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 Prado Dam Seepage - In January 2005, due to preceding storm activity that produced near -record water levels behind Prado Dam, the water surface elevation behind the dam peaked at 527.4 feet above sea level. On January 13, the U.S. Army Corps of Engineers (USACE) discovered seepage from the downstream face of Prado Dam. The seepage was located in an area that was under construction to build new outlet works as part of an overall flood control improvement to Prado Dam. As a precautionary measure, Corona city officials evacuated over 800 homes below the dam and Orange County officials relocated campers in the Canyon RV Park because of their proximity to the adjacent floodplain. To decrease the amount of water behind Prado Dam, the release of water was increased from 5,000 cubic feet per second (cfs) to 10,000 cfs to reduce the level of water being held to 505 feet. In addition to the increase in water release, USACE began holding back floodwaters upstream at both the San Antonio Dam in Los Angeles County and Seven Oaks Dam near Redlands to reduce the inflow of water to Prado Dam. As the water level was lowered, the hydraulic pressure on the dam abutment subject to seepage was reduced. When the water was reduced to 505 feet on January 17, USACE was able to start the reconditioning of the dam in order to be ready for subsequent inflows to the dam. Oroville Dam - Oroville Dam in northern California is the tallest earthen dam in the United States and holds more than 1.1 trillion gallons of water in Lake Oroville, the second-largest manmade lake in California. Because of unusually high rainfall in early 2017, Lake Oroville began reaching its storage capacity and dam operators released a flow of water down the main spillway (basically a concrete ramp) and into the Feather River. This high-water flow surged over the side of the spillway and began eroding the soil underneath, causing the concrete of the spillway to break apart, and operators were forced to shut down the release of water. Continuing rainfall runoff caused the dam to begin overtopping the emergency spillway, which consisted of a concrete berm on the inside of the dam, with a bare soil hillside on the other side. As the water flowed down this hillside, the soil eroded away much faster than expected. If enough of the hillside washed away, the wall of the dam would be unsupported and could break apart, suddenly releasing a major portion of Lake Oroville down the Feather River. On February 12, officials ordered the evacuation of 188,000 people in the Feather River Basin throughout Butte, Yuba and Sutter Counties, until the hillside was stabilized and sufficient water drained from the lake on February 14. Dams and Reservoirs Affecting the City of Santa Ana The City of Santa Ana owns ten water reservoirs at eight locations throughout the City, ranging from a capacity of 1 million gallons to 7 million gallons. Some of these reservoirs store water in above ground tanks while others store water primarily at or below ground level. For above ground tanks, collapse brought on by earthquake, acts of sabotage or other catastrophic failure may result in (in addition to the loss of water supply) substantial localized flooding; and damage to buildings, vehicles, properties, roadways, utility equipment or other structures, and injury to any persons, in the path of the spilling water. For tanks storing water primarily at or below ground level, failure would not cause widespread surface flooding, damage, or injuries. However, failure of these reservoirs may cause (in addition to the loss of water supply) damage to underground utility lines, seepage into the foundations of nearby structures, and sub -surface erosion to roadways and other properties in the area. The City also owns a 140-foot tall -elevated water tank, storing up to 1 million gallons of water, in the area of 14t' Street and the 5 Freeway. Sudden failure of this structure could result in collapse onto neighboring buildings immediately adjacent to the tank, including an elementary school, and/or localized flooding in the immediate area. There are three large dams that are outside of the City but are capable of creating substantial flooding in the City in the event of catastrophic failures. These include Prado Dam, Villa Park Dam and Santiago Creek Dam and these are described below. Prado Dam — Prado Dam is a flood control and water conservation project constructed and operated by the U.S. Army Corps of Engineers. The dam is located on the Santa Ana River approximately 30 miles upstream of the Pacific Ocean and 20 miles upstream of the City of Santa Ana. The dam embankment is located in Riverside County on the north side of the 91 Freeway, approximately 2 miles west of the City of Corona and just east of the 78