SAN FRANCISCO (AP) — The sudden collapse of spillways at the nation’s highest dam has raised alarm among those building, running and regulating big dams around the world because it seemed to come with little warning the spillways were on the verge of failing, dam experts investigating the crisis at California’s Oroville Dam said Thursday.
February’s breakup of the main spillway and then the backup spillway at the 770-foot-high (230 meters) Oroville Dam stands as an “extremely significant” event among dam disasters and near-disasters in modern U.S. history, said John France, an engineer leading the investigation by two national trade associations representing dam-safety and dam-engineering professionals.
The significance was only partly due to the size of the Northern California dam, France said.
Unlike most dam failures, which happen in flooding and after signs that a structure is being overwhelmed by water, “what happened was a surprise,” France said.
“I’m sure there are lessons to be learned,” he said. “I’m confident they’re going to be significant and change the practice of dam safety engineering in the country and perhaps in the world.”
The national Association of State Dam Safety Officials and the United States Society on Dams created the independent panel to try to identify the operational and physical failures that made the two spillways at Oroville Dam give way.
Authorities ordered the evacuation of nearly 200,000 people below the dam on Feb. 12 with an hour’s notice, fearing the disintegration of the spillways and the hillside around them could lead to massive, uncontrolled releases of water. Residents were allowed to return home in two days.
The dam holds back the state’s second-largest reservoir and is an anchor of the state’s complex north-south water delivery.
Federal and state agencies that manage and oversee Oroville Dam also are conducting separate investigations into the spillway failures.
State officials with the Department of Water Resources, which runs the half-century-old dam, at times have pointed to the torrents of runoff pouring into the dam at the time of the crisis. But the amount of water streaming down the two flood-release spillways when they began to collapse was relatively small.
The state water agency and others involved are cooperating with the independent probe by the dam groups.
The independent probe will cover everything from the preliminary work leading up to Oroville’s construction in the 1960s to this winter’s spillways failures, members of the investigation team said.
“We recognize how significant this incident is,” said Dan Wade, another member of the investigating team and a program director for the San Francisco Public Utility Commission.
Dam professionals around the country, many of them working with dams as old or older than Oroville, want independent and thorough answers on the Oroville crisis, he said.
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