Monte Carlo Simulation of Post-Fire Flood Hazard Probabilities

Significant portions of the headwaters of the Cache La Poudre River in northern Colorado were burned during the Cameron Peak Fire in 2020, burning a total of 200,000 acres. In 2012, the High Park Fire burned 85,000 acres at lower elevations within the basin. As large wildfires become more frequent across the western U.S., the resulting landcover changes are expected to have significant effects on flood risk in the impacted watersheds. Hydrologic models used for flood risk estimates and real-time flood risk management are typically calibrated to historical data, and may not reflect changes to basin conditions. Therefore, adjustments to the hydrology model parameters are necessary when performing post-wildfire risk assessments. The challenge is model parameter adjustment is not straightforward due to the dependency on burn severity and watershed characteristics. In addition, there is limited guidance on how much parameter adjustment may be necessary to accurately capture the post-wildfire enhanced peak flow and volume changes. Using flood events before and after the High Park Fire, we attempted to assess flow enhancement due to wildfire effects by modeling pre- and post-wildfire conditions from the High Park fire in the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS), incorporating snowmelt and rain-on-snow hydrology. The calibrated pre- and post-fire HEC-HMS models were applied in a Monte Carlo simulation using HEC’s Watershed Analysis Tool’s (HEC-WAT) Flood Risk Analysis mode to study changes in flow enhancement over a range of precipitation return intervals. The Monte Carlo simulation samples precipitation frequency, storm shapes, and hydrologic model parameters such as antecedent soil moisture and snowpack conditions in order to capture both natural variability and knowledge uncertainty of flood risk. The results of this study may be used to better assess post-wildfire flood risk in this basin and demonstrate methods to re-parameterize HEC-HMS models under post-fire conditions for planning and real-time applications.