Author(s): Chad E. Bailey; Donald G. Booth
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Keywords: No Keywords
Abstract: Milltown Dam, located on the Clark Fork River upstream of Missoula, Montana, is currently being removed under the U. S. Environmental Protection Agency's Superfund program to remediate a local groundwater arsenic plume. The impacts of dam-removal-induced scour of reservoir sediments on downstream surface water quality and upstream infrastructure (including 5 bridges) are of concern. Therefore as part of the Milltown Dam removal design, evaluation and management of sediment scour was carefully considered in a predictive sense to minimize impacts to both downstream surface water quality and upstream infrastructure. The computational models “HEC-6, Scour and Deposition in Rivers and Reservoirs” and HEC-RAS were used to analyze predicted sediment scour and transport resulting from removal of Milltown Dam and to estimate the effectiveness of various Best Management Practices (BMPs) to reduce potential water quality impacts and to protect upstream infrastructure. The evaluation determined that staging reservoir drawdown to take advantage of dilution provided by high flows combined with using cofferdams and a bypass channel to isolate higher metals concentration sediment from flowing water should adequately protect downstream water quality while pier and abutment modifications were required to protect upstream infrastructure. A comprehensive monitoring plan is also being conducted to evaluate dam removal and other construction impacts on downstream water quality. The monitoring plan identifies warning levels for constituents of concern which, if exceeded, may trigger changes in monitoring frequency, operational controls or implementation of additional BMPs. Comparison between modeled and measured sediment transport results to date demonstrates that a one-dimensional model was a good predictor of the total amount of scour but was less accurate in predicting scour extent and timing.
Year: 2009