The Fault Line and the Dams 

Lake Temescal Dam sits directly atop the perilous Hayward Fault. How safe is it, and the nearby Tilden Park Dam at Lake Anza?

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Size matters when it comes to dams. Lake Temescal holds about 200 acre-feet at capacity, or about 98 Olympic-sized swimming pools worth of water. Lake Anza holds 268 acre-feet of water at capacity, or about 132 pools worth. In comparison, the tallest dam in the United States — Northern California's giant Oroville Dam — holds 3,537,577 acre feet of water at capacity, or roughly 1.7 million Olympic-sized swimming pools of water. But while these local dams are not remotely close to the size of Oroville Dam, they are still considered dangerous if they were to release uncontrolled water.

Shortly after the East Bay Regional Park District appears to have created the dam assessment found online, a major dam-related failure changed the course of the park district's plans to evaluate its dams. In early February 2017, a spillway at Butte County's Oroville Dam was damaged by heavy flows, prompting the evacuation of more than 180,000 downstream residents at risk of flooding. Following the incident, the state legislature passed a law requiring all California dam owners to submit an emergency action plan for each dam they own. These plans detail where flooding would occur if a dam was damaged, who would respond in case of an emergency, and how they would do so.

Owners of dams with a "high" downstream hazards rating were required to submit an emergency action plan by January 1, 2019. Vanya said the district submitted plans for Lake Temescal and Tilden Park Dams. The district was unwilling to provide a copy of those plans. Citing fears about terrorism, state law says that emergency action plans should be treated as confidential. "In order to keep this information from individuals with improper motivations, who could use the information maliciously to expose a dam's vulnerabilities and to disrupt a critical emergency response," the law states, "it is in the state's interest to limit public access to this information."

When the East Bay Express submitted a public records request seeking a copy of the newly-submitted Lake Temescal Emergency Action Plan from the agency to which the plan was submitted, the California Office of Emergency Services, a spokesperson declined to release a copy, saying the plan is still in draft form. The state is currently experiencing a back-log in reviewing and approving all these plans, Division of Safety of Dams spokesman Orrock said.

Both plans included an inundation map of which areas would flood if there was for any reason an uncontrolled release of water. These plans are being made public as they are approved, and can be viewed online at the Division of Safety of Dams website. The Office of Emergency Services has approved both of the inundation maps, Vanya said.

The approved inundation map for Lake Temescal shows that a wide area could flood if the dam failed. Immediately west of the dam, the map shows flooding north of Highway 24 and south of Chabot Road. But starting at College Avenue, the inundation area expands southward, as far as 20th street, with many surface streets flooding. The map represents an approximation, and does not go into detail about how many feet of water different areas could see. As per the law the state passed following the Oroville incident, people acquiring property in inundation zones are required to be informed of that risk. Meanwhile, the inundation map for Tilden Park Dam shows water fanning westward in two main strips near I-80 and the San Pablo Lytton Casino.

The agency returned the Lake Temescal emergency action plan to the park district for revisions, Vanya said, but has yet to return the Lake Anza plan. But the two plans were only required to outline what will be done in the case of an emergency. Neither one addressed what changes need to be made to either dam — if any — to prevent it from failing in the first place.


Meanwhile, the 2017 East Bay Parks document found on the agency's website outlined a list of actions the park district could take to address earthquake safety, including evaluating its dams for earthquake risk. It allotted a timeline of one to three years for this assessment. Parks spokeswoman Jennifer Vanya said she could not confirm how official the two-year-old draft document's conclusions were, adding that the district was unaware the document was publicly available.

Compared to retrofitting a house, if a dam needs fixes to make it safe in an earthquake, it requires a much larger study to come up with a proper retrofit scheme, said Khalid M. Mosalam, the director of UC Berkeley's Pacific Earthquake Engineering Research Center. And given how long ago many of the dams were built, he thinks it might be worth taking a look at some of these older dams.

"Something that was designed many, many years ago, at least a careful assessment considering nowadays knowledge seems like a must," he said.

Consider the case of some other dams in the East Bay — the East Bay Municipal Utility District reservoirs, which provide drinking water to millions of customers. Most of these dams shore up reservoirs that are larger in capacity than Lake Anza and Lake Temescal, including the San Pablo Dam, the Briones Dam, the Upper San Leandro Dam, and the Chabot Dam. Following a request from the Division of Safety of Dams, the utility district went through the process of evaluating all of its dams for earthquake safety and upgrades were made where necessary, said Nelsy Rodriguez, a public information officer with the district. Chabot Dam, built by Andrew Chabot, the same builder as Lake Temescal Dam, got a makeover thanks to this process. Though it was not expected to fail in a quake, it was determined that it might settle, losing three feet of height during a major earthquake, which would require it to be drained and then repaired. It underwent retrofitting between 2016 and 2017, and afterward, it passed the Division of Safety of Dams' inspection "with flying colors," Rodriguez said. San Pablo Dam also got some updates as a result of this process, said Tom Boardman, an engineer with the utility district. Today, all of East Bay MUD's dams are considered safe to withstand a strong quake.

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