Estimated probabilities and volumes of postwildfire debris flows, a prewildfire evaluation for the upper Blue River watershed, Summit County, Colorado

Abstract

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 rainfall intensity and duration cannot be known in advance. However, hypothetical scenarios based on empirical debris-flow models are useful planning tools for conceptualizing potential postwildfire effects. A prewildfire study to determine the potential for postwildfire debris flows in the upper Blue River watershed in Summit County, Colorado, was conducted in 2009 by the U.S. Geological Survey, in cooperation with the Town of Breckenridge, to provide Breckenridge with a relative measure of which subwatersheds might constitute the most serious debris-flow hazards. Potential postwildfire debris-flow probabilities and volumes for nine primary watersheds tributary to the upper Blue River and 50 subwatersheds located within and adjacent to the primary watersheds were estimated by using empirical debris-flow models. An assumption in the debris-flow models was that a moderate to severe wildfire affected 100 percent of the forest and shrub stands within the area. Three postwildfire precipitation scenarios were used to represent a range of likely precipitation scenarios that could occur shortly after a wildfire: a 2-year recurrence, 1-hour-duration rainfall; a 10-year recurrence, 1-hour-duration rainfall; and a 25-year recurrence, 1-hour-duration rainfall. All of these precipitation scenarios resulted in debris flows from the hypothetically burned watersheds. Subwatersheds with the lowest postwildfire debris-flow probabilities tended to have large areas of alpine and subalpine vegetation or sparse forest cover that would be minimally affected by wildfire. Subwatersheds with the highest probabilities tended to be steep, heavily forested, and relatively small in drainage area. Subwatersheds with the smallest estimated postwildfire debris-flow volumes tended to have small drainage areas, relatively little forest cover, less rugged topography, or were located in alpine and subalpine areas. Subwatersheds with the highest estimated debris-flow volumes were those with the largest drainage areas. The subwatersheds with the greatest potential postwildfire and postprecipitation hazards are those with both high probabilities of debris-flow occurrence and large estimated volumes of debris-flow material. The high probabilities of postwildfire debris flows, the associated large estimated debris-flow volumes, and the densely populated areas along the creeks and near the outlets of the primary watersheds indicate that Indiana, Pennsylvania, and Spruce Creeks are associated with a relatively high combined debris-flow hazard.