Crater in Oroville Dam spillway will continue to grow, officials warn

State engineers gingerly began releasing water again through the damaged Oroville Dam spillway Wednesday in a controlled test to see how much water the scarred facility could handle, as reservoir levels continued to climb behind the critical flood-control structure.

With dozens of engineers monitoring from strategic locations around the dam's exterior, a sheet of white water began cascading down the 3,000-foot-long spillway a little after 3 p.m. Within seconds of hitting the damaged area, a crater about 250 feet long, a portion of the water turned into a long ribbon of mud as sediment that had been trapped inside the hole ran down the concrete spillway.

Misdirected by the jagged chunks of concrete lying in the spillway, some of the water left the chute and ran down an adjacent hill.

California Department of Water Resources officials said they planned to open the spillway for more than two hours Wednesday, ramping up the water release to about 20,000 cubic feet per second. That's considered a “light spill,” engineers said, and was less than half as much water as the spillway was releasing when the giant scar was discovered Tuesday and releases were shut down.

Officials said the spillway would be shut down again later Wednesday so engineers could perform a thorough inspection. They expected the test spill would cause additional erosion, but the question was whether the additional damage would be at acceptable levels.

“We'll see if anything has changed or anything has moved as far as concrete or materials that were under the spillway,” said department spokesman Eric See.

The concrete spillway was built on top of rock and various soils. It sits beside the main earthen dam that holds back Lake Oroville, the second-largest reservoir in California.

Agency officials said they didn't expect to release an assessment of the test spill until Thursday morning.

Officials with the Department of Water Resources, which operates the dam, stressed again Wednesday that they believe the dam itself is safe and doesn't pose a threat to downstream populations, a view echoed by outside experts consulted by The Sacramento Bee.

“We do not believe there's an imminent danger to the dam, or the flood control ... gates that we operate, or the public,” said Acting Director Bill Croyle.

Depending on the results of the spillway test, the agency could resume water releases from the spillway, as a short-term remedy, even if it means creating further erosion in the chute. The alternative, which is considered less preferable, is to let Lake Oroville continue rising until water begins cascading in an uncontrolled fashion over the emergency spillway at the north end of the dam. The emergency spillway – which has never been used – is unlined, so water would flow uncontrolled down a hillside and onto unpopulated land filled with trees and other material that could wind up in the Feather River below the dam and pose problems downstream.

“That is a last resort,” said department spokesman Chris Orrock.

Fixing the massive hole will be expensive, though it is too early to say how much it will cost, Croyle said. “Pushing water down that chute is going to continue to erode that chute,” he said. “We wouldn't be surprised if by the end of this wet season, much of the lower portion of the spillway has eroded away.”

With the main spillway shut off, the reservoir had taken on a whopping 174,000 acre-feet of water in the 24 hours ending Wednesday afternoon. On an average February day in recent decades, Lake Oroville would normally take in about 7,000 acre-feet, state records show.

The lake level was about 50 feet below the lip of the dam, and about 30 feet below the emergency spillway, at 2 p.m. Wednesday, state figures show. Lake levels were increasing Wednesday at a rate of about 1 foot every two hours. The reservoir remained about 13 percent empty. That's the most water the lake has contained in February at any point during the past 30 years, state records show.

Lake Oroville, in Butte County, is a central piece of California's government-run water delivery network. Completed in 1968, the 770-foot dam is the tallest in the United States. It can store 3.5 million acre-feet of water, which is divvied out through the year for agricultural and drinking water needs across a vast area of California. Much of Southern California's drinking water is stored in the reservoir.

The dam also is a critical piece of the state's flood-control infrastructure, protecting downstream communities including Oroville, Marysville and Sacramento from major flooding. The Feather River is the Sacramento River's largest tributary.

Croyle, DWR's acting director, said it was not yet clear what caused the crater to form. Three recent inspection reports for the dam – one from 2014 and two from 2015 – noted no visible signs of deficiencies along the chute.

Gary Leese and his girlfriend, Beth Bello, were among the first people this week to see that something was seriously wrong. On Tuesday morning, they hiked down the hillside adjacent to the spillway, something they'd done many times over the years. Leese said he knew something was amiss when they came upon a giant, angry splash of water instead of the normal gentle waterfall that runs down the sloping structure.

“I knew there was something up just because of the load roar it was making,” Leese said. “That's when we walked a little closer and saw the plume of water coming up in the air, and we kept seeing the fragments of concrete shooting up in the air.”

The couple soon was asked to leave by state employees.

After the problem was discovered, Department of Water Resources engineers gradually reduced the flows before shutting off the releases altogether. At that point, they started releasing water through smaller outlets at the dam, including a power plant. But those outlets can release only a fraction of the volume of water flowing into the lake – about 83,000 cfs at midday Wednesday. Before the damage, the lake was releasing more than 40,000 cfs.

At current rates, the agency said the lake has enough room to absorb three days of inflow. Water would pour out of the emergency spillway if the lake got too close to the lip of the dam.

The possibility of an emergency spill prompted a chain reaction among dam managers in the Feather River watershed. Officials have maxed out releases from New Bullards Bar Reservoir on the Yuba River, which flows into the Feather below Oroville Dam, said Joe Forbis, chief of water management for the Army Corps of Engineers in Sacramento. The thinking is that New Bullards may need room to capture additional runoff if water gushes down the emergency spillway.

So far, all runoff modeling points to the Sacramento Valley being able to withstand such a scenario without major flooding, Forbis said. “The flows in the Feather and the Yuba, they're well within their channel capacity, regardless of what flows are coming out of Oroville,” he said.

The state also is increasing releases from Thermalito Afterbay, a small downstream reservoir, to avoid a “drastic reduction” of flows into the Feather River. That's important, engineers said, because saturated levees below the dam could slump and fail if flows are suddenly shut off.

Dale Kasler: 916-321-1066, @dakasler