In Oroville, a caravan of trucks and cement mixers race to repair dam spillways ahead of new storms

Tensions were running high Friday in Feather River Basin communities, where residents were growing increasingly frustrated with conflicting messages from state officials about the threats posed by a new series of storms headed for the imperiled Oroville Dam reservoir.

Two days ago, California Department of Water Resources officials insisted 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 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 1,200 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 next week.

As rain fell hard on this community of 16,000 people, Dorman Lard, a resident and local independent contractor, sat by himself in a coffee shop early Friday morning, watching a seemingly endless line of mixers and trucks loaded with massive boulders speeding past toward the stricken spillways.

“State officials are trying not to create a panic,” Lard, 85, said. “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.”

State officials are now draining the lake at a rate of about 80,000 cubic feet per second – slightly slower than the 100,000 cubic feet per second rate they used for much of the week. 

The trouble at the key reservoir in the State Water Project, which sends Northern California supplies to the urban Southland, started on Feb. 7, after state officials increased releases of water through the dam’s main spillway to offset inflows of rainfall. The releases were halted after they discovered the hole in the main spillway.

At the time, state officials emphasized in press releases that “the dam is sound, and no imminent threat to the public exists.”

Stopping water releases, however, caused the lake level to rise. On Feb. 11, the water elevation rose above 901 feet, causing water to flow over a concrete weir and erode a deep fissure in the emergency spillway, which is a simple earthen slope.

Fearing that the erosion would undermine the weir and release a wall of water on the valley below, officials ordered more than 100,000 area residents to evacuate on Sunday. (The evacuation order was lifted on Tuesday.)

Ever since Sunday, crews have been laboring around the clock trying to stop the erosion, which some fear could be exacerbated by warm rains and strong winds expected to rake the area next week. They are also racing to repair two roads at the emergency spillway, one of which was washed out.

The crisis fractured into an urgent new front on Friday, as state officials, under threat of the impending deluge, mobilized a small army of construction crews, sandbaggers, dredging equipment operators, barges and cranes to protect the underground Hyatt Power Plant at the base of the dam and remove sediment and debris including uprooted trees from its diversion dam gates.

A cavern the size of two football fields was dug out of the bedrock below the lake in the 1960s to house the plant designed to pump water or generate power. If severely damaged, officials said, the power plant has the potential to create severe human, cultural, environmental and economic impacts including the loss of drinking water.

louis.sahagun@latimes.com

For California news, follow @LouisSahagun on Twitter.

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