The independent forensic team (IFT) investigating the Oroville Dam spillway incident last February has released its interim findings, and they indicate physical inspections alone are not adequate to identify risks and manage safety.
The team has progressed its investigation of physical factors and human and organizational factors since it published its preliminary findings on May 5.
At this time, the IFT believes the service spillway chute failure most likely initiated by the uplift and removal of a section of the slab in the chute downstream of Station 33+50 shortly after 10 am on Feb. 7. The underlying moderately to highly weathered rock and soil-like materials was directly exposed to high-velocity spillway flow. This flow rapidly eroded the foundation materials, removed additional chute slab sections in both upstream and downstream directions, and created the erosion hole that was observed at 12:30 pm on Feb. 7.
The IFT believes that four physical factors were most likely involved in the observed damage at the emergency spillway:
- New damage and/or deterioration of slab repairs that resulted in more potential flow disturbance locations and more flow into the foundation that during the prior spillway discharge events
- Possible expansion of relatively shallow void(s) under the slab due to long-term erosion under the slab
- Corrosion of the rebar across the concrete cracks or joints
- Reduction in anchor capacity due to erosion around anchors beneath the slab or anchor corrosion where anchors were not properly encapsulated in grout.
Three lessons to be learned have been identified so far:
- Physical inspections are not sufficient to identify risks and manage safety. At Oroville Dam, more frequent physical inspections would not likely have uncovered the issues which led to the spillway incident.
- Comprehensive periodic reviews of original design and construction, taking into account comparison with the current state of the practice, are needed for all components of dam projects. “The IFT has not seen any indication that such a review for the service spillway at Oroville Dam has ever been conducted since original construction,” the report says.
- Compliance with regulatory requirements is not sufficient to manage dam owners’ and public risk. “Current PFMA and risk analysis processes are … focused on uncontrolled release of reservoir water and generally do not include development of scenarios for non-release incidents that can result in … significant impacts to the owner and public.”
The IFT reviewed extensive documentation related to the geology and subsurface conditions of the spillway foundations; design and construction of the spillways; inspection, evaluation, maintenance and repair of the spillways; the California Department of Water Resources dam safety program management; and other regulatory framework for the project. The team also studied photos and videos of the spillway and post-incident forensic field investigation reports related to the failure and post-failure conditions.
The IFT’s final report will be issued in the fall of 2017.