Page - 166 - in Book of Full Papers - Symposium Hydro Engineering

5.2. EMERGENCY SPILLWAY Prior to the February 2017 spillway incident, the geology of the right abutment of the dam, including the hillside downstream of the emergency spillway crest structures was fundamentally mischaracterized and misunderstood by DWR, its consultants, DSOD, and FERC. Although geologic data from the time of original design identified the presence of areas of relatively deep strongly weathered rock, over the history of the project DWR’s geologists had come to believe that the hillside downstream of the emergency spillway crest structure consisted of shallow soil cover over non-erodible rock, and this was accepted by all others involved, without a critical review of the available information. Hence, the erosion and headcutting that occurred when the free-flow emergency spillway overtopped were not expected. Once the incident initiated with the failure of the service spillway, DWR attempted to find a “sweet spot” in balancing the risks of operating the service and emergency spillways, with the goal of releasing limited flows down the service spillway to both reduce ongoing service spillway erosion and prevent Hyatt Powerplant flooding, while also not overtopping the emergency spillway weir. These multiple constraints and objectives resulted in very difficult decisions, which were further influenced by a number of changing conditions and differing perceptions and viewpoints. Along the incident management timeline, there were particular points when decisions were made to limit discharge through the service spillway, even though the risks associated with powerplant flooding were clearly diminishing. This ultimately resulted in flow over the emergency spillway weir. In general, the IFT believes that all decisions during the incident were made with the best of intentions, but in some cases were against the advice of a number of civil engineering and geological personnel. In limiting service spillway discharge to reduce the likelihood of powerplant flooding, the additional dam safety risk associated with the use of the emergency spillway may not have been appropriately considered. 5.3. ORGANIZATIONAL, REGULATORY, AND INDUSTRY FACTORS The February 2017 Oroville Dam spillway incident can be viewed as a “textbook” case of a major dam incident, in terms of typifying the extent of contribution from human factors, including organizational, regulatory, and industry factors. Through a historical trajectory involving decades of somewhat complex interactions and effects of human and physical factors, numerous warning signs of the spillway chute failure were missed, and many barriers, which were intended to provide “checks and balances,” were overcome to eventually produce the spillway chute failure and emergency spillway damage. 166