CS19B - Concurrent Session 19B: Getting to Know Our Dams

Marin Water provides water to over 190,000 customers situated in California’s coastal mountains just north of San Francisco. Approximately 75 percent of the water supply comes from rain collected in seven district-owned reservoirs with accompanying dams. The seven dams have construction dates ranging from 1872 to 1979, and include six earthen and one concrete dam. Ensuring the aging infrastructure is still performing as designed is a full time job.

Dams are inspected monthly to collect field data, which includes measuring seepage from weirs and leakage pipes, measuring water levels within the dam with piezometers, looking for evidence of rodents, clearing vegetation on upstream and downstream faces, and examining spillways for obstructions and visual defects. In 2005, a significant crack was observed on the elevated concrete spillway at one of our dams. Through coordination with Division of Safety of Dams (DSOD), extensometers (crack meters) were installed to track vertical and horizontal movement. Since installation, the crack meters have revealed a seasonal expansion-contraction pattern within a globally shifting trend. The current movement trend leads us to believe that repair or replacement of the spillway portion is inevitable, but close monitoring until then is imperative. Annually, dam movement is quantified using traditional land surveying methods, and compared against historical trends.

Following the Oroville Dam failure, DSOD instructed us to perform condition assessments on three of the spillways that are constructed in a similar way to Oroville. The first phase of assessment included a desktop review of as-built drawings, boring logs, design features, inspection reports and repair records paired with a physical inspection of the spillway surface. Spillway inspection revealed non-structural defects, which were all addressed in a subsequent repair project. During the repair project, an unforeseen circumstance arose at the bottom of one of the spillways when a void was discovered near the standing water level of the flip bucket. After notifying DSOD of the situation, ground penetrating radar was used to conclude the void was non-structural, and repairs were made.

The second phase of assessment, which focuses on spillway conditions beneath the concrete, is currently underway. Through a combination of ground penetrating radar, core sampling and CCTV inspection, we are hoping for a better understanding of structural conditions. Information gained during this process will be used to ensure public safety and prioritize future dam-related projects.