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Climate Change 17.2 IMPACTS ON HAZARDS OF CONCERN The following sections provide information on how each identified hazard of concern for this planning process may be impacted by climate change and how these impacts may alter current exposure and vulnerability to these hazards for the people, property, critical facilities, and the environment in the planning area. 17.2.1 Dam Failure Climate Change Impacts on the Hazard The California Fourth Climate Change Assessment identifies expected changes to rainfall and winter storm patterns. On average, changes in California’s annual precipitation levels are not expected to be dramatic; however, the increase in frequency and intensity for the largest storms (atmospheric rivers) may pose increasing risks to San Mateo County critical facilities, including dams. Dams are designed partly based on assumptions about a river’s flow behavior, expressed as hydrographs. Changes in weather patterns can have significant effects on the hydrograph used for the design of a dam. If the hygrograph changes, it is conceivable that the dam can lose some or all of its designed margin of safety, also known as freeboard. If freeboard is reduced, dam operators may be forced to release increased volumes earlier in a storm cycle in order to maintain the required margins of safety. Such early releases of increased volumes can increase flood potential downstream. According to the California Department of Water Resources, flood flows on many California rivers have been record-setting since the 1950s. This means that water infrastructure, such as dams, have been forced to manage flows for which they were not designed. The California Division of Dam Safety has indicated that climate change may result in the need for increased safety precautions to address higher winter runoff, frequent fluctuations of water levels, and increased potential for sedimentation and debris accumulation from changing erosion patterns and increases in wildfires. According to the Division, climate change also will impact the ability of dam operators to estimate extreme flood events (California Department of Water Resources, 2008). Dams are constructed with safety features known as “spillways.” Spillways are put in place on dams as a safety measure in the event of the reservoir filling too quickly. Spillway overflow events, often referred to as “design failures,” result in increased discharges downstream and increased flooding potential. Although climate change will not increase the probability of catastrophic dam failure, it may increase the probability of design failures, leading to the need for dam spillways to be designed to handle larger volumes of water over long time periods. Exposure, Sensitivity and Vulnerability The following summarizes changes in exposure and vulnerability to the dam failure hazard resulting from climate change:  Population—Population exposure and vulnerability to the dam failure hazard will likely increase as a result of climate change.  Property—Property exposure and vulnerability to the dam failure hazard will likely increase as a result of climate change.  Critical facilities—The exposure and vulnerability of critical facilities are likely to change as result of climate change. Dam owners and operators are sensitive to the risk and may need to alter maintenance and operations to account for changes in the hydrograph, increased frequency of atmospheric river events and increased sedimentation. 17-11