Down go the Oroville spillway walls in controlled blasts
If you’re expecting a quick and easy answer on what caused the spillway failure at Oroville Dam, think again.
The leader of the independent forensics team studying the Oroville crisis said Thursday that the crack in the dam’s main flood-control spillway likely was caused by a combination of problems.
“We do anticipate there will be multiple contributory factors, no single factor,” said dam safety consultant John France in a conference call with reporters.
France, an independent consultant from Denver, added that it will take months before his team gets to the bottom of what went wrong in Oroville, comparable to the lengthy probes that follow major airplane or train crashes. His team will delve into the earliest design work performed on Oroville Dam, which was completed in 1968.
“We’re looking at the full span of the dam’s life, including its gestation period,” he said.
He said investigation is being closely watched by dam operators worldwide.
“This investigation and what we find is going to be very important to the dam safety community,” he said. “I’m sure there are lessons to be learned in here.”
France and the other five consultants on his team, in a preliminary analysis released last month, cited 24 possible causes for the spillway failure, including a faulty drainage system, variations in concrete thickness and corrosion in the structure’s rebar.
The panel said a surge in water releases may have contributed to the initial rupture in the 3,000-foot-long spillway. It also said the spillway wasn’t properly anchored to the underlying bedrock, and it found signs that previous repairs may have lacked “durability and effectiveness.”
France said “there are some (potential causes) that stand out more than others,” but he declined to identify them.
The forensics group’s preliminary findings dovetail with analyses performed by the U.S. Army Corps of Engineers and a separate board of consultants advising the state on repairing the fractured spillway and the dam’s emergency spillway.
California Department of Water Resources officials have said that although the forensics probe is in its early stages, they plan to take the group’s analysis into account as the repairs go forward.
Dam officials “adopted design measures to mitigate any of the challenges that the forensics team identified as possible contributors (to the crisis),” DWR engineering chief Jeanne Kuttel said last week. That includes thicker concrete and “state-of-the-art drains,” she added.
France said the forensics team hasn’t been briefed on the details of the repair plan, but he’s confident that the design team “understands the factors that we listed.”
Demolition on the lower portion of the main spillway began May 20 and is expected to last through late June. At that point, reconstruction work will start. DWR officials said they expect to have the spillway operational by November, the start of the next rainy season. Next year the state plans to replace the upper portion of the spillway even though it wasn’t damaged.
Kiewit Corp. of Omaha, Neb., is the lead contractor on the $275 million repair job. State officials expect to be reimbursed for much of the cost by the Federal Emergency Management Agency. The various water agencies that store water in Lake Oroville, the largest reservoir in the State Water Project, are expected to pick up any costs not covered by the federal agency.
The Oroville crisis began when the main spillway fractured Feb. 7. Dam operators began limiting water releases to contain the damage, even as a heavy rainstorm rolled into the region.
Lake levels rose so high that water poured over the emergency spillway, which consists of a concrete lip atop a hillside, for the first time ever Feb. 11. A day later, engineers found that the hillside had eroded so badly that the structure might collapse. That prompted the evacuation of 188,000 downstream residents. The situation eased after dam operators ramped up water releases from the main spillway, which lowered lake levels and halted the flow of water over the emergency spillway.
Kiewit is working to fortify the hillside. Among other things, it will embed a vertical “cutoff wall” into the hillside to arrest erosion if the spillway is ever used again.