Deconstructing The Oroville Spillway Situation

Capital Public Radio's environmental reporter Amy Quinton and Insight host Beth Ruyak explore and deconstruct the complexities of the ongoing issues at Oroville Dam, the nation's tallest dam.

The recent onslaught of rain in Northern California has caused unprecedented damage to the dam's spillway system. The saga began Feb. 7 after dam operators noticed a hole in the main spillway. Then, on Feb. 11, the dam's emergency spillway had to be utilized. On Feb. 12, residents downstream from the dam were told to evacuate as officials anticipated the failure of the auxiliary spillway. 

By Feb. 14, the danger had dissipated and residents were told they could return to their homes. As of Feb. 17, officials warned that crews are still in emergency mode, and are still working to fix the spillway system. Residents were also told they may be asked to evacuate again if conditions change.

Dam Terminology Translated

In the midst of the crisis, two key phrases have emerged: emergency spillway and auxiliary spillway. Much has been made about the damage to the main spillway, which is the root cause of the problem, but further, unexpected damage to the dam's second, backup spillway has further exacerbated the crisis.

According to Quinton, emergency spillways and auxiliary spillways are one and the same. The auxiliary spillway functions as a backup in case the main spillway cannot properly function.

Spillways: The Fine Line Between Order and Chaos

Emergency spillways, which are designed to carry extra flow over them, are very rarely used, according to UC Davis Civil and Environmental Engineer professor Jay Lund.

Emergency spillway usage, in fact, is so rare that Feb. 11 marked the first time in Oroville Dam's nearly 50-year history that the auxiliary backup had been used.

Getting to the Root of the Matter

So, what, exactly happened - and could still happen - with the dam's spillways?

"You can see where the ground at the base of the spillway started to erode," Quinton says. "And I am assuming that the fear was that it [the water flow] could continue and there could possibly be a failure in the concrete. There would have been a kind of caving in or dropping down [of all] this [water] and I've heard it described for engineers that it would be basically a 30-foot wall of water coming down and that was their major concern..."

Quinton also says that "what we need to be aware of with the infrastructure here" is that "these are aging systems that need to be updated ... [and] dam inspections need to happen."

Potential Scenarios

"[A spillway failure] would cause some serious damage and some serious flooding down the Feather River and even to the I-5 and Highway 70 corridor," Quinton says. "One thing to point out is that we have the Yolo Bypass here in Sacramento; you know we're lucky in the sense that the water has somewhere to go and that Sacramento is such a large river. And so that water probably would not flood Sacramento."

Dam Safety

During the crisis, the Department of Water Resources has not taken an official stance on the situation. Reports of conflicting statements have been made throughout the saga.

"You know, the Department of Water Resources has said over and over again that they think the dam is safe. They don't expect the dam to fail. You know, this is just happening with the spillway," says Quinton.

Quinton continues, "but I think we're all in a state of wait-and-see."

The Outlook is Cautiously Optimistic

As of Thursday morning, Department of Water Resources acting Director Bill Croyle said lake levels had gone down to 868 feet from the spillway-overtopping high of 901 feet.

"And then, of course, we have the upcoming storms," says Quinton. "How much can the reservoir lose [in a few days]?"

More storms forecasted for the weekend have dam officials planning ahead for contingencies, though. Friday, Saturday, and Sunday are predicted to bring as much as two inches of rain in the Oroville region and four to six inches of rain to the foothills, according to the National Weather Service.

"So officials want to be sure whatever decision they make today not only gets them beyond [this weekend but also into coming weeks] because there won't be much room in between [storm systems]," says Ruyak.

Evacuees Are Not Alone

Resources for residents affected by the Oroville Dam crisis can be found at www.capradio.org. Updates on resources as well as on the crisis will be periodically given on www.capradio.org.

- Capital Public Radio Staff