I blogged about the risk of dam failure a couple of years ago, but the topic is in the news again, with the possible failure of the Oroville Dam in California hitting the headlines. This 770-foot high earthfill embankment dam—the tallest dam in the country—was built in the 1960s to supply water, generate power, and control flooding. Behind it, Lake Oroville—California’s second largest reservoir—has a capacity of 3.5 million acre-feet. (One acre-foot equals 325,900 gallons.)
Earthquakes and flooding can trigger dam failures, both large and small, but there is usually some underlying deficiency that is a causative factor, such as:
- Inferior construction materials and/or techniques
- Geological instability or earthquake shaking
- Poor maintenance
- Human, computer, or design error
- Internal erosion (earthen dams particularly)
- Seepage causing sinkholes in the dam
Unlike some of the dams discussed in my previous blog, Oroville is not particularly old, but its problems have been caused by a combination of unusually heavy precipitation and flaws in its spillways. The dam actually has two spillways—both of which failed. One is a concrete spillway used for the controlled release of water to prevent the lake from overfilling. The other is an auxiliary spillway that lets water pour onto the hillside in an uncontrolled flow if the level in the lake becomes too high, like the overflow in a bathroom sink.
As the level of the lake rose, operators used the dam’s main spillway to discharge water at the rate of about 50,000 cubic feet per second (1,400 m3/s), and the flow catastrophically eroded the concrete channel, sending chunks of masonry flying and creating a rapidly growing hole in the structure of the spillway.
With water filling the lake faster than it could be discharged and the spillway compromised, the auxiliary spillway—which had never been called into service since the dam became operational in 1968—was clearly going to be needed. While it was being cleared of trees and undergrowth, the dam’s operators had no choice but to continue using the main spillway, even though further damage to it was inevitable.
When the water started pouring over the auxiliary spillway on Friday, February 10, the earthen hillside below began to experience erosion that spread uphill toward the concrete lip at the head of the auxiliary spillway. The flow of water down the damaged main spillway was therefore increased in an attempt to slow the erosion of the hillside. This lowered the lake sufficiently to stop the flow of water into the auxiliary spillway, but at the cost of yet more damage to the main spillway.
If the lip at the head of the auxiliary spillway were undermined by the erosion and collapsed catastrophically, up to 30 vertical feet of Lake Oroville would surge through the resulting gap to inundate the communities in the valley downstream. For that reason an evacuation order was issued for the communities in low-lying areas along the Feather River Basin in Butte, Yuba, and Sutter counties. Reportedly, almost 190,000 people in the vicinity were evacuated as a precaution, but have now been allowed home.
At present both the main and auxiliary spillways are holding, and the water level in the dam is falling at a rate of 3-4 inches per hour. The goal is to reduce the water level by 50 feet so the repairs can be made to stabilize the structure of the dam. Meanwhile, the main spillway has to continue discharging water at a high rate while its structural safety is not yet clear. The situation at Oroville, while improving, is still far from resolved and further precipitation is expected.
Oroville is not the only dam placing people and property at risk. Most notably in Iraq, the Mosul Dam upstream of Bagdad on the River Tigris, has been described by the United States Army Corps of Engineers as “the most dangerous dam in the world.” It is unstable because it was built on gypsum, which dissolves in contact with water. If it were to collapse catastrophically, casualties could run into many hundreds of thousands. Dam breaches, although rare events, do occur. The worst incident of this type in U.S. history was the collapse of the South Fork Dam in Johnstown, Pennsylvania, in 1889, which cost more than 2,200 lives. What kind of event will it take for the issue of dam safety to garner the attention and resources it needs?
