PNNL

Abstract

Pronounced stratification and peaking operation, typical of reservoirs impounded by high-head dams, result in complex current patterns. These cause disorientation in downstream migrating salmon and interfere with downstream passage. Structural and operational modifications such as installations of curtains, surface withdrawal, draw down, and selective withdrawal are often considered to alter the stratification and modify the currents to enhance the movement of fish toward the forebay where they may be collected effectively. Effectiveness of design modification in deep reservoirs is highly dependent on site-specific hydraulic and meteorological conditions, and numerical models are the tools of choice in design and selection of the optimum alternative. Although most hydropower reservoirs exhibit a vertical-longitudinal 2-D current structure, 3-D flow patterns are prevalent in reservoirs with multiple branches, and they occur near the power intakes as well. Simulation of these currents requires a 3-D hydrodynamic resolution. However, high-resolution hydrodynamic models, coupled with heat balance and water quality, have extensive computational demands and are unsuitable for iterative application or long-duration runs. An efficient strategy was developed where a vertical-longitudinal 2-D heat balance model (BETTER) was used to generate rapid, year-long simulations of temperature and stratification in the reservoir. The predicted temperature distribution provided initial conditions for focused application of the 3-D hydrodynamic model (EFDC) to predict current patterns during the fish migration seasons only. A Lagrangian particle tracking technique was used to rank the effectiveness of each alternative in terms of guiding fish to the forebay, thereby improving potential for success. Selective surface withdrawal was found to be the most effective way to improve currents for enhancement of fish passage and help manage in-lake and discharge temperatures at Round Butte Dam in Lake Billy Chinook, Oregon.