Part iii-d – Dam Failure

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There are two flood control dams that control flood flow in the Santa Ana River. The Seven Oaks Dam is located on the river in the upper Santa Ana Canyon about eight miles northeast of the City of Redlands in San Bernardino County. This dam was completed in 1999. The Prado Dam is about 40 miles downstream of the Seven Oaks Dam. Approximately 47 billion gallons (145,600 acre-feet) of water can be stored in the Seven Oaks reservoir and 61 billion gallons (187,600 acre-feet) in the Prado Dam, reservoir.

In the event of Prado Dam failure, floodwaters would flow through the Santa Ana Canyon on its way to the Pacific Ocean. The flood would range from about 3,000 feet wide in the canyon to over 15 miles wide downstream at the Santa Ana Freeway (Interstate 5). The flooding would impact over one million people and 110,000 acres.

Most of the CCCD service area lies within the Santa Ana River floodplain. The Santa Ana River begins near the Mt. San Gorgonio summit in the San Bernardino Mountains at an elevation of over 8,000 feet. Its course drains over 90 miles through both the San Bernardino and Santa Ana ranges; 27 miles of the river course flows through Orange County to the Pacific Ocean. The Santa Ana River drainage basin covers 2,450 square miles and is the largest basin in southern California. This was once considered the “worst flood threat west of the Mississippi River.” However, with the Main Stem mitigation project on the dams and flood control channels, the flood threat has been decreased.

lower santa ana river mouth
Santa Ana River Project

Exerts from the Orange County Flood Control Division

Lower Santa Ana River Improvements: Improve 23 miles of existing channel from Weir Canyon Road to the Pacific Ocean include channel widening, improvement to the existing Greenville-Banning Channel located parallel to the river near the coast, relocation of Talbert Channel ocean outlet and construction of rock jetties and derrick stone jetties at the mouth of the river, and bridge modifications to accommodate the widened channel. In the Santa Ana Canyon area, construction will be limited to levee extension and a dike to protect a mobile home park. The construction in the Santa Ana Canyon Area also include the bank protection on the south side along the SR-91(upstream of SAVI Ranch) and on the north side upstream of Weir Canyon Road. In the future, the stretch of channel from Coal Canyon to the Green River Golf Course entrance bridge will be improved.
Lower Santa Ana River Project Features

photo 1 - concrete lining of river bank, photo 2 - waterfowl on the sar

  • Improve 23 mile channel from Prado Dam to the Pacific Ocean

  • Restore/Enhance 92 acre (8-acre mitigation) wetlands

  • Estimated Cost: $367 million

  • Acquire 1,123 acres of canyon lands to ensure safe releases from Prado Dam and provide open space habitat

  • Modify 37 bridges

  • Landscaping and esthetic treatment

  • Sucker Conservation Program

  • Habitat Management Plan

  • Cooperation with the SARI proponents

Santa Ana Mainstem Mitigation Project

The Santa Ana Mainstem Project extends approximately 75 miles along the Santa Ana River from the upper canyon in the San Bernardino Mountains downstream to the Pacific Ocean at Newport Beach. The project provides urban flood protection to growing communities in Orange, Riverside, and San Bernardino counties. The system is designed to provide various levels of flood protection ranging from 100 to 190 years for areas most susceptible to damages from flooding.
The Mainstem Project includes the following:

Seven Oaks Dam in the upper Santa Ana River Canyon, to control a 350-year flood event at the dam site

5.4 miles of trapezoid-shaped channel and 18 sediment basins inside the channel of San Timoteo Creek in the cities of San Bernardino, Loma Linda, Redlands, and Colton

Delineation of the 100-year floodway and floodway fringe between Seven Oaks Dam and Prado Dam

Local authorities now manage this area according to guidelines established by the Federal Emergency Management Agency (FEMA)

Modifications to the existing federal flood control levees at Mill Creek to restore the original Standard Project Flood level of protection

Construction of a 100-year level of flood protection channel on the Oak Street Drain in the City of Corona

Changes in the existing Prado Dam to provided a 190-year level of protection

Channel improvements along Santiago Creek in Orange County provided 100-year-level flood protection

Construction of the lower Santa Ana River channel provided 190-year level flood protection

Enhancement of 84 acres of degraded marshland at the mouth of the Santa Ana River for endangered species and the restoration of eight acres of marshland for wildlife habitat

Construction of the Mainstem project started in 1990 and various stages have been phased-in based on budget approval and appropriations, engineering requirements, safety and environmental scheduling windows.

Figure 2 - - Santa Ana Mainstem Map

army corps oc flood

Fourteen of the fifteen CCCD sites were in the Santa Ana River Mainstem Project improvement area. The Santa Ana River Mainstem Project is a perfect example of a successful federal and multi-county joint hazard mitigation project.

Local History of Dam Failure

The CCCD service area has never been impacted by a major dam failure. However, on January 13 and 14, 2005, Prado Dam had seepage from the dam. The following are news articles and personal recollections of the incident:
A great deal of water escaped into the sea during big storms in 2005. The Jan. 13-14, 2005, deluge underscored the risk of flooding. It triggered seepage in a dike behind the dam, resulting in evacuations of some Corona residents as well as pupils at Riverdale and Woodsboro elementary schools in Anaheim Hills. OC Register
On January 14, 2005, after days of heavy rain, water began seeping through an earthen extension. Authorities released water in order to relieve pressure and sent a flood warning to areas downriver of the dam. Over 3,000 residents were evacuated from their homes for nearly twenty-four hours for fear of flooding. The gymnasium at Corona High School was converted by the American Red Cross into a temporary shelter. Wikipedia
CORONA, Calif. Authorities are evacuating 500 homes and 330 mobile homes this morning because of a possible leak in the Prado Dam east of Los Angeles. The neighborhood is near the city of Corona and north of the 91 freeway.

Corona police Sergeant Jerry Rodriguez says a series of rainstorms caused water level to rise, to the point where the dam is releasing 10-thousand cubic feet of water per second.

He says the evacuation is a "precautionary measure.” KESQ News
The following is the personal recollection of Glorria Morrison, City of Huntington Beach Emergency Services Coordinator on January 13 and 14 incidents in 2005. (Glorria Morrison is the plan contractor.)
Cities and other public agencies were all notified of the Prado Dam January 2005 incident by the Orange County Operational Area Emergency Operations Center (EOC). They were told that the heavy rains had caused a potential problem at Prado Dam. Huntington Beach is at the mouth of the Santa Ana River and parts of the city are at sea level and some areas below sea level. 75% of the City is in the Prado Dam Inundation Zone including Golden West College. That equates to 150,000 people. The Huntington Beach EOC had completed a Natural Hazard Mitigation Plan in 2004 and one of the hazards researched was on Prado Dam. The maps and information from the plan were reviewed and duplicated on the day of the incident. The Power Point slides and maps used at the Hazard Mitigation Community Forum were set up in the EOC. The Hazard Mitigation Plan, Prado Dam section was studied. We learned that if Prado Dam failed that it would take 8 hours for the water to arrive in Huntington Beach and it could reach heights of 8 feet in depth. The Huntington Beach EOC was activated.
High level management representatives were activated from the following departments:

Police Chief and key department personnel (responsible for alerting and warning the public and evacuations)

Fire Chief and key department personnel (responsible for emergency medical, fire suppression, search & rescue, and hazardous materials)

Public Works Director, City Engineer and key department personnel (responsible for infrastructure (water, sewers, roads, flood channels, etc), and debris cleanup)

Marine Safety Chief

EOC personnel

Not much information was available from the Army Corps of Engineers. The city of Corona in Riverside County was evacuating areas of the city. An American Red Cross shelter was set up for their evacuees. The city of Huntington Beach EOC decided to develop an Incident Action Plan to evacuate impact areas and work with cities at higher elevations to shelter evacuees. The plan would only be activated if the situation worsened and the Army Corps of Engineers ordered evacuations. The Huntington Beach EOC personnel were determined to be ready to activate the plan at a moment’s notice; to be prepared to activate and brief the Director of Emergency Services, City Council, all EOC personnel as well as Police, Fire, Public Works and Marine Safety field units. All personnel were taken out of training sessions and remained in fire and police stations. The city was prepared to respond but fortunately, the Army Corps of Engineers repaired the problem and never had to order an evacuation. The incident was informative for all EOC and public safety personnel and created an awareness of the threat Prado Dam poses to the coastal areas of Orange County. The Hazard Mitigation Plan was invaluable information during this incident!

prado dam Peak Elevation Distance and Time

The following are examples of “distance from the dam” and “peak elevation times” to assist in planning evacuations. Evacuation areas would include all residents in low-lying areas.

Table -Prado Dam Water Movement Timeline

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