At Oroville Dam on Friday, it was all about maintaining a delicate balance.
For days, officials have been rapidly reducing the water level of the massive reservoir after both dam spillways sustained major damage. Now they are expressing confidence that the situation was stable even as rain fell.
Earlier this week, the damaged spillways prompted the evacuation of more than 100,000 people downstream as officials worried the emergency spillway could collapse, prompting a catastrophic flood.
That disaster was averted because officials were able to use the main concrete spillway to reduce reservoir water levels.
On Friday, officials reduced the rate of water release as workers continued repairing a damaged spillways and clear debris from a hydroelectric plant.
The state Department of Water Resources engineers will decrease the flow of water in the Oroville Dam's main spillway from 80,000 cubic feet per second to 60,000 cfs by Saturday morning, giving crews space to dredge debris from a pool at the bottom of the spillway, said Bill Croyle, the acting director of the DWR.
Engineers had been pumping water out of the lake at 100,000 cfs for several days to make room for incoming storm runoff and to keep the lake from overflowing as it did over the weekend. That overflow badly eroded an emergency spillway and sent debris flowing into a pool at the bottom, forcing the closure of an underground hydroelectric plant.
“This reduction in flow will allow us to work on the debris pile in the spillway,” Croyle told reporters at a news conference. He estimated that 150,000 cubic yards of sediment and debris were in the pool.
The other focus by dam workers is the eroded emergency spillway, Croyle said. Rain began falling again Thursday and it’s not expected to stop until the middle of next week at the earliest.
The heaviest showers are expected Monday and could drop up to 10 inches of rain onto the mountains and foothills that drain into the reservoir, the National Weather Service said.
The storms probably won’t produce enough runoff to exceed the lake's capacity, Croyle said.
Hundreds of thousands of tons of rocks and concrete slurry have been dropped into four fissures that threatened a retaining wall of the emergency spillway on Sunday.
Rain falling onto the slurry and a small stream that had formed on the hillside Friday did not worry engineers, he said.
The Oroville Dam sits above Oroville and dozens of communities. When the emergency spillway failed last Sunday, officials issued a frantic evacuation order, saying that spillway could give way within an hour.
The spillway didn’t collapse, and residents were allowed to return to their homes within a few days.
Tensions were running high Friday in the region, where residents were growing increasingly worried about a new round of storms and whether those would prompt new evacuations.
For days, DWR officials assured that the incoming storms were expected to be weaker than the ones that bloated the lake a week ago, pushing torrents of debris-laden water over the dam’s main and emergency spillways, damaging both so badly that numerous communities downstream were forced to evacuate.
So hearts sank among some residents Friday when the National Weather Service issued a revised forecast warning that the largest storm in that atmospheric river could drop as much as 12 inches of warm rain between Sunday and Tuesday in the area surrounding the site where crews have been dumping 120,000 tons of boulders and cement per hour onto deep fissures gouged into the unpaved earthen emergency spillway of the nation’s tallest dam.
Periods of warm rain and strong winds are expected to rake the area into the week.
As rain fell hard Friday in Oroville, Dorman Lard, an independent contractor, sat by himself in a coffee shop early Friday morning, watching mixers and trucks loaded with massive boulders speeding past toward the stricken dam.
“State officials are trying not to create a panic,” said Lard, 85. “They have to walk a very fine line, because there’s so much uncertainty. For us, however, it’s a bad dream.
“How many evacuations can we withstand? One is plenty,” he said.