Q: Where is Oroville Dam?
A: The earthen dam is located a few miles northeast of Oroville in Butte County, Calif., 65 miles north of Sacramento. Lake Oroville is California's second-largest reservoir, and the tallest dam in the United States. The reservoir is capable of holding 3.5 million acre-feet of water. An acre-foot is equivalent to about 326,000 gallons, or enough water to cover an acre of land with a foot of water.
Q: What does Oroville Dam do?
A: Built in 1968, the dam captures and stores rain and snowmelt that washes in from the mountains in the Feather River watershed, a vast area of the Sierra Nevada that spans along the state's eastern border from nearly Truckee to Lassen Volcanic National Park. The dam regulates flows on the Feather River, the Sacramento River's largest tributary. The structure provides flood control for the eastern edge of the Sacramento Valley. The dam's main outlet outside of the flood season is a hydroelectric power plant at the base of the dam.
The lake also provides critical storage for the State Water Project, which serves Central Valley farms and urban water agencies from the Silicon Valley to as far south as San Diego. One third of Southern California's water supply comes from the State Water Project, whose members _ water agencies including the mammoth Metropolitan Water District of Southern California _ financed the dam's construction and pay for upkeep. The dam is managed by the California Department of Water Resources.
Q: Why is there a suddenly a crisis?
A: To make room in the reservoir for flood protection, officials had been releasing water from the dam's main spillway, a 3,000-foot concrete span built on soil and rock that leads to a channel that flows into the Feather River. A huge gash was discovered on the bottom half of the chute on Tuesday.
Fearing the concrete would further erode and make the spillway inoperable, dam operators dialed back flows to avert further damage. With stormwater and runoff from the snowpacked Sierra rushing in last week, the lake climbed to the point that water began overtopping an emergency spillway built just north of the damaged main spillway. The emergency spillway had never been used in the 48-year history of the dam.
Unlike the main spillway, the emergency releases flow down a hillside that is not lined with concrete. Water that reaches its lip _ a 1,700-foot span of concrete _ pours uncontrolled down the hill. Engineers had frantically tried to avoid topping the emergency spillway because they were worried that the flows would scrape huge amounts of trees and debris into the Feather River, creating problems downstream.
On Sunday afternoon _ just hours after officials declared the worst of the crisis over _ engineers saw a gash beginning to form in the hillside beneath the lip of the emergency spillway. The concern is that the erosion could spread and undercut the upper 30 feet of the spillway, forcing a collapse. It collapse would release 30 feet of water held by the reservoir, which experts say could inundate much of Butte and Sutter counties. Nearly 200,000 people who live in the flood plain below the dam were ordered to evacuate.
Q: What's the risk?
A: Officials say the earthen face of the dam, which is separated by a hillside from both the main and the emergency spillways, has not been compromised. But experts interviewed say that if the emergency spillway collapses, it wouldn't be much different than a total dam failure. The hillside could quickly erode and empty the lake.
The massive release of water could blow out levees along the Feather River and inundate Sacramento Valley communities that are homes to tens of thousands of people, including Oroville, Marysville and Yuba City. The city of Sacramento also could be affected, though experts said a levee breech along the Feather likely would relieve pressure in the system enough that Sacramento's flood infrastructure could absorb the influx.
Q: What is the state doing to minimize the risk?
A: State water officials doubled the releases from the damaged main spillway Sunday night to lower the reservoir and stop flows from going over the emergency spillway. The hope is that they can drain about 20 to 30 feet of the lake before the next series of storms hits, perhaps as early as Wednesday night.
In the meantime, crews are using helicopters to drop rocks into the eroded section of hillside below the emergency spillway to bolster it. Even if the crisis is averted in the short term, there are two more months of California's rainy season that the spillways would need to handle. Plus, officials say there are 2.8 million acre-feet of snow-water content still waiting to melt in the Sierra above the reservoir this spring and summer.
Q: Have there been problems at the dam before?
A: Inspectors at Oroville Dam found "minor" cracks in the dam's main spillway in 2009, according to a Sacramento Bee review of state inspection reports. The reports indicate the damage was repaired the following year.
Department of Water Resources engineer Kevin Dossey said Friday that additional cracks had appeared in the main spillway as recently as 2013 but were repaired.
The last major incident at Oroville Dam occurred in 2009. Workers were testing 6-foot valves that move water from the reservoir into the Feather River through two tunnels in the dam's power plant, the primary outlet outside of rainy season, when the spillway is operational. When workers opened one of the valves, the flow of water caused a suction force so great it blew out a bulkhead separating the workers from one of the tunnels. Two workers were nearly sucked in, and survived by clinging to a damaged railing.
The following year, the California Occupational Safety and Health Administration cited six violations that contributed to the July 22, 2009, accident, including five considered "serious." OSHA found, among other things, that an "energy dispersion ring" designed to eliminate the suction force had been removed two months earlier and never replaced.
It also found that the dispersion ring was damaged in 1968 and never repaired. A University of California, Davis study in 1993 advised DWR that, as a result, flow through the valves should be "severely limited" for safety. OSHA levied fines totaling $141,375.
A federal inspection in 2010 concluded that Oroville Dam needed a comprehensive earthquake safety assessment, but no significant flaws were found in the dam itself.
