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Relationship between rainfall intensity and debris-flow initiation in a southern Colorado burned area
Friedman, Evan Q. Santi, Paul M.
Friedman, Evan Q.
Santi, Paul M.
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2019
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Abstract
Wildfire impacts on vegetation, soils, and resulting hydrologic processes often result in debris-flow activity in mountainous areas, particularly in response to intense rainfall events that follow. Rainfall thresholds for debris-flow initiation in burned areas have been studied in a variety of settings. It has been proposed that short duration, high-intensity rainfall events are responsible for debris-flow initiation in burned areas. The timing of these responses relative to rainfall intensity peaks is not well understood, leaving uncertainty regarding the duration of intense rainfall necessary to trigger debris flows, which is pertinent to debris-flow initiation thresholds and processes, as well as potential risk mitigations. This study evaluates timing of debris-flow initiation relative to various periods of rainfall intensity at a burned area in Colorado. Detailed local monitoring data from the first intense rainfall events following the 2010 Medano Fire are presented and provide insight into temporal correlations between rainfall intensity peaks and debris-flow initiation. Findings indicate that flow arrivals were within approximately 10 minutes after the beginning of periods of peak short duration rainfall intensity and as soon as 6 minutes after the first rainfall of a storm event was recorded, suggesting that short periods of intense rainfall were responsible for initiation. Tests for equality of variances and means indicated that rainfall intensity values of up to 1-hour duration differentiated between storms that triggered debris flows and those that did not, while average storm intensity did not distinguish between them. The return intervals of storms that triggered debris flows in the Medano Fire burned area burned areas evaluated was as low as 2 years; however, short-period rainfall intensity within the triggering storms were similar in magnitude and correlated temporally with debris flows.
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