Page - 56 - in Book of Full Papers - Symposium Hydro Engineering

channel, removing some of the excavator cabin and roll bar, and adjusting the schedule to move the excavator. These actions were extremely important to preventing a major secondary impact to the utility, the statewide electric power grid, and public safety. 5.2 CRITICAL DECISIONS There were continuous discussions by technical teams, OICT management, and DWR’s Acting Director regarding “what if” scenarios, go/no go criteria for use of the FCO Spillway chute, reservoir inflow and releases, and forecasts of reservoir elevations. DWR’s high priorities were responding to public safety, protection of the Hyatt Powerplant and power lines, continued use of the FCO Spillway chute, and avoidance of the use of the Emergency Spillway, if possible. Setting criteria for each of these priorities was very difficult and there was little room for error and little time for decisions. Monitoring and reevaluation of conditions and criteria for decision points was also difficult due to the nature of the changing conditions. Considering the original weather forecasts, reservoir routing, current FCO Spillway chute and Feather River channel conditions, the expectation was to not allow Lake Oroville to exceed elevation 295.2 m (900 ft) or 0.3 m (1 ft) below the top of the Emergency Spillway crest. Thus, FCO Spillway releases were scheduled at 1,841 m3/s (65,000 ft3/s) February 10 at 1500 hours. DWR and its regulators were very concerned with the erosion of the FCO Spillway chute and the adjacent slopes. The erosion had not stabilized and there was concern that the foundation erosion and the concrete slab failure may migrate up the chute and threaten the operation of the FCO, leaving only the Emergency Spillway to manage the Feather River Basin’s large runoff for the remainder of the record-breaking wet season. Due to concerns about additional erosion in and around the FCO Spillway chute and near the power lines and the closest tower, the FCO Spillway release was decreased to 1,559 m3/s (55,000 ft3/s)) late on February 10. This spillway release rate was selected to limit the degradation of the FCO Spillway chute, yet prevent the Emergency Spillway from being used. On February 11, Lake Oroville inflows significantly exceeded the forecast. The storm had unexpectedly stalled over the Feather River Basin watershed, causing a peak inflow of 5,395 m3/s (190,435 ft3/s). This inflow exceeded DWR’s original expectations by approximately 850 m3/s (30,000 ft3/s). At this point, greater FCO Spillway releases would be necessary to avoid flow over the Emergency Spillway. However, because of the concerns discussed above, the decision was made not to increase the flows through the FCO. In addition to the FCO Spillway safety concerns at Oroville Dam, downstream levee capacity concerns also played into making this decision. On February 11, water began flowing over the crest of the Emergency Spillway and revised monitoring and contingency plans were quickly developed. Personnel were continually reminded that decisions and actions must stay within the OICT command and control structure for consistency and full cooperation with all those involved with this incident. The OICT and regulatory agencies were also 56