NORTHERN CALIFORNIA – The California Department of Water Resources on Thursday made public a memo from the independent forensics team, which outlines a list of potential physical factors that may have contributed to the Lake Oroville gated (main) spillway failure on Feb. 7.
“This memo outlines the potential causes of the main spillway failure, which will be thoroughly investigated by the forensics team over the next few months,” said DWR Acting Director Bill Croyle. “Although this is preliminary information, we are taking it into consideration as we develop plans to reconstruct the spillway.”
The memo, which is posted below, includes 24 candidate physical factors for the service spillway damage, ranging from variations in slab thickness to tension reinforcement, corrosion and failure of reinforcing bars across cracks, unfiltered drains, groundwater pressure, issues with the foundation and cavitation, among other things.
Four possible factors are suggested for the damage to the emergency spillway: significant depth of erodible rock and soil in orientations that allowed rapid headcutting toward the crest control structure; hillside topography that concentrated flows and increased erosive forces, facilitating headcut
formation; insufficient energy dissipation at the base of the spillway crest; and absence of erosion protection downstream of the crest structure.
The forensics team is assigned to prepare an independent report on the actual contributing factors that led to the failure of the main spillway at Lake Oroville.
As outlined in the letter dated Feb. 13, 2017, the the Federal Energy Regulatory Commission requested that DWR complete a forensic analysis aimed at determining the cause of the main spillway failure and determining if the failure mode could occur again.
The Association of State Dams Safety Officials and the United States Society of Dams assembled the independent team of experts in April that could conduct a forensic evaluation.
The Federal Energy Regulatory Commission also requested that the Board of Consultants and the California Division of Safety of Dams be allowed to review and comment on the ongoing investigation, including possibly requesting additional information or analysis be completed by the forensic team regarding the analysis.
In a conference call briefing on May 4, the team presented a preliminary list of candidate physical factors that could have potentially contributed to the initial failure mechanisms at the main spillway to DWR, the independent Board of Consultants, the Federal Energy Regulatory Commission and the California Division of Safety of Dams.
The purpose of this list is to outline the potential causes of the spillway damages, to ensure that DWR considers these factors in the new design.
DWR expects to receive the team’s final report in fall of 2017 which will outline what caused the failure.
Consistent with DWR protocol, every memo from the forensics team will be posted in the same format it was received, with critical energy infrastructure information, or CEII, redacted as needed. CEII is considered a security risk, as it contains design details about the Oroville Dam facilities.
050517 Oroville Dam spillway report by LakeCoNews on Scribd