A large-scale flume experiment to determine wave dissipation and transformation over coastal vegetation under extreme hydrodynamic loading (a HYDRALAB IV project)
Scientific Context and Theoretical Framework
The multiple services provided by salt marsh ecosystems to the local, regional, and global community are being increasingly recognised and valued. At the same time, however, such ecosystems are under increasing pressure from sea-level rise, changes to wind wave climates, and more direct anthropogenic pressures such as grazing, which alter their functioning and potentially threaten their survival over the longer term. One important function of salt marsh vegetation is that of providing a storm surge buffer. A growing body of empirical studies have highlighted the potential role of vegetation in reducing hydraulic forces (current/wave loading). However, these insights have largely been based on physical scale models in relatively small and narrow flumes, affected by scaling problems and edge effects, which hinders the applicability of results to situations encountered in the field. Other studies have used numerical modelling to determine the impact of vegetation on hydraulic forces, but this suffers from the difficulty of accurately representing vegetation (plant and canopy) structure (the canopy is complex and the same density can be derived from very differing degrees of clumping of vegetation).
Previous field measurements of wave energy reduction under typical storm conditions are also questionable, as observations have been restricted to average (rather than extreme) tidal inundation depths and incident wave conditions (with limited information on the actual plant loading conditions). Furthermore, relationships between vegetation characteristics (density, flexibility, height, etc.) and physical parameters (water depth, incident wave energy, and observed wave attenuation) are non-linear. As it is not clear whether the coastal protection function is maintained under intense storm impacts, it is often neglected in the planning/management of coastal protection schemes.
The Hydralab Experiment
To build coastal vegetation into coastal protection schemes, the dynamics that control the sustainability of such an approach need to be fully understood. 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 acquire fundamental knowledge on the wave dissipation process over vegetation under controlled circumstances 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. This scale-appropriate setting will allow wave dissipation across salt marsh vegetation to be studied in realistic inundation and wave energy conditions. It will thus allow the determination of:
- The relationship between water level / wave height and wave damping across a raised marsh platform with intact and degraded (50% original height) vegetation cover for a range of incident wave heights;
- Thresholds that control transitions from regimes of energy reduction to regimes of energy transmission and marsh breakup (the former being dominated by drag and turbulence, the latter by vegetation canopy 'overtopping' and/or vegetation flexing); and
- The impact of canopy degradation (e.g. through grazing) on the sea defence function of salt marshes.
This study will make a significant contribution to the pressing issues of 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. The realistic and prototype scale data obtained will provide a sound basis for the development of new design and safety concepts for vegetated foreshores as storm buffers. Integration of the results into European efforts on ecological safety concepts should be seamless.
To read more about the construction of the experiment and the preliminary findings, see the main project website.
Acknowledgement: The work described in this publication was supported by the European Community's 7th Framework Programme through the grant to the budget of the Integrating Activity HYDRALAB IV, Contract no. 261520.
Disclaimer: This document reflects only the authors' views and not those of the European Community. This work may rely on data from sources external to the HYDRALAB IV project Consortium. Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Community nor any member of the HYDRALAB IV Consortium is liable for any use that may be made of the information.
Links to team member pages
- Cambridge Coastal Research Unit, University of Cambridge: Dr Iris Möller. Project Lead. Responsible for the overall management of the project, expert on wave transformation over saltmarsh surfaces. Dr Tom Spencer. Saltmarsh geomorphologist. Sedimentation and erosion patterns in response to wave action.
- University of Hamburg, Applied Plant Ecology Group: Dr Kai Jensen. Expert on saltmarsh ecology. Franziska Rupprecht. Organisation and overseeing of excavations of marsh sections for experimental test section construction; observations on stress response of marsh vegetation. Dr Antonia Wanner. Saltmarsh ecology.
- NIOO-CEMO: Dr Tjeerd Bouma. Expert on marsh ecology and drag measurements on vegetation; observations of seedling performance under stressed conditions.
- Unit for Coastal and Marine Systems, Deltares: Dr Bregje van Wesenbeeck. Liaison with sea defence engineers at Deltares (NL) for translation of scientific results into management practice guidance.
- ForschungsZentrum Küste, Hannover, Germany: Dr Stefan Schimmels, Dr Maike Paul, and Matthias Kudella. Experiment logistics support and expertise on hydrodynamic-ecology linkages.
Figure 1: Sketch of experimental setup
Figure 2: Excavation of marsh turf (photo: F. Rupprecht, July 2012)
Figure 3: Loading of marsh turf sections for transport to Hannover (photo: F. Rupprecht, July 2012)
Figure 4: GWK wave tank in Hannover (photo: I. Möller, June 2012)