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Gong W.,Sun Yat Sen University | Gong W.,Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering | Shen J.,Virginia Institute of Marine Science
Continental Shelf Research | Year: 2011

The increase of salt intrusion in recent years in the Modaomen Estuary, one of the estuaries of the Pearl River Delta in China, has threatened the freshwater supply in the surrounding regions, especially the cities of Zhongshan, Zhuhai in Guangdong Province and Macau. A numerical modeling system using nested grids was developed to investigate the salt transport mechanisms and the response of salt intrusion to changes in river discharge and tidal mixing. The steady shear transport induced by estuarine circulation reaches maximum and minimum, respectively, during neap and spring tides, while the tidal oscillatory transport shows an opposite pattern. The net transport is landward during neap tides and seaward during spring tides. The salt intrusion length responding to constant river discharges generally follows a power law of -0.49. The dependence of salt intrusion on tidal velocity is less than that predicted by theoretical models for exchange flow dominated estuaries. The response of salt intrusion to change in tidal velocity depends largely on river discharge. When river flow increases, the impact of tidal velocity increases and the phase lag of response time decreases. The asymmetries of salt intrusion responding to increasing and decreasing river discharge (tidal velocity) are observed in the estuary. © 2011 Elsevier Ltd. Source


Gong W.,Sun Yat Sen University | Gong W.,Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering | Maa J.P.-Y.,Virginia Institute of Marine Science | Hong B.,South China University of Technology | Shen J.,Virginia Institute of Marine Science
Ocean and Coastal Management | Year: 2014

Based on the salinity and current data from an intensive field survey carried out in the Modaomen Estuary, Pearl River Delta, China, this study reveals that the behaviors of the two major components of salt transport flux (i.e., the advective salt flux caused by mean advective flow and the tidal salt flux caused by tidal diffusive processes) are quite different at different stations along the estuary during a dry season. During the neap tide, the tidal salt flux and the advective flux are both important in driving a net landward salt transport at a downstream station, and the tidal salt flux is the primary component of landward transport at the station where the salt intrusion limit occurs, whereas the advective salt flux is more important at the station in the mesohaline region and at the estuary mouth. This situation changes during the following meso and spring tides, i.e., the advective salt flux becomes more important than the tidal salt flux. When an enhanced river flow and a strong down-estuary wind both occur at the spring tide, the surface seaward advective salt flux increases significantly, whereas the tidal salt flux is also enhanced, becoming the main contributor to the landward salt transport. The salt transport pattern is of great significance in the regional environmental management for salt intrusion prevention. An optimal freshwater releasing time from the upstream reservoir should be chosen at the spring tide. The implementation of an engineering project to prevent saltwater entering the Modaomen mainstem from the Hongwan Waterway (a branch of the estuary) should be based on a thorough understanding of the local salt transport process. © 2014 Elsevier Ltd. All rights reserved. Source


Gong W.,Sun Yat Sen University | Gong W.,Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering | Wang Y.,Nanjing University | Jia J.,State Oceanic Administration
Journal of Asian Earth Sciences | Year: 2012

In recent years, the increased pressure of saltwater intrusion in the Modaomen Estuary, one of the outlets in the Pearl River Delta (PRD), China, has threatened the freshwater supply for the surrounding regions. The estuary has complex geometry and bathymetry and branches into three waterways, Madaomen Waterway, Hongwan Waterway, and Hezhou Waterway, entering into the coastal sea. In this study, a three-dimensional baroclinic model (EFDC) was used to investigate salt transport among these branches and saltwater intrusion in the mainstem of the estuary. The salt transport at selected cross sections was decomposed into three components: advection, steady shear, and tidal oscillatory, following the method of Lerczak et al. (Lerczak, J.A., Geyer, W.R., Chant, R.J., 2006. Mechanisms driving the time-dependent salt flux in a partially stratified estuary. Journal of Physical Oceanography 36, 2296-2311). Results from this study indicate that the Hongwan Waterway serves as a salt source for the mainstem of the estuary, especially during spring tides, while the Hezhou Waterway mainly acts as a salt sink for the mainstem. The down-estuary wind increases the steady shear transport in the Modaomen Waterway and the advection transport in the Hongwan Waterway, which alters the saltwater intrusion in the estuary. Closure of the Hongwan and the Hezhou Waterways could result in a 20% decrease of saltwater intrusion in the estuary. These results provide scientific basis for water resource management in the region. © 2011 Elsevier Ltd. Source


Xiaoming S.,Sun Yat Sen University | Xiaoming S.,Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering
Acta Geologica Sinica | Year: 2011

In the present study, the modified Sverjensky-Molling equation, derived from a linear-free energy relationship, is used to predict the Gibbs free energies of formation of crystalline phases of a-MOOH (with a goethite structure) and a-M2O3 (with a hematite structure) from the known thermodynamic properties of the corresponding aqueous trivalent cations (M3+). The modified equation is expressed as \G JM FX=≤MKAAG iji^+bMyX+PMyxyiu 3*, where the coefficients aMyX, bMyX, and fiMyX characterize a particular structural family of MVX (M is a trivalent cation [M3+] and X represents the remainder of the composition of solid); j^is the ionic radius of trivalent cations (M 3*); AG /,MVX^ the standard Cibbs free energy of formation of MVX; and AG ^M3* is the non-solvation energy of trivalent cations (M34). By fitting the equation to the known experimental thermodynamic data, the coefficients for the goethite family (a-MOOH) are C MFA=0.8838, AMFA=-424.4431 (kcal/mol), and pMyx=\\5 (kcal/ mol.A), while the coefficients for the hematite family (a-M2O 3) are aMvx=U468, bMyx=S14.9573 (kcal/ mol), and /?MFX=278 (kcal/mol.A). The constrained relationship can be used to predict the standard Gibbs free energies of formation of crystalline phases and fictive phases (i.e. phases that are thermodynamically unstable and do not occur at standard conditions) within the isostructural families of goethite (a-MOOH) and hematite (a-M2O3) if the standard Gibbs free energies of formation of the trivalent cations are known. Copyright © 1999-2011 John Wiley & Sons, Inc. All Rights Reserved. Source


Gong W.,Sun Yat Sen University | Gong W.,Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering | Jia L.,Sun Yat Sen University | Jia L.,Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering | And 2 more authors.
Continental Shelf Research | Year: 2014

Huangmaohai Estuary is a micro-tidal funnel-shaped estuary, located along the southwestern side of the Pearl River Delta complex. Variations of sediment transport patterns under different conditions of river discharge and tidal mixing are investigated by using field measurements and data analysis during both dry and wet seasons, respectively. The intratidal variation of sediment dynamics is largely controlled by the tidal asymmetry. The typical pattern of 25-hour mean sediment transport during the dry season is that the transport is landward in the channel and seaward on the shoals. A bifurcation pathway of sediment transport shows that sediments are imported from the East Opening and exported through the Middle Opening. However, this pattern can be altered by mixing processes and river discharge. Enhanced mixing or increased discharge can result in a predominantly seaward transport. Conversely, weak mixing can result in an emphatic landward transport. In general, the sediment transport is closely associated with the morphological evolution in the estuary. © 2014 Elsevier Ltd. Source

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