A Screening-Level Hec-Hms Model For Post-Fire Runoff Emergency Assessment

Since 2000, wildfires have burned an annual average of 7 million acres, which is more than double the annual acreage burned in the 1990s. Wildfires pose extreme risk to life and property while burning which continues after they are extinguished as elevated flood and debris flow hazard. Changes to the landscape and soil properties after a wildfire, such as the absence of vegetation and the reduction in water infiltration, can cause significant increases in flood magnitude alongside the likelihood of dangerous debris flows during post-fire rain events. Hydrologic models help forecast dynamically changing risk to people and infrastructure downstream of burn scars.

In this study, the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) was used to develop a screening-level model of the South Fork of the Tule River watershed in California for post-fire runoff analysis. Post-fire flood events were simulated for a period after the Windy Fire, which occurred in September of 2021. Simulations of the 1/10, 1/25, and 1/100 AEP floods (10-, 25-, and 100-year storms) were run to predict changes in peak discharge and post-wildfire debris yield. For the highest burned subbasin, model results showed a pre- and post-fire difference in peak discharge of 1,100 cfs, 1,700 cfs, and 4,000 cfs and a post-fire sediment yield of 56,000 tons, 92,000 tons, and 220,000 tons for the 1/10, 1/25, and 1/100 AEP floods, respectively.

Evaluation of model parameterization will help inform future development of screening-level flood risk management models for post-wildfire clear water, sediment, and debris flow simulations to reduce response time in future emergency situations. The data, methods, workflows, and lessons learned during the development of this screening-level hydrologic analysis are currently being tested on additional watersheds. These resulting workflows will be automated to provide a quick starting point for field engineers and scientists who can then apply their regional skill for refined calibration and validation.