After four days of relentless pounding on Oroville Dam, its operators dialed back water releases on the heavily damaged main spillway Thursday, even as forecasts show another “atmospheric river” poised to strike the region early next week.
Feeling confident they’ve created sufficient empty space in Lake Oroville for the time being, state Department of Water Resources officials said they reduced spillway outflows so they could address another looming challenge: restarting the dam’s hydroelectric plant, which can release additional water when operational. The plant was shut down last week after concrete and other debris from the spillway accumulated at the bottom of the dam, backing up water to the front door of the hydro facility.
DWR acting director Bill Croyle said the water level in the reservoir has receded enough since Sunday that it’s safe to reduce the spillway releases to allow crews to clear debris from the Feather River below the dam.
“The threat level – it is much, much, much lower than what it was on Sunday,” he told reporters at DWR’s regional office a few miles from the dam.
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Croyle said the main spillway does not appear to have suffered significant new erosion since Sunday, when outflows were nearly doubled to 100,000 cubic feet of water per second in a desperate effort to drop the lake’s level and avert a potentially catastrophic collapse of the dam’s adjacent emergency spillway.
Sunday afternoon, fearing failure of the emergency structure was imminent, officials ordered the evacuation of nearly 200,000 downstream residents from Butte, Sutter and Yuba counties. They were allowed to return home Tuesday, after dam operators were able to lower lake levels below the emergency spillway lip, and conditions stabilized.
Despite forecasts of a substantial new rainstorm Monday and Tuesday, officials said they felt urgency to begin work on the hydro plant, even if it required dialing back releases. DWR spokesman Chris Orrock said debris that had accumulated at the bottom of the main spillway the past few days was starting to interfere with the power plant’s outtake pipes. That would make it harder to restart the plant.
Croyle said releases from the main spillway would be held at 80,000 cfs, a decline of about 20 percent, so crews could start removing debris.
When operational, the hydro plant could release up to 14,000 cfs, Croyle said. “That adds another dial to our equation,” he said. He also said he doubted the plant could be reopened in time for Monday’s storm.
Independent experts interviewed said the strategy makes sense. The reservoir should be empty enough to handle the inflow from the “atmospheric river” forecast for Monday and Tuesday, and getting the power plant working again will prove vital to navigating the rest of winter and into spring, when snowmelt from the Sierra Nevada will generate heavy runoff.
“They think they’ve got the reservoir under control,” said Jeffrey Mount, a senior fellow at the Water Policy Center at the Public Policy Institute of California. “They’ve got the breathing room.”
The lake was down to 866 feet Thursday, a drop of 35 feet since Sunday. To leave sufficient room for flood control, Lake Oroville should stand at no more that 850 feet this time of year, according to U.S. Army Corps of Engineers regulations.
Joe Countryman, a former Army Corps official, added that DWR needs the power plant running to augment water releases this spring.
“They’re thinking down the road,” said Countryman, a member of the Central Valley Flood Protection Board. “It’s very, very important to make that (power plant) functional, no doubt.”
Countryman said the releases down the main spillway still will be considerable. “It’s not like they’re cutting it back to zero; 80,000 (cfs) is a lot of water.”
Reducing the outflows provided a better look at the damage to the main spillway, revealing significant gaps in the retaining walls on either side of the 3,000-foot-long concrete chute, as well as a ravine that has been carved out of the hillside east of the structure.
A DWR incident report dated Feb. 11 suggests that heavy rainfall running down adjoining hillsides may have contributed to the problem at the main spillway after its initial fracture. “Flowing water was diverted toward the adjoining hillsides, effectively eroding and undermining the spillway causing a section to collapse,” said the report, which was obtained by the Los Angeles Times. However, DWR spokesman Chris Orrock said the report doesn’t pinpoint the underlying cause of the fracture, and a full investigation will take months.
The storm that rolled into the Oroville region late Wednesday didn’t bring significant rain, and a second storm forecast to begin early Friday isn’t supposed to deliver much of a punch, either. But the National Weather Service reported that a warm, heavy storm known as an atmospheric river is expected to hit Monday and Tuesday.
“It is looking like the system for next week is trending wetter and warmer,” said Michelle Mead of the National Weather Service in Sacramento.
Mead said the rainfall, nonetheless, is expected to be only half as heavy as last week’s storm. The weather service said Oroville could expect up to 1 inch of rain Thursday and another half-inch Friday. The seven-day outlook calls for 10 inches of rain, most of it coming Monday and Tuesday, when temperatures will turn warmer.
While not as bad as last week, when total rainfall approached 20 inches in the Feather River Basin, Mead said the reservoir can expect a significant amount of new water. “Enhanced inflows are expected,” she said.
Last week’s storm swamped Lake Oroville with peak inflows of 191,000 cubic feet per second just as a massive crater opened in the main spillway. That fracture temporarily stalled efforts to push water out of the reservoir, allowing the lake to rise to the point that it topped the lip of the emergency spillway early Saturday for the first time in the dam’s history. A day and a half later, inspectors found major erosion in the hillside just below the concrete apron of the emergency structure. That sparked concerns the spillway would collapse, prompting the evacuation order Sunday afternoon.
The large crews working to reinforce the emergency spillway with tons of rock and concrete continued their efforts Thursday, despite periods of heavy wind and rain. Croyle said one of the three eroded spots was patched by noon Thursday. The other two spots were 25 percent and 69 percent completed, he said.