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Dam Safety Incident Report: Computerized rainfall-runoff model for Benson Creek

Publication number Date Published
15-11-002January 2015
VIEW NOW: Dam Safety Incident Report: Computerized rainfall-runoff model for Benson Creek (Number of pages: 62) (Publication Size: 2506KB)

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Author(s) Walther, Marty
Description On the evening of Thursday, August 21, 2014, a rainstorm hit the recently-burned Benson Creek watershed causing considerable flood damage. By the next day, State Highway 153 was closed 6 miles south of Twisp, and three of the five Wenner Lakes in Finley Canyon were empty.

There were no rain gauges or stream gauges in the Benson Creek watershed to measure what actually happened, so a rainfall-runoff model was compiled to estimate what probably happened. The development of the computerized rainfall-runoff model for the Benson Creek watershed and some preliminary model results are the subject of this report.

What happened on August 21st? Why did a modest storm cause so much damage? Model runs for the August 21st storm indicate the post-fire runoff flows may be on the order of 7 to 8 times the estimated pre-fire flows for the same storm event. Model runs also estimate that the post-fire runoff flows from the August 21st storm exceed the estimated pre-fire runoff flows from a 1,000-year storm event.
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Keywords report, dam safety