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Here's the Technical Reason the Oroville Dam Spillways Burst

On Sunday, officials at the California Department of Water Resources noticed massive holes in both the main and emergency spillways of the Oroville Dam, the tallest dam in the country, in an utterly avoidable accident according to environmental groups. 

An estimated 200,000 people were evacuated from their homes in the northern part of the state, after officials notified them in a very frightening alert stating this was not a drill and that 30-foot-tall “wall” of water could flush the valley in a matter of minutes. An evacuation alert like this always creates chaos, but officials had no choice because they knew the burst could happen and could do nothing about it. 

The emergency spillway is a 1,700-foot-long concrete structure that leads into a heavily wooded flood plain before entering the river. It was built 50 years ago as a last resort for officials should they ever need it. The aging infrastructure had several small cracks and unstable eroded land beneath, it was destined to fail. 

When environmental groups pointed out the potential for disaster in 2005, they said it would destroy transmission lines and existing development in the valley. Government officials said it was too expensive and deemed it unnecessary to restore. 

Construction would have cost at least $100 million, said Ronald Stork, senior policy staff of Friends of the River. “They told us not to worry. All was good. Everything was fine. It’s all safe,” Stork told the Washington Post. “First of all, they’re not supposed to fail. That’s not what we do in a first-world country. We don’t do that. We certainly don’t do that with the nation’s tallest dam. An auxiliary spillway isn’t supposed to cause lots of havoc when it’s being used.” 

In the next ten years, the state would experience widespread drought, putting the project lower and lower on the state’s to-do list. 

So, when Oroville experienced 25 inches of rain in a four-month time period this winter — its average annual rainfall is only 31 inches — the officials had a lot more water to deal with than they predicted. The lake began spilling over the dam and the main spillway was already at capacity. So, the officials had to use the decades-old emergency spillway for the first time ever. Just as the environmental groups had predicted, the land at the end of the emergency spillway eroded, and as a result, both the main and the emergency spillways experienced large bursts. 

The main spillway was a stable structure, but when the land beneath it gave out, a crater formed. Then shortly after, under the weight of 12,000 cubic feet of water per second (that’s just three percent of what officials thought it could handle), the emergency spillway cracked and a 200-foot-long, 30-foot-deep hole formed within the structure — that’s roughly the size of a football field. But, as shocking as it may seem, both the holes in the main and emergency spillways could have been avoided. 

Concrete is a composite of water, sand, gravel, bits of glass, and even recycled concrete. So, as we see with roadways and bridges, it tends to crack under pressure and water gets into those cracks and disintegrates the material, usually causing potholes. In the case of this emergency spillway, it was a gaping, hazardous, pot hole-monster ready to destroy the nearby homes in the area. 

As the 2005 motion submitted to the Federal Energy Regulatory Commission from three environmental groups — Sierra Club, South Yuba River Citizens League, and Friends of the River — suggests the emergency structure needed to be reinforced and rerouted from possible erosion to also protect the main spillway. This involves laying down a large steel cage using rebar. The concrete is then set and it acts as a shield against compression while the steel bars acts as reinforcement against tension. In the case of the emergency spillway, the concrete didn’t have a steel reinforcement so, it cracked uncontrollably from the tension of the erosion and washed away. 

But, officials claimed they had never heard the reports and that the spillways always met Federal Energy Regulatory guidelines and were deemed safe. 

Lester Snow, the Department of Water Resource’s director from 2004 to 2010, told the Mercury Register that he did not recall specific information about the debate over the emergency spillway 12 years ago. “The dam and the outlet structures have always done well in tests and inspections,” he said. 

Now, helicopters are throwing boulders into the hole of the main spillway as a band-aid precaution and the emergency spillway can’t be used at all. 

Officials are still declaring the event an emergency and with an expected six more inches of rainfall over the next weeks, they are holding their breath as they watch the spillway deplete the lake hoping that by some miracle, the neighboring community will be saved.



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