Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/7/1997
Publication Date: N/A
Interpretive Summary: A water supply dam is being designed for construction in central North Carolina. A 500-foot wide spillway is being designed to safely pass flood waters over the top of the dam. The dam will be constructed using roller compacted concrete, so the spillway surface will consist of a series of concrete steps. A model study was performed to determine the best step size eand to determine if the spillway walls could come closer together or converge. If the walls can converge, a significant cost savings is possible, since less rock needs to be excavated and less concrete is needed for the downstream stilling basin. A scale model of the dam was constructed and different step sizes and wall convergence angles were tested. The 3-foot high step size was found to reduce the energy of the flowing water and to reduce the length of the stilling basin required. Several wall convergence angles were examined, but converging the spillway from 500 feet twide to 400 feet wide was the recommended shape. Model Study recommendations will provide a construction cost savings of between 1 and 2 million dollars for this 20 million dollar project. These results should be of interest to Federal, State, and private dam builders and owners.
Technical Abstract: Randleman Lake Dam, a proposed 102-foot high roller compacted concrete (RCC) dam with earth sections on both abutments, is being designed for construction on the Deep River located in central North Carolina. Upstream features require a 500-foot wide spillway to pass a Probable Maximum Precipitation (PMP) storm outflow of 200,000 cfs or 400 cfs/ft of spillway. .A converging spillway is considered to reduce the stilling basin width, reduce the excavation quantities, and reduce the concrete volume in the stilling basin. Studies were conducted using a two-dimensional, 1:40 scale model to determine the optimum step height, and a three-dimensional, 1:40 scale model to study the effects of converging the spillway walls. Three-foot high steps were found to be effective in providing energy dissipation to reduce the stilling basin length to 100 feet, while providing good flow characteristics over the full range of spillway discharges. A 20.9 degree angle of convergence of the chute walls displaye excellent performance with 25-foot high sidewalls that allows the stilling basin to be reduced to a 440-foot width. A 32.5 degree angle of convergence that allowed the stilling basin width to be reduced to 400 feet also performed well. Substantial savings in construction costs were realized based on the model study results.