The Background: Saltmarshes as Sea Defences
Salt marsh ecosystems are important to the local, regional, and global community for many reasons, amongst others for providing a buffer against waves and tidal currents. At the same time, however, such ecosystems are under increasing pressure from sea-level rise, changes to wind wave climates, and more direct human pressures such as grazing, which alter their functioning and potentially threaten their survival over the longer term. A growing body of studies suggests that marsh vegetation reduces hydraulic forces (current/wave loading) on landward lying sea defences. Existing knowledge about this, however, comes from field observations during only a limited amount of conditions and from models in relatively small and narrow flumes, affected by scaling problems and edge effects. This means results are not applicable to situations, particularly potential storm surge conditions, encountered in the field. In turn, this means that the sea defence function of saltmarshes is often neglected in the planning/management of coastal protection schemes – but this experiment aims to change that.
The Experiment – What is planned?
To build coastal vegetation into coastal protection schemes, we need to understand how it is affected by, and how it affects, waves. The Cambridge Coastal Research Unit (CCRU), in collaboration with Deltares (NL), the University of Hamburg (Germany), the Netherlands Centre for Estuarine and Marine Ecology (NIOO-CEME, NL) and the University of Hannover (Germany) has obtained funding under the EU Hydralab IV call to conduct an experiment on how waves change when they travel across submerged saltmarsh vegetation. The experiment will be carried out in the large wave flume (Großer Wellenkanal, GWK) of Forschungszentrum Küste (FZK) in Hannover, one of the largest experimental flume facilities worldwide. For this project, a 200 m2 test section of marsh turf, excavated in July 2012, will be exposed to a range of wave conditions and water depths in GWK in a series of experimental runs in the autumn of 2013. Conditions will range from those encountered during an average high tide to those encountered only very rarely during a storm. The controlled conditions and the scale-appropriate setting will, for the first time, allow us to find out about:
- The relationship between water level / wave height and wave damping across a saltmarsh for a range of waves; and
- Thresholds that determine at what point (e.g. particular wave energy or water levels) the saltmarsh becomes inefficient in reducing wave energy or breaks up under the impact of the waves.
This study will address a key issue in coastal defence and protection in the context of sea level rise and potential increases in extreme wave events on many of the world’s shorelines. If all goes well, the realistic and prototype scale data will provide a sound basis for the development of new design and safety concepts for vegetated foreshores as storm buffers. The integration of the results into European efforts on ecological safety concepts should be seamless.
This work was supported by a Hydralab IV EU grant (HyIV-FZK-07) and a grant from The Isaac Newton Trust, Trinity College, Cambridge.