The Oroville Dam spillway incident was caused by a long-term systemic failure of the California Department of Water Resources (DWR), regulatory and general industry practices to recognize and address inherent spillway design and construction weaknesses, poor bedrock quality, and deteriorated service spillway chute conditions.
The incident cannot reasonably be “blamed” mainly on any one individual, group or organization.
Those are the chief conclusions of a 584-page final report by the Independent Forensic Team assembled to investigate causes of the February 2017 failure of the dam, which is the highest in the United States. (Read "Highest Dam in the U.S. Faces Threat.")
The report says that during service spillway operation on Feb. 7, 2017, water injection through both cracks and joints in the chute slab resulted in uplift forces beneath the slab that exceeded the uplift capacity and structural strength of the slab, at a location along the steep section of the chute.
The uplifted slab section exposed the underlying poor quality foundation rock at that location to unexpected severe erosion, resulting in removal of additional slab sections and more erosion.
Responding to the damage to the service spillway chute necessitated difficult risk tradeoffs while the lake continued to rise, the report says. The resulting decisions, made "without a full understanding of relative uncertainties and consequences," allowed the reservoir level to rise above the emergency spillway weir for the first time in the project’s history, leading to severe and rapid erosion downstream of the weir and, ultimately, an evacuation order affecting around 200,000 people.
The report builds on an interim memo from September 2017 that identified physical causes of the incident. The California Department of Water Resources, which is responsible for the dam, says it is in the process of reviewing the report, and intends to incorporate the latest findings into its ongoing efforts.
In May, the state agency began planning for a comprehensive needs assessment of the entire Oroville complex to identify any changes that need to be made to bolster dam safety. An evaluation of the agency's dam safety program is already underway. With the forensic report's findings, organizational issues also may be addressed.
The report says that no single root cause of the Oroville Dam spillway incident exists. It also says there was no simple chain of events that led to the failure of the service spillway chute slab, the subsequent overtopping of the emergency spillway crest structure and the necessity of the evacuation order.
"Rather, the incident was caused by a complex interaction of relatively common physical, human, organizational and industry factors, starting with the design of the project and continuing until the incident."
The report finds that the inherent vulnerability of the service spillway design and as-constructed conditions reflect "lack of proper modification of the design to fit the site conditions." Almost immediately after construction, the concrete chute slab cracked above and along underdrain pipes, and high underdrain flows were observed. The slab cracking and underdrain flows, although originally thought of as unusual, were quickly deemed to be “normal,” and as simply requiring on-going repairs. The report finds that repeated repairs were ineffective and possibly detrimental.
The report outlines broad lessons to be learned by the dam safety community:
• In order to ensure the safe management of water retention and conveyance structures, dam owners must develop and maintain mature dam safety management programs which are based on a strong “top-down” dam safety culture. There should be one executive specifically charged with overall responsibility for dam safety, and this executive should be fully aware of dam safety concerns and prioritizations through direct and regular reporting from a designated dam safety professional, to ensure that “the balance is right” in terms of the organization’s priorities.
• More frequent physical inspections are not always sufficient to identify risks and manage safety.
• Periodic comprehensive reviews of original design and construction and subsequent performance are imperative. These reviews should be based on complete records and need to be more in-depth than periodic general reviews, such as the current FERC-mandated five-year reviews.
• Appurtenant structures associated with dams, such as spillways, outlet works, power plants and so on, must be given attention by qualified individuals. This attention should be commensurate with the risks that the facilities pose to the public, the environment, and dam owners, including risks associated with events which may not result in uncontrolled release of reservoirs but are still highly consequential.
• Shortcomings of the current Potential Failure Mode Analysis (PFMA) processes in dealing with complex systems must be recognized and addressed. A critical review of these processes in dam safety practice is warranted, comparing their strengths and weaknesses with risk assessment processes used in other industries worldwide and by other federal agencies. Evolution of “best practice” must continue by supplementing current practice with new approaches, as appropriate.
• Compliance with regulatory requirements is not sufficient to manage risk and meet dam owners’ legal and ethical responsibilities.
The forensic team included John W. France, PE, D.GE, D.WRE – Team Leader and Geotechnical Engineer; Irfan A. Alvi, PE – Hydraulic Structures Engineer and Human Factors Specialist; Peter A. Dickson, PhD, PG – Engineering Geologist; Henry T. Falvey, Dr.-Ing, Hon.D.WRE – Hydraulic Engineer; Stephen J. Rigbey – Director, Dam Safety at BC Hydro, and Geological Engineer; John Trojanowski, PE – Hydraulic Structures Engineer.