Rehabilitation of the auxiliary and principal spillways on South River Dam 10A, Augusta County, Virginia

Abstract

Abstract. In 1963, the USDA Natural Resources Conservation Service (formerly Soil Conservation Service) built South River Dam 10A, known locally as Mills Creek Dam, in Augusta County, Virginia, as a significant hazard dam. This multi-purpose structure included both flood control and municipal water supply. By 2005, the hazard classification of the dam changed to high because of the downstream development and the increased risk to public safety from a dam breach. The vegetated auxiliary spillway did not have the capacity, stability, or integrity to pass the probable maximum flood (PMF) event without breaching. There were also several problems with the principal spillway pipe outlet and riser. Rehabilitation of the auxiliary spillway included lowering the crest elevation to increase capacity and armoring the surface with articulated concrete blocks to achieve integrity and stability. Since poor water quality prevented use of the municipal water supply, the water supply component of the dam was removed. The original riser of the principal spillway was abandoned and a new riser was installed. The principal spillway pipe was slip-lined and the concrete outlet structure was replaced by a riprap plunge pool. Cooperation with multiple Federal, state, and local entities was an essential part of this project. The dam is located on USDA Forest Service administered land but is owned by Augusta County. A threatened plant species was found downstream of the site. The lake is a put-and-grow fishery; and there was a flood-monitoring gage in the dam embankment. NRCS completed rehabilitation of this dam in 2013.