When operators of Oroville Dam suddenly ordered evacuations on Sunday, it focused a big spotlight on a crucial piece of California’s flood-control infrastructure – spillways.

California is home to more than 1,500 dams, and all of the major ones have spillways to release water in big floods and relieve pressure on the dam itself. Some of these spillways are old; some have never been used before. Some are lined with concrete. Some aren’t. Many are too small to handle the sorts of floods California faces now and in the future.

All of them have been overlooked – until now.

“California has a very good division of dam safety, but we also have a large number of dams, and a chronic problem of underinvesting in flood control,” said Jay Lund, an engineering professor who directs the UC Davis Center for Watershed Sciences. Spillways will now get more attention, he said, but only because the unfolding crisis.

At Oroville, the tallest dam in the United States, the concrete-lined main spillway worked properly for 48 years – until it didn’t. On Feb. 7, after heavy flow down the spillway, water managers noticed that the water had dug a hole in the structure, tossing chunks of concrete into the air.

To relieve pressure on the main spillway, Oroville’s operator, the California Department of Water Resources, decided to let the reservoir rise in elevation until water spilled over the concrete lip of the emergency spillway, also known as the “auxiliary spillway.” The dam’s operators had never before used this spillway, an earthen hillside. When they did, it caused quick erosion – so much so that DWR officials feared the structure could fail entirely, unleashing a torrent of water. At that point, they ordered a mass evacuation downstream.

DWR officials had previously been warned that the emergency spillway was unreliable, but apparently they never imagined a scenario where they would be forced to use it. Now, with more storms approaching, DWR is left with three compromised options to get water out of Lake Oroville – the main spillway, the emergency spillway and the dam’s hydroelectric generators. “All three of those are unreliable,” said Lund.

We have a large number of dams, and a chronic problem of underinvesting in flood control. Jay Lund, an engineering professor who directs the UC Davis Center for Watershed Sciences

Oroville is not the only big dam in California with an unlined emergency spillway. The New Melones Dam, which plugs up the Stanislaus River east of Modesto, holds up to 2.4 million acre feet of water, about one third less than Oroville. Its emergency spillway is carved out of a hillside to the west of its 625-foot-high dam.

“There’s a difference between that spillway and the one at Oroville,” said Todd Hill, Mid-Pacific branch chief of the dam safety division at the U.S. Bureau of Reclamation, which operates New Melones. Hill said the New Melones’ spillway cuts into bedrock instead of a forested hillside, greatly lessening the erosion risk.

Yet like some other emergency spillways in California, the one at New Melones has never been tested. Since it was completed in 1970, the reservoir has yet to reach a level requiring the emergency spillway to be used, according to Hill.

South of New Melones, the New Don Pedro Dam holds back the Tuolumne River, with a capacity slightly above 2 million acre feet of water. As of Wednesday, the reservoir was 98 percent full, and managers were releasing as much water as safely possible to create space in the reservoir before a storm system was expected to arrive within hours.

Calvin Curtin, spokesman for the Turlock Irrigation District, which manages New Don Pedro, said the district expects the coming storm to be a cold one, producing snow in the Sierra instead of rain that would further swell the reservoir. If that forecast is wrong, with heavy rain instead of snow, the reservoir could rise and overtop the main spillway, causing some flooding downstream. Yet it’s unlikely it would come close to what happened during the flood of early 1997.

During that flood, more than 70,000 cubic feet of water per second poured out of the reservoir, including down the unlined emergency spillway. The torrent scoured out dirt and trees, destroyed a road and dumped debris in the river channel downstream. Afterward, the district had to remove “thousands and thousands and thousands of cubic yard of dirt and debris from the river,” said Curtain.


Despite that damage, Curtain said the emergency spillway “functioned exactly as it was supposed to.” Downstream? It was a different picture.

In and around Modesto, hundreds of homes were flooded, along with three sewage treatment plants, which gushed raw sewage into the Tuolumne River. While use of the emergency spillway prevented water from destroying New Don Pedro Dam, the levees downstream were easily overwhelmed by the 70,000 cfs surging from the reservoir.

Lund, the UC Davis engineer, said the 1997 flood demonstrates how dam spillways and the channels below them must be designed in unison. “In the case of New Don Pedro, the channel is way undersized,” said Lund. He called it “criminal” there isn’t more channel capacity near Modesto, a city of more than 200,000 people.


Climate change poses another challenge for dam operators and their spillways. Many of these dams are decades old and were not designed with outlets and spillways capable of handling the expected mega-storms of the future. Operators of these dams are stuck between competing demands to maximize water storage and also save space for flood control. They work under operating manuals that don’t account for recent trends, such as the Sierra snowpack melting earlier in the year.

Some of these dams are getting upgrades, albeit slowly. Upstream of Bakersfield, home to 360,000 people, the U.S. Army Corps of Engineers is preparing this year to launch a five-year, $500 million upgrade of the Lake Isabella dam and spillway, to better protect it from earthquakes and megafloods. “Given the large population downstream of Isabella Lake as well as significant dam safety issues at the dam, urgent action is needed to address deficiencies and reduce risk,” says the environment impact statement for the project.

Above Sacramento, the Corps of Engineers expects this year to finish construction on a new $900 million auxiliary spillway for the 62-year-old Folsom Dam. Once completed, that spillway will allow operators to quickly release water down the American River – creating new flood space in the reservoir – if they see a major storm approaching.

Lund said the new spillway is the kind of project that can help communities become more resilient to climate change, even as they continue to depend on decades-old infrastructure. “The storms we are preparing for at Folsom are much different than when it was built,” he said.

The Sacramento Bee’s Ryan Sabalow contributed to this report.

Stuart Leavenworth: @sleavenworth

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