The Evolution Of Flow-sediment Series And Channel Responses In The Lower Yellow River After Operation Of Xiaolangdi Reservoir

The operation of reservoirs could change the flow-sediment conditions in the lower river,which have great impact on the response of channel.The Xiaolangdi Reservoir is the key project that is closest to the lower Yellow River,controlling the entrance of flow-sediment series into the lower channel.After ten years of operation,Xiaolangdi Reservoir have played an important role in decreasing channel deposits and shaping normal channel of the lower reaches.Based on the previous research,this thesis takes the Xiaolangdi Reservoir as example.The evolution the flow-sediment series and channel response after operation of the Xiaolangdi Reservoir is summarized,and the relationship between channel response and flow-sediment conditions is studied.Main achievements are as follows:After 15 years of operation of the Xiaolangdi Reservoir,the flow ratio in flood period is less than the non-flood period and the sediment amount entering the lower channel declined obviously.During the flow and sentiment regulation,the maximum flow process maintained for a period of time,and the period of middle flow increased.The variation trend of flow-sediment series showed an obvious decreasing trend in the last 65 years from 1950 to 2005.After the operation of the Xiaolangdi Reservior,the flow tendency become stable and the sediment obviously decreased.The multiple time scale analysis of flow-sediment series show that the principle cycle of flow and sediment in Huayuankou station is 22 and 9 years,respectively.The variation tendency and cycle of Huayuankou station in lower reach are similar with that of Tongguan station in middle reach,which means the operation of Xiaolangdi Reservoir associated with Sanmenxia Reservoir has little impact on the multiple time feature of flow-sediment series in lower reach.The erosion and deposition feature of in lower channel after operation of the Xiaolangdi Reservoir and regulation of flow-sediment are summaried.The variation of cross section and its related parameters as well as bankfull discharge are studied.The results show that the lower channel maintained stable erosion after operation of reservoir.During the 18 times of flow and sediment regulation,the erosion amount only contributes about 21.85%of the all erosion in lower channel,which is not the main part of channel erosion and the erosion effect after 2007 began to decease.After the operation of reservoir,the cross section in typical stations shows the cutting of riverbed,accompanied by local broadening.The bankfull discharge after operation of reservoir also increased,and the average value in 2007 arised to 4000 m3/s.The stepwise regression and BP neural network have been used to study the response relationship between flow-sediment conditions and bankfull parameters,such as bankfull area and bankfull discharge.The results show that the bankfull area and bankfull discharge has obvouis relationship with previous flow-sediment series,which lasts for 4-6 years.Considering the flow and incoming sediment coefficient,the bankfull discharge is fitted with these two factors based on 5 years of moving average values.The fitted expression could basically reflect the variation of bankfull discharge.Considering multiple factors,new prediction model of bankfull discharge are estabilished based on stepwise regression and BP neutral network.In terms of overall prediction accuracy,the BP neutral network model>stepwise regression>fitted formula,but the BP model is weak in prediction of extreme flow and more wide range of data are needed to train the model.