The Approach Of Performance And Mechanism On Branching Channel In Estuary Of Yangtze River

The evolution of branching channels in the estuary of Yangtze river is the result which in a specific environment the reciprocal change happens in numerous nature factors in the coordinate of time and space. The mutual change about numerous nature factors are effect on the profile of channels and process of evolution about channel branching. Then, for adapting to the variety of nature factors, the branching channels will change their morphology. As a significant brace of nature factors, limitations to sustain the change of factors are complicated in each of branching channels. According to quantifying the magnititude of nature factors, we can attain a series of variation, which is called environment capacity reflecting the limitation to brace nature factors. Cognizing and approaching the function between branching channels and nature factors contribute to understand evolution process of branching channels, to master a series of problems on branching, to clarify the influence of branching on motion, morphology and sediment. And it is also beneficial for enriching theory on estuary, supporting the construction of channel, harbor and management. Based on these thoughts, the branching channels are divided into five sections containing upper segment and lower segment in north harbor, upper segment in south harbor, north channel and south channel. Select some appropriate indexes and calculate to analyze the characteristic of time and space on environment capacity for approaching the mechanism about evolution of branching channels:1. The trend of change on profile is similar between upper segment in north harbor and south harbor. Environment capacity in these channels is stable. There is a "W" section appearing in process of evolution, which is related to the swing of main stream and difference between ebb and flow path. The ratio of river width and depth has a different trend of change. Water surface slope in upper segment in north harbor decreases, and fluctuates in upper segment in south harbor. The average of river width ratio in south harbor is biggest, means a similarity between upper and lower section. Environment capacity in lower segment in north harbor is not stable. There is a break point in a period from 80s to 90s. Water surface slope and ratio of river width and depth are different before and after that point.2. In recent 50 years, north channel developed constantly and environment capacity has a great change. South channel growed slowly and change of environment capacity is not stable, undergoing four processes to form the Jiangya'nansha. There is a break point in a period from 80s to 90s. Water surface slope and ratio of river width and depth are different before and after that point. The ratio of river width in south channel is smallest meaning a great difference between upper and lower section.3. The evolution of branching channels in Yangtze river includes four periods: embryo, prototype, development and stability. Then the period of prototype, the extent of environment capacity is small, river length is less than 25km, ratio of river width is less than 0.4, channel volume of 0m depth ranges from 1.6-2km3, tidal prism ranges from 1-2km3. When the period of development, the extent of environment capacity is large, river length is greater than 36km, ratio of river width ranges from 0.4-0.5, channel volume of 0m depth is greater than 2km3, tidal prism is greater than 2km3. When the period of stability, the extent of environment capacity is stable, river length ranges from 25-36km, ratio of river width is greater than 0.5, channel volume of 0m depth is less than 1.6km3, tidal prism is less than 1-2km3.4. Human activities has a great effect on the nature factors of branching channels. Environment capacity has changed great since the Yangtze Estuary Channel Regulation Project. Channel volume of north channel after 90s decrease 25.63% than before that. And tidal prism after 90s decrease 44.22% than before that. Furthermore, construction of big dam and grain for green policy in Yangtze basin have an influence on water and sediment.