Abstract
This study investigates the complex and still insufficiently understood interactions between ocean currents and offshore wind farms (OWFs), with a focus on local-scale hydrodynamic effects near individual wind turbine foundations. Despite growing interest in the environmental impacts of OWFs, empirical field data on local-scale current dynamics within wind farms remain sparse. This technical report describes the results from a field campaign, which was conducted within the Anholt OWF in the Kattegat over a 9-day period in August 2024. The field campaign employed high-resolution measurements of current velocity, acoustic backscatter, temperature, and salinity using Acoustic Doppler Current Profilers (ADCPs) and CT (Conductivity, Temperature) sensors placed upstream and downstream of an OWF monopile. The study was motivated by the need to calibrate and validate hydrodynamic models that simulate flow dynamics, vertical mixing, and stratification in OWF environments. The measurements showed that wind forcing dominated the observed variations in water mass properties and currents, complicating efforts to isolate turbine-induced effects. However, data collected downstream of the monopile revealed notable a persistent near-bottom shear layer, alongside enhanced acoustic backscatter suggesting enhanced turbulence or sediment transport. Despite the brief sampling time window, the observations captured a broad range of physical processes (e.g., wind and tide driven currents and episodic variations in the temperature and salinity record), demonstrating the potential of such local area datasets for validating and refining numerical models. From a modelling perspective, comparisons with results from a hydrodynamic flume model (FlexSem) showed good agreement between simulated and measured flow characteristics, particularly when incorporating monopile drag parameterizations. We conclude that short-term campaigns, while informative, are insufficient for capturing the full variability of OWF impacts. Therefore, we recommend longer-term monitoring, the deployment of denser ADCP arrays, and complementary shipbased surveys using vessel mounted (VM) ADCPs and CTD (Conductivity, Temperature, Depth) systems.