Indian Valley Dam is a zoned earthfill dam constructed between 1973 and 1974 with a maximum height of approximately 220 feet above a streambed at the downstream toe. The crest is approximately 30 feet wide and 970 feet long. During construction of the dam, several faults, which juxtapose terrace deposits against bedrock, were mapped in the foundation excavation and the right abutment.
The faults were considered inactive at the time; however, fault activity was questioned during a Potential Failure Mode Analysis (PFMA) in 2005. In response, the District performed a series of fault studies to better characterize the potential fault hazards. The studies were inconclusive with respect to location, length, and age of potential faulting, and the faults were classified as conditionally active under California Division of Safety of Dams (DSOD) criteria.
SAGE performed a detailed peer review of the fault studies and concluded that the likelihood of obtaining data that conclusively shows the fault to be inactive to be low to very low. We recommended that the District proceed with the seismic stability evaluation of the dam assuming the fault to be conditionally active, and assisted the District with DSOD and FERC interactions.
SAGE participated in the updated PFMA session and FERC Part 12D Safety Inspection for the dam, outlet works, spill-way, and powerhouse. The work included reviewing the previous PFMA , Part 12D reports, and other background information, in order to update the classification of Potential Failure Modes (PFMs) identified in the original PFMA session.
SAGE performed an updated seismic stability evaluation of the dam. The evaluation was performed using the seismic source characterization that had been peer reviewed by SAGE, as discussed above. SAGE developing four sets of spectrally-matched acceleration time histories representative of the 84th percentile Mw 7.3 event on the Bartlett Springs fault. Due to the proximity of the fault to the dam site, the deterministic response spectra were modified to include directivity effects in the long period range.