Spatiotemporal Evolution Of Erosion And Deposition And Its Dynamic Mechanism In The Littoral Zone Of Active Yellow River Estuary

The estuarine and coastal area is a sensitive area of land-ocean interaction,which is also one of the most economically and ecologically valuable environments on the earth.In recent decades,under the background of global climate change and increasing intensified human activities,the amount of water discharge and sediment load in the basin has decreased sharply,and the evolution pattern of erosion and deposition in estuarine and coastal areas has gradually changed significantly,facing the challenge of changing from deposition to erosion.It has become the focus of many scholars,management departments and society to deepen the understanding of the evolution process of erosion and deposition in estuarine and coastal areas,explore the dynamic mechanism of erosion and deposition evolution,and protect the sustainable development of estuarine and coastal areas.In recent years,due to climate change and the intervention of anthropogenic activities,the littoral zone of Yellow River Estuary(YRE)is also facing the threat of coastal erosion.Therefore,it is urgent to understand the evolution process of erosion and deposition in the YRE,and exploring the dynamic mechanism of erosion and deposition evolution.In this paper,the YRE is taken as the research object.Combined with the data of water discharge and sediment load,remote sensing images,hydrological and sediment measured in-situ data,and bathymetric measurements data,the temporal and spatial evolution of erosion and deposition in the YRE and its influencing factors are systematically and quantitatively studied.The Delft 3D model is used to establish the hydrodynamic model of the YRE.The influence of tidal dynamics and different wind conditions on the development process of tidal shear front is considered.Combined with the distribution of suspended sediment concentration(SSC)retrieved from remote sensing images,the dynamic mechanism of spatial-temporal difference of erosion and deposition in the YRE was systematically discussed.The main results and conclusions are as follows:(1)A digital elevation model(DEM)of bathymetric topography is established to study the temporal and spatial evolution of erosion and deposition in the littoral zone of the Yellow River EstuaryBased on the bathymetric measurements data of the YRE from 1997 to 2018,the submerged delta digital elevation model(DEM)was established by using Arc GIS.The results show that the submerged delta has undergone significant deposition with a total sediment volume of 5.36×10~8 m~3 from 1997 to 2018.On inter-annual timescale,morphological evolution of the submerged delta has gone through three sequential stages:slow erosion(1996-2002),rapid accretion(2002-2007),slow accretion(2007-2018).From the perspective of spatial variability,the accretion trend occurred in the Active Yellow River Mouth submerged delta,while the erosion appeared in the Abandoned Yellow River Mouth submerged delta and the Gudong littoral zone,respectively.The depo-center changes with the position of the river mouth.The erosion/deposition centers are all located within the 10 m isobath,mainly elliptical,and the long axis is roughly parallel to the 10 m isobath.(2)Quantifying the relationship between the volume change of the YRE and the amount of sediment load,and discussing the river dynamic factors driving the evolution of erosion and deposition in the YREAccording to the analysis of the water discharge and sediment load data of Lijin hydrological station from 1950 to 2018,which is found that the sediment load of the Yellow River into the sea water decreases in a step-by-step manner.Since 2002,the Yellow River has implemented Water Sediment Regulation Scheme(WSRS),and the water discharge and sediment load have increased.However,the annual sediment load is only 1.35×10~8 t/yr,about 13.1%of the previous(1950–1986)level,and the YRE is in a state of deposition.In 2016-2017,a large area of submerged delta was eroded due to the interruption of the WSRS.After the flood control operation was implemented in2018,the submerged delta deposited rapidly.There is a close positive correlation between the volume changes of YRE and the amount of sediment load entering the sea,and the critical sediment load for maintaining the submerged delta stability is different at different stages.An estimation of 1.65×10~8 t sediment load should be kept from 1997to 2002,and about 1.09×10~8 t from 2002 to 2018.In addition to study results show that the evolution of YRE is not only influenced by WSRS,but also by sediment grain-size composition and the position of river mouth outlet.(3)Establishing a hydrodynamic model of the YRE based on the Delft 3D model,investigating the response of tidal shear front development and shear strength distribution to different wind conditionsA three-dimensional hydrodynamic numerical model for the YRE was constructed to systematically analyze the development process of tidal shear fronts,as well as the characteristics of plane shape and vertical structure.In plane,the shear front in the YRE developed from the active estuary and gradually moved toward the abandoned estuary.During the propagation process from north to south,the distance to the shore of the shear front became larger and larger.Vertically,the width of the shear front zone increases slightly from the surface to the bottom.The shear strength of the shear front decreases from the surface to the bottom.The bandwidth of the shear front is narrow in the early stage of development,and then gradually widened with time,and finally disappears with the disappearance of the shear front,and the shear strength of the shear front shows a decreasing trend throughout the development process.When the wind direction is the same as the inner tide flow direction of the shear front,the shear strength of the shear front is strengthened and the duration increases;when the wind direction is opposite to the inner tide flow direction of the shear front,the shear strength of the shear front is weakened and the duration decreases;Under strong wind conditions,when the wind direction is the same as the inner tide flow direction of the shear front,the shear front duration is significantly increased,the vertical structure is destroyed,and the shear strength of the shear front is weakened.When the wind direction is opposite to the inner tide flow direction of the shear front,the shear front is difficult to form.(4)The control effect of the development of tidal shear front on the evolution pattern of'summer deposition and winter erosion'in the YRE is revealed from the perspective of ocean dynamicsIn summer,the Yellow River has a large amount of sediment load into the sea,and the counterclockwise circulation system of the residual current on the east side of the active river mouth has a strong restriction on sediment,which makes it difficult to transport it to the Gudong littoral zone.In addition,the strong IEOF type shear front under the action of the southerly wind in summer is more likely to make sediment to deposit in the current estuary.Some of the sediment transported along the coast from the north to the south was transported to the outer sea at the abandoned river mouth,and the other part was transported to the southwest side of the abandoned river mouth,which is highly consistent with the spatial range of the distribution of suspended sediment concentration retrieved from remote sensing images.Therefore,the YRE is dominated by deposition in summer.In winter,the amount of sediment load entering the sea of the Yellow River is reduced,and the residual current direction is generally from north to south with a large flow rate.In addition,due to the strong IFOE type shear front under the effects of the northerly wind in winter,the sediment is more likely to be transported to the abandoned river mouth after entering the sea from the active river mouth,and then transported to the outer sea under the action of the northeastward residual current.In addition,the strong tidal dynamic at the Abandoned Yellow River Mouth in winter forced the mouth of the estuary to continue to erode,and the sediment after erosion was continuously transported to the outer sea.Therefore,the YRE formed the pattern of erosion and deposition evolution of‘summer deposition and winter erosion'in time,and formed the erosion and deposition difference of‘Active Yellow River Mouth deposition,Gudong littoral zone and Abandoned Yellow River Mouth erosion'in space.In conclusion,based on multi-source data,this paper systematically studied the temporal and spatial evolution of erosion and deposition and its dynamic mechanism in the YRE,explored the evolution process and features of tidal shear front under different wind conditions,and revealed the effect of river dynamics and ocean dynamics factors on the temporal and spatial evolution of erosion and deposition in the YRE.This study can provide scientific guidance for the sustainable development of the YRE and the defending erosion of the delta,and help to deepen the understanding of the dynamic geomorphology process in the global estuary and coastal areas.