Wind Wave Attenuation over Saltmarsh Surfaces: Preliminary Results from Norfolk, England
Keywords:Coastal defence, wave energy, wave recording, marsh surface hydrodynamics
An array of three bottom-mounted pressure transducers (placed approximately 200 metres apart along a shore-normal transect centred on the sandflat/saltmarsh transition) was used to measure changes in wave characteristics across sandflat and saltmarsh on the Norfolk coast, UK. Pressure readings were taken at a frequency of 5Hz over periods of 5 and 7 minutes at different times during the tidal cycle over a range of tides between September 1994 and May 1995. The time-series were corrected to offset attenuation with depth of the high frequency fluctuations. A comparison of surface waves computed in this way with observations made using a video camera showed a significant positive correlation. Analysis of all 54 records showed a consistent energy decrease of between 47.4 % and effectively 100% across the saltmarsh section of the transect. This differed significantly from the much lower wave energy reduction (1.9 to 55.3%) across the sandflat section of the transect. Reduction in wave energy and significant wave heights was only weakly related to water depth across the sandflat, but more strongly related to water depth across the saltmarsh. The results suggest that saltmarshes are extremely effective in buffering wave energy over the range of water depths and incident wave energies investigated here. The increased surface roughness of saltmarshes is likely to be most effective in reducing wave energy at low to intermediate water depths or during conditions of high incident waves.