Response Of Sediment Transport To Cascade Reservoirs In The Lancang River

The Lancang-Mekong River is one of the world’s most significant rivers.The section of the river in China,called the Lancang River,is rich in hydropower resources.Cascade reservoirs have been built and are to be developed in the Lancang River.While effectively utilizing the hydropower resources,cascade reservoirs cause changes in river sediment fluxes and siltation in the reservoir area,which affect the sustainable use of hydropower,changes in the ecological environment and management of the international river.This is not only a worldwide problem,but also a hot spot of international concern.This work explored the response of sediment transport to the construction of cascade reservoirs on the mainstream of the Lancang River by means of inventions,prototype observations and theoretical analysis.The main results are as follows:(1)Based on the long-term hydrometric data from the Gajiu hydrological station and Yunjinghong hydrological station,the changes of the runoff process and sediment flux in the mainstream of the Lancang River were analyzed using various methods such as Mann-Kendall trend test,wavelet analysis,dimensionless double mass curve and abrupt change point analysis.The sediment fluxes at both stations significantly decreased with the construction of cascade reservoirs and the abrupt change points were determined.Compared with the pre-dam period from 1964 to 1993,the average annual sediment flux at the Gajiu and Yunjinghong station has dropped by about 95.5% and 98.8% respectively from 2009 to 2019.The intra-annual distribution of runoff at both stations tends to be uniform due to the influence of cascade reservoirs,and the total annual runoff decreases,which is mainly related to the decrease in precipitation under the influence of climate change.(2)The depth integrating method is widely used for sediment measurement at hydrological stations on the Lancang River.This method cannot measure the sediment concentration in the near-bed layer,resulting in an error in the measured sediment flux.Based on the formula of suspended sediment transport rate in the form of elementary function,a correction method of suspended sediment flux was proposed.Moreover,a stratified water sampling device based on the vacuum principle and a multi-point synchronous water sampling device were invented to help collect stratified water samples on the vertical line.Taking the Mengsheng hydrological station on the Lancang River as a pilot,a comparison test of the suspended sediment transport rate between the depth integrating method and the six-point method was carried out.The results show that the sediment transport rate of the depth integrating method is 7.1% ～ 23.1% smaller than that of the correction method considering the near-bed sediment transport rate,and the average is 15.0% smaller.The combination of the new sampling device and the correction method can significantly improve the accuracy of the suspended sediment flux.(3)Samples of bed materials along the Manwan reservoir and the Jinghong reservoir on the Lancang River were collected,and the grain size characteristics of the bed materials and its variation along the reservoir were analyzed based on laser diffraction particle size measurement and calculation of characteristic parameters.The relationship between representative particle Reynolds number of bed materials and flow parameters with boundary conditions was established.Furthermore,a stochastic model of the variation of the number of suspended sediment particles with distance in the process of reservoir sedimentation was constructed.The non-uniform sediment deposition probability and the reservoir characteristic velocity were introduced to determine the single particle transfer intensity coefficient in the model.Based on this stochastic model,the law and mechanism of sediment sorting and sedimentation in the reservoir were revealed,and the variation of the grain size characteristics of the collected bed materials was explained theoretically.(4)Based on the hydrometric data from the main hydrological stations on the the Lancang River,using the sediment balance method and Brune model,the trap efficiency of Manwan reservoir,Dachaoshan reservoir and Jinghong reservoir was calculated to be 69.0%,68.0% and66.0%,respectively.The sedimentation volume and storage capacity loss of these reservoirs in different periods were also calculated.Furthermore,a reservoir desiltation method based on the sediment funnel in the form of bivariate normal distribution and the sediment delivery pipe in the form of inverse hyperbolic tangent curve was invented.The sediment funnel and pipe are smooth everywhere,which has the advantages of low resistance and not easy to block.The sediment is automatically discharged under the action of water pressure with low energy consumption.This method can not only improve the efficiency of reservoir sediment flushing to alleviate the siltation of the reservoir,but also contribute to improving the ecological environment downstream.