Effects of Beaver Dam-Break Floods on Downstream Channels in King County, Washington
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Beaver dam failures were responsible for 13 deaths between 1984 and 2003 and can cause damage to infrastructure and property (Butler and Malanson, 2005). In the Puget Lowland of western Washington State, the unique geologic setting with beaver ponds on steep-sided uplands, surrounding flat alluvial plains of rivers like the Snohomish or Tolt Rivers, creates an environment where beaver dams that fail perched high atop these uplands can erode huge volumes of materials from the channels and valley sidewalls (up to 119,000 cubic yards). While public officials in western Washington have taken an interest in the failures after damage to property and infrastructure has occurred, little to no information exists quantifying the geomorphic response in the channels in this region. With recent failures backing up culverts and wiping out roads and trails, interest has been building in predicting the downstream effects of these failures in King County. Using LiDAR data from before and after the most recent failure events in the Snoqualmie Valley Trail, Deer Creek, Tate Creek, Evans Creek, Preston dam, and Tolt River dam stream channels in King County, the depth of degradation and aggradation in the channel and the volume of sediment evacuated from within 100-feet of the channel are estimated. Kennar et al. (2017) was used to calculate beaver pond volume for use in the peak discharge equations from dam breach peak discharge equations cited by the Washington State Department of Ecology (2007). Peak discharge calculations were compared to the expected 100-year flood events for each of the channels to compare the magnitude of the floods in the failures and their role in forming the channel. Using correlations between different channel and dam failure parameters (peak discharge, erosional volume, pond volume, and channel length), equations were developed that can be useful for public officials to predict the expected response downstream of these beaver dam failures. Further, understanding the magnitude of the flood that might be expected to enter these channels and intersect culverts or other infrastructure crossings can help officials plan for beaver dam failures. While this study did not focus on the role of the geology of the downstream channels, the strongest predictor of erosional volume in the channel and valley sidewalls is the average peak discharge and the channel length. Since a longer channel should produce more erosional volume, a multiple regression analysis using pond volume and average peak discharge concluded a significant correlation to the volume of material eroded from the channel with a correlation coefficient of 6.85*10-3 as well as the maximum elevation lost in the channel with a correlation coefficient of 4.28*10-2. The flows from these failure events were calculated to be anywhere from 2.5 to 40 times the magnitude of the 100-year events. These calculations and measurements show these beaver dam failures could be important in sizing channels and could also play a significant role in incising the valleys of these streams in the Puget Lowlands. A better understanding of the frequency of these dam failure events will help to more precisely be able to predict the associated hazards.