A numerical study of the temperature dynamics at McNary Dam

High summer water temperatures have caused increased juvenile fish stress in fish passage facilities at McNary Dam. The need to better understand the main mechanisms that generate high temperatures and result in subsequent harmful stress to downstream migrating fish motivates this study.Most numerical studies of temperature dynamics in reservoirs are based on one- or two-dimensional models. McNary Dam forebay is characterized by complex three-dimensional flow patterns and unsteady heat exchange between the atmosphere and water. An unsteady three-dimensional non-hydrostatic model is used in this paper to predict the hydrodynamics and thermal dynamics in the forebay and turbine intakes of McNary Dam. This model is based on the Reynolds Average Navier–Stokes equations, using a Boussinesq approach, with a standard k–ɛ model to solve the flow field. The thermal model takes into account the short and long wave radiation and heat convection at the free surface, which is function of air temperature and wind velocity.The predicted temperature is compared against a 24 h field data set for a warm day in 2004. Simulated and measured temperature profiles in the forebay and within the gatewells show reasonable agreement. An additional simulation studies the inclusion of a thermal curtain upstream of the turbine intakes. Numerical results indicate that the thermal curtain reduces gatewell temperatures potentially increasing survival for migratory salmonids at the dam.