New aerial footage shot by a drone shows the main spillway at Oroville Dam in Northern California, wrecked during heavy winter storms in February, slowly being rebuilt.
Hundreds of construction workers from Kiewit Corp., of Omaha, Nebraska, the lead contractor on the job, are working around the clock with huge dump trucks, cranes, excavators, bulldozers, concrete pumps and other equipment to demolish and rebuild the 3,000-foot-long main spillway, and shore up the emergency spillway.
The goal of the $500 million project to rebuild enough of the main spillway — which is as wide as 15 lanes of freeway — by Nov. 1 so that it can be ready for heavy rains this winter. The entire job is scheduled to be finished in 2018.
The latest footage was shot Wednesday.
Last week, the Oroville Dam Spillway Incident Independent Forensic Team, an independent panel of dam experts, concluded that poor design and construction in the 1960s, combined with a failure to perform adequate repairs, led to the spillway failure. The report noted that water rushing down the spillway during raging storms penetrated cracks and voids in the structure, building up pressure underneath it, and eventually forcing up one of the huge concrete slabs.
The surging water then tore away at other slabs, and at the rock and earth below the spillway.
The report noted the spillway wasn’t built thick enough originally, wasn’t anchored adequately and was built on rock prone to erosion. It noted that cracks had appeared in the general area of the failure before Feb. 7, when the event occurred, “and these deteriorated and repaired locations may have been vulnerable to damage during high-velocity spillway flow.”
The California Department of Water Resources, which owns the dam, issued a statement Sept. 5 commenting on the report.
“Protecting public safety is the state’s top priority, and we are committed to applying lessons learned from Oroville,” it noted.
The forensic team’s first report was on engineering issues. The team plans to release a report on the human failures connected with the incident later this fall.
Meanwhile, the administration of Gov. Jerry Brown has begun a review of the nearly 100 dams with similar spillways around California, and several bills aimed at improving dam safety were pending in the Legislature on Friday.
Oroville Dam is the tallest dam in the United States, at 770 feet high. It is a critical part of California’s water system, providing water to farmers across the Central Valley and millions of people from San Jose to San Diego.
In February, water flowing down the main spillway during raging winter storms that ended California’s five-year drought ripped a huge hole in that concrete structure. State dam officials closed the gates on the main spillway then to assess the damage, allowing Lake Oroville, California’s second largest man-made reservoir, to fill to to the top.
As water flowed over the dam’s emergency spillway, it violently eroded the base of the hill on which it was built, leading authorities to evacuate 188,000 people over fears that it could collapse and possibly send a wall of water onto Oroville, Marysville, Yuba City and other towns below, which could have killed thousands of people.
To stave off disaster, officials reopened the gates on the main spillway. The waters further tore its concrete apart, but the lake level dropped, allowing the public to return, and averting what could have been one of the worst dam catastrophes in American history.
On Friday, Lake Oroville was 43 percent full. Its water level now has been dropped to 170 feet below where it had been in February, leaving lots of space for winter rains to fill the reservoir before any water would go down the new spillway.
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