Part iii-d – Dam Failure

Dam Failure in California

Overview of Dams

What is a Dam?

A dam is a barrier preventing the flow of water or loose solid materials (such as soil or snow); or a barrier built across a watercourse for impounding water. Dams are artificial barriers, which are 25 feet or more in height or have an impounding capacity of 50 acre-feet or more.

Advantages of Dams

Dams are important because they provide water for drinking, industry, irrigation, fishing and recreation, water for hydroelectric power production, water for navigation in rivers, and other needs. Dams also protect people by reducing or preventing floods. The primary purpose of this report is to discuss the Prado and Seven Oak Dams and the flood protection and flood threat they provide to the Coast Community College District service area.

Causes of Dam Failure

Dam failures can result from a number of natural or man-made causes such as earthquakes, erosion of the face or foundation, and improper sitting of the dam, rapidly rising floodwaters, and structural/design flaws. Dam failure can result in severe flood events to lower-lying areas. A dam failure from the Seven Oaks and/or Prado Dams could severely impact most of coastal Orange County and the CCCD service area.

Impacts of Dam Failure

A dam failure may cause loss of life, damage to property, and other ensuing hazards, as well as the displacement of persons residing in the inundation path. Damage to electric generating facilities and transmission lines as well as sewer and water facilities could also impact life support systems in communities outside the immediate hazard areas.

A catastrophic dam failure, depending on the size of the dam and the population downstream, could exceed the response capability of local communities especially overtaxing the public safety personnel and resources. Damage control and disaster relief support would be required from other local government agencies, private organizations, the Orange County Operational Area, the State of California, and possibly the Federal government.

Identifying Dam Failure Hazards

Dam failure is the uncontrolled release of impounded water from behind a dam. Flooding earthquakes, blockages, landslides, lack of maintenance, improper operation, poor construction, vandalism, and terrorism can all cause a dam to fail. Dam failure causes downstream flooding that can affect life and property.

Profiling Dam Failure Hazards

Dams and reservoirs of jurisdictional size are defined in the California Water Code Sections 6000 through 6008. There are currently more than 1,400 dams of jurisdictional size in California. Approximately 1,250 of these dams are under the jurisdiction of California’s Department of Water Resources, Division of Safety of Dams. Dams and reservoirs owned by the federal government are not subject to state jurisdiction except as otherwise provided by federal law. In California, there are currently 149 dams owned by federal government agencies such as the United States Forest Service, Bureau of Reclamation, Army Corps of Engineers and the U.S. Military.
Los Angeles leads the state as being the county with the most jurisdictional-size dams, with 100 dams. The county of Sonoma is second behind Los Angeles with 63 dams. Del Norte County is the only county in the state having no dams of jurisdictional size.
The term “dam failure” encompasses a wide variety of circumstances. Situations that would constitute a dam failure vary widely, from developing problems to a partial or catastrophic collapse of the entire dam. Potential causes of a dam failure are numerous and can be attributed to deficiencies in the original design of the dam, the quality of construction, the maintenance of the dam and operation of the appurtenances while the dam is in operation, and acts of nature including precipitation in excess of the design, flood and damage from earthquakes. Water over-topping the dam crest is a common cause of failure in earth dams. Overtopping will cause erosion and the dam crest and eventual dam breach. Piping of each dam is another common form of failure. Piping is a form of erosion that occurs underground caused by rodent burring and the presence of extensive root systems from vegetation growing on and around the dam.
In the past 50 years, there have been only a small number of dam failures in California. The most catastrophic dam failure in California’s history is that of the infamous St. Francis Dam in Los Angeles County, which failed in March 1928 shortly after construction of the dam was completed. This failure resulted in the deaths of more than 450 people and destruction of nearly 1,000 homes and buildings. Numerous roads and bridges were also destroyed and/or damaged beyond repair. The Division of Safety of Dams came into existence as a direct result of this catastrophe. Other significant dam failures in California’s history include Baldwin Hills in 1963 and the near-failure of the Lower San Fernando Dam in 1971.

Assessment of State Vulnerability and Potential Losses

No assessment of state vulnerability or potential losses is available in the State Hazard Mitigation Plan.

Assessment of Local Vulnerability and Potential Losses

Information related to community vulnerability and loss assessments may be found in Local Hazard Mitigation Plans. Local planning departments have access to the state’s inventory of inundation maps, which are kept on a server and published annually as DVDs.

Types of Dams

There are four general types of dams: Arch, Buttress, Gravity (Prado), and Embankment (Seven Oaks). Each of these types of dams has different failure characteristics.

Arch Dams

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