Water And Sediment Condition Changes Cause The Middle Reaches Of Yangtze River Channel Adjustment Responses

The Three Gorges reservoir is the largest reservoir in the world, which has attracted wide concern at home and abroad since construction. Affected by the extreme weather and human activities recent years, great changes have occurred in the water and sediment conditions in the reservoir station of this reservoir. The application of the Three Gorges Reservoir brings great benefits to economics and flood control, but the mode that dam operates intercepts large amounts of discharged sediment and water, which causes river bed erosion, water level decline and other adjustment response in the downstream.This paper uses Mann-Kendall method, wavelet analysis method and multivariate nonlinear regression method. The Three Gorges storehouse main control station and its downstream from Yichang to Chenglingji are selected as objects to study the influence of the reservoir’s discharged water and sediment on the river cross-section and water level of its downstream. This paper mainly draws the following conclusions:(1) Trend and multi-time scale variation features of water and sediment characteristics exist in the Three Gorges Reservoir and its downstream. Operation of the Three Gorges reservoir has little influence on downstream runoff series. This is shown in the following aspects:outbound trend of runoff change is not obvious; first and second main periods of the annual flow changing in upstream and downstream are close. This indicates that downstream runoff is mainly influenced by the variation of runoff in the upstream and the construction of reservoirs in the upstream doesn’t quite influence the annual runoff in the downstream. The reservoir has a greater impact on the sequence of the sediment load and sediment concentration in the downstream. This is shown in the following aspects. The inflow and outlet sediment are significantly reduced, resulting in a significantly reduce in the amount of sand in the downstream. The first main periods of annual sediment load and annual sediment concentration variations are not consistent in the upstream and downstream; the lower the downstream, the larger the first primary periods are. But the second main periods are close. So the construction of the reservoir extends the first main periods of annual sediment load and annual sediment concentration variation.(2) After the filling of the Three Gorges reservoir, the river sections from Yichang to Chenglingji make adjustment in response to the changes in the incoming water and sediment conditions. The flat beach area of the section tends to increase and the stability of the section shows different characteristics. The section of Yichang reach remains stable while other sections broaden laterally or deepen vertically. Although there are some increases in the transverse, longitudinal, and overall stability coefficients of the sections, this still cannot meet the requirements of the stability of the section. The sections are still in a transverse, longitudinal, and overall adjustment process.(3) The stability of the river section in the downstream of the Three Gorges reservoir is influenced by the water and sediment of the Three Gorges Reservoir and the water and sediment accumulation in the early stages. A formula is established on how the overall stability coefficients of the sections of Zhicheng station, Shashi station, Jianli station are influenced by the water and sediment of the Three Gorges Reservoir and water diversion ratio and sediment diversion ratio of the three estuaries of Jin River. The stability of the sections in the lower reaches can be increased by changing the water and sediment conditions of the Three Gorges Reservoir and water diversion ratio and sediment diversion ratio of the three estuaries of Jin river.(4) The application of the Three Gorges Reservoir results in the water level adjustment in the downstream. The change of water level of average annual flow in downstream stations shows a trend and multi-time scales features. Multivariate nonlinear regression methods are applied and obtain a formula for the impact of the Gorges Project and changes of river-lake relationship on the water level of Zhicheng station, Shashi station and Jianli station.