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the length of heat waves.52 By the end of the century most of the region will average six heat waves a year, with the average longest heat wave lasting ten days. In addition to the more frequent occurrence and duration of heat waves, they are expected to occur in months the region historically hasn’t experienced extreme heat. Historically, extreme heat occurs between July and August, but in the future extreme heat will be an issue the region faces in both the Spring and Fall.53 Additional Hazards The hazards outlined in this chapter represent those that pose the greatest impacts to the Bay Area region as a whole. However, there are other hazards that may cause localized impacts or may pose less of a threat to the region due to lesser impacts or have lower likelihoods of occurring. They may be discussed in more detail in Local Hazard Mitigation Plans, as appropriate. These are discussed briefly below. These hazards, and many more, are characterized in the 2013 California State Hazard Mitigation Plan.54 5.10. Dam Failure The dams built in the Bay Area over the last 150 years were built without seismic or government regulation. Dams can be damaged by large storms and the associated runoff, an earthquake, slope failures, or a terrorism event. While dam failure is rare, their failure can be catastrophic, destroying downstream structures and killing people, while reducing water supply to the Bay Area until the dam is rebuilt. In the 1970s, the state mandated the development of maps showing potential inundation areas due to dam failure. However, the methodology of these maps was limited and they have not been updated since, so they are generally no longer used. Additionally, when a dam is known to have a failure potential, the water level is reduced to allow for partial collapse without loss of water, as required by the State Division of Safety of Dams. Dam owners are required to routinely inspect their facilities and reevaluate their safety in light of current engineering and seismology, and many Bay Area dams have been retrofitted because of this. There has never been a dam failure in the Bay Area. However, the potential property losses from catastrophic failure are enormous, considering the amount of development within potential inundation zones. Additionally, a dam is most likely to fail as a result of an earthquake, which would lead to its own catastrophic property damage. 52 Cayan, D., et al. (2009) 53 California Climate Change Center (2006) 54 http://hazardmitigation.calema.ca.gov/plan/state_multi-hazard_mitigation_plan_shmp