Three-dimensional numerical modelling for studying smolt migration in regulated rivers

Licentiate thesis, 2005

This licentiate thesis considers the numerical modelling of natural river flow for studying the importance of hydraulics on the outmigration of juvenile salmon and trout (smolt). The construction of power plants and dams in Swedish rivers has led to disturbances in the migration of the fish. Consequently, losses of fish have been compensated by setting out hatchery fish. However, an increasing respect for the environment and the risk of impoverishing the, for each river unique, wild salmon stocks has raised the demands for increased natural reproduction. In the rivers used for power production that still have spawning, smolt are often led by the main stream of the river through turbines or spillways leading to a certain amount of mortality. It is desirable to lead the smolt through the safest passageways to minimize losses and to increase the stocks of wild salmon. To do this, knowledge of both fish behaviour and capabilities as well as knowledge of the flow pattern at each site is important. The three-dimensional modelling of flows in two rivers reaches upstream the hydro power plants of Stornorrfors in Umeälv and Sikfors in Piteälv is described in this study. The modelling is carried through using a commercial computational fluid dynamics (CFD) package. The models are validated against measured velocities achieved by acoustic Doppler profiling. The qualitative agreement between measured and simulated velocities is generally good in the areas where smolts have been observed during outmigration. The quantitative agreement is also fair for the Sikfors models while large discrepancies exist in the models of Umeälv. To model smolt migration and to estimate passage through spillways at different flow conditions a particle tracking model is introduced. The smolts are modelled as passive neutrally buoyant particles and turbulent spreading is introduced by a stochastic model. The modelled trajectories of the particles agree well with observed positions in the horizontal plane and by comparing trajectories at different depths secondary flow effects on fish diversion can be shown. The need of well-resolved and accurate bed topography to achieve good modelling results is underlined in the study. Also, the importance of choosing cell types for the computational mesh is shown and discussed.