Water-sediment Variation Of Three Gorges Reservior And The Yichang-Chenglingji Channel Responses

After the operation of the Three Gorges Reservoir, serious erosion occurred in the downstream river channel because of the release of clear water, which had some influence on channel navigation condition, the relation between the Yangtze River and Dongting Lake and so on. Changes of water-sediment series in and out the Three Gorges Reservoir and the Yichang-Chenglingji channel responses to water-sediment series released from the Three Gorges Reservoir are studied in this paper. The main findings and conclusions are summarized as follows:(1) The change features of water-sediment series in and out the Three Gorges Reservoir are analyzed and their trends, variations and periods are obtained. It shows that most of water-sediment series in and out the Three Gorges Reservoir have tendency change in recent 60 years and these trends will continue in the future. The Three Gorges Reservoir has greater adjustments to the sediment concentration, the sediment transport rate, the coefficient of incoming sediments and the annual distribution of water and sediment than to the runoff. The variation years of water-sediment series in and out the Three Gorges Reservoir are almost the same with the operation years of large reservoirs of the upper Yangtze river basin, the starting years of the main human activity and the typical hydrological years. The Three Gorges Reservoir has some influences on the periodic variation characteristics of mean value series and annual distribution series. The average variation amplitude of water and sediment index is brought out, which shows that the relationship of discharge is well but the relationship of the sediment, the water and sediment combination and the annual distribution of water and sediment are bad between the inflow station and the outflow station.(2) The change features of the Yichang-Chenglingji channel cross-section profile parameters are analyzed and their trends, variations and periods are obtained. It shows that the bankfull discharge, the bankfull area, the bankfull average depth and the bankfull maximal depth all show increasing trends while the bankfull width-depth ratio show decreasing at all the four stations. In future, All these trends will continue except the bankfull discharge and the bankfull area of Jianli station have no obvious charge trend. The variation years of the cross-section profile parameters of all the four stations are almost the same with the variation years of water-sediment series out the Three Gorges Reservoir. The first dominant period between the cross-section profile parameters and the water-sediment series out the Three Gorges Reservoir is a difference of five years, which shows that the cross-section profile parameters are related to the previous five years’water-sediment series out the Three Gorges Reservoir. The channel erosion parameter is brought out and The Yichang-Chenglingji channel began to erode from 1980s and the channel erosion became serious since the 21st century.(3) By using multiple regression method, formulas between the bankfull discharge, the bankfull cross-section profile parameters, the channel erosion parameter with water and sediment parameters released from the Three Gorges Reservoir and the flow diversion ratio of three outlets along Jingjiang River are founded at Yichang, Zhicheng, Shashi and Jianli station. The formula between all the channel erosion parameters at all the four stations with water and sediment parameters released from the Three Gorges Reservoir and the flow diversion ratio of three outlets along Jingjiang River is founded:By decreasing the discharge, improving the sediment concentration and increasing the water and sediment process coordination released to the downstream river channel as well as enlarging the flow diversion ratio of three outlets along Jingjiang River, the Yichang-Chenglingji channel erosion could be slowed.(4) Critical control condition of the Yichang-Chenglingji channel erosion is found: channel erosion begins channel erosion becomes seriousBy the joint operation of the upstream reservoirs and the Three Gorges Reservoir and the regulation of the flow diversion of three outlets along Jingjiang River, the serious erosion of the Yichang-Chenglingji channel would be slowed.