| Due to the impoundment of the Three Gorges Reservoir(TGR),velocity and sediment carrying capacity decreased,leading to a large amount of sediment deposition volume,phosphorus retention and enriched phosphorus nutrients in the reservoir area.Therefore,eutrophication of water body and water bloom occurred frequently in some tributaries.The change of hydrodynamic condition is the key factor causing the eutrophication of water body and frequent occurrences of water blooms in tributaries.It is very urgent and important to reduce sediment deposition and total phosphorus(TP)retention by improving the hydrodynamic condition in reservoir area.However,the study on controlling sediment flux,TP flux and TP retention rate in the tributaries and the whole reservoir area by optimizing pool level is not carried out.In addition,sediment deposition in the reservoir usually occurs after the change of pool level,and the water and sediment inflows.But,for the sediment issues of the TGR,little attention has been paid to the delay phenomenon in the process of riverbed evolution.Consequently,in order to investigating the influence mechanisms of pool level on sediment deposition,sediment and phosphorus transport processes in the reservoir area.A delayed response model of sedimentation was established considering sediment inflow and pool level,which is used to study the delayed laws of sedimentation in the TGR during the flood season.Based on measured water quality data during 2008–2017in the TGR,flux variation and retention effect of TP were analyzed in detail.A 3D hydrodynamic-water quality model in the Xiangxi Bay,a typical tributary of the TGR,was established by using the Environmental Fluid Dynamics Code(EFDC).Effects of cumulative amplitude and daily amplitude of pool level,starting time of tide-type operation and other factors on TP flux and TP retention rate in the Xiangxi Bay were investigated by the 3D hydrodynamic and water quality model.Taking account of sediment and TP inflows of tributaries,a 1D sediment and water quality model in the whole TGR area was proposed,aiming to conduct the research on tide-type operation for decreasing the sediment deposition and TP retention in the whole TGR area.Summary,major results were shown below.(1)Due to the effects of sediment inflow and pool level on the sedimentation,a formula for calculating the sediment-weighted average pool level is proposed.A delayed response model involves accumulated sedimentation and sediment-weighted average pool level is established.Based on the model,delayed laws of sedimentation in the TGRduring the flood seasons are comprehensively analyzed.The result indicates accumulated sedimentation in the TGR during the flood seasons are closely related to the 5 years'linearly superimposed pool level.Sedimentation in the flood season is affected not only by the current year's sediment inflow and pool level but also by the sediment inflows and pool levels of the previous 4 years.(2)The statistical model of TP flux and sediment flux in different periods was established,indicating that there is a good correlation between TP flux and sediment flux.Taking no account of TP inflows of tributaries,average annual TP flux and retention rate of the TGR were 82300 t and 49.65%during 2008–2012,respectively.During 2013–2017,the average annual TP flux and retention rate decreased significantly,to about 49100 t and 8.81%,respectively.TP flux reduced about 41.3%because of the opertioan of upstream cascade reservoirs,phosphorus pollution control and other factors.(3)A 3D hydrodynamic-water quality model in the Xiangxi Bay,a typical tributary of the TGR,was proposed.Based on the 3D model,phosphorus transport process of Xiangxi River Bay was simulated by adopting the tide-type operation mode.The study found that the effect of variation amplitude of pool level on phosphorus transport in Xiangxi Bay is more obvious than starting time of tide-type operation,and tide-type operation has significantly timeliness.Sometimes,the larger variation amplitude of pool level,the effect of tide-type operation is not better.Variation amplitude of pool level should be set according to the actual operation of the reservoir and the desired effect.(4)The influence of variation of pool level on TP flux in Xiangxi Bay decreases gradually from downstream to upstream.In order to reduce the TP retention in the bay as much as possible,designs of tide-type operation schemes should obey some basic principles.Starting time of tide-type operation can be appropriately delayed(advanced)when the variation amplitude of pool level is large(small).(5)Taking account of sedment and TP inflows of tributaries,a 1D sediment and water quality model in the whole TGR area is established,which includes the main stream(about 760km)from Zhutuo to dam site and 56 tributaries,such as Jialing River,Wujiang River,Xiaojiang River,Tangxi River,Meixi River,Daning River and Xiangxi River and so on.Results show that total sediment inflow of the TGR was 14239.11×10~4t during 2015–2017,and the Yangtze River,Jialing River,Wujiang River and tributaries were 8440.16×10~4t,1595.81×10~4t,543.16×10~4t and 3659.9×10~4t,respectively,accounting for 59.27%,11.21%,3.81%and 25.70%of the total sediment inflow.Total TP flux of the TGR was 12.546×10~4t during 2015–2017,and the Yangtze River,Jialing River,Wujiang River and tributaries were 8.275×10~4t,1.256×10~4t,2.183×10~4t and0.832×10~4t,respectively,accounting for 65.96%,10.01%,17.40% and 6.63% of the total TP flux.(6)Based on the 1D sediment and water quality model in the whole TGR area,sediment and phosphorus transport process in the TGR was simulated by adopting the tide-type operation mode.Results indicate that tide-type operation can reduce sediment deposition and TP rentention in the TGR area.During 2015–2017,when the tide-type operation was adopted,sediment outflow and sediment export ratio of the TGR increased about 17.13%and 1%?3%,resepctivly,and sedimentation decreased about3%compared with the actual operation.Moreover,TP flux outflow increased about3.39%,and TP rention flux and rention rate decreased about 13.77%and 2%?4%.Obeying the basic characteristics of tide-type operation,relatively large variation amplitude of pool level is more conducive to improving the sediment outflow and reducing the TP retention in the TGR.Results can provide new ideas for optimal operation and water pollution control of the TGR. |
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