Dam Safety Incident Report: Benson Creek Rainfall-Runoff Model Results, November 2015
Dam Safety Incident Report: Computerized rainfall-runoff model for Benson Creek, January 2015
Dam Safety Incident Report - Benson Creek Flood, August 2014
Heavy rains in fire area collapse dams - August 22, 2014
The third and final report on the 2014 Wenner Lakes dams incident is now available. The Benson Creek watershed, where the dams were located, suffered considerable damage by wildfires during the summer of 2014. And then on August 21, 2014 there were heavy rains, causing significant flood damage. By that evening, only two of the five dams remained standing. Three collapsed in the storm.
There were no rain gauges or stream gauges in the Benson Creek watershed to measure what actually happened during the August 21st storm. Ecology dam safety engineers have compiled a rainfall-runoff model to estimate what probably happened at each of the Wenner Lakes dams. The findings from these analyses is the subject of this third and final report, available at:
The final report discusses the model run which, done in conjunction newly-developed guidance for burned watershed hydrology calculations, allowed for a close examination of the overtopping that occurred at the Chalfa and Rabel dams, and the impacts of the eroded spillway at the Hawkins Dam. These three dams are part of the inventory of dams that Ecology regulates and inspects (dams that store 10 or more acre-feet of water).
Model runs for the August 21st storm indicate the post-fire runoff flows were 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 storm exceed the estimated pre-fire runoff flows from a 1,000-year storm event.
There were high runoff flows and numerous mudslides throughout the area. The rain clogged spillways with stormwater debris. The incident provides a very dramatic illustration of the impacts that wildfire can have on dams.
Forest fires can affect burned-area soils by reducing the effective ground cover, reducing the amount of soil structure, and forming water repellent layers. These changes reduce the amount of rainfall that soaks into the soil, and also increase the speed of runoff across the soil surface.
The Wenner Lakes are located southeast of Twisp in Okanogan County. These earthen dams in Lower Finley Canyon are in an area burned in the 2014 summer wildfires. The fires destroyed trees and groundcover that otherwise would have held back stormwater. The loss of groundcover created more and faster runoff.
Going from north to south (upstream to downstream), here is information on each of the affected dams:
Note: An acre-foot is the volume of water that will cover an area of one acre to a depth of one foot. This is equivalent to 43,560 cubic feet, or 325,850 gallons. A football field 300 feet (100 yards) long and 160 feet wide has an area of 1.1 acres.
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