Jianzhong Ge

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.

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.

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.

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.

Morphodynamic processes of the Elbe River estuary, Germany: the Coriolis effect, tidal asymmetry and human dredging

The Digital Elevation Model (DEM) based on the historical sea-charts and on-site hydrological records were used to examine the morphological change of the Elbe River estuary. The results show that siltation predominated in the tidal flat in the northern estuary, with a net siltation rate of 1.8 cm·a−1 during 1927–2006. In contrast, a continuous erosion prevailed in the main river channel, south of the estuary, with a net erosion rate of 2.5 cm·a−1 in the same time. In addition, a seaward shift of the estuarine island has happened with the old island coalescing to the northern tidal flat and new one emerging through siltation process. The tidal asymmetry via ebbing flow (maximum at 140 cm·s−1, and average at 76 cm·s−1) prevailed in the tidal flat, meaning continuous aggradation northwestward, while flooding flow (maximum at 100 cm ·s−1, and average at 67 cm·s−1) dominated in the main river channel with deepening thaweg at south, showing a landward sedimentation via the tidal pumping processes. This dextral extension of the estuarine morphology is due to the Coriolis force, leading to the inconsistent directions of in-out flows, which enables to facilitate the estuarine siltation. Human dredging prevailing in the estuary has dramatically altered the nature of the silted river channel to erosional since the last century. This is characterized by a net erosion rate of 3.2 cm·a−1 derived from the DEMs mapping, but only partially accounting for the dredging amount of 1994–2006, when the total dredging volume was 67 × 106 m3, equal to 5.9 cm·a−1.

Contributions of different tidal interactions to fortnightly variation in tidal duration asymmetry

The general framework for identifying tidal duration asymmetry proposed by Song et al. (2011) is extended to express fortnightly variability in duration asymmetry. The extended metrics are verified and studied using observed sea-level data at 481 stations worldwide. The results reveal that fortnightly variability is universal and that duration asymmetry can be stronger during neap tide than during spring tide. The fortnightly variability in duration asymmetry is primarily induced by three types of tidal interactions: interactions within the principal tidal constituents, interactions between high-frequency and principal tidal constituents, and interactions between long-period and principal tidal constituents. Among these interactions, the first type is most important at most of the stations and is related to the form number F. The contributions of different interactions can be quantified using their frequencies, amplitudes and phases. Global patterns of the fortnightly variation are illustrated using TOPEX/Poseidon altimetry data. The findings show that remarkable fortnightly variation in the tidal duration asymmetry occurs in most open oceans and is significant around an amphidromic point. The metrics derived in this study can be used to examine any time-varying characteristics in tidal asymmetry (not limited to duration asymmetry) by selecting a suitable frequency threshold. This article is protected by copyright. All rights reserved.

Innovative Engineering Solutions and Best Practices to Mitigate Coastal Risk

Engineering solutions are widely used for the mitigation of flood and erosion risks and have new challenges because of the expected effects induced by climate change in particular sea level rise and increase of storminess. This chapter describes both active methods of mitigation based on the reduction of the incident wave energy, such as the use of wave energy converters, floating breakwaters and artificial reefs, and passive methods, consisting of increase in overtopping resistance of dikes, improvement of resilience of breakwaters against failures, and the use of beach nourishment as well as tailored dredging operations.Existing coastal management and defense approaches are not well suited to meet the challenges of climate change and related uncertanities. Professionals in this field need a more dynamic, systematic and multidisciplinary approach. Written by an international group of experts, Coastal Risk Management in a Changing Climate provides innovative, multidisciplinary best practices for mitigating the effects of climate change on coastal structures. Based on the Theseus program, the book includes eight study sites across Europe, with specific attention to the most vulnerable coastal environments such as deltas, estuaries and wetlands, where many large cities and industrial areas are located. * Integrated risk assessment tools for considering the effects of climate change and related uncertainties* Presents latest insights on coastal engineering defenses* Provides integrated guidelines for setting up optimal mitigation measures* Provides directly applicable tools for the design of mitigation measures* Highlights socio-economic perspectives in coastal mitigation

Chapter 3 – Innovative Engineering Solutions and Best Practices to Mitigate Coastal Risk

Engineering solutions are widely used for the mitigation of flood and erosion risks and have new challenges because of the expected effects induced by climate change in particular sea level rise and increase of storminess. This chapter describes both active methods of mitigation based on the reduction of the incident wave energy, such as the use of wave energy converters, floating breakwaters and artificial reefs, and passive methods, consisting of increase in overtopping resistance of dikes, improvement of resilience of breakwaters against failures, and the use of beach nourishment as well as tailored dredging operations.Existing coastal management and defense approaches are not well suited to meet the challenges of climate change and related uncertanities. Professionals in this field need a more dynamic, systematic and multidisciplinary approach. Written by an international group of experts, Coastal Risk Management in a Changing Climate provides innovative, multidisciplinary best practices for mitigating the effects of climate change on coastal structures. Based on the Theseus program, the book includes eight study sites across Europe, with specific attention to the most vulnerable coastal environments such as deltas, estuaries and wetlands, where many large cities and industrial areas are located. * Integrated risk assessment tools for considering the effects of climate change and related uncertainties* Presents latest insights on coastal engineering defenses* Provides integrated guidelines for setting up optimal mitigation measures* Provides directly applicable tools for the design of mitigation measures* Highlights socio-economic perspectives in coastal mitigation

Mapping sea surface velocities in the Changjiang coastal zone with advanced synthetic aperture radar

Range Doppler velocities derived from the Envisat advanced synthetic aperture radar (ASAR) wide swath images are analyzed and assessed against the numerically simulated surface current fields derived from the finite volume coastal ocean model (FVCOM) for the Changjiang Estuary. Comparisons with the FVCOM simulations show that the European Space Agency (ESA) Envisat ASAR based Doppler shift anomaly retrievals have the capability to capture quantitative information of the surface currents in the Changjiang Estuary. The uncertainty analysis of the ASAR range Doppler velocity estimates are discussed with regard to the azimuthal and range bias corrections, radar incidence angles, inaccuracy in the wind field corrections and the presence of rain cells. The corrected range Doppler velocities for the Changjiang Estuary area are highly valuable as they exhibit quantitative expressions related to the multiscale upper layer dynamics and surface current variability around the East China Sea, including the Changjiang Estuary.