Decision Framework to Determine Potential Sediment Impacts and Analyses for Reservoir Sediment Sluicing and Dam Decommissioning
* What are the potential sediment impacts from sediment sluicing and dam decommissioning?
* How does the size of the reservoir and sediment load affect the level of modeling and data collection needed to address sedimentation questions associated with sediment sluicing and dam decommissioning projects?
* How does reservoir size and sediment volume influence the ability of current data collection methods and analysis tools to detect and quantify impacts of downstream sediment redistribution following sluicing or dam decommissioning?
Need and Benefit
Reclamation designers, reservoir operators, permitting agencies, and stakeholders need a tool to match the level of reservoir sediment analysis and certainty needed for a given project with the scale of the sediment issue. This would be applicable to projects involving prediction of sediment erosion and transport related to reservoir sluicing and flushing. This would also be applicable to predicting sediment levels associated with design of pumping plants or other water intake structures downstream of Reclamation reservoirs (releasing sediment either in conjunction with reservoir sluicing or dam removal).
Reservoir sedimentation is a national and international issue that has the potential to reduce reservoir storage capacity, thus reducing the potential for delivery of water and power generation. All reservoirs are expected to lose some storage to sedimentation, but the rate at which it happens varies considerably. Sediment that is delivered from upstream sources can also contain contaminants that pose a risk to water quality. The cost of dealing with reservoir sediment management can be high; thus, learning if reservoir sluicing is a feasible option for maintaining reservoir capacity could be very useful in future years to maintain power generation and water delivery capability.
Reclamation dam outlets were designed to accommodate 100 years of reservoir sedimentation. Presently, 14 percent of Reclamation dams are more than 90 years old. In 20 to 30 years, about one-third of Reclamation dams will be more than 100 years old and likely will have accumulated their 100-year sediment volume. Predictive tools for reservoir sedimentation and sluicing are limited in applicability because of the complex processes involved in reservoir sediment erosion and redistribution. Reclamation has one technical guideline available for reservoir sedimentation analysis tools that was produced by the Sedimentation and River Hydraulics Group in 1982. Equivalent information from the 1982 guideline and from a textbook by Morris and Fan has been duplicated in the new Erosion and Sedimentation Manual, produced in November 2006. Neither of these manuals provides nontechnical users a framework or guideline for determining level of analysis needed, significance of sedimentation issues, or certainty that can be attained with analysis tools. Historically, reservoir sluicing operations have not been well documented, so it is difficult to apply lessons learned to further refine currently available tools. There is a need to improve predictive tools so Reclamation can better manage sedimentation in reservoirs and extend reservoir capacity life.
The idea would be to develop a decision tree that would work for the full range of reservoir sizes managed by Reclamation. In other words, the steps would be applicable for a range of dams with small and large sediment accumulations. The staff of most permitting agencies does not have extensive experience with dam removal or reservoir sluicing operations projects. Therefore, they often need guidance about what should be required to assess and permit the proposed actions. If this tool could be adopted by the National Committee on Sedimentation, it could be a reference and guideline for Reclamation to refer to when working with local agencies, permitting agencies, and stakeholders to determine types of sediment analysis needed for decisions on reservoir sediment management.
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This information was last updated on April 16, 2014
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