1 Modeling of the Thermal Discharge from a Power Generating Facility into a Tidal- Like Hydropower Controlled Pool on the Connecticut River

Vermont Yankee uses a sophisticated cooling system that varies from an open cycle, once-through cooling water configuration to a closed cycle, cooling tower configuration that recirculates the cooling water. Its cooling system operation is based on river flow and temperature. In support of a request to increase discharge temperature limits during the summer period pursuant to a 316(a) of the Clean Water Act, this study was conducted to assess the thermal discharge from the plant and its effects on the River using a state-of-the-art, general curvilinear boundary fitted hydrothermal computer simulation model. The unique aspect of this model application was the time varying pool flow of the receiving water caused by hydropower generating activity. The flow varied on 1- to 4-day time scales with the diurnal being the most predominant. The pool response did vary widely during different measurement periods, however, sometimes with tidal (diurnal) regularity and other times with the diurnal component being only a small portion of larger multi day variations. A field program collected data from a set of continuous monitoring thermistors from May through October 2002 in the river with primary focus on the plant thermal plume. The data also included river flows and water temperatures obtained from permanently deployed instruments located both upstream and downstream of the plant and used in Vermont Yankee’s operations. The model calibration and verification exercises use