Item 26 - Adoption of the City of Santa Ana Emergency Operations Plan

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City of Santa Ana Emergency Operations Plan Part I Basic Plan Figure 70: Atmospheric Rivers The science behind atmospheric rivers An atmospheric river (AR) is a Flowing column of condensed water vapor in the atmosphere responsible for producing significant levels of rain and snow, especially in the western United States, When ARs move inland and sweep over the mountains, the water vapor rises and cools to create heavy precipitation. Though many ARs are weak systems that simply provide beneficial rain or snow, some of the larger, more powerful ARs can create extreme rainfall and floods capable of disrupting travel, inducing mudslides and causing catastrophic damage to life and property. Visit www.research.noaa.gov to learn more. Geography and Geology Orange County's 510,000 acres are mainly mountainous terrain from the northeast to the southeast and floodplain in the central and western section. These high mountains to the east and north of the County trap eastern -moving winter storms and prolong the rainfall. Some mountain peaks in Orange County receive more than forty inches of precipitation annually. The rainwater moves rapidly down the steep slopes and across the coastal plains on its way to the ocean. The County's rapid growth and transformation from an agricultural community to an urban community has changed flood control from containing large flows from the mountains and hills, to control of additional runoff produced by development of the plains. Although there is a countywide system of flood control facilities, the majority of these are inadequate for conveying runoff from major storms. Essentially all of the City of Santa Ana and most of Orange County is built out leaving little open land to absorb rainfall. The lack of open land forces water to remain on the surface and rapidly accumulate. In -fill building is a common practice in many areas. Developers tear down an older home, replacing the home with a larger structure covering more of the lot. Another potential source of flooding is "asphalt creep." The street space between the curbs of a street is a part of the flood control system. When water leaves property and accumulates in the street, it is directed toward the storm drain system. The carrying capacity of the street is determined by the width of the street and the height of the curbs along the street. Often, when resurfacing streets, a one to two-inch layer of asphalt is laid over the existing asphalt. This added layer of asphalt subtracts from the design capacity of the street to carry water. Thus, the original engineered capacity of the entire storm drain system is marginally reduced over time. 70