Field-Scale Study of Rock Scour in an Unlined Spillway

Scour of rock is a critical issue for dam safety because excessive scour can result in high remediation costs and lead to dam or spillway failure and loss of life. As such, there has been a critical need to understand and quantify the scouring process in a reliable manner. However, previous studies of rock mass erodibility have been performed exclusively using scaled laboratory models under idealized flow and geologic conditions, making the extrapolation of results to actual dam and spillway sites challenging. Collection of high-resolution field prototype data has historically been elusive, and accordingly, a key focus of this project was to obtain a full-scale data set from an existing spillway to further understand scouring processes in rock. To this end, two instrumented artificial three-dimensional (3D) rock blocks were installed in an unlined rock spillway channel at a damsite in northern California. The blocks were hydraulically plucked during two separate flood events, which provided real-time data on hydrodynamic pressures acting on the blocks, as well as the block displacement during the plucking process. Here, we present observations captured during the flood events and a theoretical interpretation of the data using observations from a complementary physical hydraulic model. The observed block displacement behavior agrees well with those obtained from the physical hydraulic model study for blocks with similar kinematic constraint conditions. Additionally, theoretical estimates of 3D block erodibility thresholds using a block theory framework show reasonable agreement with the observed conditions under which the blocks were eroded in the field, providing an improved methodology to capture 3D site-specific geologic structure for evaluation of block plucking in rock scour.