Water & Drought

February 27, 2017 11:13 AM

Giant chasm revealed as water stops flowing at Oroville Dam

Water stopped cascading down Oroville Dam’s fractured main spillway Monday, revealing a gaping wound from a beating that lasted nearly three weeks.

Dam operators gradually scaled back water releases to zero over a six-hour period, providing breathing room for construction crews trying to clear debris from a badly choked Feather River channel and restart the dam’s critically needed hydroelectric plant. The shutdown is expected to last about a week, which the state Department of Water Resources hopes will be enough time to bring the plant back into operation.

“This is a big day; there’s going to be a lot of changes,” said DWR Acting Director Bill Croyle.

With the spillway no longer obscured by giant fountains of spray, engineers in hard hats and fluorescent vests watching from adjoining hillsides got an eye-popping look at what was left of the enormous structure. The initial sideways gash that opened Feb. 7 had morphed into a monster. Erosion consumed a good chunk of the lower third of the 3,000-foot-long concrete chute. Water that got misdirected by the pothole had carved enormous caverns in the hills on both sides of the structure.

However, it didn’t appear that the spillway erosion had grown upward, toward the spillway gates or the dam itself, a nightmare scenario that could have rendered the spillway inoperable and plunged California’s second-largest reservoir into crisis mode again.

The initial crack prompted a one-day shutdown of the spillway. DWR resumed water releases, but not quickly enough to prevent the reservoir from filling up with inflow from a heavy storm and spilling over the top of the nearby emergency spillway. That emergency structure, which had never been used before, lasted barely 36 hours before engineers realized that the hillside beneath its concrete lip was seriously eroding, prompting fears of a collapse that would send a giant “wall of water” into downstream communities. That sparked the evacuation of 188,000 residents in Butte, Yuba and Sutter counties.

Potential disaster was averted when dam engineers doubled the outflow from the damaged main spillway, to 100,000 cubic feet of water per second. That brought lake levels down and halted the flow of water over the emergency spillway. Evacuees were allowed to return home two days later. In the days since, levels at Lake Oroville have fallen to about 838 feet, or 63 feet below the top. Water flows over the main spillway were reduced to about 50,000 cfs, and on Monday down to nothing.

Shutting off the main spillway brought into view the massive mound of concrete, rubble and debris that has formed in the channel at its base. The eroded material has raised channel levels to the point that the dam’s hydroelectric plant can’t function. DWR will spend several days in an intensive dredging operation to try to bring the river level back to normal.

With the main spillway closed, Croyle said, lake levels will begin rising. But with little rain in the immediate forecast, he said he was confident his crews could make a big dent in the debris mound without risk of the reservoir overtopping the emergency spillway.

Croyle said it’s almost certain the main spillway will have to be used again this winter and spring. But having the power plant back in operation will relieve some of the pressure. It can release about 14,000 cfs. That would be enough to handle about half of the inflow from the huge Sierra snowpack that’s expected to begin melting into the reservoir in the next few weeks.

Shutting off the main spillway required something of a balancing act. Croyle said engineers were afraid that shutting off water too quickly could cause levees in Oroville and other downstream communities to slump and weaken. A sudden shutoff also could strand fish in the Feather River.

As it was, officials with the National Marine Fisheries Service urged DWR to ramp down the releases more gradually to protect Chinook salmon, sturgeon, steelhead and other fish that ply the Feather River. “Rapid reduction in the river water elevation can result in mortalities,” the agency said in a Friday letter to state and federal officials.

That sparked protests from Republican Rep. Doug LaMalfa, whose district includes Oroville. His spokesman Kevin Eastman said the agency was “putting fishery needs over public safety.”

DWR declined to follow all of the fisheries agency’s suggestions, but Croyle said his engineers would ensure fish were protected and flows on the Feather River wouldn’t fall too low too quickly. Water is being released from a downstream pool, the Thermalito Afterbay, to compensate for the lack of water coming out of the dam.

“There’s a step-down graph that we have with an awful lot of engineering behind it to make sure that we can make those changes, minimize the threat to our levee system and the environment, our fish resources,” he said. He said state biologists are being sent to spots downstream to rescue fish that get stranded.

Related stories from The Sacramento Bee

Croyle and Butte County Sheriff Kory Honea said helicopters and drones will be swarming the dam to help with inspections. They reiterated that the area is a no-fly zone for privately owned drones. Honea said the restrictions will remain in place until mid-May.

Oroville Dam shuts down main spillway, crews begin damage assessment

Shutting off the main spillway brought into view the massive mound of concrete, rubble and debris that has formed in the channel at its base. The eroded material has raised channel levels to the point that the dam’s hydroelectric plant can’t function. DWR will spend several days in an intensive dredging operation to try to bring the river level back to normal.

Paul Kitagaki The Sacramento Bee

See the damage evolve: Oroville spillway erosion over time

As seen from the air and ground level, the gaping hole in the spillway was first photographed by The Bee on Feb. 8, the same day that inspectors were sent via tether down into the hole. Water was released later that same day. Since then, we have documented the erosion caused by the force of water.

Produced by Sue Morrow The Sacramento Bee

Ryan Sabalow: 916-321-1264, @ryansabalow

 
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