What is dam failure?
Dams built in the Bay Area over the last 150 years were constructed using then current construction techniques and seismic knowledge of the time, and many without the benefit of government regulation. Dams built to hold the water in reservoirs can be damaged due to a huge storm and associated runoff, an earthquake, slope failures, or a terrorism event. Understanding the impact of a dam failure is critical for two reasons: (1) their catastrophic failure can cause many fatalities and destroy homes and other structures downstream from the facility, and (2) the storage capacity is lost and not recovered until the dam is rebuilt.
During the 1971 San Fernando earthquake shaking caused a major slide of the top thirty feet of the Lower San Fernando Dam. The dam was very close to completely failing. Eighty thousand people living downstream of the dam were immediately ordered to evacuate. At the time, there were no dam failure inundation maps available showing the areas which would be affected by a dam failure, and there were no planned evacuation procedures to follow.
How does dam evacuation planning take place?
As a result of the near failure of the Lower San Fernando Valley Dam, the Dam Safety Act was passed into law. This new law required dam owners to create maps showing areas that would be flooded if the dam failed. The State approved the maps and distributed them to local governments, who in turn adopt emergency procedures for the evacuation and control of areas in the event of a dam failure. The law required that each map be produced only once, without any requirements for updating. Furthermore, the scenario used to create the maps constrained the results to only a worst case situation that does not fit the historical evidence of how and why dams fail.
The maps were developed using engineering hydrology principals and represent the best estimate of where the water would flow if the dam completely failed with a full reservoir. The inundation pathway is based on completely emptying the reservoir and does not include run-off from storms. Had the maps have been developed more recently, different assumptions and map-making methods would have been used. In addition, dam inundation maps do not always indicate the depth of inundation and may represent only an inch of water over some inundation areas. In 1995, ABAG aggregated these maps into a single regional map.
Development downstream of dams, and upgrades to older dams have altered the inundation area of a dam, but the law does not require dam owners to update these maps and no new information is available on inundation areas. These maps still provide the best available estimate of the general location and extent of dam failure inundation areas. More detailed maps and dam failure hazard information may be available from the individual dam owner.
What is the likelihood of dam failure?
No quantitative probability information exists for the Bay Area dam failure hazard, in part because when a dam in known to have a failure potential, the water level is reduced to allow for partial collapse without loss of water as required by the State Division of Safety of Dams and by safety protocols established by dam owners. For example, the SF PUC is currently operating Calaveras Reservoir at less than 30% of capacity to avoid a catastrophic release of water. Thus, the probability of failure resulting in damage is approaching zero.
Dam Failure Inundation Maps
Source: This map is summarized by ABAG from maps prepared by dam owners and filed with California Office of Emergency Services to meet the requirements of SB 896, 1972. More info.
Interactive | Static Dam Failure Inundation Map – by city or region
Google Earth – Export map | Download Google Earth
Perkins, Moy and Lesser, 1988. Liability of Local Governments for Earthquake Hazards and Losses. Association of Bay Area Governments, Oakland, CA, pp. 114-116
California Government Code section 8589.5
Joint Committee on Seismic Safety of the California Legislature, 1972. The San Fernando Earthquake of February 9, 1971 and Public Policy, CA, pp. 14-37.
Association of Bay Area Governments, 2010. Taming Natural Disasters: Multi-Jurisdictional Local Hazard Mitigation Plan for the San Francisco Bay Area. http://quake.abag.ca.gov/mitigation/
Additional Information on Dam Failure
The Story of Hydraulic Fill Dams in California
Upper San Leandro Reservoir showing original hydraulic fill dam and newer replacement dam. Source – J. Perkins, ABAG
In hydraulic fills, materials are mixed with water and pumped to the fill location where they are poured into place. As the water drains, the sand settles in distinct layers that are prone to liquefaction failure. In the 1971 San Fernando earthquake, shaking and resulting liquefaction caused a major slide of the top thirty feet of the Lower San Fernando Dam. This hydraulic-fill dam was very close to completely failing. Eighty thousand people living downstream of the dam were immediately ordered to evacuate. Most hydraulic fill dams were deemed to be unsafe and have been replaced with other types of dams (usually rolled earth dams in the Bay Area). Various other standards for dam structures have been improved and applied.