Experimental study on the effects of sediment size gradation and suspended sediment concentration on the settling velocity, ws

The settling velocity of non -uniform sediment is of great importance for the sediment transport on coasts and estuaries. Although existing settling velocity formulas have already considered factors such as the Suspended Sediment Concentration (SSC), temperature, salinity, turbulence, and the effect of size gradation on the settling velocity, more studies are needed. In this study, the fiber optic sensors and Particle Track Velocimetry (PTV) for high SSC were used to measure the settling velocity of two size gradations with different SSCs in a settling column in laboratory. The size gradation curves and settling velocities of these two non-cohesive sediment with SSCs ranged from 0.1 kg/m3 to 50 kg/m3 were obtained. When the SSC is less than or equal to 0.1 kg/m3, the settling velocity is close to the grain size distribution weighted average settling velocity. With the increase of SSC, the distance between sediment particles decreases, and the mutual influence between different components become important, resulting in a decrease in the settling velocity of large-sized particles and an increase in the settling velocity of small-sized particles. This phenomenon is more pronounced for natural sediments. Based on the experimental results, a particle size distribution correction coefficient was proposed to fit the fine-grained settling velocity in quiescent water considering size gradation and sediment concentration.