Virginia Ranch Dam is located on French Dry Creek, about 9 miles upstream of its confluence with the Yuba River. It is about 20 miles northeast of the City of Marysville and about 8 miles northeast of the Town of Browns Valley. The dam forms Merle Collins Reservoir, which is also shown as Collins Lake on current maps.
The dam was completed in October 1963 and first filled between December 1963 and January 1965. Virginia Ranch Dam is 152 feet high and has a nominal crest length of 2,800 feet. The reservoir capacity is 57,000 acre-feet, and the reservoir occupies 975 acres.
During preparation of the FERC Part 12D report, the Independent Consultant retained SAGE to perform an updated seismic source characterization and associated seismic stability evaluation.
SAGE performed an evaluation of potential seismic sources in the vicinity of the dam. The evaluation focused on several fault traces within the Bear Mountains fault zone that previous paleoseismic studies have shown to exhibit evidence of Quaternary fault activity. The closest known Quaternary fault to the dam is the Swain Ravine fault, at 6½ km to the west. Updated “Next Generation of Ground-Motion Attenuation Models” for the western United States (NGA-West2) were used to deterministic response spectra for the site.
SAGE participated in the updated Potential Failure Modes Analysis (PFMA) session for the dam. The work included reviewing the previous PFMA, Part 12D reports, and other background information to update the classification of the PFM identified in the original PFMA session.
SAGE also performed an updated seismic stability evaluation of the dam. The seismic stability and potential for seismically induced deformations at Virginia Ranch Dam were evaluated using updated ground motion estimates and simplified procedures. The updated stability evaluations were performed using undrained strengths in the saturated clayey core and drained strengths in the moist and free-draining materials. The conservative deformation evaluation showed the potential for up to 3 to 5 inches of deformation at the dam crest. These deformations are considered acceptable and will not lead to significant cracking or piping at the dam. The available freeboard of 14 feet is adequate to accommodate the postulated movements, should they occur.