Item 26 - Updated Hazard Mitigation Plan

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Hazard Mitigation Plan | 2022 Flood Hazards - 74 - According to the Vulnerability Assessment Report (2020), flooding can cause significant harm to buildings, people, and infrastructure. Floodwater can be deep enough to drown people and may move fast enough to carry people or heavy objects (such as cars) away. Flooding can be caused by heavy rainfall or long periods of moderate rainfall, or clogged drains during periods of rainfall. In rare instances, a break in a dam, water pipe, or water tank can also cause flooding. Additionally, heavy periods of rainfall can stress levee systems, and overtopping can lead to catastrophic flooding. Storm drainage systems throughout the city collect stormwater runoff and convey water to prevent flooding, although these systems are typically designed based on winter storms recorded in the past and may not be designed to accommodate more intense storms. The levee system along the Santa Ana River is designed for a 190-year flood and may also not be able to accommodate a more intense storm or heavy rainfall over a short period of time. Scientists project that climate change will increase the frequency and intensity of floods within Santa Ana, although total annual precipitation levels are not expected to change very much. The northern, southern, and western portions of Santa Ana are within the 100-year or 500-year flood zone, or an area with reduced flood risk due to a levee. The portion of Santa Ana east of Broadway is outside of the flood zone. Due to a projected increase in frequency and intensity of rainstorms, flooding could occur more frequently, leaving major roadways and highways, bridges, and railroads highly vulnerable to damage. While parks and open space can absorb stormwater, park facilities such as restrooms and pedestrian paths can be damaged by floodwaters, rendering the parks unusable. Other buildings east of Main Street are also vulnerable to flooding, including commercial buildings in Downtown Santa Ana, industrial buildings in western Santa Ana near the Santa Ana River, and over half of school buildings, including the Santa Ana Community College. Many of the community services are also highly vulnerable to flooding, such as wastewater services that can back up due to an increase in wet weather flow and roadways that can become impassable from flood waters, disrupting public transit services, emergency medical response, and public safety response. Disruption of these services could be detrimental to both residents and businesses. Community services, such as emergency medical response, are highly vulnerable to human health hazards, such as vector-borne diseases, that can overwhelm and create shortages of facilities, equipment, pharmaceuticals, and personnel if health care workers become sick or if supply chains are disrupted. Santa Ana River and Santiago Creek According to the Public Safety Element of Santa Ana’s General Plan (2010), the City has two major drainage courses with potential for significant flooding: Santiago Creek and the Santa Ana River. The Santa Ana River, with its normally dry riverbed and broad engineered channel and armored levees, does not appear to present a significant flood hazard. However, the Santa Ana River has a long history of overflowing its banks and flooding in the surrounding areas. Another reference document concerning flooding is the Santa Ana Storm Drain Master Drain (2018) which is referenced in the Mitigation Actions Matrix. To reduce this risk, the U.S. Army Corps of Engineers (Corps) is constructing the $2.2 billion Santa Ana River Mainstem Project which includes raising the height of Prado Dam and constructing new gate-works allowing for controlled water releases up to 30,000 cubic feet per second and constructing channel improvements between Prado Dam and the ocean. Given the progress to date, the Corps’ project provides 100-year flood protection for Orange County. When completed, the project is designed to provide 190-year level protection from flooding due to the Santa Ana River. Located in Riverside County at the head of Santa Ana Canyon, Prado Dam is a single purpose dam designed to reduce the flood risk for metropolitan area in Orange County.