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Dam Failure 8.1.4 Secondary Hazards Dam failure can cause secondary hazards of landslides, bank erosion, and destruction of downstream habitat. Dam failure may worsen the severity of a drought by releasing water that might have been used as a potable water source. A loss of water supply could exacerbate the wildfire hazard by hindering an impacted area’s ability to fight fire. 8.2 HAZARD PROFILE 8.2.1 Past Events The only recorded dam failure in San Mateo County was the failure of a small dam in the community of El Granada in 1926. According to the 2018 State of California Multi-Hazard Mitigation Plan, there have been nine failures of federally regulated dams elsewhere in the state since 1950. Overtopping caused two of the nine dam failures in the state, and the others were caused by seepage or leaks. The most catastrophic event was the failure of the St. Francis Dam in Los Angeles County, which failed in 1928 and killed an estimated 450 people. If a dam is determined to be unsafe, the California Department of Water Resources Division of Safety of Dams (DSOD) requires reduction of the water level to allow for partial collapse without catastrophic loss of water. The state’s most recent dam emergency occurred in February 2017 when the Oroville Dam in Butte County was on the verge of overflow. The dam’s concrete spillway was damaged by erosion and a massive hole developed. The auxiliary spillway was used to prevent overtopping of the dam, and it experienced erosion problems also. Evacuation orders were issued in advance of a potential large uncontrolled release of water from Lake Oroville, but such a release did not occur. After this incident, state officials ordered that flood-control spillways be reinspected on 93 California dams with potential geologic, structural or performance issues that could jeopardize their ability to safely pass a flood event. The San Andreas Dam near Millbrae and San Bruno was one of the dams reinspected. 8.2.2 Location List of High-Hazard Dams According to DSOD, 24 dams are in San Mateo County. Twelve of these, plus another nearby in Santa Clara County, could endanger lives and property if an uncontrolled release or catastrophic failure occurs. Table 8-1 lists dams with potential to endanger lives and property in the County. Their locations are shown on Figure 8-1. The Lower Crystal Springs Dam is the largest dam in San Mateo County, making it a higher priority for regulation and preventative maintenance by county, state, and federal officials. This dam impounds water to form the Lower Crystal Springs Reservoir, which serves as a water supply for San Francisco and most cities in San Mateo County. Although located directly on the San Andreas Fault, the dam survived both the 1906 San Francisco earthquake and 1989 Loma Prieta earthquake. In 2010, DSOD inspected the Lower Crystal Springs Dam to investigate effects of an 8.3 magnitude earthquake and determined dam failure to be a low probability. Despite this low probability, the County and dam owner, San Francisco Public Utilities Commission, are seeking to enhance safety and quality of the dam. Significant upgrades to the dam and a nearby overpass bridge occurred between 2010 and 2015 to restore maximum storage capacity of the reservoir. The project involved widening the spillway, raising the parapet wall, and replacing the stilling basin with a new and larger facility (San Mateo County OES, 2015). 8-3