Pingxing Ding

Changes in progradation rate of the tidal flats at the mouth of the Changjiang (Yangtze) River, China

Progradation rates of tidal flats at the Chiangjiang River mouth, China, over different periods were examined, based on surveys and historical records. Rapid progradation after the Holocene sea-level rise began about 2000 years ago because of an increase in riverine sediment. The tidal flats at the river mouth have grown at a rate of about 5 km2/year and the southern mainland shoreline in front of the river mouth has advanced at a rate of 17 m/year. In recent decades, the lateral progradation rate at the frontal area has been from tens to hundreds of meters per year. The evolutionary process of the tidal flat is highly episodic under this general tendency of progradation. Spring–neap cyclicity shows clearly in calm seasons while the bare flat surface is sensitive to windy conditions. Different seasonal patterns occur between the marsh and the bare flat because of the influence of vegetation. Spatial changes in sedimentation rate are also striking, which could be attributed to differences in sediment distribution and hydrodynamics. The changeability of sediment surface shows a shoreward reduction from the subtidal area to the high marsh, likely due to the shoreward decrease in water energy and submergence time as well as the protective effect of marsh vegetation. It is predicted that the general progradation could be greatly slowed when the sharp reduction in riverine sediment caused by the Three Gorges project and the South-To-North Water Diversion will coincide with the rapid relative sea-level rise.

Saltwater intrusion into the Changjiang River: A model-guided mechanism study

[1] The Changjiang River (CR) is divided into a southern branch (SB) and a northern branche (NB) by Chongming Island as the river enters the East China Sea. Observations reveal that during the dry season the saltwater in the inner shelf of the East China Sea flows into the CR through the NB and forms an isolated mass of saltwater in the upstream area of the SB. The physical mechanism causing this saltwater intrusion has been studied using the high-resolution unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM). The results suggest that the intrusion is caused by a complex nonlinear interaction process in relation to the freshwater flux upstream, tidal currents, mixing, wind, and the salt distribution in the inner shelf of the East China Sea. The tidal rectification, resulting from the interaction of the convergence or divergence of tidal momentum flux and bottom friction over abrupt topography, produces a net upstreamward volume flux from NB to SB. With river discharge the net water transport in the NB is driven through a momentum balance of surface elevation gradient forcing, horizontal advection, and vertical diffusion. In the dry season, reducing the surface elevation gradient forcing makes tidal rectification a key process favorable for the saltwater intrusion. A northerly wind tends to enhance the saltwater intrusion by reducing the seaward surface elevation gradient forcing rather than either the baroclinic pressure gradient forcing or the wind-driven Ekman transport. A convergence experiment suggests that high grid resolution (∼100 m or less) is required to correctly resolve the net water transport through the NB, particularly in the narrow channel on the northern coast of Chongming Island.

Morphological response of tidal marshes, flats and channels of the outer Yangtze River mouth to a major storm

Systematic morphological changes of the coastline of the outer Yangtze River mouth in response to storms versus calm weather were documented by daily surveys of tidal marshes and flats between April 1999 and May 2001 and by boat surveys offshore during this and earlier periods. The largest single event during 1999 to 2001 was Typhoon Paibaian, which eroded the unvegetated tidal flat and lower marsh and led to accretion on the middle-to-upper marsh and in the subtidal channel. The greatest erosion of 21 cm occurred at the border between the marsh and the unvegetated flat due to the landward retreat of the marsh edge during the storm. Strong waves on the flats increased suspended sediment concentration by 10–20 times. On the upper marsh, where the frequency of submergence by astronomical tides is only 3%, Typhoon Paibian led to 4 cm of accretion, accounting for 57% of the net accretion observed over the 2-yr study. Typhoon Paibian led to 4 cm of accretion, accounting for 57% of the net accretion observed over the 2-yr study. Typhoon Paibian and other large storms in the 1990s caused over 50 cm of accretion along the deep axis of the river mouth outlet channel. During calm weather, when hydrodynamic energy was dominated by tides, deposition was centered on the unvegetated flats and lower, marsh with little deposition on the high marsh and erosion in the subtidal channel. Depositional recovery of the tidal flat from typhoon-induced erosion took only several days, whereas recovery of the subtidal channel by erosion took several weeks. A conceptual model for the morphological responses of tidal marshes, flats, and subtidal channels to storms and calm weather is proposed such that sediment continually moves from regions of highest near-bed energy towards areas of lower energy.

Eco-Morphological Problems in the Yangtze Estuary and the Western Scheldt

This paper compares the Yangtze Estuary in China and the Western Scheldt Estuary in The Netherlands by their morphodynamic and ecological systems, their engineering works and estuarine management issues, and the major challenges in studying them. Physically speaking, the two estuaries are very different. The Yangtze Estuary is much larger and much more influenced by the upstream river than the Western Scheldt. Yet, they also have a number of morphological and ecological features in common. Both estuaries have a multi-channel system and extensive intertidal flats and wetlands with ecologically valuable flora and fauna. These eco-morphological systems are influenced by similar societal developments and human activities. Examples of the latter are engineering works and dredging activities for improving and maintaining the navigation channels, and shoreline management activities including land reclamations and setbacks. The fundamental eco-morphological phenomena that remain to be analysed and understood are the same for the two estuaries and will be discussed in this paper.

An integrated East China Sea???Changjiang Estuary model system with aim at resolving multi-scale regional???shelf???estuarine dynamics

A high-resolution numerical model system is essential to resolve multi-scale coastal ocean dynamics. So a multi-scale unstructured grid-based finite-volume coastal ocean model (FVCOM) system has been established for the East China Sea and Changjiang Estuary (ECS–CE) with the aim at resolving coastal ocean dynamics and understanding different physical processes. The modeling system consists of a three-domain-nested weather research and forecasting model, FVCOM model with the inclusion of FVCOM surface wave model in order to understand the wave–current interactions. The ECS–CE system contains three different scale models: a shelf-scale model for the East China Sea, an estuarine-scale model for the Changjiang Estuary and adjacent region, and a fine-scale model for the deep waterway regions. These three FVCOM-based models guarantee the conservation of mass and momentum transferring from outer domain to inner domain using the one-way common-grid nesting procedure. The model system has been validated using data from various observation data, including surface wind, tides, currents, salinity, and wave to accurately reveal the multi-scale dynamics of the East China Sea and Changjiang Estuary. This modeling system has been demonstrated via application to the seasonal variations of Changjiang diluted water and the bottom saltwater intrusion in the North Passage, and it shows strong potential for estuarine and coastal ocean dynamics and operational forecasting.

Influence of the Deep Waterway Project on the Changjiang Estuary

Using the improved 3-D ECOM model with a high resolution grid and incorporating various dynamic factors, the impact of the deep waterway project on the saltwater intrusion in the Changjiang Estuary was analysed. Salinity in the South Branch is shown to be influenced by saltwater intrusion downstream and from spillage of brackish water from the North Branch upstream. In the North Channel, the saltwater intrusion had been alleviated distinctly after the deep waterway project, because the dykes of the project blocked off the southward drift of the brackish water plumes under forcing by the northerly monsoon and the Coriolis force. The saltwater intrusion in the project area was intensified at the upper section and alleviated at the lower section. In the South passage, the saltwater intrusion was intensified as the background salinity increased and the river discharge decreased. The deep waterway project had an obvious impact on saltwater intrusion in the Changjiang Estuary.

Estimation of critical shear stress for erosion in the Changjiang Estuary: A synergy research of observation, GOCI sensing and modeling

Simulating the sediment transport in a high-turbidity region with spatially varying bed properties is challenging. A comprehensive strategy that integrates multiple methods is applied here to retrieve the critical shear stress for erosion, which plays a major role in suspended sediment dynamics in the Changjiang Estuary (CE). Time-series of sea surface suspended sediment concentration (SSC) were retrieved from the Geostationary Ocean Color Imager (GOCI) satellite data at hourly intervals (for 8 h each day) and combined with hydrodynamic modeling of high-resolution CE Finite-Volume Community Ocean Model (CE-FVCOM) to estimate the near-bed critical shear stress in the clay-dominated bed region (plasticity index > 7%). An experimental algorithm to determine the in situ critical shear stress via the plasticity index method was also used to verify the GOCI-derived critical shear stress. Implemented with this new critical shear stress, the sediment transport model significantly improved the simulation of the distribution and spatial variability of the SSC during the spring and neap tidal cycles in the CE. The results suggest that a significant lateral water exchange between channels and shoals occurred during the spring flood tide, which led to a broader high-SSC area in the CE throughout the water column.

Morphological Impact of the Construction of an Offshore Yangshan Deepwater Harbor in the Port of Shanghai, China

Abstract Yangshan Deepwater Harbor, located in the Qiqu Archipelago adjacent to Hangzhou Bay, is the new deepwater harbor of the Port of Shanghai. Its construction, which began in 2002, entails three types of engineering projects: closing a series of inlets, land reclamation, and dredging. After the construction of harbors 1, 2, and 3, these engineering projects caused a series of morphological changes in the area. Because of the serious sedimentation in the harbor area, further construction of the planned harbors has been stopped for the time being. Research on the sedimentation and its causes is urgently needed in order to decide whether to continue construction of more harbors. In this paper we analyze the morphological changes in the harbor area using bathymetric data collected from 1998 to 2010. Since 2004 bathymetry in the area has been surveyed every year, making it possible to analyze the spatial and temporal variation of sedimentation–erosion in detail. The analyses provide a good insight into how the morphological changes are related to the various projects of the harbor construction and how the changes develop over time. It is shown that in the year immediately after an inlet was closed accretion occurred on both sides of the closure. The sedimentation rates decrease significantly in the following years. Accretion accelerated from 2007 to 2009 but decreased thereafter. The results of the analyses indicate that routine dredging will be necessary to maintain the requisite 15-m depth requirement for berths, but a regular dredging routine is feasible. For a better understanding of the mechanisms responsible for the observed morphological changes a numerical model based on Delft3D is used to simulate the hydrodynamic and sediment transport processes. The model results provide insights into how the morphological changes are related to the changes in hydrodynamics induced by the closures of the inlets.

Impact of Sea Level Rise on Storm Surges around the Changjiang Estuary

ABSTRACT Zhao, C.; Ge, J., and Ding, P., 2014. Impact of sea level rise on storm surges around the Changjiang Estuary. The potential impacts of sea level rise (SLR) on storm surge around the Changjiang Estuary and the Hangzhou Bay are investigated using a shallow-water circulation model ADCIRC coupled with a spectral wave model SWAN. The validated model is applied to two typical typhoons under three scenarios: 1.0 m SLR, 0.483 m SLR and present sea-level condition. In consideration of interactions of tide, waves and surge, the impact exerted by SLR on tide and waves are also discussed. The migration of the amphidromes generated by SLR causes the co-phase lines to defect relatively counterclockwise near the Changjiang Estuary. The amplitude of tide increases slightly at the inner mouth of the Changjiang River, and more notable increases are presented at the northern part of the Changjiang River Mouth. The amplitude of tide decreases in adjacent areas of the Hangzhou Bay. The wave heights respond to the sea level rise in a non-linear and spatially non-uniform manner. Comparing the maximum wave height between each scenario, the wave height increase is found to be significant in shallow areas due to the increase in water depth. The breaking locations of the wave shift shoreward. The general patterns in wave height change are approximately determined by the typhoon path and topography. As for changes in surge and elevation, time series of water surface curves and peak value distribution are all analyzed. The curves of surges and water elevations accelerate slightly relative to that of the control scenario. The surges are not very sensitive to the SLR and the variations in elevation could mainly attribute changes in tide, with the changing extents ranging from a few to a dozen centimeters. Taking the value of SLR into account, the peak of elevation near shore could experience a significant increase in the future. All the properties respond to sea level rise in a non-linear and spatially non-uniform manner.

Seasonal changes in coastal dynamics and morphological behavior of the central and southern Changjiang River delta

Seasonal changes in sea level, tidal range, wind, riverine discharges, nearshore SSC (suspended sediment concentration) and bed-level of intertidal flat at 4 different sites were shown. In addition, the statistical relationships between the dynamics and the behavior of the sediment surface were examined. The average intertidal elevation seems negatively correlated to sea level while positively correlated to nearshore SSC. The effect of wind on seasonal cycle of average intertidal elevation is not evident although wind is an important factor governing short-term erosion/accretion events. The influence of riverine discharges on seasonal cycle of deltaic intertidal flats is masked by other factors. It is concluded that seasonality on mudflats is more complicated than on beaches.