No single root cause was found to be responsible for the failure of the auxiliary spillway at Oroville Dam in California, US, in February 2017 according to the final report of the independent team tasked with investigating the incident – instead it stated that the incident was the cause of ‘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 Oroville Dam Spillway Incident Independent Forensic Team (IFT) submitted their final report compiling the team’s conclusions and opinions regarding the conditions, actions and inactions that contributed to the damage of the service spillway and emergency spillway at Oroville Dam to the DWR on 4 January. The report is an expansion on the findings released in the team’s interim reports, with the addition of detailed supporting evidence and human factors not previously addressed.
Summarizing the report, the team states that during service spillway operation on February 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 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.
The report states that the seriousness of the weak as-constructed conditions and lack of repair durability was not recognized during numerous inspections and review processes over the history of the project.
It said there were many opportunities to intervene and prevent the incident, but the overall system of interconnected factors operated in a way that these opportunities were missed.
It also offered a number of lessons to be learnt from the incident – both for DWR specifically and for the dam industry as whole.
A full copy of the report can be found here. A breakdown of the report and the lessons learned will be published in the February print edition of International Water Power & Dam Construction.