Turbulent ship wakes and their spatiotemporal extent

Licentiate thesis, 2021

Shipping activities occur in almost every part of the global oceans and in intensely trafficked shipping lanes there can be up to one ship passage every ten minutes. All these ships impact the marine environment in different ways through pollution or physical disturbance. This thesis is focused on the turbulent ship wake, a physical disturbance from ships and previously overlooked as an environmental impact.

When a ship moves through water, the turbulence induced by the propeller and hull, will create a turbulent wake that remains and expands after the ship passage. The turbulence in the wake will govern the spread of contaminants and affect gas exchange in the wake water, physically perturb plankton, and potentially impact local biogeochemistry through increased vertical mixing.

To be able to assess the environmental impact of ship-induced turbulence in areas with intense ship traffic, knowledge of the spatiotemporal extent and development of the turbulent wake is necessary. The aim of this thesis is to increase that knowledge, by conducting in situ observations of turbulent ship wakes, which can be used to estimate the spatiotemporal extent of the turbulent wake.

By using a collection of methods, the thesis work has resulted in a first estimate of the spatiotemporal extent of the turbulent ship wake, based on more than 200 field observations of different real-size ships in natural conditions. The observed turbulent wakes showed large variation in their spatiotemporal extent, and further studies are needed to fully disentangle how environmental conditions and vessel specifications affect the intensity and extent of the turbulent wake.

The results and experiences gained from the in situ observations, give an indication of the complexity entailed in characterising the development of the turbulent wake, and provide valuable input regarding the relevant parameters and spatiotemporal scales to include in future studies. The work of this thesis constitutes the first step in addressing the knowledge gap regarding the environmental impact of ship-induced turbulence and can be used as a road map for further studies within the field.