New stress for battered spillway
Water released from Oroville Dam will test channel’s ability to handle spring snowmelt
Water began gushing down the mangled spillway at Oroville Dam on Friday in what state officials said was the start of a weeklong test to see if the sheared-off chute and the carved-out hillside around it could sustain even more pummeling as flows into Lake Oroville increase during the spring snowmelt.
The floodgates opened at 11 a.m., after the spillway was dry for nearly three weeks, and within an hour the flow had reached a brisk 50,000 cubic feet per second. By the early afternoon, a cascade was flying off the busted concrete spillway, racing through a yawning crater dug out of the earth last month, and joining the Feather River below.
Bill Croyle, acting director of the California Department of Water Resources, which runs the dam, said water will continue to spill for five or six days. The move was necessary, he said, because the elevation of Lake Oroville — the state’s second biggest reservoir — had risen about a foot in the previous 24 hours, to 864 feet above sea level.
State officials are trying to get the level to 835 to 838 feet above sea level and to keep the lake well below 901 feet — the point at which water would begin
pouring over the dam’s emergency spillway, which is also compromised.
About 16,000 cubic feet per second was flowing into the lake Friday from the surrounding mountains, which are piled high with snow after a series of winter blizzards. Reservoir levels are expected to rise more next week as another storm rolls through.
“At the moment, I need to get some water out of this reservoir, so as long as we don’t see catastrophic loss of a lot of concrete then we’re going to need to roll through this,” Croyle said. “If we need to deploy another corrective measure we can do it.”
A gaping hole was first detected Feb. 7 on the main spillway of the nation’s tallest dam, forcing operators to reduce the flow, which at the time was moving at the same rate — about 3.2 million pounds of water a second — as managers ramped it up to on Friday.
Within days of the initial spillway failure, the lake water rose and began pouring over an emergency spillway that had never been used and onto a bare hillside below. The hill eroded quickly, causing the state to warn of a catastrophic collapse and temporarily evacuate nearly 200,000 people downstream.
At that point, dam managers were forced to continue using the main spillway, worsening the damage. The panorama of destruction revealed when the floodgates were finally closed on Feb. 27 was shocking.
Croyle said the current weeklong spill will be the first of up to three lengthy releases before June 1, as snow melts in the mountains and flows into Lake Oroville. The sooner the state can stop using the spillway, the sooner it can start fixing it.
“If we have a mild, cool spring and early summer then maybe we can get away with two spills, including the spill today,” Croyle said.
The reservoir’s Hyatt Power Plant, which had been pumping 12,900 cubic feet per second of water out of the reservoir, was shut off during Friday’s procedure. Managers need to make sure the hydroelectric plant will work simultaneously with the spillway.
The releases mean the Feather River below will rise between 10 and 15 feet, officials said, a concern for downstream farmers who saw banks along the river collapse the last time the spillway flows swelled and then subsided.
Croyle said managers will monitor the effect the water has on the damaged hillside near the main spillway, as well as the debris flow into the river and the downstream erosion. He urged anglers, boaters and people who live by the river to be aware that the river will become deeper and swifter.
Meanwhile, engineers and geologists are scrambling to figure out how the crippled chute can possibly be fixed by the end of the year, in time for the next rainy season. Croyle said a team of engineers, geologists and others are working up various potential schemes, even as 170 state employees and 500 contractors work 24 hours a day to strengthen what remains.
Crews have reinforced the main spillway with concrete and bolted it to the ground, and every crack on the chute has been sealed, Croyle said. Meanwhile, tons of concrete and boulders are shoring up the hillside below the emergency spillway, though the state hopes not to use it again.
Through February, the work has cost the state $100 million, or about $4.7 million a day. Although Croyle said the state has “mitigated the major concerns,” the state will probably have to spend at least twice as much in the coming months.
Drill rigs, concrete mixers and a rock-crushing plant are being mobilized in a staging area at the site, while haul roads are being shored up in preparation for the work ahead, which is expected to begin after repair plans are finalized within the next two weeks, Croyle said.
“There are a whole bunch of different options” to repair or replace the main spillway, he said. “Some of them are really interesting.”
Jay Lund, a professor of civil and environmental engineering at UC Davis, said the key over the next three months is for the state to use the spillway as little as possible, with just enough releases to handle the melting snowpack.
“The biggest risk to me could be that the operation of the gated spillway would cause continued erosion of rock into the river that would block the hydropower outlets,” forcing the power plant to remain shut down and unable to release water, said Lund.
He said officials will have to decide whether to rebuild, modify or shore up the existing spillway — or build a new one parallel to the old one.
“If you have eroded everything that can be eroded, then maybe that’s your new spillway,” he said.