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Seattle City Light restarts turbines after rockslides at Diablo Dam

A rockslide slammed into the back of the dam powerhouse during heavy rains last Wednesday. Another slide took out support poles Thursday.

SEATTLE — After four days, Seattle City Light said it restarted the Diablo Dam Powerhouse after another rockslide last Thursday.  The utility said the restart happened Monday afternoon. 

City Light said the first slide was a few weeks ago when large rocks at the north end of the building slid and punctured the roof. A rockslide on Wednesday, Oct. 27, also damaged part of the facility. The powerhouse building was evacuated and turbines were shut down to prevent the spillage of any water. 

Another rockslide Thursday was big enough that it tore away the slide-protection fencing and supporting poles and cables, which KING 5 cameras could see from across the valley. The damage there was at the opposite end of the building from the first slide.

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On Friday, a structural engineer, dam safety engineer and two engineers from their geotechnical contractor evaluated the site. They said the slide was mostly superficial and did not pose a threat for releasing water. They are working on mitigation efforts and plan to scale the side of the cliff, which means removing any loose rock they can find.

They went to what's known as the bi-lateral market, used to mitigate load and resource fluctuations when some 112 megawatts were offline.

Slides are a known risk on the near-vertical cliff behind the powerhouse, which dates back to the 1930s. Signs and gates warn people to stay away from the back of the powerhouse.  

City Light employees have been reaching out to KING 5 since Friday morning, expressing concerns about safety because of the slides. The Local 77 of the International Brotherhood of Electrical Workers, which represents workers on the hydroelectric project, told KING 5 those safety concerns have not been raised with the union.

A photo provided by Seattle City Light shows a steel door bashed in at the bottom by fallen rocks and debris. Though the utility said employees were not endangered, one worker who reached out last week said the slide barely missed two of his friends.

RELATED: Diablo Dam powerhouse damaged after rockslide, Seattle City Light says