TY - JOUR

T1 - On the effect of structure-induced resistance and mixing on inflows into the Baltic Sea

T2 - A numerical model study

AU - Rennau, Hannes

AU - Schimmels, Stefan

AU - Burchard, Hans

N1 - Funding Information: The work for this study was carried out within the framework of the international QuantAS Consortium (Quantification of water-mass transformation processes in the Arkona Sea), which is partially funded by the QuantAS-Off project (QuantAS-Impact of Offshore Wind Farms) by the German Federal Ministry of Environment , Nature Conservation and Nuclear Safety (BMU) . We are indebted to Karsten Bolding (Asperup, Denmark) to keep GETM up and running and Holger Janssen (Rostock, Germany) for providing the data on wind farm distributions. We are further grateful for the intense scientific discussions on structure-induced mixing which we could have with Ole Petersen (Hørsholm, Denmark), Alfred Leder (Rostock, Germany) and Mark Markofsky (Hannover, Germany). We are finally grateful to two anonymous referees for their constructive and helpful comments.

PY - 2012/2

Y1 - 2012/2

N2 - 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 - 0.3 psu. 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 0.02. psu which is more than one order of magnitude smaller than the standard deviation of the bottom salinity change.

AB - 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 - 0.3 psu. 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 0.02. psu which is more than one order of magnitude smaller than the standard deviation of the bottom salinity change.

KW - Numerical modelling

KW - Stratified flow

KW - Structure-induced friction

KW - Turbulence closure models

KW - Western Baltic Sea

UR - http://www.scopus.com/inward/record.url?scp=84855197138&partnerID=8YFLogxK

U2 - 10.1016/j.coastaleng.2011.08.002

DO - 10.1016/j.coastaleng.2011.08.002

M3 - Article

AN - SCOPUS:84855197138

VL - 60

SP - 53

EP - 68

JO - Coastal engineering

JF - Coastal engineering

SN - 0378-3839

IS - 1

ER -