Wave dynamics over saltmarsh surfaces

Coastal marshes constitute a delicately balanced system between hydrodynamic forcing on the one hand and ecological, sedimentological, and morphological responses on the other. Equally, however, the marsh surface itself exerts an influence on tidal currents and incident waves. Saltmarshes can significantly dissipate the hydrodynamic energy available for erosion, re-suspension and transport of sediment (Möller et al. (1999 and 2002)). Knowledge of the feedback mechanisms by which vegetated marsh surfaces achieve such energy dissipation, and the thresholds at which these dissipative controls are exceeded, is a key factor in understanding the morphological response of marshes to sea-level rise and associated changes in incident wave type/energy.

The aim of this research is to quantify changes in significant wave heights and total spectral energy across densely vegetated macro-tidal saltmarshes of different morphological characteristics (Möller et al. (1999, 2002), Möller and Spencer (2003)). It has already been shown that marsh surfaces are capable of dissipating more than 90% of the incident total spectral wave energy over short (several 10s of meters) distances, although attenuation appears highly spatially and temporally variable. While spectral summary parameters (e.g. significant wave height, total spectral energy) provide useful indicators of the energy dissipation potential at the marsh-wide scale, information on the mechanisms by which such dissipation takes place can only be obtained if wave pathways over marsh surfaces and changes in the wave spectra themselves are considered.

Bottom-mounted pressure sensor (left) and data logging equipment for wave recording (right), Tillingham, Dengie Peninsula, Essex, UK.

After an initial study on the open coast marshes of Stiffkey, North Norfolk (Möller et al., 1996), a more extensive dataset of wave spectra was collected for marsh surface inundation events (Sept 2000 - Jan 2002) at two sites on the Dengie saltmarshes, eastern England (southern North Sea), using a network of 21 bottom-mounted pressure sensors, covering both intertidal mudflat and saltmarsh surfaces (Möller and Spencer, 2003). Further research on the Dengie marshes is ongoing and an in-depth analysis of changes in wave spectra over a range of distances across mudflat and saltmarsh surfaces and at a cliffed marsh edge has provided insight into the temporal and spatial variations in wave characteristics across the mudflat to saltmarsh transition zone (for both cliffed and smooth marsh edge configurations).

The project also investigates the potential effect of the observed patterns / re-distribution of spectral energy and peak wave period shifts on marsh morphology and uses the field observations to validate and extend numerical and conceptual models (see, e.g., Möller et al., 1999). Improved models of this kind are required, amongst others, to (a) evaluate marsh surface responses to changes in environmental forcing consequent upon global climate change, and (b) assist in the formulation of 'design rules' for the restoration and re-creation of degrading marsh systems.

References:

• Möller, I. and Spencer, T. (2003) 'Wave transformations over mudflat and saltmarsh surfaces on the UK East coast - Implications for marsh evolution'. Proceedings of the International Conference on Coastal Sediments '03, Florida, USA
• Möller, I., Garbutt, A. & Wolters, M. (2001) Managed realignment of sea defences and the re-creation of saltmarshes in south-east England. In: Green, R.E., Harley, M., Spalding, M. & Zöckler, C. (eds) Impacts of Climate Change on Wildlife. pp 40-43. Royal Society for the Protection of Birds, Sandy.
• Wolters, M., Bakker, J.P., Bertness, M.D., Jefferies, R.L., and Möller, I. (2005) Saltmarsh erosion and restoration in south-east England: squeezing the evidence requires realignment. Journal of Applied Ecology, 42 (5), 844-851.
• Smith, G.M., Thomson, A.G., Möller, I. and Kromkamp, J.C., 2004, Using Hyperspectral Imaging for the Assessment of Mudflat Surface Stability. Journal of Coastal Research: Vol. 20, No. 4, pp. 1165-1175
• Möller, I. and Spencer, T. (2002) Wave dissipation over macro-tidal saltmarshes: Effects of marsh edge typology and vegetation change. Journal of Coastal Research SI36, 506-521.
• Möller, I., Spencer, T., French, J.R., Leggett, D.J., Dixon, M. (2001) 'The sea-defence value of salt marshes - a review in the light of field evidence from North Norfolk'. Journal of the Chartered Institution of Water and Environmental Management 15, 109-116.
• Möller, I., Spencer, T., French, J.R., Leggett, D.J., Dixon, M. (1999). Wave transformation over salt marshes: A field and numerical modelling study from North Norfolk, England. Estuarine, Coastal and Shelf Science 49, 411-426.
• Moeller, I., Spencer, T., and French, J.R. (1996) Wind wave attenuation over saltmarsh surfaces: Preliminary results from Norfolk, England. Journal of Coastal Research 12(4), 1009-1016.

Collaborators

Part of this research has been carried out in collaboration with Dr Han Winterwerp, WL Delft Hydraulics within the EstProc project led by HR Wallingford Ltd.

Funding

This project has been funded by the Environment Agency and DEFRA.