The Impact Of The Shelf Circulations On The Saltwater Intrusion In The Changjiang Estuary

In this thesis, making use of the three dimensional ECOM model and field data, shelf circulations in the East China Sea and the saltwater intrusion in the Changjiang Estuary are simulated. Based on the results, the impact of the shelf circulations in the East China Sea on the saltwater intrusion in the Changjiang Estuary is studied by three numerical experiments. The main fruits are as following.1. A three dimensional, high resolution, hydrodynamic and salinity numerical model in the Changjiang Estuary and its nearby areas is built up by using of the three dimensional ECOM model. Several field-measured elevation, current and salinity data are used to validate this model, and the result is reasonable.2. Based on the model which takes into account various dynamical factors, shelf circulations in Yellow Sea and East China Sea in winter are simulated exactly. The results show that, the Taiwan Warm Current flows northeastwards along the Min-Zhe Coast. Meanwhile, its direction turns to east twice obviously. It can arrive at the Changjiang Estuary. While the Subei Coastal Current flows southeastwards and the strength of its surface currents is bigger than the bottom currents. For the back pushing of the Changjiang River diluted water, current velocity is reduced gradually in the south of 32°N.3. The results of control numerical experiments show the saltwater intrusion in the North Branch is most serious, South Passage is the second. For Salt spilling over from the North Branch, salinity of the South Branch and its reaches presents a "high-low-high" saddleshaped pattern during spring tide. The salinity front induced by saltwater intrusion inclines northwestwards round the river mouth by the Coriolis force. In the North Branch and the South Branch, the saltwater intrusion at the north side is stronger than the south. The saltwater intrusion at the lower layer is stronger than the upper layer by the baroclinic pressure gradient force.4. The Taiwan Warm Current eliminated by omitting the residual water level and the baroclinic pressure gradient force at the river valley off the Changjiang Estruary and its north area, which weaken the ligulate front off the Changjinag Estruary. The Subei Coast Current is intensified and oppresses the Changjiang diluted water turn to south. Salinity front off the North Branch move to south, which increases the salinity in the North Channel and makes great high salinity water enter the North Branch causing the increment of the saltwater spilling over from the North Branch. Salinity in the South Passage is decreased for its diluted water is intensified.5. In no wind case, the Subei Coastal Current vanishes. The diluted water from the North Branch is intensified leading to the decrement in North Branch and the Subei Coast. The salinity front off the North Branch is weakened obviously, which decreases the saltwater spilling over from the North Branch. Because its resistance is weakened, the range of the Taiwan Warm Current becomes broad and it can deepened to flow northwards. The direction of the expansion of the Changjiang River diluted water turn to north, which cause the increment of the salinity in the South Passage, the North Passage and the Min-Zhe Coast. The salinity in the east of 122.5°E is decreased a little by the diluted water from the North Branch and the North Channel.6. Shelf circulations can be eliminated by omitting the wind stress, the residual water level and the baroclinic pressure gradient force off the Changjiang Estuary. The numerical experiment shows that, river flow from the North Branch and the North Channel is increasing, which makes the salinity front near the North Branch move to east. The salinity in the Subei Coast and the North Branch is obviously decreasing. Rive flow from the South Channel is decreasing, causing the increment of the salinity in the South Channel and the Min-Zhe Coast. Because the Taiwan Warm Current is eliminated and the southeastward expansion of diluted water from the North Channel is increased, the salinity near 122.5°E off the Changjiang Estuary is decreased.The numerical experiments show that, the shelf circulations off the Changjiang Estuary play an important role in saltwater intrusion and the salinity distributions off the estuary, so it is a noticeable dynamic factor.