1A3: Rehabilitating Hibbs Island to Preserve Norris Dam Environmental Flows
July 13, 2022
Civil Works and Dam Safety
By: Boualem Hadjerioua, Madison Bowling, William Deichert, Robert Feiel, Matt Yeager, Kyle Walker, Jeffrey Ogden, and MeLisa Dixon Hibbs Island in Norris, TN, is located on the Clinch River at about 1 mile downstream from Norris Dam, which is owned by the Tennessee Valley Authority. Norris Dam impounds water for a 110-MW hydroelectric powerhouse that was completed in 1936. The island connects the two sections of the Norris reregulating weir, a low dam built to regulate the release flow from Norris Dam. The main purpose of the weir is to regulate water flow during off-generation periods from Norris Dam through a series of low-level pipes that ensure a constant minimum flow downstream. This minimum flow is paramount for the local fishing attraction, and the weir function is a design parameter in the flow rates that helps control flooding and promote public safety. Throughout the years, high flow releases from Norris Dam (such as spilling) have resulted in high river currents that have caused part of the nearly 10.6-acre island to erode and deteriorate. If no action is taken soon, there is a high chance that Hibbs Island will continue to erode and could eventually disappear. Thus, the functionality of the reregulating weir could be at risk because the flow could then pass through the area between the weir sections. Tennessee-based Mesa Associates, Inc. conducted an evaluation of Hibbs Island to create a solution regarding the current deterioration of the island and its impact on the reregulating weir stability. The options explored for protecting the weir were to build a wall across the island, install a berm between the weirs, rebuild Hibbs Island to the original condition, or make no changes to the island (the do-nothing option). Mesa determined that immediate action should be taken to save the island in order to guarantee the proper functionality of the weir. The recommended approach to guarantee long-term functionality of the weir is to install a berm across the center of the island between the existing concrete weir abutments. To assure water flow is directed over the reregulating weirs instead of free flowing over the island, the berm should be installed to an elevation created by high spilling flows from Norris Dam Installation of the berm across the island will redirect all flow of the river over the weirs to maintain the designed intent of the weirs. Eliminating water flow over the island will decrease the erosion of the center portion of the island and extend the useful life of the reregulating weir. The recommended approach will be implemented by TVA in FY2022. Preserving Hibbs Island is the most effective solution for saving the existing reregulating weir and guaranteeing environmental flow for several miles of riverine aquatic life. Authors: Dr. Boualem Hadjerioua is Director of Integrated Water Resource Management, Madison Bowling is an Administrative Intern, William Deichert is Senior Civil Engineer, Robert Feiel is Senior Civil/Structural Engineer, Matt Yeager is the Civil/Structural Department Manager, and Kyle Walker is Senior Civil/Structural Engineer, all are with Mesa Associates. Jeffrey Ogden is Program Manager, Reservoir Release Improvements and Melissa Dixon is Senior Civil Engineer with the Tennessee Valley Authority.