Investigating Instabilities with HEC-RAS Unsteady Flow Modeling for Regulated Rivers at Low Flow Stages

Dams are used throughout the United States for generation of electricity and flood control. The Tennessee River Valley’s regulated river system extends through Tennessee and parts of Kentucky, Mississippi, Alabama, Georgia, North Carolina, South Carolina, and Virginia. Of the dams in this area, 49 are managed by the Tennessee Valley Authority (TVA). The TVA forecasts and monitors the activities related to these dams and others in the area on an around-the-clock basis to ensure maximum generation potential is attained and to prevent flooding of the surrounding cities. To best forecast hydrology for the regulated river system, the TVA chose to upgrade the forecasting center to include HEC-RAS models for high and low flow simulations. These simulations can better guide TVA engineers to the best route when releasing water from the dam in the case of an emergency flooding situation or on a daily basis. Building a HEC-RAS model for the TVA reservoir system is a large undertaking and model stability issues arise from different possible causes. In this paper, the best methods of stabilizing modeled reaches on a regulated river system during low flows were compiled in logic flow charts, which can be of general assistance to modelers when stabilizing HEC-RAS simulations for regulated river systems. The flow charts were created and explained, with multiple options for stability described and analyzed throughout the creation process. They were based on results where stability issues were regularly due to cross sectional spacing and the addition of base flow through the upstream boundary and lateral inflows. Other potential methods were also considered for applicability in a model such as this, including the addition of pilot channels and increasing the theta weighting factor. Using the flow charts created, HEC-RAS modelers should gain a better understanding of stability issues in a river system and what causes these problems.