As Oroville Dam operators worked into the night Sunday to try to prevent collapse of the emergency spillway, independent engineering experts said the state has few options at its disposal.
The collapse of the emergency spillway likely would cause catastrophic flooding along the entire Highway 70 corridor, inundating Oroville, Marysville and much of Sutter County, top engineering and flood-control experts said late Sunday. The emergency spillway – which sits adjacent to the main spillway that fractured Tuesday – is a critical part of the structure holding water in Lake Oroville, California’s second-largest reservoir.
At 5 p.m. Sunday, Mike Smith, a spokesman for the California Department of Forestry and Fire Protection, said “as much as 30 vertical feet of the top of the spillway could fail.”
A failure of that magnitude likely would cause billions of gallons of water to cascade down a wooded hillside that sits below the lip of the emergency spillway.
“You look at 30 feet times the area of the reservoir,” said Nicholas Sitar, a civil engineering professor at UC Berkeley. “That is how much water is going to come out. That is a huge volume of water.”
Unlike the main spillway, which is lined in concrete, the emergency spillway releases water directly onto a rocky, wooded hillside. Sitar said the amount of damage such a failure would cause largely would depend on the composition of the rock on the hillside beneath the emergency spillway. It is possible that rock would continue to hold water back.
“If they are lucky, it will arrest itself because they hit a rock rim,” Sitar said.
If no such “rock rim” can stop the flow of water following the collapse, tens of thousands of homes along the Highway 70 corridor likely would be inundated.
“That’s gone,” said Joe Countryman, a member of the Central Valley Flood Protection Board and a former engineer with the U.S. Army Corps of Engineers. “I’ll tell you right now, that’s gone. If they lose that 30 feet, that’s gone.”
Oroville Dam releases water into the Feather River, the Sacramento River’s biggest tributary; the two meet just north of the city of Sacramento. Even if the emergency spillway collapses, Sacramento likely would be spared from major flooding, said Jeffrey Mount, former director of the Center for Watershed Sciences at UC Davis. That’s because the heavy flows on the Feather likely would breach levees upstream, before the two rivers meet, relieving pressure on the Sacramento River.
To mitigate the risk of emergency spillway failure, dam officials Sunday night started releasing water from Lake Oroville’s crippled main spillway at a rapid pace – about 100,000 cubic feet per second. Countryman said those flows in themselves should not overwhelm the Feather River levees and channels downstream, which are rated to handle up to 150,000 cfs.
The massive releases caused Lake Oroville levels to drop rapidly, and by 9 p.m. Sunday water had stopped coming over the top of the emergency spillway.
Concern about the stability of the hillside beneath the emergency spillway grew Sunday afternoon, when dam engineers spotted erosion. Chris Orrock, a spokesman for the California Department of Water Resources, said the source of the erosion likely was the water flowing over the concrete lip of the emergency spillway, as opposed to seepage from the lake side.
With the water flow over the emergency spillway stopped, officials should be able to start assessing the nature and extent of the damage. Late Sunday, helicopter crews were dumping boulders on the hillside to provide additional stability.
But stabilizing the emergency spillway doesn’t necessarily mean an end to problems at the troubled dam, a critical source of water for much of Southern California.
The state was able to lower levels in the reservoir by releasing huge amounts of water over the damaged main spillway. The question is whether those releases caused caused so much erosion that the main spillway becomes inoperable, Countryman said.
The problems at the dam started with the main spillway, which developed a crack Tuesday that by Sunday had eroded to the point that the bottom portion of the 3,000-foot span had largely washed away. As the main spillway continues to erode, its gates could be compromised, leaving the state with few options for releasing water from the dam.
That, too, is a catastrophic scenario.
Lake Oroville is fed by the vast, snowpacked Sierra watershed that continues to send runoff into the reservoir. And more storms are on the way, perhaps as early as Wednesday night. Without a release valve, the lake level would continue to rise. Eventually, it would hit the emergency spillway again.
Orrock said Sunday night that he did not know how much further damage was done to the main spillway by releasing water so quickly.
“I’m sure it’s going to be severe,” he said.
The morning will reveal how well the damaged main spillway held up under such powerful flows, and whether it can be relied upon to release that level of water through the rest of the rainy season, said Jay Lund, director of the Center for Watershed Sciences at UC Davis.
“The success of this strategy should be evident tomorrow morning after dawn,” he said Sunday.