Page - 55 - in Book of Full Papers - Symposium Hydro Engineering

As a result of these extreme efforts, the Hyatt Powerplant was successfully protected from the unprecedented high backwater caused by the incident. The six turbines in the Hyatt Powerplant can only produce power when connected to the State’s electric power grid. They also must be spinning and generating power in order to release water from the reservoir to the Feather River channel. The assessment of DWR’s power lines and towers connecting the Powerplant to the power grid became a high priority since the loss of the power lines would prevent the ability to discharge up to 479.9 m3/s (16500 ft3/s) from the reservoir once the water dropped below the sill of the FCO. Monitoring and assessment teams became aware that continued erosion on the right (north) side of the FCO Spillway chute could compromise at least one of the transmission towers on the right side of the chute. The risk of losing this one tower was increasing and the impact could have resulted in the failure of numerous towers and power lines. This in turn increased the safety risks for responders and the possibility of losing the power grid connection to the powerplant for months, if not years. The OICT Commander ordered additional monitoring and risk assessments of this transmission tower as well as contingency and mitigation plans. It was determined that the tower power lines must be cut, and the tower removed to protect responders, prevent failure of multiple towers, and reduce the threat to the other high-voltage electric power in the area. These actions were approved by the OICT Commander, scheduled into working Actions Plans, coordinated with cooperating partners and the public, and then successfully implemented. Mitigations measures were then necessary to plan, design, and install shoe-fly power lines over the FCO Spillway chute away from the erosion threats. This process was completed with a high level of coordination with OICT Planning, Operations and Logistics teams, the FCO Spillway Strike Team, the Hyatt Powerplant Strike Team, the Emergency Spillway Strike Team, contractors, and the other power utility (Pacific Gas and Electric, PG&E). This work was successfully completed just as sufficient debris was removed from the Feather River channel to allow the water elevation to drop low enough for safe reoperation of the Hyatt Powerplant. This allowed DWR to begin releasing additional water from the reservoir and reduced the need to use the FCO Spillway for the remainder of the 2017 runoff season. 5.1.6 Electric Power Grid Considerations An additional 230 kV electric power line, which is critical to California’s electric power grid, crossed the Oroville Dam facility just downstream of the confluence of the FCO Spillway chute and the Feather River in the area of a large rock debris pile in the river channel. Debris removal was critical to getting the Powerplant back on line, but the use of tall equipment working in proximity to the transmission lines was an extremely high risk. Mitigation alternatives developed and implemented to protect the power lines and the electric power grid, which included flooding the barge and allowing it to sit lower in the water, drawing down the water elevation in the river 55