A disaster worse than Katrina would threaten Sacramento if an Auburn Dam were built
The catastrophe of New Orleans was man-made. Levees were built to withstand only Category-3 hurricanes and improvements were deferred. When Katrina overwhelmed the levees much of the city was destroyed and many residents died.
An even worse man-made disaster would threaten Sacramento if an Auburn Dam is built. Because there are no more good dam sites in the region, the original Auburn Dam was being built at a risky location within the Bear Mountain fault zone. When the northward extension of this fault ruptured near Oroville in 1975, causing a magnitude 5.7 earthquake, construction was halted in Auburn.
The U.S. Bureau of Reclamation, which builds and operates many large dams, concluded that the 685-foot high, thin-arch concrete dam they had started to build would not withstand an earthquake of the size predicted by these studies. So, they abandoned that dam and proposed a new, massive and terribly expensive concrete gravity dam instead.
Some people will make a lot of money if an Auburn Dam is built, not only from the construction project, but also by selling water and hydro-electric power, and building housing developments. Because of these special interests, the issue of restarting an Auburn Dam never dies.
An Auburn Dam would improve flood protection for Sacramento, but would cost much more than improving the Folsom Dam and levees. Nonetheless, Representative John Doolittle threatens to renege on his promise to the late Representative Robert Matsui to upgrade the Folsom Dam. Representative Doris Matsui still supports upgrading the Folsom Dam, but Representatives Dan Lungren and George Radanovich have joined Doolittle to push for building a massive Auburn Dam, without acknowledging its dangers.
The canyon walls allow for a large dam near Auburn only within the Bear Mountain fault zone. Before construction was started in the 1970s, this fault was thought to be inactive. The Oroville earthquake revealed otherwise. The redesigned dam could be moved only a short distance off a fault strand, though, leaving the dam in danger.
The Department of Defense identified the Folsom Dam as a top potential terrorist target among U.S. dams because of the large urban population in its flood path, therefore leading the Bureau of Reclamation to close the Folsom Dam Road to limit access by terrorists.
An Auburn Dam would hold more than twice the volume of the Folsom reservoir, which would add to the flood, so a terrorist attack or earthquake could release a much greater torrent on Sacramento. Donald C. Rose, a member of the Association of Engineering Geologists Seismic Hazards Committee, wrote a letter on March 22, 1976, to the chairman of that committee, saying: “If Auburn Dam should fail and release some or all of its 2.3 million acre-feet reservoir downstream into Folsom Dam’s smaller reservoir, the Folsom Dam’s earthfill flanks could then be overtopped. All told, several million acre-feet of water would then be released into the American River and head toward Sacramento in a wave about 100 feet high.”
Even a partial failure of an Auburn Dam would rupture the Folsom Dam, causing a lesser flood that still would be so severe that Folsom Dam Road would be closed.
A Bureau of Reclamation study in 1980 confirmed this scenario, predicting that a failure of an Auburn Dam, when full, would send a nightmare wave surging across the Sacramento metropolitan area, topping the Folsom Dam within five minutes, then sweeping 70 feet over the Nimbus Dam. It would hit the federal building on Cottage Way in one hour and 40 minutes, peaking at 46-feet deep. The flood would hit the Capitol building in two hours and six minutes, peaking at 40 feet. Such deep and turbulent flood waters likely would overtop and destroy many houses, lowering chances of rooftop survival.
Except for a few areas of high ground in North Highlands, Citrus Heights and Carmichael, the flood would extend from Roseville and Verona in the north, to the Yolo bypass in the west (including all of West Sacramento), to Walnut Grove and Elk Grove in the south, and Mather Air Force Base and Folsom in the east.
The hurricane evacuations of New Orleans and the coastal regions of Texas and Louisiana took days. Can you imagine trying to escape from a metropolitan area of 900,000 people in less than two hours?
Hurricane Katrina killed about 1,200 people in the gulf states. The Indian Ocean tsunami of December 2004 killed more than 214,000 people in many countries. A 1974 University of California, Los Angeles engineering study estimated that 255,860 people would drown if the Folsom Dam failed. With today’s population, the death toll would be much higher if an Auburn Dam were breached.
Destruction of the Folsom Dam by terrorists would require lots of carefully placed conventional explosives or a “suitcase” nuclear bomb. Chances of a successful attack might be small, but the consequences are so great that the Folsom Dam Road was permanently closed. An Auburn Dam could be redesigned to minimize risk of attack, but that would raise costs even more. The terrorist risk is real, but difficult to evaluate. The likelihood of destructive earthquakes at the dam site has been studied, though.
Possible quakes at an Auburn Dam
After the Oroville quake, geologic consultants to the Bureau of Reclamation estimated that a maximum credible earthquake could reach a Richter scale magnitude of 6.5, and they used this value to design a concrete gravity dam to replace the planned, fragile thin-arch concrete dam (see the U.S. Geological Survey Web site at http://ca.water.usgs.gov/archive /reports/auburn/review.html).
The U.S. Geological Survey used more recent data to estimate that a Richter-scale magnitude 7.0 earthquake is possible at the Auburn site. Slippage along a fault during a 7.0 earthquake can reach three feet, compared to only nine inches for 6.5 magnitude earthquake. Ground accelerations during a 7.0 magnitude earthquake can exceed one G (the force of gravity), three times greater than for 6.5 earthquake, tossing unfastened objects into the air. The concrete gravity dam certainly is under-designed for the newer understanding of the maximum credible earthquake.
Filling a reservoir can trigger a quake
The science of earthquake prediction is too new to say whether a quake might strike an Auburn Dam in its lifetime. We do know from past experience that filling reservoirs can trigger earthquakes. The weight of water increases stress on pre-existing faults. Seepage of water along rock fractures lubricates faults so they break again at lower stress levels. This is called “reservoir-induced seismicity.”
The 1975 Oroville earthquake on the northward extension of the Bear Mountain fault zone likely was triggered by filling the reservoir behind the Oroville Dam. Because of similar geology at the Oroville and Auburn sites, a government-funded study (http://ca.water.usgs.gov/archive/reports/auburn/induced.html) estimated a 2 percent to 30 percent chance that filling a reservoir in Auburn would trigger a large earthquake.
Even if a reservoir-induced earthquake doesn’t break an Auburn Dam, the ground shaking from a magnitude 6.5 to 7.0 earthquake would cause much damage in Auburn. Older historic buildings in Sacramento probably would be damaged, too.
Breaking a dam by shaking it
After the 1975 Oroville quake, the Bureau of Reclamation ran computer analyses of how the proposed concrete gravity dam would respond to a magnitude 6.5 earthquake, concluding that the dam would survive without a catastrophic breach. The more recent estimate of magnitude 7.0 makes this conclusion obsolete.
Mathematical models that simulate a mechanical response of dams to earthquakes are very complex, with lots of variables whose values aren’t very well known. In 1988, John F. Hall, of the Civil Engineering and Applied Mechanics division at the California Institute of Technology, found that earthquake responses of real dams during real quakes and “forced-vibration” tests compared poorly to behaviors predicted by computer models. Because the Bureau of Reclamation computer models for an Auburn Dam used too small an earthquake (and the models are uncertain), we don’t know whether any dam near Auburn could survive a maximum credible earthquake.
Too great a risk
Without a large earthquake or terrorist attack, a well-designed concrete gravity dam in Auburn will stand for a long time. But should we be betting the lives of almost a million Sacramento citizens on a quake or attack never occurring? If a dam must be built, build it somewhere else, not in or near the Bear Mountain fault zone. It is just plain irresponsible to propose building the sixth highest dam in the United States in an active fault zone right above a major population center. Even filling the reservoir could trigger an earthquake bigger than a dam can withstand.
Opponents of an Auburn Dam have been accused of using scare tactics. But it’s not the tactics that have the “scare” in them: It is the very real possibility of a terrible man-made catastrophe occurring in Sacramento, now underscored by the tragedy of New Orleans. The public has the right to know the dangers, as well as the benefits. Elected officials must place public safety above all other concerns.
Even if the likelihood of failure of a dam in Auburn is closer to 2 percent than 30 percent, that 2 percent corresponds to a one in 50 chance of disaster. Hundred-year storms happen. Earthquakes happen. Terrorist attacks happen. Dam failures happen. The risk for catastrophe posed by a massive dam near Auburn should not be ignored.
Upgrading the Folsom Dam and levees will give Sacramento improved flood protection faster, and cheaper, than building an Auburn Dam. The risks of building an Auburn Dam are too great. Public officials who promote the benefits of a dam but do not consider the risks along with the costs are derelict in their duties to the citizens they serve.