ESTIMATING POTENTIAL POST-WILDFIRE DEBRIS-FLOW CHARACTERISTICS IN MOUNTAINOUS AREAS OF COLORADO, USA

Debris flows resulting from rainfall on recently burned, rugged, forested areas create potential hazards to life, property, infrastructure, and water resources. The location, extent, and severity of wildfire and the subsequent storm intensity and duration cannot be known in advance; however, hypothetical scenarios based on empirical debris-flow equations are useful planning tools for conceptualizing potential post-wildfire effects and developing pre-wildfire mitigation strategies. A pre-wildfire study to determine the potential for post-wildfire debris flows in the upper Blue River watershed in Summit County, Colorado, was conducted to provide the Town of Breckenridge with a relative measure of subbasins that might constitute the most serious debris-flow hazards and threats to the municipal water supply. Potential post-wildfire debris-flow probabilities and volumes for 51 subbasins were estimated by using the empirical equations of Cannon and others (2010, GSA Bul., v. 122, p. 127-144) which are based on watershed geomorphic characteristics, soil properties, forest cover and fire extent, and precipitation characteristics.

The upper Blue River debris-flow model simulations assumed that a moderate to severe wildfire affected 100 percent of the forest and shrub areas within the subbasins. Three post-wildfire precipitation scenarios were used to represent a range of likely precipitation scenarios that could occur shortly after a wildfire: 1-hour duration rainstorms of 2-, 10-, and 25-year recurrence intervals. All rainfall scenarios predicted debris flows from the hypothetically burned subbasins. Subbasins with the highest post-wildfire debris-flow probabilities tended to be steep, heavily forested, and have relatively small drainage areas. Subbasins with the highest estimated debris-flow volumes were those with the largest drainage areas. The subbasins with the greatest potential post-wildfire hazards are those with both high probabilities of debris-flow occurrence and large estimated volumes of debris-flow material. Evidence of previous debris flows was subsequently found in subbasins predicted to have the highest probability of post-wildfire debris flows validating use of the empirical equations for predictive purposes.