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Dam Failure 8.2.3 Frequency Dam failure events are infrequent and usually coincide with or follow events such as earthquakes, landslides and excessive rainfall and snowmelt. Although the recent Oroville event raised public concern about dam failure, the probability of such failures remains low in today’s regulatory environment. The single recorded dam failure in the planning area—in El Granada in 1926—represents a frequency of about one event in 100 years. All dams face a “residual risk” of failure, which represents the risk that conditions may exceed those for which the dam was designed. For example, dams may be designed to withstand a probable maximum precipitation, defined as “theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographical location at a certain time of the year” (Hansen, 1982). The chance of occurrence of a precipitation event of a greater magnitude than that represents residual risk for such dams. This in turn represents a theoretical probability of future occurrence for a dam failure event, though the probability of an event exceeding the assumed maximum is not generally calculated as part of dam design. 8.2.4 Severity Dam failure can be catastrophic to all life and property downstream. California’s Division of Safety of Dams has developed a hazard potential classification system for state-jurisdiction dams, as shown on Table 8-2. This system is modified from federal guidelines, which recommend three-tier classification. The California system adds a fourth hazard classification of “extremely high.” Dams classified as extremely high hazard may impact highly populated areas or critical facilities or have short evacuation warning times (California Division of Safety of Dams, 2017). Table 8-2. State of California Downstream Hazard Potential Classification Hazard Category Direct Loss of Life Economic, Environmental, and Lifeline Losses Low None expected Low and principally limited to dam owner’s property Significant None expected Yes High Probable (one or more expected) Yes, but not necessary for this classification Extremely High Considerable Yes, major impacts on critical facilities or property Source: California Division of Safety of Dams, 2017 8.2.5 Warning Time Advance Warning of Failure Warning time for dam failure varies depending on the cause of the failure. Events of extreme precipitation or massive snowmelt can be predicted in advance, so evacuations can be planned with sufficient time. In the event of a structural failure due to earthquake, there may be no or limited warning time. The USGS Earthquake Hazards Program has several dam-safety related earthquake programs, including dam-specific earthquake monitoring programs in California to help monitor safety concerns following seismic events. San Mateo County and its planning partners have established protocols for emergency warning and response through its adopted emergency operations plan. The San Mateo Department of Emergency Management maintains copies of the most recent dam EAP and inundation maps, and it has used this information to plan notification needs for downstream areas in the event of a failure (San Mateo County OES, 2015). 8-7