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

Home Browse Search

City of Santa Ana Emergency Operations Plan Part I Basic Plan Characteristics of Flooding Two types of flooding primarily affect the Orange County region: Riverine flooding and urban flooding. In addition, any low-lying area has the potential to flood. The flooding of developed areas may occur when the amount of water generated from rainfall and runoff exceeds a storm water system's capability to remove it. Riverine Flooding Riverine flooding is the overbank flooding of rivers and streams. This flooding typically results from large-scale weather systems generating prolonged rainfall over a wide geographic area. Flooding occurs in hundreds of smaller streams, which then drain into the major rivers. When the accumulated water exceeds the carrying capacity of the waterway, the water overflows out across the floodplain. Urban Flooding As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization of a watershed changes the hydrologic systems of the basin. Heavy rainfall collects and flows faster on impervious concrete and asphalt surfaces. The water moves from the clouds, to the ground, and into streams at a much faster rate in urban areas. Adding these elements to the hydrological systems can result in floodwaters that rise very rapidly, peaking with violent force. Further, portions of the County not inundated by river overflow during a 100-year event could be subject to flooding from overflow of water drainage facilities currently inadequate for carrying the 100-year discharge. Local Flood Hazard The Santa Ana River, flowing through the City of Santa Ana to the Pacific Ocean, is Orange County's greatest flood threat. The River enters Orange County from the Prado Dam, just across the Riverside County line adjacent to the 91 Freeway. The Prado Dam collects rainfall from across a 700 square mile watershed. Additionally, Santiago Creek enters the northeastern corner of the City and flows west into the Santa Ana River. The Creek flows through the Villa Park Dam, which collects rainfall from across an 81 square mile watershed. Despite extensive efforts at flood control protection, it appears that portions of the City which would not be inundated by river overflow during a 100-year event could be subject to flooding from overflowing of storm drains and other water drainage infrastructure that are presently inadequate for carrying the 100-year discharge. The East Garden Grove-Wintersburg Channel is one of the underlying channel systems of the Santa Ana River floodplain, which passes north -south through the west end of the City near the border with the City of Garden Grove. This drainage system does not have the capacity to contain a 100-year flood and the channel banks and levees become overtopped at several locations, causing recurring flooding in severe storms. Flood damages along this Channel affect residential, commercial, and industrial development within the cities of Santa Ana, Westminster, Garden Grove, Huntington Beach and Fountain Valley. The East Garden Grove-Wintersburg Channel was constructed in the early 1960s as a mixture of earthen, riprap, and concrete -lined trapezoidal sections with short reaches of concrete rectangular and covered box facilities. It was designed to carry 25-year peak discharge which was the design standard at the time the channels were constructed. With urbanization growth throughout Orange County, the existing capacity has become deficient and needs to be improved to convey a 100-year peak discharge. The hundreds of homes in the downstream segment of the channel system could be subjected to an estimated 8-foot depth of flooding if a 100-year storm event occurred today. The winter storms of 2005 in this area severely eroded the maintenance roads and levee banks. Today, this Channel is considered Orange County's most flood -prone location. Other flood control channels within the City of Santa Ana and subject to flooding during severe storms include the Greenville -Banning, Santa Ana -Delhi, Santa Ana -Santa Fe, Santa Ana Gardens, Southwest Tustin, and Lane Channels. Flood History The infrequency of very large floods further obscures the County's flood hazard. For some particularly disastrous storms, a false sense of security prevailed following long periods of mild or semi -arid years. 73