188,000 Californians unable to go home
Disaster averted, but crews trying to shore up spillways of Oroville Dam
OROVILLE, CALIFORNIA Tens of thousands of Californians faced an indefinite stay in shelters as engineers worked for a second day yesterday to fix the United States' tallest dam before more storms sweep the region.
The Oroville Dam is 235m-high and holds more than 3 trillion litres of water. It was built between 1962 and 1968.
Crews were working to shore up an overflow channel and drain the reservoir at the Lake Oroville dam, but the authorities gave no sign of when it will be safe for people to go home.
The authorities said they had averted the immediate danger of a catastrophic failure at the dam that could unleash a wall of water three storeys tall on towns below.
On Monday afternoon, crews dropped large bags filled with rocks into a gap at the top of the emergency spillway to rebuild the eroded hillside.
The main spillway, a separate channel, is also damaged because part of its concrete lining fell apart last week.
Both spillways are to the side of the dam, which itself is not compromised, engineers said.
Last Tuesday, operators noticed turbulence in the water rushing down the spillway's chute at a rate of 1.41 million litres a second, The New York Times reported.
They discovered that a large portion of the concrete had been washed away.
It is not yet known what caused the damage, but one culprit is cavitation, or tiny bubbles of water vapour that can form in high-velocity water, said Mr Blake P. Tullis, a professor of civil engineering at Utah State University.
When the bubbles collapse, they create tiny shock waves that are strong enough to damage concrete, he said.
The erosion in the main spillway led to the decision last Saturday to use the emergency spillway for the first time since the dam was completed.
Emergency spillways are designed for use only in exceptional conditions, when the reservoir is rising so fast that there is a risk it will top the dam itself.
Late on Sunday, about 188,000 residents were ordered to evacuate their homes in the Feather River valley below the dam, 105km north of Sacramento, Reuters reported.
"We're doing everything we can to get this dam in shape so they can return and live safely without fear. It is very difficult," California Governor Jerry Brown told reporters.
Ms Yolanda Davila, 62, who is now in an evacuation centre, said the authorities should have warned residents near Oroville much sooner.
The Washington Post reported that in 2005, three environmental groups warned state and federal officials about what they believed was a problem with the Oroville Dam's emergency spillway - the same one that was at risk of collapsing this week, after storms caused the adjacent reservoir to swell.
But federal officials determined that nothing was wrong with the emergency spillway, and said it "would perform as designed".
They also said sediment resulting from erosion would be insignificant. - WIRE SERVICES