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Silicon Valley Reservoir Must Be Drained Because of Earthquake Risk to Dam, Federal Safety Agency Says

Ron Brackett

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2-25-20Major California Reservoir to be Drained Due to Earthquake Risk

Federal regulators have ordered that a major reservoir be emptied in the Silicon Valley because of earthquake fears.

At a Glance

  • The risk of the Anderson Dam collapsing during an earthquake is "unacceptably high."
  • Santa Clara County water officials have been ordered to drain the reservoir by Oct. 1.
  • If the dam fails when the lake is full, an 8-foot wave could reach San Jose.

Federal regulators have ordered California officials to drain a reservoir because its dam could collapse during a major earthquake and flood towns and cities from San Francisco Bay to Monterey Bay.

The 240-foot earthen dam on the Anderson Reservoir in Santa Clara County poses too great of a risk, the Federal Energy Regulatory Commission has decided, and the lake must be drained by Oct. 1.

"It is unacceptable to maintain the reservoir at an elevation higher than necessary when it can be reduced, thereby decreasing the risk to public safety and the large population downstream of Anderson Dam," wrote David Capka, director of FERC’s Division of Dam Safety and Inspections, in a letter to the Santa Clara Valley Water District last week.

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Anderson Reservoir, which is less than 20 miles southeast of San Jose, sits alongside the Calaveras Fault. More than 10 years ago, according to the San Jose Mercury News, an engineering consultant warnedthat a 6.6 magnitude quake centered on the fault directly at the reservoir, or a 7.2 quake centered one mile away, could cause the dam to fail.

Studies found that if the dam failed when the reservoir was full, a 35-foot wall of water would slam into downtown Morgan Hill within 14 minutes. Within three hours, an 8-foot wave would reach San Jose.

The Valley Water District, which is Silicon Valley's chief water supplier, began the Anderson Dam Seismic Retrofit Project in 2012 as a permanent fix. Construction was supposed to begin in 2022, but the district said it has had trouble getting necessary permits from other government agencies. The project is now expected to cost $563 million, according to the Mercury News.

"Valley Water agrees with the Federal Energy Regulatory Commission that it is vital we reduce the dam safety risk and move as quickly as possible with the Anderson Dam Seismic Retrofit Project," Valley Water CEO Norma Camacho said in a statement Monday.

However, Camacho added, "The demand to empty Anderson Reservoir could result in unsafe consequences."

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The reservoir has been kept at 45% of capacity since last October. At that level, the reservoir "provides a buttressing effect" for the dam’s intake structure, Christopher Hakes, a Valley Water dam safety official, said in a Dec. 31 letter to FERC. "Lowering reservoir levels beyond the current level would decrease the structural reliability of the intake structure" and its protection against earthquakes.

Camacho said draining the reservoir also could cause fish die-offs and significantly impact sensitive native fish, amphibians, reptiles, wetlands and riparian habitats.

Water supplies would also be reduced and the district might have to find sources outside the county, her statement said, especially if California experiences another drought.

In his letter, FERC's Capka wrote, "Until full remediation is completed, the dam safety risk at this project is unacceptably high. Your actions to date do not demonstrate an appropriate sense of urgency regarding the interim conditions at the project."

FERC and the state of California have cracked down on dam safety since the near-failure of the Oroville Dam Spillway in 2017,which required the evacuation of nearly 200,000 people in Butte County.

State officials required dam owners to prepare emergency plans that would include the steps they'd take to address potential flood emergencies and inundation maps. Earlier this month, a state audit found that emergency plans have been approved for only 22 of the 650 California damsclassified as high or extremely high hazard risk to downstream populations.

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