Unlined Auxiliary Spillway Design

Earth and rock-cut auxiliary spillways have been successfully utilized in the southeastern United States to pass large, infrequent storm inflows. However, as land became more costly, methods other than weir equations and bulk length were needed to estimate spillway flow rates and stability. The United States Department of Agriculture (USDA) SITES computer program predicts erosion that may occur in an unlined spillway during a design storm such as the Probable Maximum Precipitation (PMP). In recent years, the SITES program has been utilized on numerous new dam projects ranging in height from 25 feet to in excess of 100 feet to provide a better estimate of auxiliary spillway capacity and to estimate the amount of erosion that may occur during a given storm event. The use of SITES will be compared with older earth-cut spillway technology (TR-52) for several representative dam projects recently completed or designed in the state of Georgia. In addition to the utilization of SITES on new large dam projects, SITES has been successfully utilized for the rehabilitation of small existing dams and associated spillways. For these smaller projects, SITES was used to reduce the spillway size and thus reduce the impact on adjacent property owners. The use of SITES, including the subsurface exploration and data required for analysis, as well as the site-specific benefits, will be discussed. 7 pp., 1 table, 2 figures, 4 references.