State officials plan to stop releasing water down the mangled main spillway at Oroville Dam on Friday, allowing workers to begin months of round-the-clock repairs to the chute and to an emergency spillway that is also badly damaged.
The main spillway's gates will begin closing at 9 a.m. on Friday and slowly pinch off the outflow from the reservoir until they are fully shut at 1 p.m., said the California Department of Water Resources.
Bill Croyle, the acting director of the department, said he is confident the reservoir is low enough to handle snowmelt this spring and summer with no further use of the spillway.
"We are making gradual changes to outflows and river levels and will continue to adjust as we balance community concerns, regulatory requirements and Mother Nature," Croyle said.
Croyle said discharges will be increased through the hydroelectric plant at the bottom of the dam to prevent the water level in the reservoir from rising too fast. A contingency plan, allowing for another spillway discharge, is in place in case the spring melt produces more water than expected.
Contractors are expected to work around the clock all summer and fall to fix the damage. The goal is to make sufficient repairs on the crippled spillways by Nov. 1 in case there are early-season storms.
Croyle acknowledged that a good deal of the work to the spillways, which send water down the Feather River, will have to be done in 2018, after another rainy season.
The damage to the main spillway was detected Feb. 7 as operators released water from Lake Oroville during this year's historic rainstorms. A pothole quickly grew into a crater before part of the concrete chute collapsed.
As the flow was reduced in the main spillway, water in the reservoir rose and began pouring over the dam's emergency spillway, a bare hillside that had never been used. The water pouring over the dam's concrete apron eroded the hillside, prompting the evacuation of 180,000 people downstream amid fear of a catastrophic flood.
A forensics team hired by the Department of Water Resources outlined a series of problems with the original main spillway design, including too little concrete, compacted clay being used to fill depressions in the underlying rock, inadequate patch jobs and excessive leakage.
The contractor hired to do the rebuild, Kiewit Infrastructure West Co., has begun road construction, slope stabilization and other work around the spillways, but the effort is expected to intensify next week. The agencies that buy water from Lake Oroville are expected to pay a portion of the $275.4 million contract for the repairs, along with the federal government.
The public can watch construction work through a live feed from two cameras at www.parks.ca.gov/live/lakeorovillesra_spillway.
(c)2017 the San Francisco Chronicle
