Study On The Freshwater Resource Near The Easten Hengsha Shoal And The Prediction Model Of Saltwater Intrusion In Changjiang River Estuary

The saltwater intrusion has always been an important issue of study in the Changjiang (Yangtze) River Estuary. In recent decades, many scholars have made remarkable achievements. With the development of the research techniques and the diversification of the research method, the study of saltwater intrusion in the Changjiang River Estuary turns to be progressively refined. Based on the observed data and the numerical model, the freshwater resource on the north of the reclamation Project of Eastern Hengsha Shoal has been discussed. A set of rapid prediction of saltwater intrusion forecasting system for the three important reservoirs (Qingcaosha, Chenhang, Dongfengxisha) is established and applied.Water depth is the basis data for the numerical model, which is crucial for the accuracy of the model. However, most of the depth data is given on the base of theoretical lowest tidal level. In order to realize the transformation of different datum, the following works have been done. The improved3-D estuarine coastal and ocean numerical model ECOM si was used to simulate the water elevation in the Changjiang River Estuary and its adjacent sea. The harmonic constants of the main tidal constituents are calculated by the software T_Tide. According to the algorithm in the "Specification for hydrographic survey"(GB12317-1998), and considering the8main tidal constituents M2,S2,N2,K2,Ki,O1,Pi,Q1and3shallow water tidal constituents M4,M6,MS4, the distributions of the theoretically lowest and highest tidal levels were presented. Using the numerical model and designing high grid resolution, the shortages of spatial asymmetry of limited tidal stations are overcome effectively, and the calculation accuracy of the theoretically lowest and highest tidal levels are improved. The data of water depth based on the theoretically lowest tidal level observed in2009are converted to those based on85national height data, enabling them to be used more effectively.With the development of Shanghai, the contradiction of shortage of land resources becomes more prominent. Eastern Hengsha Shoal is one of the main extensive inter-tidal flat. The Eastern Hengsha Shoal reclamation Project started in 2003, which is now in progress for the possible use of agriculture. This region locates at the river mouth where the saltwater intrusion occurs very frequently, therefore, the freshwater source is a key issue for the demands of watering and life. Several observations along the northern side of the Eatern Hengsha shoal indicated that, the huge Changjiang river runoff could overcome the saltwater intrusion, and the freshwater does exist around this region. Understanding the strong interaction between the river and ocean in this region could help to better predict the occurrence of freshwater zone around the Eastern Hengsha shoal. Hence in this study, a modified3-D numerical ECOM-si was used to simulate the freshwater zone and further analyze its formation mechanism under the situation of project completed in dry season. It is found that even in dry season, the freshwater zone still exists. The river discharged freshwater tends to concentrate on the northern side of the Eastern Hengsha shoal as influenced by the Coriolis forcing, which is in favor of forming a freshwater zone. Since the baroclinic forcing declines with the increase of tidal mixing, the peaks of salinity have phase differences with spring and neap tides. It is most possible to get freshwater during middle tide period after spring tide. Compared the residual field, salinity field before and after the project, we can get the conclusions that the water diversion ratio of the North Channel increases while the ratio of the South Channel decreases; the stagnation point in the North Passage moves downstream about9km; the salinity turns to increase on the north of the North Channel and decrease on the south of the North Channel.The Qingcaosha reservoir, Chenhang reservoir and Dongfengxisha reservoir supply water for about25million people, are the most extremely important water sources in Shanghai. Nevertheless, since located in the estuary, these three reservoirs are always affected by the saltwater intrusion in dry season. According to statistics, Qingcaosha reservoir was affected11times saltwater intrusion in2011. The three dimensional variation data assimilation was used in the prediction system of Changjiang River saltwater intrusion. This system can provide rapid salinity forecast in the estuary. Verified by observational data, the prediction accuracy increase significantly, and the prediction system has applied from November2013to February2014. The accuracy of predictions is confirmed to be good and the error analysis is also be done. Then the possible reasons which caused the error were analyzed. According to the specific case of the Yangtze River estuary salinity monitoring network, the setting of observations error covariance matrix was optimized.