| Summary: | In the near future, the operating conditions of hydropower plants in the Nordics are expected to change, inorder complement expansion of intermittent power production such as solar- and wind-power. This in turnmight increase the amount and frequency of short-term flow regulations events, so called hydropeaking. Inrecent years, hydropower’s effect on local ecosystems has drawn much attention. Especially hydropeakinghas been shown to affect the ecosystems and induce morphological changes in the reach downstreamhydropower plants. In order to assess the potential impact of a change in operating conditions, differentnumerical models can be used to examine the downstream reach when subject to potential future operatingconditions. In-flow parameters, such as depth and velocity, can be predicted by using numericalapproximations of the Navier-Stokes equations in either one-, two- or three-dimensions. These modelingapproaches have different advantages and disadvantages. In this work the uncertainties that arise naturallyfrom field measurements and numerical modelling, and how it might affect in-flow parameters, specificallyvariables frequently used in ecohydraulics, with special emphasis on 2D models was investigated. The flow ina regulated reach was here modelled with the open-source hydrodynamics solver Delft-3D. A stretch of theriver Umeälven in northern Sweden was modelled based on bathymetry measurements from a multibeamsonar. The model was calibrated with measured water levels and validated with ADCP velocitymeasurements. The results show that the model is significantly dependent on the calibrated roughness.Additionally, it is shown that 2D models might be insufficient for some ecohydraulical applications. Lastly, adiscussion on the merits of one-, two and three-dimensional models is presented.
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