With a height of 770 feet, California’s Oroville Dam is the tallest in the U.S., and it’s still teetering on the edge of disaster.
On Feb. 2, parts of the dam’s spillway began to erode. Chunks of concrete broke loose, exposing the soft clay below to the rushing force of the water. An evacuation order issued Feb. 12 by the Butte County Sheriff’s Department made more than 180,000 people leave the area.
It took until March 22 before the evacuation order was lifted. Meanwhile, crews worked around the clock to reinforce an emergency spillway just west of the original concrete one.
In spite of all the efforts underway, the threat isn’t going away yet. Oroville Dam itself wasn’t at risk of breaking. Unlike Hoover Dam (Nevada), which is 50 feet shorter, or Fontana Dam (North Carolina, 480 feet), Oroville isn’t a concrete arch wedged against the weight of the water. It’s an earthen “embankment” type dam. This makes it less vulnerable to earthquake damage.
But an event that must be prevented at all costs is water overflowing its top. In that case, the flow would quickly tear a breach into the dam. The enormous water pressure would keep opening the breach, allowing the lake to drain completely. A catastrophic flood would result. Not only would it wipe the eight towns evacuated in February off the map. Most likely, the flood wave would make it all the way down the Feather River to where it joins the Sacramento.
The city of Sacramento itself could see a deadly flood. In 1889, an embankment dam broke upstream of Johnstown, Pa., and caused exactly that kind of disaster.
To prevent this, every dam in California has a fortified spillway to allow for controlled releases of water. That’s the one that broke in Oroville, sending a wave of debris downstream that put the power station out of order. Farther away from the dam, an emergency weir overflowed, draining water into the unlined emergency spillway. Rapid erosion raised fears of a major breach, which could almost be as bad as a complete dam failure.
The task at hand is to repair the main spillway before rainy season starts in the fall. At the same time, the emergency spillway is being cleared of trees and receiving a concrete liner, an additional safety measure in the race against the clock.