Sacramento, Calif. — The California Department of Water Resources (DWR) 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.”
In a conference call briefing on Thursday, 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 (BOC), the Federal Energy Regulatory Commission (FERC), and the California Division of Safety of Dams (DSOD).
Following are the candidate physical factors potentially contributing to service spillway damage:
- Thinning of the chute slab above herringbone drains; these locations can promote cracking.
- Large variations in slab thickness.
- Limited slab reinforcement consisting of one layer of light reinforcement in the top of the slab.
- Lack of continuous tension reinforcement across slab joints.
- Corrosion and failure of reinforcing bars across cracks.
- Slab joints with insufficient keys or lack of keys.
- Slab placement sizes which were too large to control cracking.
- Lack of waterstops in slab joints.
- Hydraulic pressures and flows transmitted beneath the slab sections through open cracks and joints.
- Increase in spillway discharge shortly before slab failure.
- Plugging or collapse of drains or collector pipes, including potential plugging by tree roots.
- Flow into the foundation that exceeded the capacity of the drain pipes, including possible flows from areas adjacent to the chute.
- Lack of redundancy in collector drains.
- Unfiltered drains; the gravel envelope may not serve as a filter.
- Herringbone drains crossing joints in the slab.
- Weathered rock and completely weathered rock that is soil-like material as slab foundation, without appropriate modification of the chute slab design, resulting in potentially erodible material beneath the slab and lack of foundation bond with concrete; the weathered rock and completely weathered rock appears to be associated with geologic features such as shear zones, and the degree of weathering changes relatively rapidly between some areas of the chute slab.
- Less rigorous foundation preparation, resulting in lack of foundation bond with concrete.
- Extended drought impacts on foundation materials.
- Insufficient anchorage, due to limited anchor development in the concrete, short anchor length, inadequate grouting or grout strength, and/or installation in weak foundation material.
- Relatively high spillway flow velocities in the lower chute for higher spillway discharges.
- Lack of durability and effectiveness of slab repairs.
- Spalling and/or delamination of concrete at slab joints.
- Groundwater pressures; although current evidence suggests this may not have been a significant factor.
- Cavitation; although preliminary analysis suggests this may not be a significant factor.
Following are candidate physical factors potentially contributing to emergency spillway damage:
- Significant depth of erodible rock and soil in orientations that allowed rapid headcutting toward the crest control structure; these materials also appear to be associated with geologic features such as shear zones.
- Hillside topography that concentrated flows and increased erosive forces, facilitating headcut formation.
- Insufficient energy dissipation at base of the spillway crest.
- Absence of erosion protection downstream of the crest structure. 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 posted the Forensics Team memo online at http://www.water.ca.gov/oroville-spillway/pdf/2017/Memorandum_050517.pdf.
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 (CEII) redacted as needed CEII is considered a security risk, as it contains design details about the Oroville Dam facilities.
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 February 13, 2017, FERC 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. Furthermore, FERC requested that the BOC and the DSOD 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.