Wave Transformation Over Salt Marshes: A Field and Numerical Modelling Study from North Norfolk, England

Abstract

This paper presents quantitative evidence for the effectiveness of a meso- to macro-tidal open coast salt marsh in attenuating incoming waves over a range of tidal and meteorological conditions. Field measurements of wind waves at three locations on a sand flat to salt marsh cross-shore transect showed that wave energy dissipation rates over the salt marsh were significantly higher (at an average of 82%) than over the sand flat (at an average of 29%). The difference in water depth between the sand flat and salt marsh sections of the transect was not sufficient to account for the difference in wave attenuation, indicating that an increase in surface friction is the primary reason for increased wave attenuation over the salt marsh. Comparison of the field results with a numerical model based on theoretical wave energy dissipation suggests that surface friction factors of the order of ≤0·4 are responsible for the high values of wave attenuation over the salt marsh. The results provide empirical support, both for maintaining salt marshes in front of existing sea defence lines and for creating new salt marsh as part of coastal set-back/shoreline realignment schemes.