The operators of Oroville Dam acknowledged Monday they might not be able to permanently repair the dam’s battered main spillway in time for the next rainy season, but said they’re confident the fractured structure will be usable.
Engineers expect to have this summer’s repair plans largely in place by early next week for the concrete spillway, said Bill Croyle, the acting director of the state Department of Water Resources.
“If I have anything to say about it, we’ll have a spillway to use by Nov. 1,” Croyle said, referring to the expected start of the next rainy season. “Whether that’s a permanent or temporary structure, it hasn’t yet been decided.”
Croyle’s remarks came Monday at a press briefing not far from the dam, where operators again were halting flows from the spillway to resume emergency repair work. The spillway developed a massive crater Feb. 7, triggering a crisis that prompted the temporary evacuation of 188,000 downstream residents.
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In a report to DWR earlier this month, a team of independent consultants cast severe doubt on the agency’s ability to permanently repair or replace the 3,000-foot-long concrete chute by fall. Their report became public last week.
The expert panel’s conclusions, based on a review of reports and an on-site inspection earlier this month, provided a first-ever accounting of structural and design problems that might have caused the spillway to fail. The consultants described seeing troubling amounts of water flowing from underneath the structure, even with the spillway shut off. They also expressed concern that the concrete chute is only a foot thick, and less so in some places.
In addition, DWR built the 48-year-old spillway on an uneven mountainside and in some spots used compacted clay to fill in the depressions in the rock foundation beneath the concrete. The consultants described finding evidence of “a number of repair instances” in which portions of the chute were cut away in order to “fill voids beneath the concrete.”
“This calls into question whether the portions of the slab that appear undamaged by the failure should be replaced during the restoration,” the panel wrote.
The findings contradicted statements Croyle had made during the crisis, in which he had said he believed permanent repairs to the spillway could be finished by fall. On Monday, he said a temporary fix would be OK. “I’m not bothered by that,” he said.
Monday’s briefing also served to announce that the dam’s operators were again shutting off the water flowing down the spillway. The structure had been releasing water at about 40,000 cubic feet per second since March 17. The releases would be dialed back to zero by Monday night and could stay that way for the next 15 days.
Croyle said the shutoff will enable DWR crews to dredge more debris from the Feather River channel at the base of the spillway and make additional repairs to the dam’s hydroelectric power plant. The plant was shut off temporarily Monday as well, although Croyle said it was expected to resume releasing water Monday afternoon.
The shutdown of the spillway was in some ways a matter of necessity. Lake levels have dropped to the point that the spillway could no longer release water at relatively high volumes, Croyle said. DWR engineers believe that low-volume releases can create more severe erosion problems than the more powerful releases. When the releases are high, the blast of water skips over the giant crater that formed in the structure nearly two months ago.
“We don’t feel comfortable running water over the spillway at lower levels,” Croyle said.
As it is, the pounding the spillway has taken since the crater was discovered has done extraordinary damage, both to the concrete structure and the adjoining hillside. On Monday afternoon, as engineers began dialing back flows, water continued to cascade down the spillway. Almost all of it was being redirected by the ruts in the structure into the enormous canyon that’s opened up immediately to the side of the spillway.
Water levels at Lake Oroville had fallen below 837 feet Monday, the lowest since the heavy rains began in early January. Croyle said the shutdown of the spillway would probably bring lake levels back up to around 865 feet in a matter of days. While that would be above the prescribed levels for this time of year, it would still be well below the point at which water would flow over the dam’s questionable emergency spillway.
Croyle said he expects to resume water flows down the main spillway once or twice again this spring. His goal is to shut it down for good as soon as possible so major repairs can start. Over the past few weeks, temporary shutdowns have allowed crews to shore up the structure with quick-drying concrete and other measures.
Last month damage to the main spillway, as a big storm rolled into the region, led to the evacuation crisis. After a crater formed in the main spillway, dam operators temporarily shut off the structure for inspections. Although they resumed outflows a day later, the lake rose to its highest level in history Feb. 11 and water started flowing over the emergency spillway for the first time.
A day later, engineers feared massive erosion on the hillside below the emergency spillway would cause it to collapse. That prompted the evacuations.
Croyle said the work to fortify the hillside is largely complete and the emergency spillway could be used again if lake levels rose too high. But he acknowledged that area residents would be nervous about using it.
“The sheriff doesn’t want to use it,” he said, gesturing to Butte County Sheriff Kory Honea. “The city of Oroville doesn’t want to use it.”