Updated Hydrologic Analysis of the Tennessee Valley with Regulatory Impact on Nuclear Plant Sites within the Tennessee Valley Authority

Hydraulic modeling of natural river systems poses many challenges, particularly on a large-scale. This paper provides an overview of large-scale reservoir system modeling including approximately 1,200 river miles utilizing the United States Army Corps of Engineers’ (USACE) Hydrologic Engineering Center-River Analysis System (HEC-RAS) unsteady flow model and a first of a kind design basis hydrologic approach. Tennessee Valley Authority (TVA) utilized these methods to update the design basis for its three operating nuclear plants from legacy software-based hydraulic models and the National Weather Service’s Hydrometerological Report (HMR)-based probable maximum precipitation (PMP) rainfall to a gridded precipitation-based PMP analysis and industry standard HEC-RAS. Significant challenges were overcome to accurately model numerous storms of varying magnitude with varying storm centering to determine a probable maximum flood (PMF). A gridded precipitation PMP model developed using 30–50 years of observed rainfall data provided a reliable, reproducible PMP on any sub-basin in the Tennessee River watershed. A nesting approach was employed to apply varying combinations of the PMP storm events across the watershed. Automated tools were employed to efficiently and reliably iterate the hydrologic combinations and flood routing accomplished through a HEC-RAS model utilizing more than 10,000 lines of user-defined unsteady flow rules. The Tennessee River watershed is complex and includes 32 dams. The rules are flexible and capable of computing discharge at these 32 dams for complex operations, configurations, and breach scenarios. The model incorporates detailed reservoir storage as well as calculates discharge considering tailwater effects and using variable coefficients in dam rating curve computations. The well-defined rules and automation tools allow for maximum scenario flexibility while also increasing computational efficiency. This work was performed meeting the quality assurance standards of 10 CFR 50 Appendix B.