The Oroville Dam, which supports California’s second largest reservoir and is the tallest dam in the country, is showing increasing likelihood of failure after an exceedingly wet winter rain season. This year’s rainy season has brought record rains to the state after years of drought.
The 770-foot-high dam is located in northern California 75 miles north of Sacramento, the state capital. The dam, built between 1961 and 1967, holds roughly 3.5 million acre-feet of water—an acre-foot is equal to the volume contained within a sheet of water one acre in area and one foot in depth, or, 43,560 cubic feet—and is integral to the California Water Project, the state’s expansive flood control and irrigation system that delivers water from the wetter and more sparsely-populated areas in the north to the drier and more densely-populated communities along the coast and south.
The dam, which holds back Lake Oroville, suffered a partial fracture along its emergency spillway early last week. The emergency spillway acts as a relief valve for the dam and is designed to be opened up when water levels in the dam get too high. It was used for the first time in the dam’s history this weekend. The damaged spillway is not paved over in concrete like the main spillway, but instead, exists as a wooded slope. The damage caused engineers tasked with managing the dam to slow down the release of excess water along the emergency spillway and, as a result, the reservoir reached capacity over the weekend. Use of the emergency spillway persisted to alleviate the high water levels.
Despite assurances from officials that the dam was safe and structurally sound throughout the weekend, officials announced emergency evacuation orders late Sunday as it became obvious that water being released over the emergency spillway had resulted in the steady erosion of the hillside holding the dam’s foundations.
The state’s Emergency Mass Notification system sent out the following message via email Sunday afternoon:
Emergency Mass Notification Message (2/12/17 4:20 p.m.)
This is an evacuation order. Immediate evacuation from the low levels of Oroville and areas downstream is ordered. A hazardous situation is developing with the Oroville Dam auxiliary spillway. Operation of the auxiliary spillway has lead to severe erosion that could lead to a failure of the structure. Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville. In response to this developing situation, DWR is increasing water releases to 100,000 cubic feet per second. Immediate evacuation from the low levels of Oroville and areas downstream is ordered.
This in NOT A Drill. This in NOT A Drill. This in NOT A Drill.
The dam sits above a wide swath of wooded and agricultural areas, including the communities of Oroville and Yuba City. As a result of the evacuation orders, roughly 190,000 residents were asked to leave the area to find higher ground. Engineers were able to lower the water level enough over Sunday evening to cease use of the emergency spillway, a development which will theoretically allow engineers to observe and try to repair the damages to the dam. Joe Countryman, a member of the Central Valley Flood Protection Board and a former engineer with the U.S. Army Corps of Engineers, told the Sacramento Bee, “I think between now and Thursday, when the next storm arrives, they need to get the reservoir down as low as they can. Tomorrow, they need to start grouting the hell out of that embankment to try to shut off where that leak is.”
As a result of the potential catastrophe, The Los Angeles Times reports Governor Jerry Brown has put the entire California National Guard—a force of 23,000 of soldiers and pilots—on call for the first time since the Los Angeles riots in 1992.
As of 7:00 AM Pacific Time, the dam is still in immediate danger of partial failure. We will bring more updates as they happen. For a list of evacuation sites and shelters, see the Butte County website.