In the early 1900s two hydroelectric dams were built along the Elwha River in northwestern Washington State. In order to restore the Elwha River ecosystem and native anadromous fish runs, the dams are scheduled to be removed. During and after removal, accumulated reservoir sediments are expected to erode downstream. The exposed sediments remaining after the dam removals and initial erosion will be prone to continuing secondary erosion and should be stabilized as quickly as possible. Experiments were performed on both fine and coarse sediment from underneath Lake Mills (formed by the Glines Canyon Dam) to determine strategies to stabilize the sediments under moderately high rainfall. Two slopes (5° and 15°) and two rainfall intensities (high and low) were examined along with mulch, plants, or polyacrylamide (PAM) stabilization treatments. The Revised Universal Soil Loss Equation (RUSLE) was used in comparison with experimental results to refine predictions. In both the experiments and as modeled with RUSLE, the coarse sediment was much less erosive than the fine sediment. Comparing the controls, the experiments had a similar erosion rate to the RUSLE model. For fine sediment, the mulch treatment was much more successful in preventing erosion in the experiments whereas the plant treatment was more successful as modeled with RUSLE at 5° slopes, but not 15° slopes. Mulch reduced erosion up to 99%, PAM up to 87% and plants by 33% in the experiments. A combination of stabilization treatments may be the most effective method of erosion reduction.
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1 December 2008
Predicting Secondary Reservoir Sediment Erosion and Stabilization Following Dam Removal
Ellen K. Mussman,
Darlene Zabowski,
Steven A. Acker