Effect of cohesive sediments on coastal bar morphology under waves and currents

Coastal systems are highly dynamic systems where sand and mud are transported under the complex interactions of bathymetry, currents and waves. A better understanding of the natural dynamics at the scale of individual bars is required for a fundamental understanding of the formation of coastal environments and how they will respond to changes in the future. The current research aims to characterize the relative influence of mud on the morphodynamic change of coastal bars under the combined action of waves and currents. To this end, experiments were conducted in the Total Environment Simulator, a large-scale wave-current flume facility at the University of Hull (6m x 11m, 0.4m deep). The experimental setup consisted of a circular mound of a mixture of sand and clay, placed on top of a flat sand bed in the centre of the flume. The experimental conditions were systematically varied between runs, with 4 different clay percentages of the mound, and 5 different combinations of wave height and current velocity. Flow velocity, water level and bed levels were monitored during each run, providing well-controlled bed development data over time. Preliminary observations of the mound morphology show lateral diffusion due to sediment transport perpendicular to the wave direction under the influence of gravity, and streamwise migration due to sediment transport in the direction of the flow. Increasing the cohesivity altered the relative influence of the waves and currents on the direction of sediment transport and therefore the final shape of the mound. Wave height had a greater control on the morphology with increasing clay content, since higher waves were more effective in winnowing out the clay into suspension and thereby mobilizing the sand fraction. Future analyses will focus on linking the change in direction of sediment transport under combined waves and currents to landscape evolution, to study the larger-scale implications of the observed differences in transport mechanisms.