Geologic Hazard Characterization for Metropolitan Water District’s Replacement of Casa Loma Siphon Barrel No. 1, San Jacinto, California

The Casa Loma Siphon Barrel No. 1 is a critical water supply lifeline in Metropolitan Water District of Southern California’s (Metropolitan) water delivery system. The 148-in. diameter siphon was constructed in the 1930s and is part of the Colorado River Aqueduct system. The Siphon conveys water across the San Jacinto Valley and the San Jacinto fault zone (SJFZ) in Riverside County. Evidence of cracking and leakage due to localized non-tectonic settlement was noted in the 1960s as concentrated where the siphon crosses a strand of the SJFZ named the Casa Loma fault (CLF). The existing siphon is vulnerable to failure due to tectonic and non-tectonic displacements at the fault crossing. A geologic investigation was conducted for a retrofit project designed to improve the siphon’s seismic resiliency against potential future ground deformation. Exploratory trenching at the siphon fault crossing and a synthesis of paleoseismic and fault mapping data for the SJFZ provided input to fault displacement hazard analyses, and these results plus analysis of historical settlement data provided constraints for the retrofit design. Deterministic and probabilistic coseismic fault displacements range from 5.9–14.2 ft (1.81–4.33 m) and are consistent with other paleoseismic estimates of single-event displacements along the SJFZ. Ultimately, the pipeline was designed to accommodate 12.8 ft (3.9 m) horizontal and 2.6 ft (0.8 m) vertical of coseismic surface displacement (2,475-year return period) on the CLF and 3.3 ft (1.0 m) of differential settlement for a 50-year design life.