WILD WAVE ATTENUATION OVER SALT-MARSH SURFACES - PRELIMINARY-RESULTS FROM NORFOLK, ENGLAND

An array of three bottom-mounted pressure transducers (placed approxim ately 200 metres apart along a shore-normal transect centred on the sa ndflat/saltmarsh transition) was used to measure changes in wave chara cteristics across sandflat and saltmarsh on the Norfolk coast, UK. Pre ssure 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 ti des between September 1994 and May 1995. The time-series were correcte d 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. A nalysis of all 54 records showed a consistent energy decrease of betwe en 47.4% and effectively 100% across the saltmarsh section of the tran sect. This differed significantly from the much lower wave energy redu ction (1.9 to 55.3%) across the sandflat section of the transect. Redu ction in wave energy and significant wave heights was only weakly rela ted to water depth across the sandflat, but more strongly related to w ater depth across the saltmarsh. The results suggest that saltmarshes are extremely effective in buffering wave energy over the range of wat er depths and incident wave energies investigated here. The increased surface roughness of saltmarshes is likely to be most effective in red ucing wave energy at low to intermediate water depths or during condit ions of high incident waves.