Exploration of the Mechanisms for the Low Sensitivity of Deposition Flux to Upstream Sediment Reduction in the North Passage, Yangtze Estuary

Deltas are densely populated industrialized regions,and home to important ports and navigation channels.Due to human interferences,the Yangtze Estuary has experienced a significant reduction in sediment load caused by the Three Gorges Dam(TGD),as well as adjustment to local morphodynamics by the Deep-water Navigation Channel(DNC).While the dramatic reduction in sediment triggers the increased channel erosion,the deposition flux of the DNC located at the estuary mouth has little change.To explore the physical mechanism of this phenomenon,a two-dimensional model is used to establish the relationship between sediment load and the sediment budget of channels based on the bathymetry in 2016.Model results show that the tidal reach and the inner estuary have a negative sediment budget.And the seasonal characteristics of water and sediment fluxes become less obvious downstream.Sensitivity analysis shows that the influence of upstream sediment load on deposition flux decreases along the channel,with a transition from deposition to erosion occurring in the tidal reach.For the last-level bifurcation,the annual siltation of sediment in the North Passage(NP)decreases by 4.5%with low sensitivity.This is attributed to the reduction of sediment load partially mitigated by riverbed erosion and cascade bifurcations.In addition,the lateral sediment supply,which accounts for 68%of the sediment input in the NP,is stable.Overall,this study strengthens the understanding of the relationship between sediment load and artificially deepened systems,thus allowing for better management of estuarine sediment and navigation channel.

Mechanism of the Impact of Floods and Typhoons on the Morphological Evolution of the Yangtze Estuary

In the last two decades,the Yangtze Estuary has undergone significant changes under the influence of reduced sediment inflow and estuary engineering.This study investigates the influence of floods and typhoons on sediment concentration and the morphological evolution of shoals and channels in the Yangtze Estuary.The analysis is conducted through the utilization of topographic data measured pre-and post-flood events and observations of hydro-sedimentary changes during typhoons.By using a generalized estuary mathematical model,this study examines the interplay between varying tidal ranges,tidal divisions,runoff volumes,and regulation projects on the erosion and deposition of shoals and channels in bifurcated estuaries.The results show that due to the implementation of river and waterway regulation projects,the impact of the 2020 flood on the main channel and shoal was significantly less than that of the1998 flood.The swing amplitude of the South Branch main channel decreased.However,local river sections such as the Southern Waterway of Baimao Shoal exhibited erosion.During typhoons,sediment concentration in the 20 cm above the bottom increased significantly and was closely related to wave processes,with a weakened correlation to tidal dynamics.After typhoons,high shoals in South Passage above 0 m were silted up,while the terrain on one side of the tail of Jiuduan Shoal in the downstream deep-water area was generally scoured due to strong wave action.The generalized mathematical model of the bifurcated estuary revealed that M2 tidal component contributed most to the ero sion and deposition evolution of estuary shoals and channels,with floods exhibiting characteristics of sedime ntation on shoals and erosion on channels.With the implementation of a branch rectification project,branch resistance increased,diversion decreased,and the riverbed changed from pre-project erosion to post-project sedimentation,with an increase in erosion in non-project branches.

Study on Volume Variation and Stability of Riverbed in the Yangtze Estuary

In recent years,regional floods and typhoons have occurred in the Yangtze Estuary.Changing dynamic conditions and dramatic reduction of sediment discharge in the basin are affecting the dynamic equilibrium pattern of the Yangtze Estuary.Based on the field measurement data and theoretical derivation,this paper analyzed the changing process of runoff-sediment discharge into the sea after the operation of the Three Gorges Project(TGP),and the tidal dynamics and sediment variation characteristics of the Yangtze Estuary.The erosion of South Branch mainly occurs in the channel below-10 m contour,and the riverbed volume below contours 0 m and-10 m has a good correlation with the sediment discharge of Datong Station in the previous year.On this basis,the ratio of the horizontal distance from the starting point to the section centroid below the average water level(B_c)and the water depth at the section centroid(H_c)was proposed to describe the change of the section shape.The relationships between the water-diverting ratio,the sediment-diverting ratio and the water-diverting angle,the conditions of runoff and sediment discharge from the upper reach and the characteristics of the riverway section were established,and the theoretical calculation equations of the water-diverting ratio,the sediment-diverting ratio and the diverting angle of each bifurcation were also established.

Simulation of Typhoon-Driven Waves in the Yangtze Estuary with Multiple-Nested Wave Models

Typhoon-generated waves are simulated with two numerical wave models, the SWAN model for the coastal and Yangtze Estuary domain, nested within the WAVEWATCHIII （WW3） for the basin-scale East China Sea domain. Typhoon No. 8114 is chosen because it was very strong, and generated high waves in the Estuary. WW3 was implemented for the East China Sea coarse-resolution computational domain, to simulate the waves over a large spatial scale and provide boundary conditions for SWAN model simulations, implemented on a fine-resolution nested domain for the Yangtze Estuary area. The Takahashi wind model is applied to the simulation of the East China Sea scale （3-hourly） and Yangtze Estuary scale （1-hourly） winds. Simulations of significant wave heights in the East China Sea show that the highest waves are on the right side of the storm track, and maxima tend to occur at the eastern deep-water open boundary of the Yangtze Estuary. In the Yangtze Estuary, incoming swell is dominant over locally generated waves before the typhoon approaches the Estuary. As the typhoon approaches the Estuary, wind waves and swell coexist, and the wave direction is mainly influenced by the swell direction and the complex topography.

2-D Current Field Numerical Simulation Integrating Yangtze Estuary with Hangzhou Bay

In this paper, integrating the Yangtze Estuary with the Hangzhou Bay, a 2-D velocity field model is established. In the model, fine self-adaptive grids are employed to adapt to the complicated coastal shape. The hydrodynamic equations satisfied by two contravariant components of velocity vector and surface elevation in non-orthogonal curvilinear coordinates are used. In each momentum equation the coefficients before the two partial derivatives of surface elevation with respect to variables of alternative direction coordinates have different orders of magnitude, i. e., the derivative with the larger coefficient may play a more important role than that with the smaller one. With this advantage, the ADI scheme can be easily employed. The hydrodynamic factors include tidal current, river runoff and wind-induced current. In terms of tidal current, seven main constituents in the area are considered in the open boundaries. The verifications of surface elevation process and current velocity process in the spring tide and in the neap tide show that the model can preferably reflect current fields in the area. Through the simulation of Lagrangian residual current fields in summer and in winter, the paths of the exchange of water and sediment between the Yangtze Estuary and the Hangzhou Bay are elementarily discussed.

Biogenic silica in intertidal marsh plants and associated sediments of the Yangtze Estuary

Biogenic silica （BSi） contents in the marsh plants （Phragmites australis, Scirpus mariqueter and Spartina alterniflora） and associated sediments in Chongming Island eastern intertidal flat of the Yangtze Estuary were determined. The BSi contents in P. australis, S. mariqueter and S. alterniflora varied from 25.78–42.74 mg/g, 5.71–19.53 mg/g and 6.71–8.92 mg/g, respectively. Over the entire growth season, P. australis and S. mariqueter were characterized by linear accumulation patterns of BSi. The aboveground biomass （leaves and culms） of the marsh plants generally contained more BSi than underground biomass （roots）. BSi contents were relatively higher in dead plant tissues than in live tissues which was probably due to the decomposition and the leaching of labile components of plant tissues such as organic carbon and nitrogen. Comparing with the habitats of S. mariqueter and S. alterniflora, the highest BSi content was recorded in sediments inhabited by P. australis, with an annual average of 15.69 mg/g. Overall, the intertidal marshes in the Yangtze Estuary may act as a net sink of BSi via plant uptake and sedimentary burial.

Seasonal bioconcentration of heavy metals in Onchidium struma(Gastropoda:Pulmonata)from Chongming Island,the Yangtze Estuary,China

The seasonal concentration changes of selected heavy metal Cd, Cr, Cu, Fe, Mn, Pb, and Zn in five tissues of marine gastropod Onchidium struma were studied in the Chongming Island, the Yangtze Estuary in April 2007, July 2006, September 2006, and November 2006, respectively. The results demonstrated that the bioconcentration factor of Cu （biomass/water） in all selected tissues was about 104 magnitudes, Fe and Cd were 103, Zn was 102, and Mn, Pb, and Cr were 101. Hepatopancreas was proven to be the dominant storage tissue of Cr, Cu, Mn, and Zn, whereas Fe and Pb were mainly stored in muscle and digenetic gland, and Cd was stored in vitelline gland and albumen gland. Additionally, it was found that Cu, Fe, Mn, and Zn were concentrated significantly by O. struma （whole-body） in summer or autumn, and Cd, Cr, and Pb increased slightly in spring and winter. Furthermore, the bioconcentration of Cr was nearly 2-fold higher and Zn was 1.6-fold higher in the water compared with the Water Quality Standard for Fisheries. With view of excessive amount of Pb, Cd, and Cu according to seafood standard, the consumption of O. struma might have the risk of health hazard.

Three Dimensional Baroclinic Numerical Model for Simulating Fresh and Salt Water Mixing in the Yangtze Estuary

For simulating fresh and salt water mixing in estuaries, a three dimensional nonlinear baroclinic numerical model is developed, in which the gradients of horizontal pressure contain die gradient of barotropic pressure arising from the gradient of tidal level and the gradient of baroclinic pressure due to the gradient of salinity. The Eulerian-Lagrangian method is employed to descretize both the momentum equations of tidal motion and the equation of salt water diffusion so as to improve the computational stability and accuracy. The methods to provide the boundary conditions and the initial conditions are proposed, and the criterion for computational stability of the salinity fields is presented. The present model is used for modeling fresh and salt water mixing in the Yangtze Estuary. Computations show that the salinity distribution has the characteristics of partial mixing pattern, and that the present model is suitable for simulation of fresh and salt water mixing in the Yangtze Estuary.

Distribution and accumulation of biogenic silica in the intertidal sediments of the Yangtze Estuary

Sedimentary biogenic silica is known to be an important parameter to understand biogeochemical processes and paleoenviromental records in estuarine and coastal ecosystems. Consequently, it is of great significance to investigate accumulation and distribution of biogenic silica in sediments. The two-step mild acid-mild alkaline extraction procedure was used to leach biogenic silica and its early diagenetic products in intertidal sediments of the Yangtze Estuary. The results showed that total biogenic silica （t-BSi） in the intertidal sediments varied from 237.7-419.4 μmol Si/g, while the mild acid leachable silica （Si-HCl） and the mild alkaline leachable silica （Si- Alk） were in the range of 25.1-72.9 μmol Si/g and 208.1-350.4 μmol Si/g, respectively. Significant correlations were observed for the grain size distributions of sediments and different biogenic silica pools in intertidal sediments. This confirms that grain size distribution can significantly affect biogenic silica contents in sediments. Close relationships of biogenic silica with organic carbon and nitrogen were also found, reflecting that there is a strong coupling between biogenic silica and organic matter biogeochemical cycles in the intertidal system of the Yangtze Estuary. Additionally, the early diagenetic changes of biogenic silica in sediments are discussed in the present study.

Numerical simulation of water quality in Yangtze Estuary

In order to monitor water quality in the Yangtze Estuary, water samples were collected and field observation of current and velocity stratification was carried out using a shipboard acoustic Doppler current profiler （ADCP）. Results of two representative variables, the temporal and spatial variation of new point source sewage discharge as manifested by chemical oxygen demand （COD） and the initial water quality distribution as manifested by dissolved oxygen （DO）, were obtained by application of the Environmental Fluid Dynamics Code （EFDC） with solutions for hydrodynamics during tides. The numerical results were compared with field data, and the field data provided verification of numerical application： this numerical model is an effective tool for water quality simulation. For point source discharge, COD concentration was simulated with an initial value in the river of zero. The simulated increments and distribution of COD in the water show acceptable agreement with field data. The concentration of DO is much higher in the North Branch than in the South Branch due to consumption of oxygen in the South Branch resulting from discharge of sewage from Shanghai. The DO concentration is greater in the surface layer than in the bottom layer. The DO concentration is low in areas with a depth of less than 20 m, and high in areas between the 20-m and 30-m isobaths. It is concluded that the numerical model is valuable in simulation of water quality in the case of specific point source pollutant discharge. The EFDC model is also of satisfactory accuracy in water quality simulation of the Yangtze Estuary.

Hydrodynamic and morphological processes in Yangtze Estuary:State-of-the-art research and its applications by Hohai University

This paper presents a review of the state-of-the-art research and its applications developed at Hohai University relating to the hydrodynamic and morphological processes in the Yangtze Estuary. Longitudinal, lateral, and horizontal flow circulations have been revealed based on the measurements with acoustic Doppler current profilers （ADCP）. The hydrodynamic mechanism at diversion points as well as the changing patterns of flow and sediment flux in the Yangtze Estuary has been investigated through long-term data analysis. A field survey has been carried out to detect the saltwater intrusion from the North Branch to South Branch. Different numerical models of flow motion, sediment transport, and saltwater intrusion have been developed to simulate the complicated processes and to evaluate the effects of engineering projects. The morphological processes of wetlands over a time scale of decades have been analyzed with an established database. Ideas for further research on the bio-geomorphological model system and long-term evolution mechanisms are put forward.

Tidal Level Response to Sea-Level Rise in the Yangtze Estuary

The rise of tidal level in tidal reaches induced by sea-level rise has a large impact on flood control and water supply for the regions around the estuary. This paper focuses on the variations of tidal level response along the tidal reaches in the Yangtze Estuary, as well as the impacts of upstream discharge on tidal level response, due to the sea-level rise of the East China Sea. Based on the Topex/Poseidon altimeter data obtained during the period 1993-2005, a stochastic dynamic analysis was performed and a forecast model was run to predict the sea-level rise of the East China Sea. Two- dimensional hydrodynamic numerical models downscaling from the East China Sea to estuarine areas were implemented to analyze the rise of tidal level along the tidal reaches. In response to the sea-level rise, the tidal wave characteristics change slightly in nearshore areas outside the estuaries, involving the tidal range and the duration of flood and ebb tide. The results show that the rise of tidal level in the tidal reaches due to the sea-level rise has upstream decreasing trends. The step between the stations of Zhangjiagang and Shiyiwei divides the tidal reaches into two parts, in which the tidal level response declines slightly. The rise of tidal level is 1-2.5 mm/a in the upper part, and 4-6 mm/a in the lower part. The stations of Jiangyin and Yanglin, as an example of the upper part and the lower part respectively, are extracted to analyze the impacts of upstream discharge on tidal level response to the sea-level rise. The relation between the rise of tidal level and the upstream discharge can be fitted well with a quadratic fimction in the upper part. However, the relation is too complicated to be fitted in the lower part because of the tide dominance. For comparison purposes, hourly tidal level observations at the stations of Xuliujing and Yanglin during the period 1993-2009 are adopted. In order to uniform the influence of upstream discharge on tidal level for a certain day each year, the hourly tidal level observations are corrected by the correlation between the increment of tidal level and the increment of daily mean upstream discharge. The rise of annual mean tidal level is evaluated. The resulting rise of tidal level at the stations of Xuliujing and Yanglin is 3.0 mm/a and 6.6 mm/a respectively, close to the rise of 5 mm/a according to the proposed relation between the rise of tidal level and the upstream discharge.

Sediment Deposition and Resuspension in Mouth Bar Area of the Yangtze Estuary

A comprehensive analysis is conducted based on observations on topography, tidal current, salinity, suspended sediment and bed load during the years of 1982, 1983, 1988, 1989. 1996, and 1997 in the Yangtze Estuary. Results show that the deformation of tidal waves is distinct and the sand carrying capacity is large within the mouth bar due to strong tidal currents and large volume of incoming water and sediments. Owing to both temporal and spatial variation of tidal current, deposition and erosion ore extremely active. In general a change of up to 0.1 m of bottom sediments takes place during a tidal period. The maximum siltation and erosion are around 0.2 m in a spring to neap tides cycle. The riverbed is silted during flood when there is heavy sediment load, eroded during dry season when sediment lo:ld is low. The annual average depth of erosion anti siltation on the riverbed is around 0.6 m. In particular cases, it may increase to 1.4 m to 2.4 m at some locations.

Numrical Model of Total Sediment Transport in the Yangtze Estuary

Based on the non-equilibrium suspended load transport equation, bed load transport equation and sediment transport capacity formulas derived by Don et al. , a 2-D numerical model of total sediment transport in the Yangtze Estuary is presented. In the model, the actions of tidal currents and wind waves and the effect of salinity on sediment transport are considered. An automatically generated boundary-fitted grid is used to fit the boundaries of the estuary and the boundaries of engineering projects. The verification of calculations shows that the sediment concentration, the deformation of riverbed and siltation in the channels caused by typhoons can be successfully simulated.

Calculation of Diversion Ratio of the North Channel in the Yangtze Estuary

The circumfluence around the Jiuduan Sandbank is thoroughly studied by means of the principle of least resistance in fluid dynamics. The diversion ratio of the North Channel for 1998 is calculated, which is almost the same as the field survey data. The normal and minimum diversion ratios of the North Channel after stages I , II and III regulation works as well as the future phase are obtained. The numerical results and predicted value are accurate.

Impacts of Different Dynamic Factors on the Saltwater Intrusion in the Northern Branch of the Yangtze Estuary

A 3-D baroclinic numerical model is established to study the temporal and spatial distribution of salinity in the Northern Branch(NB) of the Yangtze Estuary(YE) in response to river runoff, wind and currents. The idealized,process-oriented studies show that the lower river runoff in the dry season is the main driving factor for the spilling over from the NB to the Southern Branch(SB). The NB takes longer time to get in dynamic stable state compared with the SB, as the runoff transfers from the wet season to the dry season. The vertical stratification intensity of the NB has obvious periodic phenomena in the dry season, and the stratification strength is enhanced under the action of low runoff and north wind, which further weakens the vertical transport of water. Momentum balance analysis suggested that the landward net flux increases when considering the influence of the moderate north wind, and the landward flux will enhance if considering the boundary current during the spring tide. With the influence of the boundary current, increases the average salinity of the NB, as well as the intrusion degree from the NB to the SB.This indicates that the nearshore ocean current of the Yellow Sea is an important factor affecting the saltwater intrusion in the NB of the YE.

Tidal Flat Morphodynamic Processes of the Yangtze Estuary and Their Engineering Implications

The intertidal flats are classified as 'attached bar', 'spit' and 'isolated bar' in relation to the land, and 'broad flat', 'sharp bank' and 'eroded cliff' according to the shape of the cross-shore profile. Tidal currents on the flat are basically back and forth along the river channel banks but gyratory on the seaward side of the Chongming Island. The flow velocity on the intertidal flat is gradually reduced with increasing elevation. The river discharge strengthens ebb flows and modifies current asymmetry especially on the lower flat in neap tide, although hydrodynamics over the tidal flat is tide-dominated. The wave height on the tidal flat is normally limited to a few decimeters although it changes with water depth, slope and wind. Suspended sediment concentration over the tidal flat is typically hundreds to thousands of mg / 1. Although the delta has grown rapidly in history, the rate of growth is different in different periods. A maximum advancing rate of 330 m / a was recently found in the central part of the river month towards the sea. In view of the natural conditions, reclamation of higher intertidal flat (above the mean tidal level) in advancing coasts is suggested, which would leave broad wetlands for wild lives. In addition, some possible influences of coastal engineering projects and the future natural backgrounds of engineering under reduction in riverine sediment supply and sea level rise are addressed.

Fractal Characteristics and Prediction of Backsilting Quantity in Yangtze Estuary Deepwater Channel

Fractal interpolation has been an important method applied to engineering in recent years. It can not only be used to fit smooth curve and stationary data but also show its unique superiorities in the fatting of non-smooth curve and non-stationary data. Through analyzing such characteristic values as average value, standard deviations, skewness and kurtosis of measured backsilting quantities in the Yangtze Estuary 12.5 m Deepwater Channel during2011–2017, the fractal interpolation method can be used to study the backsilting quantity distribution with time.According to the fractal interpolation made on the channel backsilting quantities from January 2011 to December2017, there was a good corresponding relationship between the annual(monthly) siltation quantities and the vertical scaling factor. On this basis, a calculation formula for prediction of the backsilting quantity in the Yangtze Estuary Deepwater Channel was constructed. With the relationship between the predicted annual backsilting quantities and the vertical scaling factor, the monthly backsilting quantities can be obtained. Thus, it provides a new method for estimating the backsilting quantity of the Yangtze Estuary Deepwater Channel.

Impact of water and sediment discharges on subaqueous delta evolution in Yangtze Estuary from 1950 to 2010

In order to determine how the subaqueous delta evolution depends on the water and sediment processes in the Yangtze Estuary, the amounts of water and sediment discharged into the estuary were studied. The results show that, during the period from 1950 to 2010, there was no significant change in the annual water discharge, and the multi-annual mean water discharge increased in dry seasons and decreased in flood seasons. However, the annual sediment discharge and the multi-annual mean sediment discharge in flood and dry seasons took on a decreasing trend, and the intra-annual distribution of water and sediment discharges tended to be uniform. The evolution process from deposition to erosion occurred at the -10 m and -20 m isobaths of the subaqueous delta. The enhanced annual water and sediment discharges had a silting-up effect on the delta, and the effect of sediment was greater than that of water. Based on data analysis, empirical curves were built to present the relationships between the water and sediment discharges over a year or in dry and flood seasons and the erosion/deposition rates in typical regions of the suhaqueous delta, whose evolution followed the pattern of silting in flood seasons and scouring in dry seasons. Notably, the Three Gorges Dam has changed the distribution processes of water and sediment discharges, and the dam＇s regulating and reserving functions can benefit the subaqueous delta deposition when the annual water and sediment discharges are not affected.