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The role of two seagrass species in wave attenuance and coastal protection

More:  Institute 
Period: October 2007 till September 2010
Status: Completed

Institute  Top 
  • University of Southampton; National Oceanography Centre (NOCS), more, co-ordinator

Abstract:
Storm waves are a major cause of coastal erosion. It is known that seagrass can reduce the impact of waves at the coast by modifying the wave climate, and therefore reducing the impact of storms. However, the nature and degree of wave attenuance has not been investigated in detail and there is still a high need for laboratory and field studies to determine the influence of seagrass meadows on wave climate.
To date, investigations of the interactions of submerged vegetation and water movement have concentrated on unidirectional flow and crown building species. The results of these studies cannot be transferred to wave attenuation by meadow building plants. The boundary layer under unidirectional flow behaves differently from that under a dynamic and oscillatory flow in wave-dominated environments. A detailed understanding of how seagrass meadows affect the hydrodynamics and sediment dynamics on the European Shelf is lacking despite the high abundance of several species.
Along the coasts of Great Britain and the European Shelf seas the species Zostera noltii is highly abundant. It is recognised to have a decisive influence on food webs and material budgets, but its role in the dynamic regime is still poorly understood. In the Mediterranean the seagrass Cymodocea nodosa is a widely distributed species that is observed to replace Zostera noltii in various locations. It is likely that global change leads to a wider distribution of C. nodosa and based on the observations in Venice Lagoon a replacement of Z. noltii in other European regions will be possible.
This research aims at investigating the influence of the two seagrass species on the boundary layer and the nature of this influence in the water column. Seagrass influences the boundary layer and therefore is likely to alter shear stress. It is envisaged that this work will explore this dissipation in detail and determine the change of shear stress at the bed based on seagrass density.
Furthermore, seagrass will have an influence on the wave height and it is hypothesised that this influence will alter with water depth. Wave height is related to orbital velocity and with it wave energy. The orbital velocity strongly determines the wave forces that act upon engineered structures and hence a decrease can have a significant influence on the structural design. In order to improve design approaches, wave height dissipation by seagrass will be investigated as well. It is proposed to conduct field and laboratory work in order to extend and validate existing models.

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