Physically Based Model for Evaluation of Rock Scour due to High-Velocity Jet Impact

Scour of rock may occur downstream of dam spillways, as a result of the impact of high-velocity jets. The phenomenon is traditionally assessed by means of (semi-) empirical methods. These partially neglect basic physical processes responsible for rock mass breakup. Therefore, a model to evaluate the ultimate depth and time evolution of scour in jointed rock is presented. The model is based on near-prototype scaled experimental investigations of transient water pressures in artificially created rock joints and on a numerical modeling of the measured pressures. It describes two different ways of rock mass destruction, i.e., failure by instantaneous or progressive breakup of closed-end rock joints, and failure by dynamic ejection of single rock blocks. The corresponding computational methods are easily applicable to practice, without neglecting relevant physics. The basic principles are outlined and applied to the well-known scour hole at Cabora-Bassa Dam.