Suspension dynamics of sand-silt mixtures

Abstract

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.