New Technology for Estimating Plunge Pool or Spillway Scour

The Dam Foundation Erosion Study Team presents the latest progress of the dam safety research program on erosion characteristics of dam foundations. Team members include the US Department of the Interior Bureau of Reclamation, the Federal Energy Regulatory Commission, Western Area Power Administration, Colorado State University, and Pacific Gas & Electric Company. This paper describes new technology for estimating plunge pool or spillway scour. The focus of the study has been the foundation areas of concrete dams and spillway plunge pools. In contrast to existing scour estimation equations this technique extensively treats unique geology and geotechnical properties of earth materials. A credibility index incorporates the mass strength, the particle size, interparticle shear strength, and relative shape and orientation of foundation materials. These factors derive from standard borehole data and geologic investigations. The key to this technology is the relation of the credibility index, or the ability of the material to resist erosion, to the erosive power of water. This technology builds upon extensive analysis of the hydraulics of water overtopping a dam or spilling from a flip bucket spillway. Hydraulic data includes head, discharge, tailwater depth, and crest or spillway configuration. This paper summarizes the elements of the credibility index and the methods for calculating the erosive power of water. It includes an example estimate of maximum scour depth in an alluvial material. It also includes the results of a scour test simulating a fractured rock mass. The study integrates a large quantity of field and laboratory data. 12 pp., 12 references.