Wave Attenuation over Salt Marsh Surfaces
This is an on-going CCRU research interest that has been followed up in several projects, the first of which started in 1993 as a PhD project. Predictions of near-future local relative sea level rise and the cost of re-engineering ageing lines of coastal defence have stimulated interest in alternative approaches to coastal management. It is widely believed that salt marshes are able to dissipate wave energy, reduce wave heights across marsh surfaces, and thus permit the reduction of design criteria for flood defence embankments at their landward margin. The CCRU has carried out field observations at Stiffkey, North Norfolk, UK, that provide the first quantitative data set on wave energy dissipation over salt marshes in the UK. Wave energy transformations were measured over a range of tidal flooding events across a sand flat to salt marsh transect. Field measurements were then compared with one-dimensional model calculations based on theoretical knowledge of wave dissipation, providing information on the relative importance of surface friction effects in reducing wave heights across topographically complex, vegetated surfaces.
In June 2000 the CCRU was awarded an 18-month Research and Development contract by the Environment Agency to determine the natural sea defence value of UK saltmarshes. The main aim of this project is to develop improved methods for assessing wave attenuation over intertidal areas based on field measurements. Measurements are collected at a total of 21 locations on the marsh/mudflat surface for a one-year period at two sites on the Dengie Peninsula in Essex. The assessment of the natural sea defence value of saltmarshes is crucial to a successful sea defence and coastal management strategy. This is particularly important in the south-east of the UK, where isostatic subsidence and the effects of global sea-level rise interact to result in rates of relative sea-level rise of up to 4 mm/yr. In addition, a high risk of storm surges exists in the Southern North Sea and many of the sea defences in East Anglia are fronted by saltmarshes that show erosional tendencies. Reduction in width and elevation of saltmarshes reduces their effectiveness in damping waves, and increases the risk of seawall overtopping and structural undermining.
To assess and evaluate possible differences in the efficiency with which saltmarshes attenuate waves, it is necessary to establish any differences between sites with respect to these causative factors. More specifically, the objectives of this study are aimed at establishing the effect of overall marsh and mudflat topography, marsh width and inundation depths on wave attenuation. The effect of different marsh surface vegetation communities on wave attenuation is also considered.