Cohesive sediments alter coastal bar dynamics under waves and currents

Coastal systems are highly dynamic environments 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. However, many coastal environments consist of spatially varying mixtures of sand and mud, while current sediment transport predictors and empirical relations of bar dynamics do not take into account the effect of cohesive sediment. The current research therefore aims to characterize the relative influence of clay on the direction of sediment transport and the resulting 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, while keeping the total bed shear stress constant. Flow velocity, water level and bed levels were monitored during each run, and the bed was scanned before and after each experiment.

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. With increasing clay content, relatively more lateral and less streamwise transport occurred under the same hydrodynamic conditions. Furthermore, 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. These results imply that coastal and estuarine environments with spatially varying clay content will adapt differently to changing hydrodynamic conditions. In systems with a relatively high clay content, wave energy will have an important control on dynamics as it is needed to mobilize the sediment.