Simulation On The Sediment Dynamic Processes And Geomorphological Evolution Of Pearl River Estuary,Associated With Impacts Of Human Activities

Geomorphological evolution of the estuary regions are controlled by river and ocean hydrodynamics.The main factors include waves,tides and river discharge.Recently,with the development of human society,human activities including river dredging,dam construction,and land reclamation has become an important factor that impacts geomorphological evolution of estuaries.Therefore,the study of the mechanism of sedimentary dynamic process,estuaries evolution and its influencing factors provides scientific data and participate basis for rational exploitation and utilization of resources and environmental protection,and has important significance for sustainable development of estuarine and coastal areas.Pearl River Estuary(PRE)is one of the world's most complex river estuary with second large river discharge of sediment in China.Its geomorphological evolution is also affected by human activities.The impacts of human activities is not clear in pervious research.In this paper,the Delft3D model,combined with field data,are applied to simulate the sediment transport and morphological evolution of the PRE,and to explore the tides,sediment transport,the distribution of turbidity maximum and the morphological evolution of 30 years.Besides,several experiments are designed to explore the impacts of two main human activities-dams and reclamations.The result shows that:(1)Pearl River Estuary are dominated by rectilinear flow,and the strong tidal current is at the east channel during flood and at the west channal during ebb.The velocity is higher at the deep channel that that at shalow shores.Suspended sediment concentration(SSC)shows positively correlated with tidal range,indicating a strong sediment resuspension in this region.In the model simulation results,turbidity maximum zone is found mainly in the northern Lingding Bay,with a north-south strip distribution.(2)Pearl River estuary is mainly dominated by M2 and K1 consitituent.The traspost parttern of M2 and K1 is similar,the tidal wave moves parallel to the coast outside the esturay,and turns to the northwest when went in the Lingding Bay.Since the bell-shape of Lingdingyang,the amplitude of the tidal wave increases during the trasport upstream,and reach the maximum in the northern Lingdingyang maximized.M2 has a larger amplitude comparing to K1,which can reach 0.7m,and the maximum amplitude of K1 is 0.4m.(3)The distribution parttern of Lagrangian residual current is similar to Euler residual current in PRE,the distribution is mainly influenced by topography and river discharge.Lagrangian residual current in northern Ling Ding is southeast.Due to the Coriolis force and terrain,the current gradually turns to right(south)in southern Lingding Bay.Lagrangian residual current near the Neilingding island reach the maximum of about 0.15m/s.Stokes drift direction is opposite to the Lagrangian residual stream,which is affected by the topography.However,Stokes drift is very small,only 0.001-0.005 m/s.(4)The 3D model results show that the PRE bottom salinity increased significantly during flood tide,especially in the channel,what indicates the Pearl River estuary faces serious salt water intrusion problems.During the ebb period,freshwater front appears at the surface,and the stratification is more obvious,which is consistent with the measured data.The surface SSC is lower than the bottom SSC,and the dry season is higher then the wet season.PRE turbidity maximum zone exists between Qiao Island and Neilingding,and its center and the range moves upstream and downstream with the tides and seasonal changes.(5)The simulation of geomorphological evolution in 30 years shows that the evolution trend of PRE is that the deep channel will be deepper and shallow shores will be shallower.The whole PRE shows a deposition trend.Besides,around the islands such as Neilingding island and Qi'ao island are the deposition center which indicates important role these islands plays in the geomorphological evolution.(6)Due to the impact of human activities,geomorphology evolution is greatly changed.And the two main human activities,have different impacts,even opposite effects.The dam construction leads to the decrease of sedmient input which results in the erosion of PRE.However,land reclamation leads the estuary to a deposition trend.Therefore,human activities impacts on morphological evolution of estuaries are complex,and previous studies have not well explain the problem.This problem is related to the major issues of our own development,so the there is a need for more research and attention.