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Extreme post-wildfire flooding in the Colorado Front Range




Brogan, Daniel Joseph, author
Nelson, Peter A., advisor
Bledsoe, Brian P., committee member
MacDonald, Lee H., committee member
Wohl, Ellen E., committee member

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In 2012, the High Park Fire near Fort Collins, Colorado burned ~353 km2 and destroyed 259 structures. After the fire, the ungaged 15.5 km2 Skin Gulch watershed experienced two geomorphically effective floods. Here, we investigate connections among storm characteristics, flood response, and geomorphic change by characterizing the hydrometeorology, peak flood discharge, and channel changes for these two extreme events. Our specific objectives were to: 1) to quantify spatial patterns of total rainfall and maximum 15-minute intensity for each storm by using local rain gages and Doppler radar; 2) to estimate the peak discharge and the corresponding uncertainty for each event using several modeling techniques; and 3) to interpret the rainfall-runoff response for these storms in the context of spatial interaction of precipitation, burn severity, runoff, and geomorphic change. Precipitation was estimated with bias-corrected radar observations, and at-a-station, 1D, and 2D hydraulic modeling calculations were used to characterize the peak discharge for each flood, calibrated to surveyed high water marks. The first storm occurred on 6-7 July 2012, just days after the fire was extinguished, when a convective thunderstorm produced total rainfall of ~50 mm with a maximum 15-minute intensity of ~60 mm/h over a portion of Skin Gulch that was burned at high severity. The resulting flood caused considerable deposition in the channel and 2D hydraulic calculations suggest the peak discharge was 70-120 m3/s. The following summer, from 9-15 September 2013, a very unusual multi-day storm produced 279 mm, which represents a recurrence interval greater than 1000 years. The peak 15-minute rainfall intensity for this storm estimated by the radar was about 100 mm/h, although the highest 15-minute intensity recorded by a rain gage was 38 mm/h. Based on 2D flow analysis, the peak discharge for this flood was <50 m3/s. Although the peak discharge was lower than that of the 2012 flood, this flood produced comparable channel change due to its extended duration. Both events rank among the largest rainfall-runoff floods per unit area ever recorded in the continental United States, and point to the dramatic effects wildfire can have on storm hydrology and channel morphology.


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