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State report card says some local dams could fail in an earthquake

The Oroville Dam spillway overflows with runoff in Oroville, California on February 14, 2017.
The Oroville Dam spillway overflows with runoff in Oroville, California on February 14, 2017. A sheriff lifted a mandatory evacuation order in northern California, which had impacted nearly 200,000 people in an area under threat of catastrophic failure at the tallest dam in the United States. / AFP / MONICA DAVEY (Photo credit should read MONICA DAVEY/AFP/Getty Images)

California safety regulators for the first time publicly posted safety ratings for the hundreds of dams under state jurisdiction on Friday, bowing to public pressure for more transparency after the failure of the Oroville Dam spillway in Feburary.

Eight percent of dams under jurisdiction of the Division of Safety of Dams have deficiencies that keep them from being rated satisfactory, which is state’s the highest rating. Eleven of 229 dams in Los Angeles and five surrounding counties show up on the state’s list with a rating of fair or poor.

The largest two of those — Castaic Dam and Lake Perris Dam — were rated “fair” because they are vulnerable to earthquakes, said Erin Mellon, spokeswoman for the Department of Water Resources, which owns the two dams.

“Fair” means the dam can hold the amount of water it was built to hold under normal circumstances, but that it could have problems under stresses like an extra load of water, or an earthquake.

The Perris and Castaic dams are located near populated areas that could be inundated if the dams failed, so they are described as posing extremely high downstream hazards.

Perris Dam near Hemet in Riverside County is undergoing repairs, and its capacity been restricted by the state until the work is finished. The water level was lowered in 2005.

At Castaic Dam, north of Santa Clarita, the outlet towers and bridge could suffer damage in an earthquake, according to a recently-completed structural analysis by DWR.

“Between January 2017 and April 2017, DWR also installed and replaced instrumentation to monitor the left abutment of the dam for water levels and slope movement. DWR will use this new instrumentation to update our stability evaluation which will be completed around Spring of 2018,” Mellon said in an email.

Castaic Dam’s spillway is also due for a close examination, one of 93 dams in California whose geology or construction is similar to the Oroville spillway. That exam could be done in the fall, Mellon said.

Information about the number of people who could be in harm’s way after a dam failure is not yet available from the state, Mellon said. Inundation maps showing what parts of the state could be endangered are being updated and will be published when they are finished, she said.

Nine other “less than satisfactory” dams are owned by local flood control and water districts, private companies, even a homeowners association.