What to Expect When Dams Fail
Vulnerability is the susceptibility of population, human services, transportation, other infrastructure, and the economy to damage. Vulnerability is essentially the linkage between hazard and loss. It is critical to current and future hazard exposure analysis. No assessment of state vulnerability or potential losses is available at this time, but 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 California Office of Emergency Services (CalOES) server and published annually as DVDs. These DVDs are provided without cost to both governmental agencies and non-governmental parties upon request.
Based upon reported damages from past floods, the following can be considered vulnerable to current and future damage and loss if a dam fails and causes extensive flooding.
- Loss of human life.
- Livestock from drowning.
- Furnishings, equipment, personal property and basements.
- Roads and ditch lines from mud and rockslides.
- Roads and bridges due to washout, road surface and road bed failure.
- Rural water supplies (wells, springs and cisterns) due to contamination from surface water entering the supply source.
- Heat, water and electrical sources cut off due to rising water.
- Drainage from catch basins and retention ponds. If they cannot handle the volume of water, they cause back-up flooding.
- Homes are destroyed by deep, fast moving floods – characteristics of the floodway portion of the floodplain.
- Municipal water and sewage treatment plants can become inoperable if levees and retaining walls are overtopped and/or if sediment basins are flooded.
- The supply of raw water from municipal water supply reservoirs and back up water supply reservoirs can be reduced or lost due to the failure of an impounding dam.
- In essential facilities, electrical panels and circuit breakers are often installed on interior walls below the 100-year flood level. This results in loss of power when the water rises to the level of the panels.
- Water can enter otherwise protected facilities through non-flood proofed mechanical and electrical rooms and through conduits.
- Bridges, culverts and stream crossings may be unable to handle the volume, causing backup onto roads and into residential and commercial structures.
- Backup of water can be caused or increased when automobile-bodies, refrigerators and other appliances that have been discarded into streambeds hinder the natural flow of water. Backup also occurs if drainage systems have not been properly maintained.
- Backup water can enter storm sewers and cause flooding in areas not threatened by stream flooding.
- Levees constructed by the Corps of Engineers are sometimes inadequate to hold back the volume of water resulting in the failure of the structure.
- Many developed areas have failed to provide for the excessive run-off caused by concrete and blacktop ground coverage.
- Detention basins, retaining walls and berms are designed to redirect water from vulnerable areas. Flooding often results when these protective measures are not in existence or have not been properly maintained.
The 2013 California Multi-Hazard Mitigation Plan contains a section on Dam Failure, and notes that 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 of the dam crest and eventual dam breach. "Piping" of earth dams is another common form of failure. Piping is a form of erosion that occurs underground caused by rodent burrowing 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. The state Department of Water Resources (DWR) Division of Safety of Dams (DSOD) came into existence as a direct result of this catastrophe. Another significant dam failures in California's recent history is the Baldwin Hills Dam in 1963. Then came the near-failure of the Lower San Fernando Dam in 1971.
One Dam Fails and Another Hangs On, Barely
The failure of the Baldwin Hills Dam in Los Angeles led to the amendment of the California Water Code to include within state jurisdiction both new and existing off-stream storage facilities. The dam was completed in 1951 by the Los Angeles Department of Water and Power and was built on an active fault line which was subsidiary to the nearby Newport-Inglewood fault. The catastrophic failure flooded the residential neighborhoods surrounding it. It began with signs of lining failure, followed by increasingly serious leakage through the dam. After three hours the dam breached, with a total release of 250 million gallons, resulting in five deaths and the destruction of 277 homes. Vigorous rescue efforts averted a greater loss of life.
The Lower San Fernando Dam reservoir was the terminus of the main aqueduct system for Los Angeles, which supplied 80% of the city's water. Moments after the 6.7 magnitude San Fernando earthquake of 1971, only a thin dirt wall stood between 80,000 people in the San Fernando Valley and 15 million tons of water poised behind a heavily damaged dam. The 142-foot-high Lower San Fernando Dam was perilously close to failure. At any moment a strong aftershock could have triggered a disaster. As it was, residents in an 11-square-mile area were forced to evacuate, while the water behind the earthen dam was lowered, a process that took 3 days.
Damage from the magnitude 6.7 shock was so heavy that the dam could not be repaired to safely hold its water supply in the event of another large earthquake. The replacement Los Angeles Dam and Reservoir was built about 3,000 feet up the valley from the old Lower San Fernando Dam, which was reconstructed to provide a holding basin for storm water and to back up the new dam. The 1994 Northridge earthquake put the Los Angeles Dam to the test. The Northridge earthquake was almost equal in magnitude to the previous San Fernando earthquake. Ground shaking was very strong, with amplitudes among the highest ever recorded. Yet the dam showed only minor deformation and superficial cracking. Despite the intense shaking, the crest of the dam moved only 1 inch sideways and settled only 3.5 inches.
Mitigation of dam failure is constantly occurring at both the federal and state level. For example, the U.S. Bureau of Reclamation is planning to replace the longest earthen section of Folsom Lake's dam in order to mitigate earthquake damage. At the state level, officials are currently reviewing an $84 million project to remove a 106‐foot‐high dam on the Carmel River to mitigate earthquake damage and deal with flood safety issues.
Another major remediation projects currently underway is Lake Perris Dam, which is 60 miles southeast of downtown Los Angeles and 11 miles southeast of Riverside. Recreation there draws 1.1 million visitors each year, but in 2005 the DWR identified potential seismic safety risks in the foundation of the dam. While there was no imminent threat to life or property, in the interest of ensuring public safety, the water level of Lake Perris was lowered. With study work completed, the retrofitting of existing structures and construction of new features are expected to be completed in 2017.
These are just a few examples of the numerous dam mitigation projects currently being undertaken. As of July 2010, remediation was identified or under construction at 102 dams, of which 72 are HHP.