The interaction of waves with seaweed farms: wave attenuation and intra-farm hydrodynamics

Seaweed cultivation is well recognised as an industry with substantial potential for growth in Scotland. Most farms in Scotland are close to shore, growing kelp species such as Saccharina latissima on a grid system that is typically 4 hectares or smaller; development of the industry will require increasing farm size and moving further offshore. Successful growth in seaweed farming requires sound understanding of the physical and biological processes affecting farm productivity and the marine environment in which farms are located. These processes include the hydrodynamic interactions between waves and farms.

Sea waves generate water motions within the water column, which interact with the seaweed and take energy from the waves, resulting in wave attenuation as the waves propagate across the farm. Depending on the incident wave conditions, the water depth, the farm size, the seaweed coverage and its properties, the reduction in wave energy can be significant; a seaweed farm can therefore potentially provide an important coastal protection service by significantly reducing the wave energy that reaches the adjacent coastline. The use of vegetation for so-called nature-based coastal protection has received a lot of research attention in recent years, though that research has focused mainly on benthic vegetation - sea grass meadows, salt marshes and mangroves. Much less attention has been paid to wave attenuation over benthic seaweeds , and even less to suspended, near-surface seaweeds, as configured in seaweed farms.

Planning and design of interventions in the marine environment require application of hydrodynamic models that aim to predict the impact of the intervention on the wave and current climate in the area containing the intervention. Commonly used models include FVCOM (Finite Volume Community Ocean Model) for current prediction and the spectral wave model SWAN (Simulating Waves Nearshore) for waves. Such models do not resolve the fluid dynamics at fine scale; instead, for application to seaweed farms, the vegetation effects on the flow/waves need to be incorporated using semi-empirical models based on physical argument and data from experiments. Proposed formulae have already been implemented in SWAN for benthic vegetation, but corresponding formulae for near-surface, suspended seaweed have yet to be proposed.

The objectives of the proposed research are:
(i) Conduct laboratory experiments to investigate wave interaction with seaweed farms, with focus on intra-farm hydrodynamics and wave attenuation.
(ii) Based on insights and data from the experiments, develop new semi-empirical formulae for farm effects on wave attenuation, which can be implemented within coastal wave hydrodynamic models.
(iii) Implement the new formulae in SWAN and apply to several seaweed farm scenarios to investigate the potential impact of farm parameters on wave attenuation.