Re: Dam failure and flooding
Oroville residents returning to their home say they're happy the evacuation order was lifted, but understand why it was imposed.
Oroville Dam evacuees told they can return home
http://www.sacbee.com/news/state/california/water-and-drought/article132705659.html
With the crisis at the Oroville Dam stabilized for now, authorities announced Tuesday that the 188,000 people evacuated Sunday will be allowed to return to their homes but should prepare to move again if a new emergency arises.
The announcement by Butte County Sheriff Kory Honea, who ordered the mass exodus Sunday afternoon amid fears that the dam’s emergency spillway might collapse, came in a 1:40 p.m. press briefing in Oroville.
“The risks that we faced have significantly been reduced...,” Honea said. “We have concluded that it is safe to reduce the immediate evacuation order to an evacuation warning.”
The sheriff said residents could begin returning home immediately, but he urged them to remain alert for the possibility that an emergency may require another evacuation.
Despite that, officials said they are confident that the steps taken to repair the hillside along the emergency spillway – using helicopters to dump boulders into holes and pour concrete on top of the piles – have made the structure safe in the event that the emergency spillway must be used again.
They also said such a scenario is highly unlikely, with the next series of storms predicted to be much weaker than previous ones.
Authorities also revealed that inspections since Sunday showed that “there was no piping or other erosion that compromised the overall integrity of the emergency spillway” and that action taken to fill in holes and erosion from water releases have strengthened it.
Helicopters carried boulders and blocks of concrete onto Oroville's damaged emergency spillway Tuesday in advance of rains predicted for later in the week.
Oroville Dam operators say they’re ready for next key hurdle: Wednesday night’s storm
http://www.sacbee.com/news/local/article132787074.html
There’s another storm bearing down on troubled Oroville Dam, set to begin late Wednesday. But state officials say they believe the precipitation will be mild enough – and the reservoir empty enough – to handle this latest challenge.
The crisis at Oroville Dam continued to ease Tuesday. The dam’s heavily damaged main spillway was still able to expel water at 100,000 cubic feet per second, as it has around the clock since Sunday afternoon, with no apparent signs of significant new erosion. For a second day, crews worked to pack the crevice that formed last weekend in the hillside beneath the dam’s crippled emergency spillway, using helicopters and dump trucks to spread a mixture of boulders and concrete over the eroded section.
“We’re continuing to make significant gains in removing water from the reservoir, which drops the water surface elevation (and is) further reducing the risk to our situation here,” said Bill Croyle, acting director of the state Department of Water Resources, in an afternoon news briefing.
Enough progress has been made that Butte, Yuba and Sutter counties rescinded the mandatory evacuation orders issued Sunday afternoon, allowing an estimated 180,000 downstream residents to return home.
“The risks that we faced when we initiated those evacuations have been significantly reduced,” Butte County Sheriff Kory Honea said.
At Tyler Island, crews were working frantically Tuesday to patch a leaky levee in the hopes of saving around 20 homes from being inundated
Oroville Dam isn’t the only piece of California flood infrastructure under strain
http://www.sacbee.com/news/local/article132779679.html
All eyes have been on the crisis at Oroville Dam, but weeks of wet weather have put pressure elsewhere on the network of levees and dams protecting cities and farms in California’s vast Central Valley flood plain.
Almost all of the major reservoirs that ring the Valley have filled to the point that officials have cranked up releases to catch water from a storm building up off California’s coast that’s expected to hit Wednesday night.
Most of the river flows below the dams haven’t exceeded the capacity of the levees that line their channels, and independent experts say California’s flood-control network has endured the exceptionally wet winter rather well.
But some levees, including those that protect the Sacramento region, are showing signs of the strain as prolonged heavy river flows push back.
“That groaning sound you’re hearing throughout the Central Valley isn’t the dams – it’s the levees,” said Jeffrey Mount, the former director of the UC Davis Center for Watershed Sciences. “We’re stressing them pretty well right now. And, just as you’d expect, issues are starting to crop up.”
The most obvious of those near Sacramento is in the northern Sacramento-San Joaquin Delta.
Fearing a dangerous levee-cracking “flood pulse,” workers this weekend intentionally breached a levee along the Mokelumne River in the north Delta. The breach flooded the levee-ringed McCormack-Williamson Tract of farmland near Walnut Grove in Sacramento County.
A bit farther south
at Tyler Island, crews were working frantically Tuesday to patch a levee in the hopes of saving around 20 homes from being inundated from what officials described as an imminent collapse.
On Tuesday, crews used a crane to scoop huge loads of stone from a barge onto the huge crater that had formed in the levee, taking most of a gravel road with it.
“The key is we’ve got to get weight on this thing,” said Steve Mello, a Tyler Island resident and a trustee of the local reclamation district. He had no estimate when residents could return to their homes.
Mount said
he expects more trouble to pop up along Delta levees in the weeks ahead from the continual rush of powerful flows from the estuary’s two primary rivers – the Sacramento and San Joaquin.
Mount said the flows from the rivers and tributaries such as the Mokelumne have fueled powerful tides from the Pacific Ocean that are pushing back in the other direction.
“The Delta is basically in a hydrological vise right now,” he said.
With potentially months left of rain and runoff from melting mountain snow, crews are already working round the clock to patrol the levees looking for breaches along the 70 levee-ringed tracts of land that make up the Delta, said Erik Vink, executive director of the Delta Protection Commission. These tracts are commonly called “islands” because they’re surrounded by canals, sloughs and channels.
“There’s certainly a lot of vigilance right now,” he said. “People are keeping a really close eye out, and they’re prepared to shift into flood-fight mode at a moment’s notice. There’s just a lot of water out there.”
Of particular concern both in and out of the Delta is the San Joaquin River, which is flowing higher than it has in years. Some buildings along its banks west of Turlock already have flooded. On Tuesday, officials issued a flood warning along the river southwest of Manteca.
Meanwhile, a key tributary of San Joaquin River – the Tuolumne River – has strained the regional flood-protection system east of Modesto to the brink.
Locals have been nervously watching the precariously full Don Pedro Reservoir. The reservoir, which has more than twice the capacity of Folsom Lake, was less than 3 feet from being completely full on Tuesday.
Operators of New Don Pedro Dam have their fingers crossed that the approaching storm follows Tuesday’s forecast: a relatively cold storm that will dump snow in the Sierra Nevada instead of the warm rain and melting snow that could prompt huge releases at the lake. That would overwhelm the small Tuolumne River channel and flood part of Modesto.
As it stood Tuesday, forecasts pointed to the lake avoiding such problems for the next 16 days, said Calvin Curtin, a spokesman for the Turlock Irrigation District, which manages the dam.
“It will be extremely close, but ... it’s not projected to spill at this point,” he said.
Closer to Sacramento, local levees were showing signs of stress, but experts weren’t worried.
“Sacramento has a lot of protection,” said Joe Countryman, a member of the Central Valley Flood Protection Board and a former engineer with the U.S. Army Corps of Engineers.
He said the region has performed substantial levee upgrades over the years. Sacramento also is protected by a robust system of outlets that spill major Sacramento and American river flows in the massive engineered Yolo Bypass flood plain west of the city.
But Countryman added, “You have levees, you have water – you’re going to have seepage.”
That was evident Monday when crews began emergency work along the Sacramento River to cork three boils that formed along a levee near the confluence of the Feather River north of Sacramento International Airport. Boils form when water seeps under the levee and eventually pushes its way to the land side, creating a small geyser. Local flood officials said they believed they had the problem under control.
