Page - 170 - in Book of Full Papers - Symposium Hydro Engineering

SUMMARY The Oroville Dam spillway incident was caused by a long-term systemic failure of practices of DWR, its regulators, and the United States dam safety industry to recognize and address inherent spillway design and construction weaknesses, poor foundation bedrock quality, and deteriorated service spillway chute conditions. The incident cannot reasonably be “blamed” mainly on any one individual, group, or organization. During service spillway operation on February 7, 2017, water injection through both cracks and joints in the chute slab, and associated transmission of stagnation pressure under the 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 and extreme uplift pressure, resulting in removal of additional slab sections and more foundation erosion. Responding to the damage to the service spillway chute necessitated difficult risk tradeoffs while Lake Oroville 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 crest for the first time in the project’s history, leading to severe and rapid erosion and headcutting downstream of the weir and, ultimately, resulting in an evacuation order for about 188,000 people. There are a number of lessons to be learned, for both DWR and the broader dam safety community. The question is whether dam owners, regulators, and other dam safety professionals will recognize that many of these lessons are actually still to be learned. Although the practice of dam safety has certainly improved since the 1970s, the fact that this incident happened to the owner of the tallest dam in the United States, overseen by a Federal agency, repeatedly evaluated by reputable outside consultants, in a state with a leading dam safety regulatory program, is a wake-up call for everyone in the dam safety community. Challenging current assumptions on what constitutes “best practice” in this industry is overdue. KEYWORDS Appurtenant structure, chute, construction, dam, damage, design, drainage, embankment dam, emergency situation, emergency spillway, erosion, flood control, frequency of inspections, gated spillway, geology, internal erosion, Oroville Dam, performance, regulation, scouring, safety of dams, spillway, uncontrolled spillway, uplift, weathered rock 170