A series of laboratory experiments focused on the wind impact on the vertical turbulence structure in shallow water has been carried out. The turbulence characteristics in the mid-lower water column under relatively strong wave conditions are investigated. For different experimental conditions (i.e., waves only, wind only, combinations of wind and waves) in a wind-wave flume, the effects of wind and waves were investigated in detail by decomposing the total energy into different terms (i.e., wind-driven currents, wind waves, wind-induced turbulence). The results show that shallow water waves play a major role in transferring energy by non-zero wave-induced Reynolds stress (u˜w˜‾), turbulent diffusion (u^′w^′‾). The superimposed wind can further modify the energy transference due to its impact on wave asymmetry and skewness as well as through homogenizing the time-average velocity profiles. Subsequently, the impact of wind on turbulence structure was explored in detail. The most important finding is that the wind can directly influence water turbulent diffusion (u^′w^′‾) along with wave-induced turbulence. The vertical turbulence intensity (σ_w) is more sensitive to wind than the horizontal turbulence intensity (σ_u). Furthermore, the major way that wind affects water turbulence is by introducing nonlinear wind and wave interactions, which exhibit a maximum effect (∼60 % compared to the respective effect of wind and wave) at the edge of the bottom boundary layer. This study demonstrates that the wind can transfer momentum downward to mid-lower water columns even under strong waves in shallow waters, which differs from that in deep water systems.

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.

In coastal seas and estuaries, the presence of periodic forces, e.g. tides, results in the fluctuation of sediment transports. Therefore, the study of residual sediment transport in a tidal environment is more practical rather than sediment transport itself. Based on the assumption that sediment transport is proportional to an exponential power of current velocity (u⁵), a simplified analytical expression is derived for residual total load sediment transport in a tidal environment. The tidal current is represented by tidal current constituent series of M₀, M₂, S₂, N₂, M₄, MS₄, MN₄, M₆, K₁ and O₁. The expression can be adapted to quantify the sediment transport in areas with significant residual current and diurnal tidal regimes.

The Optical Backscatter Sensor (OBS) has been widely used to measure suspended sediment concentration in both field and laboratory conditions, even though it is very sensitive to many factors. The most significant factor suggested is the grain size. In order to enhance the quality of OBS data, an improved approach is proposed, based on the "mixture of linear component response" method (Green and Boon III, 1993) to account for the effect of grain size. In addition to an original sediment sample, which commonly serves as a single calibration material, an accompanying sediment sample is necessary to calibrate OBS sensors. A multi-fraction sediment model is used to predict the grain size distribution in suspension. Compared with existing methods, the improved approach does not require a sieving procedure nor the assumption that the sediment fractions of the calibrated sediment sample exhibit the same sensitivities as those of the suspended sediments. The applicability of our method has been verified by a series of laboratory experiments over silt-sand mixtures. The results show that this method successfully yields continuous concentration profiles, which agree well with the measurements using a suction method. The converted concentrations of the time-averaged OBS measurements by traditional method and by the improved method, respectively, are compared with the suction measurements. The results of linear regression analyses show that the coefficient of determination increases (e.g. from 0.55 to 0.92 for wave-current conditions) and the Root Mean Square Error decreases (e.g. from 0.97 to 0.39 for wave-current conditions). It demonstrates that improved method enhances the quality of OBS conversion. Furthermore, suggestions on selecting the accompanying sediment sample (i.e. on grain size and composition) are given.