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Follow this blog to find out about a unique set of true-to-scale experiments delivering high-quality scientific evidence for the reduction in flood and erosion risk achieved by coastal salt marsh.

The Background: Salt marshes as Sea Defences

Salt marsh ecosystems are important to the local, regional, and global community for many reasons. Amongst others, they reduce the impact of waves and erosion on shorelines. 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 overtopping risk and hydraulic forces (current/wave loading) on landward lying sea defences. Existing knowledge about this, however, has come primarily from field observations during only relatively ‘benign’ conditions and from models in relatively small and narrow flumes, affected by scaling problems and edge effects. This means results are not applicable to extreme situations, particularly potential storm surge conditions, encountered in the field. Moreover, the response of salt marsh margins to extreme hydrodynamic forcing is complex and currently not well understood. Salt marsh and adjacent tidal flat sediments contain large silt and clay fractions and host a range of plants and other organisms within and on the surface. Little is known about the response of these coastal features to hydrodynamic conditions including erosion and the damage salt marsh plants sustain under wave forcing. We need such knowledge to predict the way in which these features, and thus the degree of natural coastal protection they provide, may alter with sea level rise and climate change. This means that the sea defence function of salt marshes can be fully incorporated into the planning/management of coastal protection schemes  – this series of experiments aims to achieve just that.

If all goes well, we will provide realistic and prototype scale data to form 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.

The first Experiment – How does salt marsh attenuate waves?

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 salt marsh vegetation. The experiment was 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, was exposed to a range of wave conditions and water depths in GWK in a series of experimental runs in the autumn of 2013. Conditions ranged 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 did, for the first time, allow us to find out about:

  • The relationship between water level / wave height and wave damping across a salt marsh for a range of waves; and
  • Thresholds that determine at what point (e.g. particular wave energy or water levels) the salt marsh becomes inefficient in reducing wave energy or breaks up under the impact of the waves.

This study addresses 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. This work was supported by a Hydralab IV EU grant (HyIV-FZK-07) and a grant from The Isaac Newton Trust, Trinity College, Cambridge.

The second Experiment – How do pioneer zones stabilise the sediment?

To implement vegetated foreshores in safety concepts it is also important to understand how salt marsh vegetation stabilises the sediment, which is particularly critical in the early stages of marsh establishment and growth where erosion may also cause severe loss of vegetation. Under the lead of The Cambridge Coastal Research Unit (CCRU), in collaboration with the University of Hamburg (Germany), the Netherlands Centre for Estuarine and Marine Ecology (NIOZ-Yerseke, NL), the University of Antwerp (Belgium) and the University of Braunschweig (Germany) has obtained funding under the EU Hydralab + call to return to the large wave flume (Großer Wellenkanal, GWK) of Forschungszentrum Küste (FZK) in Hannover in summer 2018 studying the ‘Response of Ecologically-mediated Shallow Intertidal Shore Transitions to extreme hydrodynamic forcing’ (RESIST).

This experiment recognises that strong morphodynamic feedbacks exist between hydrodynamic forcing and the morphological characteristics of the tidal flat to salt marsh transition zone. By focussing on the salt marsh pioneer zone, clifflet erosion with established dense canopies, and artificial erosion protection, we address key questions on how to manage vegetated tidal flats into the future. To do so, we will focus on:

  • How extreme wave-forcing affects seedling survival of different pioneer species
  • How (and to what extent) vegetation typically present in the salt marsh pioneer zone affects erosion processes and rates under extreme forcing;
  • How small clifflets at the seaward margin of dense salt marsh canopies respond to high energy wave conditions;
  • What damage is caused by drag forces in terms of marsh plant breakage under extreme conditions; and
  • How novel artificial erosion protection / stabilisation methods may be used to protect seedlings as a restoration measure and reduce clifflet erosion as a conservation measure.

Besides understanding the processes causing erosion along salt marsh edges, this study will identify what types of species and artificial protective measures are most effective in reducing erosion / increasing stability in a particular hydrodynamic setting and thus help identify which coastal management interventions are likely to be most successful in preventing (or at least significantly delaying) the loss of these valuable natural coastal buffers in the face of climate/environmental change.

This work is supported by a Hydralab+ EU grant (alongside a Natural Environment Research Council grant (‘RESIST-UK’, 2018-2021, NE/R01082X/1) to the University of Cambridge, in collaboration with Queen Mary University of London, and the British Geological Survey).

 

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