Physical model study of rock scour in unlined rock spillways

Abstract

Scour of rock by flowing water is an integral process in the evolution of natural landscapes as well as a critical hazard for key infrastructure such as dams, spillways, bridges and tunnels. The removal of individual blocks of rock is one of the primary mechanisms by which rock scour can occur. Field investigation of a prototype unlined rock spillway site in the Sierra Nevada Mountains in northern California was used as a basis for the development of an extensive series of physical model experiments, which were complemented by theoretical, deterministic, and stochastic analyses based on 3D block theory. A scaled physical hydraulic model, loosely representing conditions at the field site was used to investigate a broad range of variables and flow conditions not readily achievable in a field setting. For the model, an instrumented 3D block mold was constructed that could be rotated with respect to the flow direction to study the influence of discontinuity orientation on block erodibility. These experiments were the first of their kind to use a non-prismatic 3D rock block, such that the full 3D kinematic conditions associated with the block geometry could be represented. The block erodibility threshold was found to be highly dependent on the flow direction. Pressure values, represented by the dimensionless dynamic pressure coefficients were determined as a function of the block mold orientation, turbulence intensity, block protrusion height, and flow velocity. Overall, the average hydrodynamic pressures on block faces were found to be adequate in the evaluation model block stability. The trajectory of the blocks through the removal process obtained from camera recordings and from proximity sensor data were plotted on whole sphere stereographic projections to analyze the specifics of the kinematic constraints for each condition. Thus, three separate block response modes were observed based on the kinematic constraint related to the block geometry and orientation.