Overview of 1. River
The main stream of the middle and lower reaches of the Yangtze River starts from Yichang and ends at the mouth of the river, with a total length of 1893km, flowing through 6 provinces and municipalities including Hubei, Hunan, Jiangxi, Anhui, Jiangsu, and Shanghai. The area along the Yangtze River is economically developed, with a high level of urbanization and rich water resources. It is the essence of the Yangtze River Basin and an important economic zone in my country. After the Yangtze River comes out of the Three Gorges, it enters the middle and lower reaches of the plain area. The main stream of the middle and lower reaches of the Yangtze River is controlled by the regional geomorphology and geological structure, and belongs to the alluvial plain river. Yichang-Hukou is an alluvial plain in the middle reaches of the Yangtze River, located in a neotectonic subsidence zone, with low terrain, small ground slope, gentle water flow, and connected with many lakes. The landform on both banks below the lake mouth is flushed along the river and lakeside plains, with large and small lake waters.
The bank slopes of the middle and lower reaches of the Yangtze River can be divided into bedrock (gravel) bank slopes, sandy bank slopes and soil bank slopes according to their material composition, with sandy bank slopes and soil bank slopes being the main ones. The general trend of riverbed composition along the course is from coarse to fine. The section from Yichang to Zhicheng is mainly composed of fine sand, medium sand and sandy pebbles. The upper Jingjiang river bed is medium sand and pebbles, and the lower Jingjiang river bed is fine sand. The riverbed below Chenglingji is basically composed of fine sand. The bed sand in the Yangtze River estuary is very fine sand and silt. The plane shape of the river is composed of micro-bend single type, meandering type and branching type, and the branching type river is the main type.
The Yangtze River is a river with small sediment concentration but large sediment transport. The sediment in the middle and lower reaches of the Yangtze River mainly comes from the inflow of the main and tributaries above Yichang and the tributaries and lakes on both sides of the middle and lower reaches of the Yangtze River. * When the sediment-carrying capacity of the river is unsaturated, the flow will pick up sediment from the riverbed and transport it to the downstream. According to statistics, the average annual sediment discharge of Yichang station and Datong station is 0.501 billion t and 0.433 billion t respectively. Since 1990, due to the construction of reservoirs in the main and tributaries and the water and soil conservation projects in the upper reaches of the Yangtze River, the annual average sediment discharge at Yichang and Datong stations has decreased compared with the annual average sediment discharge.
At present, the annual deposition of suspended sediment larger than 0.1mm in the Yichang-Datong section of the middle and lower reaches of the Yangtze River is about 25.8 million t, and the average annual deposition of sandy load is 2.7 million t. Suspended sediment greater than 0.1mm passes through the large cross section about 30 million t per year. After the initial impoundment of the Three Gorges Project in June 2003, the change of sediment greater than 0.1mm in the Yichang-Datong River section was greatly affected by the age of the reservoir and the composition of the river bed. At the initial stage of reservoir operation (11~20 years), only the section from Wuhan to Datong is slightly silted up, with an average annual rate of about 20 million t. With the trend of scouring and silting in the river reach, the amount of sediment larger than 0.1mm entering the river reach below Datong every year is close to that before the construction of the reservoir. These sediments provide abundant sediment resources for sand mining in the Yangtze River.