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Gu, J.; Zhu, J., and Lyu, H., 2021. Observation and analysis of water and salt transports in the North Branch of the Changjiang Estuary. Journal of Coastal Research, 37(3), 518–527. Coconut Creek (Florida), ISSN 0749-0208. The North Branch (NB) is the first-order bifurcation of the Changjiang Estuary and is an extremely shallow water channel. The greatest characteristic of saltwater intrusion is the saltwater spillover from the NB into the South Branch (SB). A field observation was conducted in the NB from 21 to 27 February 2017 to understand the water current and salinity variation. The residual unit width water and salt fluxes, as well as their decomposed terms, were calculated to discuss the mechanisms of water and salt transport. The tidal and vertical variations in ebb and flood duration, current velocity, and salinity during the neap-middle-spring tide were presented and dynamically analyzed in detail. The direction of the Eulerian water and salt transports and tidal pumping salt transport varied by site, vertical layer, and tidal type, whereas the Stokes drift water and salt transports were landward. During neap tide, the total residual water and salt fluxes in the upper layer of the whole NB were seaward transported because of the runoff effect; the total water and salt fluxes in the lower layer were landward in the lower-middle and lower reaches of the NB, reflecting the existence of a two-layer estuarine circulation. During the moderate tide, the total residual water and salt fluxes were seaward in the whole NB except in the lower layer in the low reaches of the NB, which were landward due to the strong baroclinic effect. During the spring tide, all the total residual water and salt fluxes were landward because some flood water flowed into the SB, resulting in an imbalance between the flood and ebb water volumes in the NB. The dynamic processes of water and salt transport from the NB into the SB during spring tide were revealed by the observation and flux decomposition method. [ABSTRACT FROM AUTHOR]

Estuarine saltwater intrusions are mainly controlled by river discharge and tides. Unexpectedly, an extremely severe saltwater intrusion event occurred in February 2014 in the Changjiang estuary under normal river discharge conditions. This intrusion cut off the freshwater input for 23 d into the Qingcaosha reservoir, which is the largest estuarine reservoir in the world, creating a severe threat to water safety in Shanghai. No similar catastrophic saltwater intrusion has occurred since records of salinity in the estuary have been kept. During the event, a persistent and strong northerly wind existed, with a maximum speed of 17.6 m s -1 , lasting 9 d and coinciding with a distinct water level rise. Our study demonstrates that the extremely severe saltwater intrusion was caused by this northerly wind, which drove substantial landward net water transport to form a horizontal estuarine circulation that flowed into the northern channel and out of the southern channel. This landward net water transport overpowered the seaward-flowing river runoff and transported a large volume of highly saline water into the northern channel. The mechanisms of this severe saltwater intrusion event, including the northerly wind, residual water level rise, landward water transport and resulting horizontal circulation, etc., were systematically investigated. [ABSTRACT FROM AUTHOR]

Qiu, C. and Zhu, J., 2015. Assessing the influence of sea level rise on salt transport processes and estuarine circulation in the Changjiang River estuary. Global sea level rise has been greatly concerned by government and society with its effects on saltwater intrusion and material transport in estuaries, which threaten freshwater habitat and drinking water supplies. A semi-implicit estuarine, coastal, and ocean model (ECOM-si) was used to assess the influences of rising sea level on salt transport processes and estuarine circulation patterns in the Changjiang River estuary, China. The variations of intruding saltwater and residual salt transport were simulated according to the variations of several sea level rise scenarios, and results in typical year and dry year were demonstrated, respectively. The modeled results are in good agreement with the observed data, and statistics show good correlation coefficient and Skill Score values. Results from the numerical experiments show that the intensity of saltwater intrusion and stratifications both increase as sea level rises, while the increments are quite distinct between each channel. Furthermore, it shows obvious interannual changes following different river discharges. The residual transport of salt was used to analyze the changes in transport processes and estuarine circulation pattern. The computation results show that the Stokes drift transport is the major mechanism for up-estuary salt transport in each channel, whereas the seaward Eulerian transport is of the same order. The flux of saltwater spillover from the North Branch into the South Branch increases with stronger Stokes transport as sea level rises. The landward salt transport is strengthened in the North Channel and may further affect the upper reaches under strengthened tidal pumping and vertical shear transport with higher sea level. The overtopping flow affected by tidal pumping is the dominant mechanism for salt exchange between the North Passage and South Passage. This may increase the salt supply from the ocean into the South Channel. [ABSTRACT FROM AUTHOR]

Saltwater intrusion typically develops in the along-channel direction but exceptions can be found in bifurcated estuaries. Based on the observational data, we found that the saltwater intrusion in the upper reaches of the North Channel (NC) of Changjiang Estuary is dominated by the lateral saltwater intrusion from a small northern outlet (denoted as NONC) of this channel. This phenomenon has severe effects on the freshwater usage in this region. To investigate the underlying mechanisms of this pattern of intrusion, numerical experiments were conducted using a well-validated model. A flux decomposing method was used to decompose the process of saltwater intrusion into several mechanisms. During the neap tide, the saltwater begins to intrude landward into the NONC through shear transport induced by estuarine circulation. During the transition period between the neap tide and the following spring tide, the saltwater that previously reached the NONC further intrudes into the NC via Lagrangian and tidal pumping transports, causing a significant salinity increase in the middle and upper reaches of the NC. During the spring tide and the subsequent middle tide, saltwater intrusion in the NONC retreats. The impacts of the topography of the NONC and the wind stress on this lateral saltwater intrusion were also evaluated in this study. [ABSTRACT FROM AUTHOR]

The observation at the Chongxi gauging station indicated the salinity of saltwater spilling over from the North Branch to the South Branch increased abnormally from November 10 to 12 in 2009 (during neap tide) and from February 11 to 12 in 2010 (during moderate tide). We found for the first time that the strong northerly wind was responsible for the above abnormal salinity increase. Previous studies indicated that the saltwater intrusion in the Yangtze Estuary is influenced mainly by the river discharge, the tide, and the wind stress, but the impacts of variations of wind speed and direction on it have not been investigated. In this study the impacts of wind stress on the saltwater intrusion were numerically simulated and the associated mechanisms were analyzed. The model results were consistent with the observed data obtained at six gauging stations during February and March in 2007 and four gauging stations in March 2008, and the abnormal salinity risings were well captured. Meanwhile, if the wind speed is reduced by half, the salinity there will be significantly decreased. Driven by the monthly mean river discharge of 11000 m/s and northerly wind of 5 m/s from January to February, the model simulated the temporal and spatial variation of saltwater intrusion. The wind-driven circulation, as well as the net water and salt fluxes from the North Branch into the South Branch, was calculated and analyzed in the cases of different wind speeds and directions. The results indicated that the intensity of the saltwater intrusion in the Yangtze Estuary is significantly influenced by the wind speeds and directions. [ABSTRACT FROM AUTHOR]

An improved 3-D ECOM-si model was used to study the impact of seasonal tide variation on saltwater intrusion into the Changjiang River estuary, especially at the bifurcation of the North Branch (NB) and the South Branch (SB). The study assumes that the river discharge and wind are constant. The model successfully reproduced the saltwater intrusion. During spring tide, there is water and salt spillover (WSO and SSO) from the NB into the SB, and tidally averaged (net) water and salt fluxes are 985 m/s and 24.8 ton/s, respectively. During neap tide, the WSO disappears and its net water flux is 122 m/s. Meanwhile, the SSO continues, with net salt flux of 1.01 ton/s, much smaller than during spring tide. Because the tidal range during spring tide is smaller in June than in March, overall saltwater intrusion is weaker in June than in March during that tidal period. However, the WSO and SSO still exist in June. Net water and salt fluxes in that month are 622 m/s and 15.35 ton/s, respectively, decreasing by 363 m/s and 9.45 ton/s over those in March. Because tidal range during neap tide is greater in June than in March, saltwater intrusion in June is stronger than in March during that tidal period. The WSO and SSO appear in June, with net water and salt fluxes of 280 m/s and 8.55 ton/s, respectively, increasing by 402 m/s and 7.54 ton/s over those in March. Saltwater intrusion in the estuary is controlled by the river discharge, semi-diurnal flood-ebb tide, semi-monthly spring or neap tide, and seasonal tide variation. [ABSTRACT FROM AUTHOR]

In a multilevel bifurcated estuary, the channels between the bifurcated branches play important roles in the exchanges of water and salt. In the Changjiang Estuary, the Hengsha Channel (HC) connects the North Channel (NC) and the North Passage (NP). In this paper, based on a two-way nesting unstructured quadrilateral grid, finite-differencing, three-dimensional estuarine and coastal ocean model, the tidal and seasonal variations in the water and salt transports in the HC were simulated, and their dynamic mechanism was analyzed. The residual water and salt transports in the HC both flow southward from the NC to the NP. In wet season, the residual water transport in the HC is 677 m3/s during neap tide and 245 m3/s during spring tide, and the residual salt transport is 0. In dry season, the residual water and salt transports in the HC are 1278 m3/s and 0.38 t/s during neap tide, respectively, and 1328 m3/s and 12.61 t/s during spring tide. Affected by the northerly wind and the southeastward baroclinic gradient force, the water and salt fluxes in dry season are much larger than those in wet season. The dynamic mechanism responsible for the water transport in the HC was numerically simulated and analyzed. [ABSTRACT FROM AUTHOR]

Vertical salinity variance (SV) budgets are derived using the isohaline Total Exchange Flow (TEF) framework, with the advection of vertical SV quantified by the dispersion of salinity classes across sections. A numerical model of the Changjiang Estuary is used to demonstrate the application of this method. The analysis is focused on the North Channel (NC) segment where strong stratification is produced during neap tides. We find that the advection of vertical SV is stronger than that of horizontal SV during early neap due to the landward excursion of stratified water. However, strong northerly winds can significantly decrease the contribution of advection of vertical SV but increase the advection of horizontal SV into the NC due to the landward transport of relatively well-mixed saline water. This process increases the seaward advection of vertical SV in the southern channels. In addition, we highlight that the vertical SV dissipation (mixing) term balances the straining term and peaks at late neap in the NC, coinciding with the maximum volume-integrated vertical SV. • Decompose the salinity variance flux into horizontal and vertical salinity variance. • Vertical variance advection is stronger than horizontal advection in the NC. • Strong north wind can decrease the contribution of vertical variance advection in the NC. [ABSTRACT FROM AUTHOR]