Costs for repairs at the troubled Oroville Dam will be "much higher" than an initial estimate of $100 million to $200 million, a state water official said Friday.
Final plans for how to repair the reservoir's massive spillway — which eroded and fractured last month, leading to a major crisis — are still weeks away, according to Bill Croyle, acting director for the Department of Water Resources.
In the interim, Croyle said, DWR must use the spillway to keep the reservoir from reaching capacity.
The release is expected to last five or six days and should lower the water level from 864 feet to about 838 feet, Croyle said. The capacity of the lake is 900 feet. If the lake level exceeds that, water will overflow and run down the facility's badly damaged emergency spillway.
"At the moment we need to get some water out of this reservoir," Croyle said. "Unless we see some catastrophic loss of concrete [on the spillway], we're going to need to roll through this."
The urgency stems from the fact that the reservoir isn't releasing water quickly enough through its hydroelectric plant alone. The facility can release water at up to 15,000 cubic feet per second through its power turbines, but water from past storms and snowmelt are flowing into the lake at a faster clip. The water level has climbed 27 feet in fewer than 20 days.
The spillway will release water at 50,000 cubic feet per second by the end of Friday, Croyle said. The rate may eventually drop to 40,000 cubic feed per second, but that's as slow as it will go, he said.
The spillway's damage is in a location where if the water were released at a pace slower than 40,000 cubic feet per second, it would drop off the end of the spillway and onto the earth beneath it, possibly causing more erosion that could endanger the dam structure itself.
Once the water level in the reservoir drops sufficiently, engineers will close the spillway gates and turn the power plant back on, Croyle said. For now, the agency won't use both simultaneously, he said.
The spillway could be used two to three times by June, Croyle said.
The massive repair operation underway at Lake Oroville began after Feb. 12, when the reservoir reached capacity and water surged onto a dirt hillside that served as the reservoir's emergency spillway. The emergency spillway eroded so badly that it threatened to undermine a concrete retaining wall of the reservoir. Fearing the concrete would give way and send a 30-foot wall of water into the valley below, officials ordered sweeping evacuations.
The erosion also knocked out the hydroelectric plant at the foot of the dam, removing another means of releasing water from the lake.
In the wake of the crisis, a small army of engineers, cleanup crews, truck drivers and others have made significant gains in patching up the dam's spillways in time for spring and summer snowmelt. The dirt hillside and eroded earth beneath the main spillway are now caked in layers of gray concrete.
"If we had to use it, we've mitigated the major concerns we have there," Croyle said.
But other issues have cropped up in the meantime. On Thursday, DWR officials announced that a vein of naturally occurring asbestos was detected at the site. The agency submitted a dust-control plan for safety.
Farther south, the yo-yo effect of water releases from the reservoir has caused dramatic ebbs and flows in the Feather River. Acres of land have been swept clean of vegetation and miles of riverbed have collapsed as a result, farmers and officials in Yuba and Sutter counties say.
It's unclear what will happen to the river when DWR shuts down the spillway again next week. The river is expected to rise as much as 15 feet in the coming days, Croyle said.