Study On Utilization Of Freshwater Resource On Nanhui East Shoal In Changjiang Estuary

Freshwater resource is one of most important resources in 21 Century. Shortage of freshwater often restricts economic development. Shanghai is short of water owing to low water quality where there are 201.2 m~3 water per person. The Huangpu River, mainly freshwater-resource, has been polluted often. In order to solve the problem of freshwater shortage and provide high quality freshwater, and keep water supply safe, it only depends on utilization of freshwater in Changjiang Estuary.Changjiang Estuary locates the area where Changjiang River meets East China Sea. In this area, the high-salinity saltwater from the sea and the freshwater from the river mixes, and saltwater intrusion occurs frequently. Changjiang Estuary is a complex system, which has three-order branches and four outlets to the East China Sea. And there are three large islands: Congming Island, Cangxing Island and Hengsa Island, and many sand banks such as Baimao sand bank, Biandan sand bank and Zongyang sand bank. In the Changjiang Estuary, in every branch, the saltwater intrusion is different from others because of the different landform, diversion ratio, and tide current. The saltwater intrusion frequently has bad effect on the development of the cities near the estuary. For example, during the winter of 1978 to the spring of 1979, the runoff of Changjiang River decreased largely, and the saltwater intrusion from the sea damaged the industry, agriculture and the daily life at Shanghai city. So it is very important to study on the mixing of salt-fresh water and saltwater intrusion in Changjiang Estuary.This paper is written for the former two purposes after doing some research work on Nanhui East Shoal. It was discussed that the saltwater intrusion and the mixing type of saltwater and freshwater in South Passage, the rules of the freshwater on Nanhui East Shoal, the causes of the freshwater, and the influence of the Three Gorge Reservoir on the freshwater resource in Changjiang Estuary based on the in situ surveys on Nanhui East Shoal and South Passage and the first-hand data obtained.This paper is composed of 5 parts.The Part 1 lists the spatial and temporal rules of saltwater intrusion on Nanhui East Shoal. Nanhui East Shoal is near the estuary mouth, and the saltwater intruding is from the sea. In every tidal cycle the salinity increases and decreases twice, the same as the tide. The maximum salinity occurs at the flood slack, and the minimum salinity occurs at the ebb slack. The salinity is higher during spring tide and lower during neap tide. The runoff of Changjiang River in wet season is different from in dry season. Affected by runoff, the salinity in Changjiang Estuary is higher in dry season than that in wet season.The salinity on Nanhui East Shoal is affected by the saltwater intrusion from the sea, and it increases from upstream to downstream. Affected by landform and Criolis's force, the water in the deep trough floods earlier and ebbs later than that on the shoal. The salinity in the deep trough is higher than on the shoal all the time. On tNanhui East Shoal, the salinity in the surface layer is lower than in the bottom layer, or the salinity is equal from surface to bottom when the water is mixed very well. But sometimes, the salinity in the surface is higher than in the bottom layer when there occurs the density current.The Part 2 discussed the mixing type of the salt-freshwater on Nanhui East Shoal. The degree of the mixing is changing during the tidal cycle all the time. Stratiform coefficient increases and decreases twice, the same as velocity and direction of water flow. The degree of the mixing increases at the beginning of flood, and reaches the maximum at the flood slack. And it decreases at the beginning of ebb, and reaches the minimum at the ebb slack.The degree of mixing of salt-fresh water has spatial character. The degree of mixing in mouth sandbar area is higher than that in the inner of estuary. And it is lower in the deep trough than on the shoal because the depth of former is larger than later.In a word, the type of mixing...