Progressive Decision-Making Process Helps Ensure Future Resiliency for San Diego’s 2nd Aqueduct Pipelines in Moosa Canyon

The San Diego County Water Authority (Water Authority) provides a safe and reliable water supply to more than 3.3 million residents through 24 member agencies. The 2nd Aqueduct pipelines serve the entire San Diego region and are considered critical infrastructure consisting of Pipeline No. 3 [72-in. (1,829 mm) steel welded pipe], Pipeline No. 4 [90-in. (2,286 mm) prestressed concrete cylinder pipe (PCCP)], and Pipeline No. 5 [96-in. (2,438 mm) PCCP] that pass through Moosa Canyon. Since 2019, these pipelines have started to exhibit vulnerabilities. Existing and ongoing threats to the pipelines where the 2nd Aqueduct crosses Moosa Creek, at the base of Moosa Canyon, include scour within the streambed, seismic effects (e.g., liquefaction, lateral spread, and potential slope instability), and other potential pipeline failure modes. The Water Authority has provided interim protections to these threats; however, if left unaddressed, these conditions may increase the risk of future failure. To mitigate these risks and ensure a state-of-good repair, mitigation of natural threats, and water supply resiliency, the Water Authority has completed a planning study to develop the project to rehabilitate, replace, relocate, and/or upgrade, as necessary, the pipelines. This paper documents the project background and pipeline threats, the range of repair and replacement alternatives identified, and the multi-step decision-making approach used for ultimately picking the “selected alternative” for implementation.