The state agency tasked with managing the safety of 1,250 dams in California has identified 93 that require a “comprehensive” assessment to be sure they can last through next year’s flood season, officials said.
After the concrete spillway at the Oroville Dam crumbled under heavy use earlier this year, the Division of Safety of Dams decided to review more than 100 dam spillways that were considered vulnerable to similar issues because of their age and capacity and size of the communities they protect, the agency said.
On Thursday, the DSOD released a list of the 93 dams that it concluded need further inspection.
“These assessments may require acquiring additional information to adequately evaluate the spillways’ ability to perform satisfactorily during a flood event,” the agency said. “It will not be known which spillways, if any, will need repairs until the comprehensive assessments are completed and reviewed by DSOD.”
When reservoir levels are high, dam spillways allow the highest volume of water to be released at one time. In dry times, reservoirs can typically manage water levels through adjacent hydroelectric plants or natural evaporation.
In Oroville’s case, the spillway was needed in February to rapidly drain water after heavy rains pushed the reservoir’s water level up to capacity. The main concrete spillway crumbled and led to a cascading set of problems that resulted in a partial failure of the dam’s emergency spillway and more than 100,000 Butte County residents down river being temporarily evacuated.
The DSOD’s call for reassessments comes as state officials are still trying to determine precisely what caused the failures at Oroville Dam. The Oroville Dam was built five decades ago, but officials noted that other dams in the state are much older.
The average age of dams in California is 70 years old, the DSOD said.
The state wants local operators to review each structure’s original design and building materials, its repair history for recurring issues, its drainage system, retaining walls and the geological makeup of its bedrock, among other elements, said Daniel Meyersohn, supervising engineer for the DSOD.
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