Dynamics And Environmental Consequences Of The Flow Oscillation In The Typical Tributary Bay Of The Three Gorges Reservoir

As an advanced equipment for the country,The Three Gorges Reservoir（TGR）has not only produced huge comprehensive benefits in flood control,power generation,navigation and water supply,but also has significant impacts on the ecological environment owing to the changes in hydrodynamics of the tributaries.Alleviating water environment problems through reservoir ecological operation,meanwhile,ensuring that the TGR produce the best possible results sustainably and efficiently,have been put forward urgent requirements in our country.Furthermore,it is not only the foundation and key to know and improve water environment in TGR,but also the technical bottleneck of reservoir ecological operation for accurately understanding the hydrodynamics and environmental effects under the influence of reservoir operation.The latest monitoring results show that density current is universal in the tributaries of TGR,in addition,flow oscillation caused by daily regulation of TGR represents a so far overlooked hydrodynamic feature of tributaries in large reservoirs.Therefore,it is of great significance to systematically study the flow oscillation and its environmental consequences in tributaries.This research adopts the methods of field monitoring,theoretical analysis and numerical simulation,focusing on the physical characteristics,formation mechanism,influencing factors and environmental consequences of the flow oscillation in Xiangxi Bay（XXB）,which is closest to the Three Gorges Dam（TGD）and has the most representative hydrodynamics.The main results and conclusions of this thesis are as follows:（1）Classification of daily regulation and characteristics of surface gravity wave in TGRThe daily regulation of TGR are summarized,namely single-peak regulation,double-peak regulation and irregular regulation,based on the wavelet transform and statistical analysis of the 2018-2019 discharge of TGR.The amplitude of WLF along the mainstream under irregular regulation was obviously smaller than that under single-peak mode and double-peak mode.The formation of surface gravity wave in the mainstream of TGR is mainly due to the sudden change of water level in front of TGD.The amplitude of surface gravity wave decreased gradually in the process of propagating upstream,besides the phase lags and the lag time changed linearly.（2）Physical characteristics of flow oscillation in XXBThe monitoring results during falling period and impoundment in 2019 showed typical double-peaked regulations.The variation of discharge can decide the amplitude of daily WLF in XXB.More interesting findings are the phenomenon of high-frequency and periodic WLF,and flow oscillation superimposed on the density currents.Based on numerical simulation,it is revealed that the persistent nature of bidirectional density currents and high-frequency flow oscillation are the two ubiquitous and dominant hydrodynamic processes in XXB.The amplitude of WLF and the oscillatory velocity()were small enough to be ignored in flood season,but large at other times.The maximum amplitude of WLF was exponentially increasing from estuary to upstream during the different operation periods（water level>170 m or<150m）.The barotropic seiche of fundamental mode（Helmholtz mode）with a period of 2 h,caused strong periodic flows with an amplitude up to 0.05-0.10 m/s and periodic WLF with an amplitude up to 0.15 m at the upstream of XXB.And that,aπ/2 phase difference between WLF andsuggested the presence of a standing wave in XXB.（3）Formation mechanism and driving factors of flow oscillation in XXBBy comparing the synchronous relationship among discharge of TGR,diurnal fluctuation of water level in front of the dam,high frequency fluctuation of water level in XXB and oscillating velocity,it is found that the daily discharge regulation drived the surface gravity wave propagating upstream in the mainstream of TGR,resulting in the WLF at the mouth and periodic flow oscillation with a period of～2 hours in XXB.The day-to-day discharge regulation forced the persistent high-frequency WLF and flow oscillation in the tributary.The oscillatory period,vertical structure and horizontal mode are inherent properties of the system,determined by the channel morphology（eg.,the length of the bay and the depth of the estuary）and friction coefficient of tributaries.Both the amplitude of WLF,were positively correlated with the intensity of daily regulation.Compared with the single-peak mode,the double-peak was more likely to stimulate strong flow oscillation.If continuous oscillation process is to be formed,the interval of water level scheduling should not exceed 2 days.The oscillation damping was from fast to slow.With the increase of oscillation intensity,it takes longer for oscillation to fully attenuate（WLF is less than 0.02 m）.The oscillation energy（E）and amplitude of WLF（A）can be quantified as:=₀0)^-6),6)is 0.061-0.076;=₀0)^-9),9)is 0.062～0.081。（4）Effect of flow oscillation on vertical turbulent mixing in XXBThe vertical turbulent mixing in XXB is determined by many factors,eg.,meteorological conditions,density current and flow oscillation.The longitudinal velocity along the channel(₇9)2)))in surface boundary layer（SBL）showed periodic variations at three different periods of 1.80 h,20 min and 1-3 s,respectively.The near-surface turbulence dissipation rate（）varied from 10^-8.27 W/kg to 10^-3.82 W/kg in summer,and from 10^-8.58 W/kg to 10^-3.50 W/kg in winter in2020.In XXB,the increase of wind speed in the afternoon was an important factor to drive the diurnal difference of surface turbulent mixing（r=0.50,P<0.01）.Surface flow current driven by density currents was an important force to enhance surface mixing,while the contribution of meteorological force was generally small,which only accounted for 27.17%in summer and58.95%in winter in 2020.The superposition of density current and flow oscillation determined the vertical structure of velocity profile.The maximum₇9)2))andreached 0.30 m/s and0.10 m/s,respectively.In addition,the stratified upper warm layer,with the square of buoyancy frequency（N²）of 10^-4 s^-2-10^-2 s^-2,restrained the development of turbulence.Inversely,the interface of density currents was energetic due to the large velocity shear.The turbulent mixing was strong in the bottom boundary layer（BBL）of XXB,and the,between 10^-10 W/kg and 10^-5 W/kg,was two orders of magnitude larger than that of the normal natural lakes and reservoirs,and changed periodicly under the influence of flow oscillation.（5）Effect of flow oscillation on thermal stratification in XXBThe heat energy budget in XXB is mainly affected by the meteorological factors and the longitudinal flow.Water temperature profile presented two significant periodic fluctuations.One period is 24 h in SBL caused by meteorological factors and the other periodic variation of water temperature in BBL,with a period of～1.83 h,was caused by flow oscillation.Water temperature variations at BBL presented the largest power spectral density（PSD）compared to SBL.Owing to riverbed friction,flow oscillation prompted the periodic turbulent mixing at BBL,and then drived the up-and-down of water temperature.Density currents can weaken the thermal stratification in tributaries by slowing down the heating process of surface water.Compared to the single-peak regulation,the double-peak regulation with a higherΔwould be more favorable to strengthen the vertical mixing,and then weaken or even destroy the thermal stratification.