Rethinking Dredging: A Quantitative Analysis of Dam Removal Techniques

The number of dams in the U.S. is estimated at over 2 million. Most are small dams, less than 4 meters high, built approximately 100 years ago. Many are structurally deficient, no longer in use, and are being removed with limited understanding of how the river system will be affected. A new approach to engineering dam deconstruction and managing the sediment impounded by small dams is needed that will reduce the ecological uncertainty about dam removal, address stake holder concerns, and reduce the economic expenditure of dredging that is associated with dam removal. Dredging sediment from behind a dam is common due to uncertainties associated with releasing it downstream. At the same time, the lack of sediment supply downstream leads to channel and back erosion and often necessitates expensive river and bank restoration projects.

This dissertation provides a direct comparison and analysis of five common methods of dam deconstruction: removal in horizontal stages from the top; removal in horizontal stages with portions of the side left in place; removal in vertical stages from the side; removal in vertical stages from the center; removal in a single-stage (also simulating dam failure), in combination with adjustments to the flow rate to simulate low flow and flood conditions as well as three sediment types scaled from typical dam removal scenarios. All of the experiments took place in a 9m long flume in the Sustainable Rivers lab at the University of Virginia.

The flow rate had the greatest impact on downstream channel behavior following dam removal, with high flows resulting in increased quantities of sediment transported out of the systems and low flow rates corresponding to instream channel deposition. The addition of silt to the sediment mixture resulted in the downstream sediment wave moving more slowly through the downstream channel under low flows. The removal of the portion of the structure that brought the elevation of the dam below the elevation of the reservoir sediment had the largest impact on the downstream channel behavior, corresponding to the various deconstruction methods. The results of this research will make dam removal more accessible by reducing or eliminating the costs associated with unnecessary dredging of uncontaminated sediments prior to dam removal.