Numerical modelling of wave-current induced turbidity maximum in the Pearl River estuary

Abstract

The dissertation describes a study of the hydrodynamics and sediment transport characteristics as well as the formation and development processes of turbidity maximum in the Pearl River Estuary under the interaction of both wave and current through field data analysis and numerical modelling. Data from a large-scale synchronous hydrographic survey carried out along the main navigational channels are used to study the sediment transport processes in the Pearl River Estuary and subsequently to analyze the formation mechanisms of turbidity maximum. The results show that turbidity maximum widely exists in the Pearl River Estuary and is not only related to the intrusion of salt water, but also to the freshwater runoff from the three western river outlets. Gravitational circulation and tidal trapping are the main causes to form the turbidity maximum in the West Channel. However, turbidity maximum in the East Channel is mainly caused by the sediment resuspension and deposition processes. Sediment input from the Pearl River outlets and tidal Stokes drift are the important factors for the formation of turbidity maximum. To investigate the horizontal characteristics of hydrodynamics and sediment transport, a depth-integrated 2D model is adopted. The model result is also verified against available measurements in the Pearl River Estuary and good agreement has been obtained. An analysis of computed residual flow shows that the Eulerian component from the non-tidal drift is the dominant one with a maximum velocity of about 0.3 m/s near river outlets, compared with that of the Stokes drift of less than 0.05 m/s. Model results also show that sediment resuspension plays an important role within tidal cycles due to the surplus sediment-carrying capacity. The sediment concentration in deep channels is smaller than that in the nearby shoals. With the background knowledge obtained from the data analysis and 2D modelling, a 3D hydrodynamics and sediment transport model is developed based on the work by Wai and Lu (1999 and 2000) to model the turbidity maximum in the Pearl River Estuary. The present 3D model has high efficiency and extended applicability through optimizing the old algorithm and taking into account the baroclinic terms in the momentum equations as well as coupling a level 2.5 turbulence closure scheme with the Navier-Stokes equations. The 3D model is validated comprehensively by comparing the computed tidal level, current, salinity and sediment concentration in a spring tide and a neap tide with available field data and good agreement is obtained. The 3D model is able to capture the formation and development processes of turbidity maximum in the Pearl River Estuary. Model results show turbidity maximum occurs during spring tides and disappears during neap tides with a cruising range of about 22 km over the sand bars in the main channels. The turbidity maximum fully develops when ebbing during a spring tide in the wet season. Gravitational circulation, tidal pumping and resuspension are the main factors in the formation of turbidity maximum in the wet season. However, local resuspension is the main cause in the dry season. To study the wave effect, a wave propagation model, developed by Chen (2001), is coupled with the present 3D hydrodynamics and sediment model. Applications in the Pearl River Estuary show that the coupled wave-current model can solve combined wave-current problems efficiently. The computed results show that the island sheltering and shoaling factors significantly influence the propagation of wave into the Pearl River Estuary. Also, the results indicate that the combined wave-current interaction only increases the sediment concentration mainly near the sand bars and in shoals, resulting in a thicker high sediment concentration vertical core in the turbidity maximum without significant modification of the general characteristics of the turbidity maximum including the location and excursion amplitude of the TM. However, the credibility of this result is yet to be verified with field measured data.