Reservoir sediment impacts on the rehabilitation of NRCS-assisted flood controldams in Virginia

摘要

Sediment storage is a vital component of an NRCS-assisted flood control dam. Because most people do not notice sediment unless there is a problem, they are unaware of its role in the design of the dam and its influence on the options for rehabilitation of the dam. Sediment accumulation directly or indirectly impacts several factors such as project lifespan, normal pool elevation, future sediment storage and flood storage capacity. Since the enactment of Section 313 of Public Law 106-472, "The Small Watershed Rehabilitation Amendments of 2000," the USDA Natural Resources Conservation Service in Virginia has prepared 16 dam rehabilitation plans. Each plan began with a survey of the amount of sediment in the reservoir. NRCS used several different techniques to collect the information. The technique used in each case depended on the reservoir characteristics. The amount of accumulated sediment was dependent on the watershed characteristics. For the suburban watersheds in Northern Virginia, the actual sedimentation rates were closer to the design sedimentation rates than those seen in the forested mountains in west central Virginia where the actual sedimentation rate was much lower than anticipated. Where there were dams in series, the dam in the upperwatershed trapped more sediment than designed while the lower dam trapped less. On some sites, the sediment volume was the limiting factor in the expected life of the structure. These different situations affected the alternatives that were available for rehabilitation.