Jackson Meadows Spillway – Nevada County, CA

The spillway at Jackson Meadows Reservoir was constructed in 1964. The spillway consists of a concrete-lined chute at the outlet of the reservoir, which is approximately 220 feet long. The spillway and chute are excavated into jointed volcanic rock. Over time, the toe of the spillway has eroded, and during large storm events, blocks of jointed volcanic rock have been dislodged, causing some erosion and undermining the toe of the spillway chute. Nevada Irrigation District retained Holdrege & Kull to perform a hazard analysis of the hillside above the spillway, evaluating the slope stability and estimating the amount of soil/rock that could enter the spillway if a slope failure occurred. H&K collected soil and rock samples for laboratory testing and mapped joints, fractures, and bedding attitudes in the volcanic rock at the site. H&K also submitted rock/soil samples to the University of Nevada Reno for large-scale shear strength testing. In addition to slope stability analysis, H&K used computer software to plot primary joint and bedding orientation. Based on the analysis, it was determined that the slopes were not prone to global instability under saturated earthquake loading and that the likely volume of debris falling into the spillway channel would not restrict flows