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.

Tidal flats are typically rich in fine-grained sediments, with tidal currents serving as the dominant hydrodynamic force. Sediment transport, mainly in the form of suspended load, is especially active in the mid- to low-tidal zones, where silt-rich sediments prevail. In these zones, variations in silt content significantly influence sediment suspension dynamics. To investigate the effect of silt content on the suspension behaviours of sand-silt mixtures under unidirectional flow, a series of experiments has been conducted in an annular flume. Sediment samples collected from silt-enriched tidal flats were remixed into seven types of sediment beds with the silt content ranging from 19 % to 79 %. Results indicate that under steady current conditions, the suspended sediment concentration (SSC) in sand-silt mixtures increased toward equilibrium. Sediment beds with 36 %–60 % silt content exhibited two-stage erosion behaviour, each marked by distinct characteristics. At low to moderate shear stress (0.18–0.74 Pa), smoother bed surfaces and hiding-exposure effects led to higher near-bed velocities and lower equilibrium SSCs. At higher shear stress levels (1.00–1.23 Pa), this behaviour shifted, with prolonged suspension development becoming the key feature. Based on the experimental findings, we further re-calibrated the equations predicting equilibrium SSC, which can be well described by a modified excess shear stress to the power of two. Subsequently, we investigated the boundary conditions that govern sediment suspension to model the observed suspension process. Building on the boundary condition prescribing a constant upward sediment flux, an asymptotic upward flux condition, which represents the delayed erosion response due to the resistance of silty bed structure, has been introduced and demonstrated as the most feasible. Furthermore, the development of SSC towards equilibrium was investigated. This process is well characterised by a time scale, which becomes notably larger for sediment beds with 36 %–60 % silt content at high velocity levels, reflecting their delayed response in suspension. These findings elucidate key mechanisms governing silt-dominated sediment erosion, enhancing predictive capabilities in modelling mixed sediment transport.