OROVILLE, Calif. (AP) — California plans to shut the shattered main spillway of the Oroville Dam for the summer on Friday, launching an all-out race to get it operational again before the next rainy season.
The spillway is the main outlet for water from man-made Lake Oroville, a half-century-old complex that includes California's second-largest reservoir and the tallest dam in the United States.
The dam's operator, the state Department of Water Resources, plans to start scaling back water releases down the main spillway at 9 a.m. Friday, closing its flood gates completely by afternoon.
The shutdown will let construction crews tackle the bulk of a $275 million project to rebuild and strengthen the 3,000-foot-long (900 meter) concrete spillway by Nov. 1.
Huge swathes of the spillway's upper reaches began crumbling and washing away in February, sending torrents of water and debris cascading down bare, steep hillside.
When a nearby hillside that made up the dam's second, emergency spillway began giving way as well, authorities on Feb. 12 ordered the evacuation of nearly 200,000 people downstream. Residents were allowed to return two days later.
Authorities have used the collapsed main spillway only sparingly since then , to keep water from once again reaching the top of the dam during one of the state's wettest winters on record.
State water officials said this week that they would use the water-release gates in the dam's hydroelectric power plant as the main way of releasing water from behind the dam during summer construction. A contingency plan calls for using the main spillway one more time if water from snow-melt in the Sierra Nevada fills the reservoir harder and faster than currently expected, the state said.
State officials "will continue to adjust as we balance community concerns, regulatory requirements and Mother Nature," acting water-agency director Bill Croyle said in a statement.
An April memo from a forensic team appointed to conduct a post-mortem on the crisis said experts were looking at possible failings in design, maintenance and inspection of the spillways.
The experts are slated to release their final findings next fall, with dam operators nationwide watching closely to see what lessons the crisis holds for other aging infrastructure.