On the effect of structure-induced resistance and mixing on inflows into the Baltic Sea: A numerical model study

Abstract

This study aims to estimate the additional mixing and dilution of dense bottom currents due to foundations of wind turbines in offshore wind farms projected in the region of the Western Baltic Sea. To some extent these offshore wind farms are planned to be build directly in the main pathways of dense bottom currents propagating into the Baltic Sea. This may have a significant effect for the Baltic Sea ecosystem. In the present study, cylindric structures are assumed for the underwater construction of the individual wind turbines, which are assembled in wind farms with typically 50–100 structures. A parameterisation of the additional mixing and friction due to a structure is developed as an extension of the k−ε two-equation turbulence closure model. Results of a high resolution Reynolds-Averaged Navier–Stokes (RANS) model of the local scale are used to calibrate this parameterisation for hydrostatic coastal ocean models. A Western Baltic Sea hydrodynamic model coupled to the extended turbulence closure model is applied in two different scenarios covering (i) weak and (ii) strong structure-induced mixing due to offshore wind farm distributions in accordance with the present (April 2009) planning situation. The scenarios are completed by two cases with unrealistically extensive wind farms simulating a theoretical future worst case scenario. By means of analysing annual model simulations, it is found that the impact of structure-induced mixing due to realistic wind farm distributions is comparably low with a typical decrease of bottom salinity in the range of 0.1−0.3psu. The annual mean bottom salinity at the outflow from the Arkona Sea through the Bornholm Channel into the direction of the Baltic Proper shows decreases due to mixing from a realistic wind farm distribution of only 0.02psu which is more than one order of magnitude smaller than the standard deviation of the bottom salinity change.