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Scientific Investigations Report 2011–5039

Prepared in cooperation with the Town of Breckenridge, Colorado

Estimated Probabilities and Volumes of Postwildfire Debris Flows, a Prewildfire Evaluation for the Upper Blue River Watershed, Summit County, Colorado

By John G. Elliott, Jennifer L. Flynn, Clifford R. Bossong, and Stephen J. Char

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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.

First posted May 20, 2011

For additional information contact:

Center Director, USGS Colorado Water Science Center
Box 25046, Mail Stop 415
Denver, CO 80225

http://co.water.usgs.gov

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Suggested citation:

Elliott, J.G., Flynn, J.L., Bossong, C.R., and Char, S.J., 2011, Estimated probabilities and volumes of postwildfire debris flows, a prewildfire evaluation for the upper Blue River watershed, Summit County, Colorado: U.S. Geological Survey Scientific Investigations Report 2011–5039, 22 p.



Contents

Abstract

Introduction

Upper Blue River Watershed Study Area

Debris-Flow Regression Models

Debris-Flow Probability

Debris-Flow Volume

Input Data for Debris-Flow Models and Assumptions

Verification of Debris-Flow Model Results

Estimated Probabilities and Volumes of Postwildfire Debris Flows

Upper Blue River Watershed Debris-Flow Probabilities

Upper Blue River Watershed Debris-Flow Volumes

Combined Relative Debris-Flow Hazard Ranking

Summary and Conclusions

Acknowledgments

References Cited

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