Potential Erosion of An Unlined Rock Spillway At New Melones Dam, Ca

New Melones Dam is located on the Stanislaus River in Calaveras County, California. The New Melones Reservoir has an unlined rock spillway that spills into Bean Gulch ~1,700 m upstream from the confluence with the Stanislaus River. The spillway has never been utilized and poses a Low Probability – High Consequence erosion risk, as asbestos-laden sediment eroded from the spillway could impact the Stanislaus River and Tulloch Reservoir, located 6.7 km downstream. The asbestos is sourced from highly erodible serpentinite rock. Even though New Melones Dam has never utilized the spillway, overland flow from precipitation and groundwater infiltration has eroded gullies into the serpentinite at the downstream end of the spillway, demonstrating the high susceptibility to erosion in a spillway flow event. We have applied a three-tiered approach to estimating erosion risk in the spillway and subsequent sediment deposition downstream: an SRH-2D sediment transport model, a 2D Annandale erosion probability model, and a 1D bedrock erosion model.

The 2D Annandale erosion probability model incorporates geologic lithology mapping, rock hardness, fracture spacing, and joint properties, which all influence the probability that any given area of the spillway will erode. We used ArcGIS Pro to generate a map of these various properties, which were then used to calculate an Annandale erodibility index (Annandale, 1996). We compared the erodibility index to the space-varying stream power for flow events ranging from the 100-year to the 1,000,000-year recurrence interval flood at different initial reservoir elevation conditions to develop a 2D map of probability of erosion within the spillway and Bean Gulch. The results show that erosion is isolated to the downstream end of the spillway and portions of Bean Gulch for all of the flow events tested. The severity of the erosion expectedly increases with increasing discharge. Preliminary data from the SRH-2D sediment transport model indicates that the majority of the eroded sediment will be deposited at the junction of Bean Gulch and the Stanislaus River, which is backwatered from Tulloch Reservoir.

We are in the process of applying a 1D time-dependent erosion model to estimate the timing and volume of bedrock erosion within the spillway. This model applies detailed fracture and rock density data to generate a fracture network at depth within the spillway. It then calculates the forces that the flow exerts on broken blocks of rock to calculate when the rock will erode. This allows us to model the upstream progression of a headcut in the spillway and approximate a volume of erosion for different flow events.