Relationships Between Wave-Induced Currents and Sediment Grain Size on a Sandy Tidal-Flat

Abstract

Studies of sedimentation on sandy tidal-flats have long acknowledged that tidal flows and waves are the most influential hydrodynamic forcing factors operating on surficial sediments. Tidal flows influence the long-term evolution of tidal-flats because of the asymmetry of tidal regimes (flood and ebb). Tidal current activity is mainly confined to channels. Outside the channels, mainly on the upper tidal-flats, tidal current velocities decrease and sediment entrainment is frequently ascribed to wave action. The role of wave-induced processes on intertidal-flat sediments is frequently stated but has seldom been investigated, apparently because of methodological constraints. In this paper we report on investigations into the role of wave processes in determining surficial sediment distribution patterns on a sandy tidal-flat at Newtownards, at the northern end of Strangford Lough, Northern Ireland (spring tidal range 3.5 m). The main aim of this paper is to investigate the potential relationship between wave-induced processes and surficial sediments without attempting to achieve a comprehensive analysis of other sedimentation. The methodological approach involves numerically modeling wave propagation and comparing this with sediment distribution on a controlled field site. The method enables the role of waves on tidal-flats to be more fully assessed. Waves were modeled at a variety of tidal levels and wave data were extracted from the model grid at positions co-incident with sediment sampling locations. Multiple correlation of sediment mean grain size with simulated wave-induced orbital velocity showed a distinctive variation in correlation coefficients according to water level. The best (and statistically significant) correlations occur when water levels occupy a vertical range between -0.15 and 1.0 m local Ordnance Level (OD). In the absence of strong tidal currents, sediment distribution appears to be best explained by waves acting at these water levels. It is inferred that at lower-than-optimum water levels, wave energy is less influential on sediment texture since (1) part of the tidal-flat is still exposed and (2) the fetch over which the waves are generated is reduced. At higher-than-optimum water levels, wave energy may be sufficient to transport sediment, but the wave penetration in the column of water is not sufficient to permit wave energy dissipation at the sediment surface. Tidal currents are dominant at low tidal elevations so outside channels wave-induced currents exert most influence on the tidal-flats.