Author(s): Frida M. Niemi; Anders G. Andersson; J. Gunnar I. Hellstrom

Linked Author(s):

Keywords: No Keywords

Abstract: The river dynamics downstream of a hydropower facility are important to understand for improving the ecological conditions. In this study, a 2D hydraulic flow model of a regulated river in the northern part of Sweden is conducted where improving the grayling habitat downstream of the hydropower facility is a specific concern. Capturing the dynamics of the river in relation to the hydropower operating conditions is vital to understand how ecological measures could improve the fish habitat in the river. To understand how modelling parameters could influence the hydrodynamics in the river, and in turn impact the habitat quality, this study investigates mesh properties, Manning bed roughness distribution and Neumann water level gradient boundary condition as an alternative to a fixed water level downstream the river. The study shows that the calculated Water Surface Elevations (WSE) in simulations with Neumann boundaries are more sensitive to changes in Manning roughness than simulations with a fixed water level boundary. The bed roughness in the upstream and downstream parts of the model, close to boundaries had the most effect on the WSE. The Neumann boundaries modeled could simulate the WSE from the steady flow case used for calibration but did not predict the WSE resulting from a transient flow case accurately according to measured data. The choice of gradient had significant influence even though the roughness defined was different. The roughness in the downstream region influences the choice of Neumann gradients in the calibration process. To find a Manning roughness distribution that is not influenced by the downstream boundary, a fixed water level boundary could be used together with velocity measurements close to the outlet and inlet. The extent of areas of the Manning roughness distribution needs to be carefully chosen especially in areas with higher velocities since this influences the bed shear stress. A gradual increase of the roughness between two calibration areas could also provide a better fit. Finally, a hybrid mesh provided greater accuracy in representing the rivers extent compared to a curvilinear mesh. The WSE, velocities, and bed shear stress remained similar between the models.

DOI:

Year: 2024