Black Butte Dam, California, Spillway Channel Head-Cut Erodibility Potential in Weathered Basalt of the Lovejoy Formation

Black Butte Dam (CA10102) is a high hazard dam located in Tehama County, California. It was designed and constructed by the U.S. Army Corp of Engineers on Stony Creek between 1950 and 1963 for flood control and water conservation. The dam is 156-feet high, 2,970-feet long earthen embankment dam with a 118-feet wide spillway section. The unlined, broad-crested weir-type spillway is excavated into bedrock approximately 50-feet clear of the left abutment of the dam. The spillway is an excavated trapezoidal bedrock channel consisting of open-jointed, weathered basalt of the Lovejoy Formation.
As part of the Semi-Quantitative Risk Assessment of the structure, the USACE risk assessment team evaluated the bedrock erosion potential of the unlined spillway. Rock erosion of unlined spillway channels is controlled by a variety of hydraulic and geological factors. The geomechanical parameters of the rock mass were evaluated to develop roughness coefficient values for analyzing the unlined spillway erodibility. Qualitative and semi-empirical quantitative metrics were applied and correlated to evidence of historic spillway bedrock erosion.