Characterising turbulent ship wakes from an environmental impact perspective

Abstract: The world’s oceans, especially coastal areas, are intensively trafficked by ships. All these ships exert pressure on the marine environment, through emission to the atmosphere, discharges of pollutants to the water, and physical disturbance through energy input. Of these impacts, energy pollution from shipping has received the least attention. Especially the impact of ship-induced turbulence in the wake, which is induced by the hull friction and propeller, and remains for up to 15 minutes. The turbulent wake can impact the spread of contaminants, affect air-sea gas exchange, physically disturb plankton, and potentially impact local biogeochemistry through increased entrainment and vertical mixing. To assess these impacts, an understanding of the turbulent wake development and interaction with surface ocean stratification, is essential. However, characterisation of the turbulent wake development in time and space, especially in stratified conditions, is challenging and requires an interdisciplinary approach.   The aim of this thesis is to advance the understanding of turbulent wake development from an environmental impact perspective. The intensity and spatiotemporal extent of the turbulent wake, and its impact, have been investigated through a combination of in situ and ex situ observations, and Computational Fluid Dynamic (CFD) modelling of ships in full-scale. The unique dataset of several hundred in situ turbulent wake observations, showed large variation in spatiotemporal extent and intensity. Wake depths can reach down to 30 m, and the turbulent intensities in the near wake are 1–3 orders of magnitude higher than generally observed in the upper ocean surface layer. In addition, during stratified conditions ship-induced turbulence entrain water from below the pycnocline, with implications for local nutrient input and primary production in the ocean surface layer. In addition, ship-passages were observed to frequently trigger large methane emissions in an estuarine shipping lane. The results highlight the importance of addressing ship-induced turbulence in marine environmental management. Intensively trafficked coastal areas should be considered anthropogenically impacted, even unnatural, with respect to turbulence. The interdisciplinary approach applied in this thesis, is a first step towards a holistic assessment of the environmental impact of the turbulent wake.