A MODEL TO EVALUATE THE EFFECT OF LOWER GRANITE DAM OPERATIONS ON THE FOREBAY HYDRAULICS AND DOWNSTREAM TEMPERATURE

摘要

Elevated temperature has negative effects on aquatic life affecting salmonid migration, behavior and physiology. In order to address the survival of fish in the system under the warmer conditions observed in the past years, the US Army Corps of Engineers seeks to identify management alternatives to reduce the impact of elevated temperature on migrating fish. A numerical tool was developed to evaluate the effect of the Lower Granite dam operations under different stratification levels and river flowrates on the hydrodynamics, upstream and downstream temperatures, and storage of cold water. The major issue regarding the prediction of thermal processes in a forebay is the coupling between hydraulics and temperature. A proper model must account for buoyancy forces caused by density differences, which are dominant at low river flowrates and strong stratification. The model comprises a powerhouse with six units, a spillway with eight bays with one bay operating as a weir, and one mile of the forebay. A circular tank was included upstream of the forebay to represent the storage volume of the reservoir. The model was implemented into the commercial CFD code FLUENT using user defined functions. The model agreed well with observations on July 3, 2015. Numerical results of 26 simulations are shown in this paper. According to the model, operations at the project influence temperature being released downstream of the project. Favorable dam operations to reduce temperature impacts on fish, strongly depend on the stratification level, mostly at low river flows. Results indicate that transients generated during operation change can be of significance particularly when spill starts operating.