Impacts of Composition on Sediment Suspensions Over Sand-Silt Mixtures Under Currents

Coastal sediments are generally mixtures of grains of different sizes, i.e. sediment compositions. Different sediment compositions would result in different sediment transport patterns, interfering with different coastal morphological features. Silt, which sizes (4–62.5 μm) are in transition zones between clay and sand, behaves differently from non-cohesive sediment and cohesive sediment. Therefore, it is essential to understand the dynamic properties of silt-sized sediment and the relevant impacts on sediment transport. This study focuses on the effects of silt fraction on sediment suspensions of mixtures bed by a series of erosion experiments using an annular flume. Totally seven sediment beds have been prepared with different silt content. The experimental results show that under small flow velocities (i.e. 0.27 m/s, just above the erosion threshold), the suspended sediment concentrations (SSCs) first decreased and then increased with the increase of silt content in the bed. The tipping point appears at 36% silt content. However, under larger flow velocities (i.e. 0.41 m/s), the SSC increased monotonously with increased silt content. Thus, under a specific velocity range (0.27–0.41 m/s), the silt content controls SSCs, while the large flow velocity can overrule the bed composition impacts. The impacts of silt content on SSCs should be considered in sediment transport modelling in the sand-silt mixed environment.