The influence of seaweed cultivation on local hydrodynamics: a case study in Bantry Bay, Ireland

Abstract

Seaweed cultivation is a globally emerging sector and is expected to show large growth in the coming decades as a sustainable marine resource. However, the influences of seaweed cultivation on local hydrodynamics and with that nutrient dynamics remains relatively

understudied. Therefore, this thesis investigates how seaweed farms modify flow pat- terns in both vertical and horizontal dimensions. The Delft3D-FM model was used to sim- ulate scenarios in an idealised flume setup and in the real-world environment of Bantry

Bay, Ireland. A modelling approach which looks at the basic influences in an idealised

flume setting includes a baseline configuration of a seaweed farm represented by vege- tation elements, followed by sensitivity studies with varying values for the stem height,

density, diameter, and drag coefficient. Results show that seaweed presence reduces surface flow velocities by up to 70% in the flume model, with the depth of maximum velocity shifting downward by approximately 10 metres in a water column of 20 metres in total. Wake lengths downstream of farms range from 55 to 92 metres, corresponding to 22 to 36% of the seaweed patch, respectively, depending on vegetation parameters and flow conditions. Sensitivity analyses indicate

that stem height has largest influence on hydrodynamic effects, followed by stem diame- ter and stem density. In Bantry Bay, similar surface flow reductions are observed, though

wake structures are less uniform due to tidal forcing and bathymetric complexity. This study offers insights into how seaweed farms interact with local hydrodynamics and provides a foundation for future work on farm design optimisation and environmental impact assessment.