Spatial-Temporal Dynamic Characteristics Of Saltwater Intrusion In The Changjiang Estuary

Measured water speed/direction and salinity data, especially the long-term salinity data at the 4 gauging stations (Yonglong, Chongxi, Nanmen, Baozhen) is adopted to give a comprehensive study on the temporal and spatial variations of the saltwater intrusion in the Changjiang Estuary. The river discharge at the Datong station and wind speed/direction at the weather station in eastern Chongming Island are also employed to analyze the observed salinity patterns. The longitudinal salinity variations in the South Branch-North Channel, the middle and lower reaches of the North Branch, and the South Passage; the lateral salinity variations in the river mouth as well as in the middle reaches of the South and North Branches; the vertical salinity variations in the South Branch, the South and North Channels as well as the South Passage are studied, respectively. The saltwater intrusion variations in different time scale such as semidiurnal, semimonthly, seasonal and annual are also investigated. Although some of the conclusions derived from those measured data are consistent with the previous ones, some new senses are obtained owing to the long-term salinity data. The abnormal salinity increasing in Chongxi is consistent with the strong northerly wind, which gives a proof that the wind stress tends to enhance the saltwater spilling over from the North Branch to the South Branch (SSO). The maximum/minimum salinity at the Yonglong is lag behind the maximum/minimum tidal range, and the mean salinity there is higher after spring tides than that after neap tides. The peak salinity at the Chongxi and Yonglong is almost present at the same day. The peak salinity at the Nanmen is 2 or 3 days lag behind that at the Chongxi. In the mouth of the North Branch, the surface salinity is decreased during the later period of flood and the salinity stratification is detected just at the flood slack during both spring and neap tides, which indicates the fresher water is derived from the downstream. This phenomenon may be the evidence of the extension of the diluted water from the North Channel to the mouth of North Branch.Based on the measured data in the winter season of 2011, the origin of the salt water in the North Channel as well as its impacts to the Qingcaosha Reservoir (QCSR) is analyzed. Under the similar river discharge, during the spring tide, the salt water in the North Channel is derived from the outer sea after the QCSR is built, while it is from the SSO before this project is conducted. The tidal inlet in the eastern Chongming Sandbank (TIECS) is connected to the North Channel. During the spring tide, the salinity at their bifurcation is higher than that in the lower reaches of the North Channel, but this abnormal phenomenon is vanished during the neap tide. This tidal inlet probable contributes to the enhancement of saltwater intrusion in the North Channel.In order to give a further study on the physical dynamics behind the mentioned abnormal phenomenon, the 3D saltwater intrusion numerical model with high resolution grids is employed. The processes derived from runoff, tide, wind stress, baroclinic gradient and mixing are included in this model, and the wet-dry method is also adopted to simulate the moving intertidal zone. The 3rd HSIMT-TVD scheme is employed to solve the advection term in transport equation, which results in mass conservation and high accuracy during the simulation. Driven by the in situ runoff, tide and wind stress, the modeled elevation, velocity and salinity in the main channels of the Changjiang Estuary were coincided with the measured data. In addition, the long term variations of the salinity were also simulated, in which the peak salinity as well as the phase of the SSO were well captured. These results indicate the model can illustrate the process of saltwater intrusion in the Changjiang Estuary.The observation at Chongxi indicated the SSO increased abnormally from November 10 to 12 in 2009 (during neap tide) and from February 11 to 12 in 2010 (during moderate tide), respectively, when the strong northerly wind was dominant, which indicates the wind stress plays an important role in effecting the SSO. The abnormal salinity risings were well captured by the model, and if the wind speed is reduced, the salinity there will be significantly decreased, which confirmed the fact that strong northerly wind is responsible for the above abnormal salinity increase. Driven by the monthly mean river discharge and wind stress, the model simulated the temporal and spatial variations of saltwater intrusion in the Changjiang Estuary. The wind-driven circulation, as well as the net water/salt fluxes and their decomposed components from the North Branch into the South Branch, is calculated and analyzed in the cases of different wind speeds and directions. The results indicate that the intensity of the saltwater intrusion in the Changjiang Estuary is significantly influenced by the wind speeds and directions.Based on the fact that the intensity of saltwater intrusion was enhanced in the North Channel during the spring tide, the impacts of topography evolution, including the construction of QCSR and the deepened TIECS, on the water sources in the North Channel and South Branch is studied, respectively. Under the averaged runoff and wind stress, the salinity is simulated before and after the QCSR project, and their difference in the water intakes of 3 reservoirs (Qingcaosha, Chenhang, Dongfengxisha) is compared. In addition, the water flux and water diversion ratio in the South and North Channels as well as in the North Branch are also analyzed. The modeled results demonstrate the saltwater intrusion is weakened during moderate tides to spring tides in the North Channel, while the SSO is slightly enhanced which gives little impacts on the water sources in the South Branch. The salinity distribution around the North Channel is explored at the flood slack during the spring and neap tides under the mean runoff and wind stress. These results show the TIECS contributes to the enhancement of the saltwater intrusion in the North Channel during the spring tide and this impact is almost vanished during the neap tide. If the TIECS is deepened, its impacts on the saltwater intrusion in the North Channel will be intensified. Furthermore, an abnormal fact, the saltwater intrusion in the TIECS is severer during the neap tide than that during the spring tide, is detected. The longitudinal residual elevation and salinity distribution in the TIECS show that the northerly wind and baroclinic gradient force are the main physical dynamics behind that abnormal phenomenon.Brackish water (chlorinity slightly > 250 ppm) in-taking of the Chenhang Reservoir (CHR) from Changjiang Estuary is an emergency method to ensure the freshwater supply in Shanghai when severe SSO is detected. Mixed with freshwater previously stored inside the reservoir, the chlorinity of output water can below the criterion. The temporal/spatial variations of chlorinity in the CHR and the way of the water intake operation are analyzed in detail when the brackish water in-taking were conducted. The 3D numerical model is adopted to simulate the distributions of current and chlorinity in the CHR when the brackish water entered it, and their responses to the different wind directions. The results show the circulations in the CHR are dominated by the inflow/outflow, wind-driven current and the density-driven current. The inflow/outflow currents are along the shore. Near the inlet, the baroclinic gradient force resulted in compensation flows at the upper layers due to the mass conservation. In the other area of the reservoir, the wind-driven currents are dominant at the upper layers and the compensate flows are formed at the lower layers. The wind stress tends to intensify the mixing in the water column. A patch of water mass with the chlorinity less than that of outflow water is formed in the northeast corner under the windless, southerly and westerly wind condition. If the artificial mixing is carried out, the chlorinity in the outlet will be decreased, and the quantity of the raw water supply will be increased.