Item 26 - Updated Hazard Mitigation Plan

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Hazard Mitigation Plan | 2022 Flood Hazards - 80 - ZONE DESCRIPTION AH Areas with a 1% annual chance of shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30 -year mortgage. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones. AO River or stream flood hazard areas, and areas with a 1% or greater chance of shallow flooding each year, usually in the form of sheet flow, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Average flood depths derived from detailed analyses are shown within these zones. AR Areas with a temporarily increased flood risk due to the building or restoration of a flood control system (such as a levee or a dam). Mandatory flood insurance purchase requirements will apply, but rates will not exceed the rates for unnumbered A zones if the structure is built or restored in compliance with Zone AR floodplain management regulations. A99 Areas with a 1% annual chance of flooding that will be protected by a Federal flood control system where construction has reached specified legal requirements. No depths or base flood elevations are shown within these zones. Undetermined Risk Areas ZONE DESCRIPTION D Areas with possible but undetermined flood hazards. No flood hazard analysis has been conducted. Flood insurance rates are commensurate with the uncertainty of the flood risk. Dam Failure Failed dams can create floods that are catastrophic to life and property as a result of the tremendous energy of the released water. A catastrophic dam failure could easily overwhelm local response capabilities and require mass evacuations to save lives. Dams typically are constructed of earth, rock, concrete, or mining tailings. Two factors that influence the potential severity of a full or partial dam failure are the amount of water impounded and the density, type, and value of development and infrastructure located downstream. Prado Dam and Reservoir, completed by the Corps of Engineers in 1941, are intended to provide flood protection to the Lower Santa Ana River basin. The earthen dam and its reservoir were designed in the 1930s to control floods of magnitudes that could be reasonably expected to occur under anticipated future development of the watershed (typically a 200-year flood). Since Prado Dam was built, however, changes have occurred in the drainage area. Historical data on rainfall and runoff, coupled with advances in predicting future flood potential, have shown Prado Dam to presently offer only 70-year flood protection. In addition, intensive urbanization within the drainage area has occurred, further complicating this problem. Another serious concern is that the existing Prado Dam and spillway could not accommodate a probable maximum flood, resulting in overtopping of the dam. \