Erosion at Dam Foundations: Plunge Pool Circulation

The stability of dams during extreme flood events is an important factor in preserving the health and safety of the public. Arch dams that overtop produce a plunging jet that has the potential to erode the foundation. The impinging jet on the foundation dislodges materials resulting in a large scour hole, or plunge pool, that may impact the structural integrity of the dam. Due to the catastrophic consequences of dam failure, it is important to understand the stability of a dam during overtopping.

The overtopping jet is usually in the form of a highly turbulent, aerated jet. When a Plunge pool does not exist at the onset of overtopping, a backwater effect will occur at the base of the dam and a scoured plunge pool will usually form. As the jet impacts the water surface and impinges into the plungepool, the impinging jet induces circulation in the plunge pool. The combination of the impacting jet on the basin floor and circulating flow in the plunge pool can potentially cause material entrainment from the abutments or pool bottom resulting in sediment transport downstream.

A cooperative study team comprised of the U.S. Bureau of Reclamation, Pacific Gas and Electric Company, the Electric Power Research Institute, the Federal Energy Regulatory Commission Golder Associates, Inc., and Colorado State University was organized to develop a state-of-the-art procedure for predicting the extent of erosion at dam foundations and abutments. The third phase of the effort characterized the magnitude and direction of the circulation patterns that develop in the plunge pool as a result of the overtopping jet. The study focuses on plunge pool circulation and velocity prediction in the plunge pool basin between the downstream face of the dam and the jet impact location.

The magnitude of the circulation velocities in the plunge pool upstream of the jet impact are important as they respond to the size of sediment that can be dislodged and transported through and out of the plunge pool. The circulation direction in the pool is indicative of the aggradation/degradation potential at the toe of the dam. The study objectives are: 1) Determine flow circulation flow patterns in a plunge pool stilling basin resulting from an impinging jet; 2) Determine the flow pattern driving forces; 3) Determine how impact velocity and air concentration influence the velocity magnitudes in the plunge pool stilling basin; and 4) Develop a velocity prediction method for estimating the magnitude and direction of circulation velocities located along the upstream boundary (downstream dam face) and the basin floor in a plunge pool stilling basin.

This paper describes a physical model consisting of a free falling, developed, rectangular jet impacting into a plunge pool. The experimental variables include the discharge, plunge pool tailwater depth, and basin width. The experimental results, data analysis, and prediction procedures are presented. 10 pp., 7 references.