San Francisco Chronicle - (Sunday)
Oroville spillway ready for first test
Storm likely to open gates rebuilt after ’17 flood scare
Crunch time may finally be here for Oroville Dam, which is expected to send water over the repaired concrete spillway this week for the first time since the structure partially collapsed in 2017 and engulfed the region in crisis.
State water officials, who manage the 770foot-high dam, said storms this week will probably force them to open the floodgates to control water levels in the reservoir, which is brimming after several winter storms and heavy snowfall in the mountains.
A storm is forecast to hit the Oroville area early in the week, with peak rainfall late Tuesday, followed by another, stronger storm later in the week.
The reservoir, on the Feather River about 75 miles north of Sacramento, is currently three-quarters full, but if the storms fill it above safety levels and the spillway is needed, dam officials are confident the chute will hold.
“We’re preparing for it,” said Erin Mellon, spokeswoman for the California Department of Water Resources. “There is still water entering the system, plus we have a lot of snowpack. We want to make sure we have adequate space in the reservoir to handle the inflows.”
The dam’s main spillway failed so spectacularly during heavy rains in February 2017 that managers turned to an emergency spillway, which poured water
over a mostly barren hillside that quickly eroded, as the deluge cascaded down the hillside.
A team of independent engineers blamed the failure on weakened concrete, poor drainage and a history of shoddy maintenance, including a failure to adequately review for problems. It raised concerns about the rest of California’s aging water infrastructure.
The department spent $1.1 billion rebuilding the complex’s two faulty spillways, pouring enough concrete to fill 372 Olympic-size swimming pools, reinforcing the concrete with 12.4 million pounds of steel rebar.
The half-mile-long main spillway, where the initial fracture occurred, is now as wide as a 15-lane freeway and averages 7½ feet thick compared with 2½ feet in the original 1960s version. It is capable of handling up to 270,000 cubic feet of water per second, way more than dam operators ever expect to release and nearly twice the capacity of the old chute, which could handle only 160,000 cubic feet per second.
The new 3,000-footlong spillway has steel pillars anchoring the structure 15 to 25 feet deep into bedrock and a modernized drainage system. The old spillway had only 5-foot-deep piles holding it in place.
The work on the 50year old dam, a primary source of drinking and irrigation water in California, was the biggest and fastest construction project in recent state history.
“The main spillway has been fully reconstructed,” Mellon said. “We've used the best engineering minds and practices in the reconstruction and we have done it under the oversight of state and federal regulators and independent experts.”
The hillside that serves as the emergency spillway has also been armored with concrete and a retaining wall. Final touches are still being worked on, but Mellon said the reservoir would have to rise 51 feet above its current level before the emergency chute would be used, a highly unlikely prospect based on this week’s forecast.
In preparation for use of the main spillway, construction equipment and a temporary haul road made of compacted dirt below the structure were removed as a precaution.
Mellon said the agency will give the public at least a 24-hour notice before using the spillway.
The Trump administration recently rejected $306 million of the $639 million California requested from the Federal Emergency Management Agency in reimbursements for the spillway repairs. It means only about a third of the cost will be picked up by the federal government.
Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@ sfchronicle.com Twitter: @pfimrite