Managing Dam Sediments: Rudimentary Sediment Budgets As a Practical Method for Assessing Risks to Aquatic Resources

Based on project scale, dam removal projects boast some of the greatest ecological benefits to aquatic resources. A single dam removal project can restore connectivity to miles of upstream habitats, re-establish sediment balance to downstream reaches, and reduce thermal impacts. While the removal of the dam structure can be fairly straight forward, resource managers are challenged by what to do with the impounded sediments from both a quantity and quality perspective. In many cases resource managers lack the information necessary to make informed decisions about how to manage sediments, which results in the costly excavation and disposal of sediments. The perceived complexity of many available analytical tools that model the transport and fate of sediments prohibits their application, but for the purpose of assessing risk to aquatic resources, these complexities can be overcome through the use of reasonable approximations of input values. Visual site inspections, pebble counts, and hydraulic models can provide the basic inputs to compute sediment loads and develop rudimentary sediment budgets. Recognizing the uncertainty in these empirical computations without calibration, the predicted sediment loads may be less useful than the relationship among loads used to identify sources and sinks in a watershed. These sediment budgets are most useful as screening tools to assess the physical or ecological risks to aquatic resources downstream. As a screening tool, rudimentary sediment budgets can be used by resource managers to determine if impounded sediments should be removed or if it is prudent to conduct additional studies and calibration to justify allowing the impounded sediment to be redistributed naturally by the river. This discussion presents methodologies for data collection, analysis, and interpretation of rudimentary sediment budgets so that resource managers can establish a framework for the evaluation of risks to downstream aquatic resources from the release of impounded clean sediments.