Poor design and construction of the Oroville Dam in California, along with inadequate state and federal oversight since its construction 50 years ago, led to a spillway collapse in February.
An independent team of dam safety experts announced their interim findings on September 5 and urged tougher safety reviews nationwide.
The Independent Forensic Team (IFT) is conducting a review of available information to develop findings and opinions on the chain of conditions, actions and inactions that in February 2017 caused the damage to the service spillway and emergency spillway at Oroville Dam, the tallest dam in the United States. The team is also considering why "opportunities for intervention in the chain of conditions, actions, and inactions were not realized."
Service Spillway Chute Failure
The IFT said that the service spillway chute failure was most likely initiated by the uplift and removal of a section of the slab in the chute downstream of an area known as Station 33+50 shortly after 10 a.m. on February 7.
(Read the interim report [pdf].)
Once the initial section of the chute slab was removed, the underlying moderately- to highly-weathered rock and soil-like material beneath the slab in this location was directly exposed to high-velocity spillway flow. The high-velocity 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 by 12:30 p.m. that same day.
Based on its work to date, the IFT concluded that the initial uplift and removal of the slab section downstream of Station 33+50 was most likely caused by water uplift pressure beneath a sufficiently large area of the slab, producing an uplift force that exceeded the uplift capacity of that particular section of slab.
Resistance to uplift is provided by a combination of the weight of the slab, the weight of the water above the slab, the structure of the slab and the uplift resistance provided by the foundation anchor system. Once the upstream end of the slab section lifted into the flow, the pressure under the slab rapidly increased and produced the slab's sudden failure.
The IFT indicated that the excessive uplift pressure was mainly due to high-velocity spillway flow injecting water into slab surface features, such as open joints, unsealed cracks over herringbone drains, spalled concrete at either a joint or drain location or some combination of these. Localized slab deterioration and repairs pre-existed in this area prior to February 7, the IFT said, and these deteriorated and repaired locations may have been vulnerable to damage during high-velocity spillway flow.
Because all physical evidence at the point where the failure initiated was destroyed, the IFT says "it will never be possible to confirm exactly where the initiating event occurred."
The IFT said that numerous lessons are to be learned in terms of current California Water Resource and industry dam safety practices, and these lessons will be developed and presented in the IFT’s final report. It shared three "higher-level lessons" that have been identified so far:
1. Physical inspections, while necessary, 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.
2. 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. These reviews should be:
• Thorough, taking advantage of all available information
• Critical and independent, rather than relying largely on the findings of past reviews
• Completed by people with appropriate technical expertise, experience and qualifications to cover all aspects of design, construction, maintenance and failure modes of the assets under consideration.
The IFT said it has seen no indication that such a review for the service spillway chute at Oroville Dam has ever been conducted since original construction. Such a review would likely have “connected the dots” by identifying the physical factors that led to the failure of the service spillway chute, including:
• Design shortcomings relative to current state of the practice
• Construction procedures, decisions and changes to the designs that exacerbated the shortcomings of the design
• Drain flows well beyond those intended in design and beyond observed drain flows at other spillways
• Subsurface geologic conditions that left portions of the spillway susceptible to uplift and subsequent foundation erosion
• Chute slab repairs that were generally limited in extent, rather than designed to reliably and durably withstand high-velocity flows, thermal effects and other loading conditions
3. Compliance with regulatory requirements is not sufficient to manage dam owners’ and public risk.
• Current regulatory requirements are generally focused on preventing failures involving uncontrolled releases of stored water. Serious incidents can occur that do not necessarily lead to uncontrolled release, but still have significant impacts on the owner and public, such as (a) limitations on an owner’s ability to control the reservoir, (b) costs of emergency management and repairs, (c) damage to, or loss of, resources and project benefits, (d) environmental damage, (e) impacts on society, (f) damage to reputation and (g) potential legal liability.
• Current risk analysis processes are also focused on the uncontrolled release of reservoir water and generally do not include the development of scenarios for non-release incidents that can result in the same impacts noted in the previous bullet point.
• Regulators have an essential role in the management of dam safety, but do not have the resources nor the primary responsibility for managing dam safety. That responsibility, both ethically and legally, rests with dam owners, and dam owners must, therefore, develop and maintain mature dam safety management programs that are based on a strong “top-down” dam safety culture.
The IFT said its work will be completed later in the fall of 2017 when a final report will be issued.