Race against rain: Water levels drop, 190k remain evacuated at Oroville Dam

Race against rain: Water levels drop, 190k remain evacuated at Oroville Dam

FERC's attorneys noted: "The emergency spillway was created to safely convey the Probable Maximum Flood, and DWR has reviewed and confirmed the efficacy of the PMF hydrologic analysis for Oroville Reservoir".

At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest USA dam, exceeding the Hoover Dam by more than 40 feet (12 meters).

On Sunday, the spillway on the dam, the tallest in the USA, began to erode, causing water levels to rise at such a rate that officials forced residents of nearby counties and cities to leave their homes and find shelter elsewhere. The Oroville Dam crisis this week, in which almost 190,000 residents were abruptly evacuated from a valley below the tallest U.S. dam, illustrates the safety risks of the Golden State's ageing infrastructure in increasingly populated areas. Marysville and Yuba County are also under mandatory evacuation. Butte County Sheriff Kory Honea called this a "critical and hard decision" that he recognised would cause significant disruption. Engineers don't know what caused the cave-in, but a Department of Water Resources spokesman told the Associated Press it appears the dam's main spillway has stopped crumbling even though it's being used for water releases. It's not clear how damage to the two spillways will affect long-term water releases from the dam.

An evacuation center has been established for Oroville residents at the Silver Dollar Fairgrounds in Chico.

Some of the dams operated by the city are those at the San Vicente and El Capitan reservoirs, Lake Morena - all in the East County - and Lower Otay Lake, just east of Chula Vista. The warning is in effect until 4:15 p.m. PST on Monday, Feb. 13. Both spillways are to the side of the dam itself, which has not been compromised, engineers said.

Gov. Jerry Brown issued an emergency order for the area, and officials immediately mobilized all state-sponsored California Swift Water Rescue Teams while dam and construction experts have been working around the clock to find a solution to fix the damage that contributed to the fear that the emergency spillway could potentially fail. Crews continue to make repairs to the structure, especially as another storm looms.

He wants the repairs finished by the start of the next rainy season in October.

Right now in California, the nation's tallest dam is facing its toughest test yet - holding back about 1.1 trillion gallons of water while one of its auxiliary spillways, which acts as a relief valve, threatens to collapse. Water poured down the hillside toward Oroville and towns further downstream.

"Basically it's going to be a triage situation".

The main concrete spillway partially collapsed last week when a huge hole wore through the concrete.

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