Earthquakes and HHP Dams
The 2013 California Multi-Hazard Mitigation Plan provides a comprehensive look at the disaster risks across the state, along with the planning and actions underway to reduce and/or address those risks. Dam owners and citizens who live, work or travel near HHP dams should consider the implications of earthquakes for the integrity and possible failure of dams. The research and assessments contained in the Mitigation Plan further underscore the need for comprehensive EAPs on all HHP dams.
While seismic activity must be taken into account in the engineering and construction of newer dams, older dams that have not been retrofitted to withstand seismic activity present higher risk. The dams do not have to be right over a fault to be at risk. Observations of damage from California earthquakes have shown that ground shaking may be locally attenuated but then be amplified farther away due to differential soil conditions and structural response to the shaking. The Mitigation Plan notes that liquefaction of soils from strong ground shaking is the greatest weakness of the system of canals, tunnels, dams and reservoirs that provide water to heavily populated areas of Southern California. The Plan further states:
During the 1971 Sylmar Earthquake, the Lower San Fernando Dam, which is upstream from a heavily populated area, was severely damaged from liquefaction. Though heavily damaged, the dam was not breached and no dam failure induced flooding occurred. Later, another dam and a reservoir were built upstream from the Lower San Fernando Dam. The San Fernando Dam, which was being used only for flood control purposes, was damaged again during the 1994 Northridge Earthquake. Several other dams have experienced damage during earthquakes. The Department of Water Resources (DWR) Division of Safety of Dams (DSOD) has been working with dam owners to periodically assess the safety of dams in their jurisdictions, and several dam owners have rehabilitated their dams.
Three major seismic hazard mitigation efforts include the East Side Reservoir Project in Riverside County, the Olivehain Dam in San Diego County and the Calaveras Dam Replacement Project. The East Side Reservoir Project includes canals, pipeworks, a new dam, and a reservoir intended to provide water to a large portion of the Los Angeles metropolitan region for up to six months should an earthquake take the California Aqueduct out of service. The Olivehain Dam and reservoir are intended to provide San Diego with water should there be interruptions of water from the Colorado River after earthquakes.
The Dam Failure section of the Mitigation Plan cites earthquakes among many causes of dam failure. Flooding, lack of maintenance, improper operation, poor construction, vandalism and terrorism also are mentioned. Negligence and poor construction may not cause a dam to fail, but could contribute to a dam failure during the stress of an earthquake. This underscores the importance of dam owners properly inspecting and maintaining their dams, and working with DSOD engineers to determine if remediation is needed.
Los Angeles leads the state as the county with the most state-regulated dams, with 100. Sonoma County is second with 63 state-regulated dams. News stories note that after the two recent back-to-back earthquakes – a relatively small one with a magnitude of 3.6, followed by a long and rolling 5.1 quake – Los Angeles was shaken by nearly 175 smaller aftershocks. It was the first time the area suffered an earthquake in excess of magnitude 5 since 1997, and it came two weeks after a 4.4 earthquake jolted residents awake.
Earthquake authorities with the U.S. Geological Survey (USGS) and other institutions believe the Puente Hills fault where these quakes were said to have occurred could produce an event that would be more destructive than the long-feared "big one" on the San Andreas fault. The Puente Hills thrust fault is dangerous because of its location, running from the suburbs of northern Orange County, through the San Gabriel Valley and under the skyscrapers of downtown Los Angeles before ending in Hollywood.
The Puente Hills fault could be especially hazardous over a larger area because of its shape. Other local faults, like the Newport-Inglewood and Hollywood, are a collection of vertical cracks, with the most intense shaking occurring near where the fault reaches the surface. The Puente Hills is a horizontal fault, with intense shaking likely to be felt over a much larger area, roughly 25 by 15 miles. That is sufficient to include some of the Los Angeles County dams, and even more dams might be reached by amplification and liquefaction extending far beyond the immediate area.
Remediation projects on dozens of dams, most of which are HHP, have been completed and even more are in progress but will take several years to complete. These improvements will help dams survive some seismic activity, but EAPs for the dams will remain critically important to help dam owners and the public understand and address the risks associated with earthquakes near HHP dams.