In a report released Wednesday, engineers assigned to investigate the February failure of Oroville Dam’s main spillway cited a variety of flaws in the 3,000-foot-long structure, including variations in the thickness of the concrete slabs, poor drainage beneath the spillway, improperly filled cracks and signs of inadequate maintenance.
The forensics team, consisting of six outside consultants, also said the spillway may have split because of an increase in water releases just prior to the Feb. 7 incident.
However, the team warned that its analysis, which consists of 24 possible causes, is preliminary. “Additional factors may be identified as the investigation proceeds,” the group wrote in a three-page memo to the state Department of Water Resources. The team is expected to make its final report this fall.
The preliminary report represents the first analysis by the team officially designated by DWR to determine what caused the dam’s main flood-control spillway to break apart. DWR was ordered to hire the forensics team by the Federal Energy Regulatory Commission, which licenses the dam and approved DWR’s selection of the team members. The team turned in its preliminary analysis to DWR last week, but the state didn’t release its contents until Wednesday.
Among the possible causes, the team took aim at DWR’s maintenance of the structure over the years, pointing to “lack of durability and effectiveness of slab repairs.”
One engineering expert said preliminary findings looked right – with one key exception.
Bob Bea, of the Center for Catastrophic Risk Analysis at UC Berkeley, said the memo lacked analysis of the institutional deficiencies that allowed the problems to persist. In past infrastructure failures, those issues accounted for almost 80 percent of why problems developed and went uncorrected, said Bea, a retired engineer whose credentials include conducting an independent investigation into why the levees around New Orleans failed during Hurricane Katrina in 2005.
Looking into both human and mechanical errors provides “the insights about how … you fix this in the future,” said Bea, who released an unsolicited report on the spillway’s fracture in April.
The forensics team’s report largely dovetails with earlier analyses by Bea, the U.S. Army Corps of Engineers and a group advising DWR on spillway repairs.
The fracture of Oroville’s main spillway prompted dam engineers to limit the outflow of water as they attempted to contain the damage to the structure. A huge rainstorm filled Lake Oroville so high that water then flowed over the adjacent emergency spillway – a concrete lip above an unlined hillside – for the first time in the dam’s history. The hillside eroded badly, prompting fears that the emergency spillway would crumble and release a “wall of water.” That triggered the emergency evacuation of 188,000 downstream residents Feb. 12.
DWR has hired Kiewit Corp. of Omaha, Neb., to rebuild the two spillways. The $275 million repair project is expected to take two years, although DWR officials have said the structures will be strong enough to handle the next rainy season, which begins in November.
Two Republican legislators – Assemblyman James Gallagher and Sen. Jim Nielsen – whose constituents were evacuated, said in a joint statement they were troubled that some of the flaws cited by the forensics team had been identified by state officials during routine inspections in past years. Those problems included cracking and the presence of trees growing too close to the spillway, which may have interfered with the drainage system.
“We will remain resolute in our efforts to explore whether or not inspection findings were adequately addressed,” the lawmakers said. Three Assembly committees will hold a joint hearing Thursday afternoon on the spillway emergency.