Unravelling the dynamic settling behaviour of silty sediments in turbulent water

Abstract

Bulk settling velocity is a fundamental parameter characterizing the overall settling behaviour of mixed sediment and representing the downward sediment flux in sediment transport modelling. However, the influence of turbulence on this parameter for silty sediment remains insufficiently understood. To address this, annular flume experiments were conducted using three sediment mixtures with silt contents ranging from 24% to 85% under bed shear stresses up to 1.0 Pa. Bulk settling velocities were estimated using both the conventional McLaughlin method and a novel process-approximating method based on iterative numerical simulations. Results reveal a distinct four-stage evolution. Under very weak turbulence, the settling velocity remains constant. As turbulence intensifies, the loitering effect dominates, reducing the settling velocity to a minimum near three times the critical bed shear stress of discrete particles. As turbulence further rises, the fast-tracking effect enhances, causing the velocity to gradually recover. Under strong turbulence, it increases markedly. Importantly, particle inertia affects how sediments respond to turbulence: finer, silt-rich sediment with low inertia transition between settling regimes at weaker turbulence and exhibit greater enhancement in settling velocity under strong turbulence, whereas coarser sediment with greater inertia responds more sluggishly, resulting in a more pronounced reduction by loitering. Finally, a formula was proposed to quantify bulk settling velocity as a function of bed shear stress for sediments with varying silt contents. These findings highlight that settling velocity in turbulent waters is not constant but varies with turbulence, underscoring the critical roles of turbulence and silt content in governing settling behaviour.