The effect of wind wakes on hydrodynamic parameters : coupling 3D DCSM-FM to WINS50 HARMONIE results

The present report deals with the way meteorological impacts of offshore wind farms (OWF’s) can be taken into account in hydrodynamic modelling. In the first version of the hydrodynamic model used for Wozep (Zijl et al, 2020), the effect of OWFs on the meteorological forcing was approximated with a uniform neutral wind speed reduction of 10% within offshore wind farm areas, which is a relatively crude assumption. Moreover, this ignores the effect of wind wakes, which can be felt far away from the wind farms, especially under stably stratified conditions in the meteorological boundary layer. In addition, the impact on other meteorological parameters such as radiation, air temperature and dew point temperature, which play a role in the exchange of heat between air and water, is ignored. In the present study, the impact of these assumptions was investigated by coupling to the HARMONIE meteorological model of KNMI, of which scenario computations exist that include the impact of OWF’s. These scenarios are part of the WINS50 project where the situation of 2019 - 2021 with and without offshore wind farms, as well as a future hypothetical 2050 upscaling scenario, was computed. The resulting meteorological data for the latter scenario is used as forcing to the 3D DCSM-FM hydrodynamic model. Before being applied, the annual average impact of OWFs on some of the meteorological parameters used to force the 3D DCSM-FM hydrodynamic model was determined. While wake effects seem smooth in the annual average sense, the instantaneous effects can be more pronounced. Under the right atmospheric conditions, wake effects can be felt hundreds of kilometres downstream from OWFs. Also, the presence of OWFs not only affects wind speed, but also forcing parameters such as air temperature, dew point temperature and radiation. However, the effect of OWFs on these other parameters has only limited impact on the hydrodynamic results. The impact of OWFs is introduced into the hydrodynamic model through two mechanisms: the presence of monopiles in the water column and changes in meteorological conditions. The impact of using WINS50 future hypothetical 2050 forcing to account for changes in meteorology due to the presence of OWFs is compared to the simplified approach with a 10% wind reduction in OWFs. • In OWF areas with temperature stratification, there is a decrease in surface temperature and temperature stratification, due to enhanced vertical mixing, with wake effect visible in downstream direction. This is mainly caused by the presence of monopiles, while the changes in meteorology counteract this. The 10% wind reduction underestimates this counteracting effect. • The presence of OWFs reduces the residual current magnitudes within the OWFs. Both the monopiles and changed meteorological conditions contribute to this. Downstream of the OWFs velocity deficits are also present, while in between OWFs increases in residual current magnitudes can occur. These changes are mainly caused by changing meteorological conditions. While the most significant pattern of impact due to changing meteorology is explained by the simplified 10% reduction approach, the use of WINS50 future hypothetical forcing results in larger areas being affected, especially surrounding OWFs. • The presence of OWFs reduces the M2 tidal amplitude with more than 10 mm in the English Channel and German Bight, with smaller decreases in Belgian and Dutch coastal waters. The monopiles account for most of this decrease, with the change in meteorological conditions counteracting this in the German Bight. This counteracting effect is absent when using a 10% wind reduction instead.