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The sea-defence function of micro-tidal temperate coastal wetlands

The sea-defence function of micro-tidal temperate coastal wetlands

Background

Global environmental change poses a growing challenge for the management of low-lying coastal environments. The challenge is to (a) recognise and quantify the ecological functions of such environments, and (b) develop management approaches that allow those functions to be maintained in the context of global change. Meeting this challenge is particularly important on micro-tidal shorelines, where the ecological sensitivity to sea level rise and changing climatic conditions (e.g. storm frequency and intensity) is likely to be high.

Previous studies have focused on tidal coasts and salt marsh or mangrove vegetation and have highlighted relationships between coastal wetland vegetation type, water depths, and observed wave energy reduction. Prior to this study, however, no data was available on the sea-defence function of coastal grasslands and reed beds, where irregular inundation by meteorologically driven storm surges dominates over tidal inundation. Thanks to a grant from the Bauer-Hollmann-Foundation (Stifterverband für die deutsche Wissenschaft, Germany) and the Royal Society (UK), this collaborative study with Dr J Mantilla-Contreras, then of the University of Greifswald (Germany), was made possible.

Aims/objectives and methodology

This study addresses the need to quantify the wave-dissipating function of micro-tidal coastal reed bed and grassland wetlands. The main aim was to determine wave energy dissipation across the unvegetated foreshore and within/above the wetland vegetation during a range of inundation and wave energy conditions.

Throughout the winter of 2008/2009, observations of wave conditions and water levels were conducted using pressure sensor technology at five locations along shore-normal transects within mudflat to i) reed bed and ii) salt grassland transitions at two field monitoring sites: the southern margin of the Kooser See (sheltered, with limited fetch (1-3 km from a N to NE direction) and shallow water depths (≤ 1 m)) and the western side of the ‘Kooser Ecke’ (much more exposed, with fetch distances of between 20 km (from a N direction) and >> 400 km (from a NNE direction) and with the 2 m depth contour located within one kilometre of the shore).

Results

A total of 61 wave records were obtained for the transect through the reed vegetation at the sheltered site on the ‘Kooser See’, and a total of 39 records for the reed transect at the more exposed site, with only three records for conditions when water depths at the exposed site overtopped the grassland cliff edge. Results are presented in Möller et al. (2011). Maximum significant wave heights were 31 cm (at the grassland margin) and maximum water depths 1.55 m (at the exposed reed bed margin). On the more exposed shores, incident wave energy was largely water depth limited, while on adjacent (less than 2 km distant), more sheltered shores, fetch limited conditions led to much lower incident energy with no significant influence of water depth.

Wave energy dissipation through reed vegetation was significant (up to 26 %m-1 within the vegetation). Its temporal variability, during onshore wind conditions and for wave height to water depth ratios > 0.1, was controlled to a large extent by water depth, rather than incident wave height, relative wave height, or wave steepness. High frequency waves were preferentially reduced in the reed bed transition zone. At the exposed site, wave energy dissipation across the open water to reed bed transition as well as within the vegetation was significantly related to water depth when winds were northerly (onshore). There was no change in wave energy dissipation (itself negligible) with depth in the open water section in front of the reeds.

This study also confirmed that, as has been shown to be the case in macro-tidal saltmarsh settings, micro-tidal grassland cliffs also result in complex wave energy transformation processes across cliffed transitions. Water depth thresholds control the transition between energy reflection from, versus energy transmission across, the cliff face.

Conclusion

The data obtained as part of this study highlighted significant differences in the sea-defence function of micro-tidal coastal habitats in different settings, suggesting significant variability in the likely response to future climatic (and sea level) changes and raising questions around how these functions might be maintained, enhanced, or restored in the context of environmental change.

On the more exposed shores, incident wave energy was water depth limited, while on adjacent sheltered shores, fetch limited conditions led to lower incident energy and no significant influence of water depth on this energy. Any future predictions of morphological or habitat adjustment under sea level rise scenarios (i.e. an increase in water depth) must take such depth-dependent process relationships into account.

The depth control on wave energy dissipation most likely resulted from vertical variations in the degree of physical obstruction (e.g. biomass) to the progression of waves through the reed vegetation. Any factors that affect this vertical variation in plant matter (such as plant – animal interactions) are thus as critical in determining the potential morphological and ecological impact of waves as considerations of the rate of sea level rise.

The implication of the threshold controls that determine wave energy reflection/transmission across salt grassland cliffs are that, under rapid sea level rise, periods of cliff erosion (under high energy wave impact but relatively low water level) are likely to be followed by periods of higher energy landward of the cliff face, with associated re-suspension of sediment and/or adjustment of vegetation composition.

Publications

Conference Publications

Möller I, Lendzion J, Spencer T, Hayes A and Zerbe S 2009 The sea-defence function of micro-tidal temperate coastal wetlands. In: Brebbia C, Benassi G and Rodriguez GR (eds) Coastal processes. WIT Press, Ashurst, 51-62.

Journal Publications

Möller I, Mantilla-Contreras J, Spencer T and Hayes A 2011 Micro-tidal coastal reed beds: Hydro-morphological insights and observations on wave transformation from the southern Baltic Sea. Estuarine, Coastal and Shelf Science 92, 424-436 [doi:10.1016/j.ecss.2011.01.016]

Related CCRU Publications

  • Friess DA, Spencer T, Smith GM, Möller I, Brooks SM and Thomson AG 2012 Remote sensing of geomorphological and ecological change in response to saltmarsh managed realignment, The Wash, UK. International Journal of Applied Earth Observation and Geoinformation 18, 57-68 doi:10.1016/j.jag.2012.01.1016
  • Spencer T, Friess DA, Moller I, Brown SL, Garbutt A and French JR 2011 Surface elevation change in natural and re-created intertidal habitats, eastern England, UK, with particular reference to Freiston Shore. Wetlands Ecology and Management 20, 9-33 [doi:10.1007/s11273-011-9238-y]
  • Feagin RA, Irish JL, Möller I, Williams AM, Colon-Rivera RJ and Mousavi ME 2010 Short communication: Engineering properties of wetland plants with application to wave attenuation. Coastal Engineering 58, 252-255.
  • Feagin RA, Lozada-Bernard SM, Ravens TM, Möller I, Yeager KM and Baird AH 2009 Does vegetation prevent wave erosion of salt marsh edges? Proceedings of the National Academy of Sciences of the United States of America 106, 10109-10113.
  • Möller I 2006 Quantifying saltmarsh vegetation and its effect on wave height dissipation: results from a UK East coast saltmarsh. Journal of Estuarine Coastal and Shelf Sciences 69, 337-351.
  • Wolters M, Bakker JP, Bertness MD, Jefferies L and Möller I 2005 Saltmarsh erosion and restoration in south-east England: squeezing the evidence requires realignment. Journal of Applied Ecology 42, 844-851.
  • Möller I and Spencer T 2002 Wave dissipation over macro-tidal saltmarshes: Effects of marsh edge typology and vegetation change. Journal of Coastal Research Special Issue 36, 506-521.
  • Möller I, Spencer T, French JR, Leggett DJ and Dixon M 2001 The sea-defence value of salt marshes - a review in the light of field evidence from North Norfolk. Water and Environment Journal 15, 109–116.

Figures

Figure 1: Location of (A) Greifswalder Bodden in the context of the Baltic Sea (▲ : water level recording station Greifswald-Eldena; ◆: automatic weather station location ‘Greifswalder Oie’) and (B) wave monitoring Site 1 and Site 2 at the southern margin of the Kooser See, Greifswalder Bodden

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Figure 2: Cliffed Baltic salt grassland at the Kooser Ecke (Germany) (photo: I Möller)

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Figure 3: Reed bed with wave/water level logging station visible, Kooser Ecke (Germany) (photo: I Möller)

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