive web map. The code used to run the model is freely available, allowing for rapid assessment of debris-flow hazards following wildfire.