Hydropower is an important renewable energy source providing flexible operating conditions that balances the increasing intermittent energy from wind- and solar power. Hydropower however, comes with other environmental challenges by altering the ecological conditions in the rivers by presenting flow conditions such as hydropeaking. To fulfill the Swedish Environmental Code and the demand from the European Water Framework directive to achieve good ecological status (GES) for all watercourses, measures to improve the ecology in the river for each hydropower plant must be presented. Investigating potential restoration measures might be both costly and time-consuming as they are often specific to the site and species involved. Hydraulic modelling of flow conditions connected with individual-based models (IBMs) to evaluate fish population growth, and behaviour, is one way to make these estimations more cost- and time-efficient. The aim of this thesis is to investigate how the results of hydraulic models can be used with IBM’s and how different assumptions in the modelling process could create deviations that could further propagate into the IBM’s. A 2D hydraulic model over a regulated river in northern Sweden is presented where the restoration focus is improving European grayling habitat.

As a first step, paper A investigates how modelling parameters could influence the hydrodynamics in the river, and in turn impact the habitat estimations. The results shows that the Neumann boundary conditions are more sensitive to Manning roughness than fixed water level boundaries, especially near upstream and downstream enclosure areas. A fixed water level boundary with velocity measurements at the inlet and outlet can minimize boundary effects on roughness calibration. Careful selection of roughness distribution areas, particularly in high-velocity zones, is crucial as it impacts bed shear stress, and velocities, which can be used for estimating feasible habitat. A gradual roughness transition between calibration areas may improve model accuracy. In a second step, paper B compares a steady-state interpolation method with transient simulations since the IBM implements steady-state simulations of different constant flows to interpolate the depth and velocities across the river for different discharge hydrographs. The goal was to assess how the limitations of a steady-state approach impact different parameters in the IBM’s and how this could affect the long-term habitat. Steady-state and transient simulations were compared, focusing on WSE, spawning habitat and bed shear stress. Steady-state models failed to capture temporal dynamics caused by neglect of time displacement and damping by assuming uniform response times across the river. The bed shear stress was under-predicted by the steady-state interpolation which could lead to inaccurate estimations of suitable spawning grounds and also, the risk of redd scouring. Uncertainties in the depths and velocities over time from the hydraulic model could propagate further in the IBM’s causing the long-term evaluation of fish population to be inaccurate especially when the flow conditions are highly varying during the years where a transient model is preferred.

Finally, paper C investigates how the dynamic flow conditions effect the bed shear stress. When no substrate data over the riverbed is available, the bed shear stress can serve as a substitute for identifying suitable spawning areas as input for the IBM. The paper presents the hydraulic conditions in the river during a period of 2 months between May and July to capture the spring flood together with some hydropeaking events during summer. The spawning grounds considering depths and velocities were presented with the maximum bed shear stress during the time period to identify and evaluate possible restoration areas. To evaluate the stability of different spawning gravels used for restoration, the Wilcock-Crowe bedload transport model was implemented in the model. The erosion and sedimentation of the added spawning gravel followed the maximum bed shear stress curves. The results can be used further to evaluate different restoration measures together with the fish population growth with IBM.