Contrast of fine sediment dynamics between shoals and channels in a microtidal estuary with mixed semi-diurnal tides

Abstract

Estuaries usually feature complex bathymetries, where shoals and channels are co-existent. Due to the differences in water depth, current, density gradient and therefore stratification, sediment dynamics on the shoal and in the channel demonstrate significant variations. In this study, field measurements were carried out during spring and neap tides in both wet and dry seasons in the Huangmaohai Estuary, a microtidal estuary located in the southwest of the Pearl River Delta. Harmonic analysis was conducted for the timeseries data of current and suspended sediment concentration (SSC) for each deployment. Sediment transport flux was decomposed into an advective component, and tidal pumping fluxes by different tidal constituents. During the neap tides, sediment transport is primarily controlled by the advective flux, whereas during the spring tides, tidal pumping fluxes become comparable to, sometimes even exceeding, the advective one. For a 25-hr period, the M1 component of SSC usually denotes the maximum SSC associated with the highest bottom stress, while the M2 component signifies the two highs of the SSC. The M4 component is generally insignificant. The M1 and M2 components can be induced by both the advection and bottom resuspension. For the resuspension part, the M1 component is mostly induced by tidal velocity asymmetry, while the M2 component is generated by tidal straining effect. Sediment transport at the shoal is mostly controlled by the advective flux and the tidal pumping due to tidal velocity asymmetry, while that in the channel is dictated by advective transport and the tidal pumping due to tidal mixing asymmetry.