Characteristics and simulation of sediment concentration in the northwestern head of Bohai Bay

Abstract

The evolution and utilization of estuarine and coastal regions are largely restricted by sediment issues. Waves and currents are the main dynamic driving forces of sediment transport in estuary and coastal areas. The study of wave-current movement and sediment transport is of great significance in both academic research and engineering practice, which has received much attention from many scholars and engineers. The northwestern head of Bohai Bay belongs to a transitional area from silty coast to muddy coast, where the sediment movement is complex. Firstly, based on the field observation data measured with a tripod system and the hydrological data in the northwestern head of Bohai Bay, the characteristics of tidal currents, waves, and suspended sediment concentration (SSC) changes under different hydrological conditions were analyzed. Sediment density distribution was detected by a γ-ray densitometer. Results show that the SSC is mainly influenced by wave-induced sediment suspension: Under light wind conditions the SSC was very low, with the peak value generally less than 0.1 kg/m3; the SSC increased continuously under the gales over 6−7 in Beaufort scale, with sustained wind action. The measured peak SSC at 0.4 m above the seabed was 0.15−0.32 kg/m3, with the average value of 0.08−0.18 kg/m3, which is about 3−6 times the value under light wind conditions. The density at the bed bottom was about 1.4− 1.6 g/cm3, and decreased to 1.02−1.03 g/cm3 at 0.1−0.2 m above the bed; there was rarely little fluid mud in the study area. Secondly, the mechanism of wave-induced SSC was studied from the view of flow structure, shear stress in the bottom boundary layer. The friction velocity was calculated by fitting the datasets of the measured logarithmic velocity profile. Comparison shows that the wave shear stresses (calculated from the bottom velocity profile) is higher than the current shear stress (calculated from the upper velocity profile). Following the Reynolds’ decomposition method, expressions of wave-induced and current-induced shear stress were derived by splitting the variables into a fluctuating component, an averaged component and an oscillatory component. Compared with uniform flow, an additional item of wave streaming leads to a higher wave-induced shear stress. Finally, by analytically solving the sediment diffusion equation, a numerical procedure was proposed for sediment concentration under combined action of waves and currents. Some key approaches for fine sediment simulation were presented, including stratification effects, hindered settling, moving bed roughness, reference concentration and critical shear stress. Under combined wave-current conditions, the combined sediment diffusivity is given by the square sum of the wave-related and current-related diffusivities, and a toe-type distribution of wave-related eddy viscosity was employed. The stratification effects are considered by introducing the turbulence damping coefficient. The approaches of hindered settling for silt were employed. The enhanced roughness due to mobile bed effects was included. Comparison shows that, these approaches can properly simulate the measured wave-induced sediment concentration. This study provides basic data for coastal protection and utilization as well as for scientific study of silty-muddy coast, which is of great significance in practice and theory.