| Rivers in the world today are usually disturbed by strong human activities.Especially,large reservoirs and large-scale river regulation projects have made unprecedented effects on channel adjustments.The Yichang-Chenglingji Reach(YCR)in the Middle Yangtze River(MYR)is an important navigation and flood-control reach in China,but recent human activities have aggravated channel deformation in this reach,which seriously affected river stability and flood-control safety.Some traditional theories of channel evolution applicable to natural rivers fail to meet the current requirements,and the corresponding methods of data analysis and numerical simulation need to be improved.Therefore,this study aims to investigate the non-equilibrium channel evolution in rivers affected by strong human activities.The main outcomes include:(ⅰ)Adjustment characteristics of channel geometry and flow-discharge capacity were quantitatively investigated in the YCR after the operation of the Three Gorges Project(TGP).The characteristic parameters under the bankfull level were selected as the research objects.Firstly,bankfull channel geometry and discharge in the Yizhi Reach(YZR:50 fixed cross-sections)and the Jingjiang Reach(JJR:173 fixed cross-sections)were determined annually from 2002 to 2017.Then the reach-scale bankfull characteristic parameters were calculated according to the reach-averaged method.The results show that bankfull characteristic parameters at section-scale varied greatly along the reach,therefore,the adjustments at a specific section cannot represent a whole river.After the TGP operation,channel degradation developed gradually from the YZR to the JJR,and the adjustments of reach-scale bankfull channel geometry in the Upper Yizhi Reach(UYR)was smaller than those in the Lower Yizhi Reach(LYR)and the JJR.It was attributed that channel adjustments in the UYR were not only affected by the incoming flow and sediment conditions,but also closely related to the composition of bed material.The reach-scale bankfull discharge increased annually in the YZR,while the value in the JJR fluctuated with time,reaching a maximum of 38,949 m~3/s in 2011and a minimum of 32,605 m~3/s in 2016.The main reason was that the adjustments of bankfull discharge in the JJR were greatly affected by the confluence of the Dongting Lake at the outlet.(ⅱ)An analysis method of non-equilibrium channel evolution for rivers downstream of a dam was proposed,which can be used to predict the channel adjustments with the changes of multiple boundaries.The main purpose of this method is to establish the quantitative relationships between the bankfull characteristic parameters and the multi-boundary influencing factors.In the YZR,the effects of the altered flow-sediment regime and the coarsening degree of bed material on the variation in bankfull channel geometry were investigated,and single-factor empirical models between them were developed.The results show that the correlations between the first influencing factor and the bankfull channel dimensions were lower in the UYR,and the coarsening process of bed material in the UYR limited the channel deformation,which led to a weaker response of channel adjustments to the altered flow and sediment regime.In addition,the correlations between the second influencing factor and the bankfull channel dimensions were relatively smaller in the LYR,owing to a lower coarsening degree of bed material in this subreach.Therefore,a comprehensive relation was developed to estimate the variation in bankfull channel geometry of the gravel-sand bed reach downstream of a dam,with both the altered flow-sediment regime and the bed-material coarsening degree being considered.The obtained correlations were higher than those obtained from the simple relation that merely considered a single influencing factor.For example,the coefficient of determination for bankfull depth was 0.96 in the UYR,while the coefficients of the single-factor models were 0.81 and 0.82,respectively.Due to the confluence of the Dongting Lake in the JJR,the correlation between the bankfull discharge and the incoming flow-sediment conditions was very weak,however,it responded well to the change of water level at the outlet boundary.Therefore,a comprehensive empirical model considering these two influencing factors was proposed.The results show that the determination coefficient of the comprehensive model(R~2=0.92)was much higher than that of the empirical model only considering the incoming flow and sediment conditions(R~2=0.16)or the variation in water level at the outlet(R~2=0.81).The comprehensive model can better predict the adjustment trend of bankfull discharge under the control of inlet and outlet boundary conditions.(ⅲ)A one-dimensional(1D)morphodynamic model has been improved to simulate the fluvial processes influenced by large-scale river regulation works.Firstly,the calculation methods of the riverbed resistance and the sediment-carrying capacity were improved;then the influence of river regulation project was further considered in the one-dimensional model.In the refined model,each node of a cross-section was labeled using a specified point code representing the zones of floodplain and main-channel with or without regulation engineering,and sediment transport and bed deformation in different zones were treated separately.In the zones without regulation engineering,both bed deposition and bed erosion would occur;in the zones with regulation works,bed deposition was allowed,while bed erosion wouldn’t occur owing to the restriction of those engineering,unless there was a deposition layer newly formed over the previous simulation period.Then the improved model was calibrated and verified using field observations of 2015 and 2016 through the application to the Jingjiang Reach in the Middle Yangtze River in China.The results show that the mean relative errors(MREs)of the calculated and measured discharge and water level at each hydrological station were between3-4%and 0-1%,and the MREs of suspended sediment concentration were less than 36%.Therefore,the model can accurately simulate the transport processes of flow and sediment;in addition,the channel scour volume without the consideration of river regulation works(4021×10~4 t)was greater than the measured value(3439×10~4 t),while the outcome considering this effect(4246×10~4 t)was in closer agreement with the measurements;moreover,the variation in channel geometry obtained from the improved model agreed better with the observed data.River regulation works would limit the channel incision,and the flow is likely to scour the other unprotected bed of a cross-section or the downstream cross-sections as it cannot carry enough sediment from the protected bed.(ⅳ)A two-dimensional model considering the influence of river regulation projects has been established,which focused on investigating the impact of river regulation works on the lateral distribution of channel evolution variables and the adjustments of planform geometry.Firstly,the grid nodes in the study area were marked with specific codes to distinguish the floodplain and the area with or without river regulation projects.In addition,the Kriging method was used to interpolate the bed-material composition of each grid node,to consider the spatial heterogeneity of bed-material composition.Then the modules of sediment transport and channel deformation were improved.It was achieved through the inverse algorithm to obtain the effective sediment-carrying capacity and the sediment concentration,to ensure that the obtained parameters can meet both the processes of suspended sediment transport and channel deformation with the effect of river regulation works.Finally,the improved model was calibrated using the measured data from the Chenjiawan-Haoxue Reach(CHR)of the MYR in2004,and verified with data in 2008.The results show that the depth,depth-averaged velocity and sediment concentration at several cross-sections were in agreement with the measured data,and the MREs of the discharge,section-averaged water level and sediment concentration at the Shashi station were less than 5.0%,1.0%and 24%,respectively.In 2008,the sediment concentration in the CHR calculated by the modified model was generally smaller than the result that didn’t consider the impact of river regulation engineering.It was mainly because that the implementation of river regulation projects limited riverbed incision in protected areas.Moreover,the improved model can better simulate the adjustment processes of central bars in the study reach,and the simulation results of channel geometry at some typical cross-sections also agreed better with the measured data when the influence of river regulation works was considered in the model. |
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