title: Impact of Windfarm Structures on a Region of Freshwater Influence
Creators: name: Eddon, DJ
Publisher: name: University of Liverpool Repository
Year: 2017

description: To date, most offshore wind farms are deployed in coastal regions including both well-mixed regions and other areas where water-column stratification plays an important role such as regions of freshwater influence and seasonally stratified shelf seas. In the UK for example, a number of offshore wind farms have been commissioned and approved in Liverpool Bay, which is a key region of freshwater influence, and approved in regions of the North Sea where the water column can stratify seasonally. While there has been significant work on determining the local environmental impacts of offshore wind farms, in particular when they relate to the structural integrity of the turbines (e.g. scour), there has been far fewer studies focusing on the water column dynamics in complex regions such as regions of freshwater influence, even though they have a significant control on the ecosystem. This project addresses both issues: assessment of the modelling techniques; and impact of offshore wind farms on the dynamics of regions of freshwater influence. This is achieved via a numerical modelling study in which a new module was developed to simulate the impacts of offshore structures in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS). The overall model was then applied to a case study in Liverpool Bay, which is a salinity driven, hyper-tidal region of freshwater influence with complex dynamics in the North West of the UK and where an increasing number of offshore windfarms have been deployed since 2003. This new module is based on a modified drag force approach, which takes both momentum and turbulence effects into account by introducing impact equations and additional terms to the governing equations of the momentum solver in POLCOMS and of the k-e turbulence model (General Ocean Turbulence Model, GOTM). The updated coupled model is then implemented for Liverpool Bay with numerical simulations covering the full year 2008 and with 55 turbines represented in the model. It is validated using mooring data over the same period at selected locations in Liverpool Bay. A yearly time series of temperature, salinity, velocity and density shows that the model predicts the hydrodynamics well, and that inclusion of the wind farms in the model results in a slight improvement of numerical predictions based on skill values. A number of sensitivity tests address the influence of the modelling techniques used on the numerical results. These include a model-model comparisons considered the relative effect of taking into account changes to momentum and turbulence equations. The results demonstrate that using only one equation change (either momentum or turbulence) overestimates the effects of a structure in the water column and that both momentum and turbulence should be considered when assessing the impacts of structures in a 3D model. Finally, numerical results from the best simulation are analysed to determine the impact of the offshore wind farms on the region of freshwater dynamics. The position of the wind farms controls the level of impacts in the region of freshwater influence. When the wind farm is positioned in an area where the ambient velocity is high, the velocity is affected. In contrast, where the velocity is lower, the salinity and temperature is more affected. The main conclusions are: even a small number of structures do have a discernible impact on the dynamics of a region of freshwater influence; the representation of offshore wind farms in the model, considering momentum and turbulence modifications to governing equations, is critical for predictions of the total impact on the water column in a 3D model.