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Two year anniversary of Oroville Spillway Crisis: Emergency spillway nears completion

An aerial overview of the finished roller-compacted concrete splashpad, while work continues on the concrete cap below the Lake Oroville emergency spillway weir at the Butte County, California site. Photo taken January 24, 2019.Kelly M. Grow / California Department of Water Resources,

Thursday marks two years since the first hole opened up in the Oroville Dam Spillway, triggering an emergency that forced the evacuation of nearly 200,000 people

The crisis started on February 7, 2017. The California Department of Water Resources (DWR) had been releasing 60,000 cubic feet of water per second, when they noticed the concrete on the spillway began to disintegrate. That first day, the hole was 30 feet deep by 180 feet wide.

DWR was releasing water to make room in Lake Oroville for heavy rain that was causing the lake levels to rise.

Due to the growing hole in the spillway, DWR was forced to temporarily stop the release of water and lake levels continued to rise. Crews resumed the water releases over the next several days, but by then rain continued to elevate the lake level faster than the water could be released. DWR released as much water as possible, further deteriorating the damaged main spillway.

On February 11, for the first time in the history of the Oroville Dam, water began to flow over the dirt hillside of the Emergency Spillway.

The next afternoon, on Sunday, February 12, the hillside had eroded to such an extent that DWR engineers and Butte County Sheriff Kory Honea began discussing the possibility that the Emergency Spillway concrete structure would collapse, sending a potentially deadly wall of water downstream.

Late in the afternoon on Sunday, February 12, 2017, Sheriff Honea issued the evacuation order for Oroville and thousands of residents downstream. They were allowed to return home two days later.

Over the last two years, thousands of construction crews have worked to rebuild the main spillway and the emergency spillway. As of November 1, 2018, the main spillway was ready for use.

Minor finishing work like sidewall back fill and site clean-up is ongoing on the main spillway

On the emergency spillway, construction crews are currently installing a concrete cap on top of the buttress and general site mitigation is ongoing including grading and hydroseeding. DWR expects to complete the entire $1.1 billion project in the summer of 2019.

There are key differences between the new spillway and the original spillway built in 1968.

The concrete on the original spillway chute had an average thickness of 2 feet 8 inches.

The new spillway chute is about three times as thick, an average of 7 feet 6 inches.

The rebar on the original spillway was 4 million pounds. The rebar on the new spillway is three times heavier, at more than 12 million pounds.

The main spillway has more than half a million cubic yards of concrete, enough to build a five-and-a-half foot sidewalk from Oroville to Amarillo, Texas.

The new emergency spillway is covered with roller-compacted concrete that looks like a giant staircase. It is one of the biggest changes during the reconstruction of the spillway project.

The emergency spillway concrete splashpad is so large, 25 football fields with endzones would fit. The large concrete steps have a minimum thickness of 10 feet. At the bottom is a new cutoff wall to stop erosion and headcutting of the downstream hillside.

Combined, the main and emergency spillways have more than 1.2 million cubic yards of concrete, enough to fill 372 Olympic-size swimming pools.

In January 2018, an independent panel of dam experts issued their report on the spillway emergency. Those experts said long-term and systemic failures by officials in California and elsewhere caused the near-disaster at the nation's tallest dam.

The independent panel of dam experts says the dam had inherent design and construction weaknesses. The report faults California's Department of Water Resources and other regulators for allegedly failing to recognize and address those problems.

Erin Mellon, spokesperson for the Department of Water Resources, said the organization remains committed to public safety.

"DWR has a long-term commitment to curating the best available science, sufficient financial investments, and a high standard of innovative expertise.," Mellon said. "In just 19 months, DWR and our contractors repaired and reconstructed Oroville’s main and emergency spillways, which stand ready to use when needed. DWR's dam safety practices, organization, and emergency action plans that facilitated their actions during the crisis continue to be supplemented, with benefits and lessons imparted to the global dam industry."


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