Simulating cross-sectional geometry of the main channel in response to changes in water and sediment in Lower Yellow River

To understand the non-equilibrium morphological adjustment of a river in response to environmental changes,it is essential to(i)accurately identify how past conditions of water and sediment have impacted current morphological adjustment of the river,and(ii)establish a corresponding simulation for non-equilibrium conditions.Based on discharge and suspended sediment concentration(SSC)as well as 82 cross-sectional data items for the Huayuankou-Lijin reach of the Lower Yellow River in the period 1965-2015,the process of adjustment of the geometry of the main channel(area,width,depth,and geomorphic coefficient),and its responses to changes in discharge and SSC for different reaches are statistically analyzed.Following this,a delayed response model(DRM)of the geometry of the main channel subjected to variations in discharge and SSC is established using a multi-step analytical model,with the discharge and SSC as the main controlling factors.The results show that the area,width,and depth of the main channel decreased initially,then increased,decreased again,and finally increased again.These features of the geometry of the channel were positively correlated with the 4-year moving average discharge and negatively with the 4-year moving average SSC.The geomorphic coefficient for the Huayuankou-Sunkou reach exhibited a trend of decrease,whereas that of the Sunkou-Lijin reach decreased initially,then increased,decreased again,and finally increased again.Except for the Huayuankou-Gaocun reach in 1965-1999,the coefficient was negatively correlated with the 4-year moving average discharge and positively with SSC.The simulated values of the morphological parameters of the main channel for all sub-reaches obtained using the DRM agreed well with the measured values.This indicates that the DRM can be used to simulate the process of response of the cross-sectional geometry of the main channel to variations in the water and sediment.The results of the model show that the adjustment of the geometry of the main channel was affected by the discharge and the SSC at present(30%)as well as for the previous 7 years(70%).The proposed model offers insights into the mechanism whereby past water and sediment influence the current morphological adjustment of the river,and provides an effective method for predicting the magnitude and trend of the geometry of the main channel under different flow conditions.

小浪底水库运用以来黄河下游时空冲淤规律与模拟

This study focuses on the Lower Yellow River(LYR),which has experienced continuous erosion since the operation of Xiaolangdi Reservoir in 1999,and its spatiotemporal variation process is complex.Based on the single-step mode of the Delayed Response Model(DRM),we proposed a calculation method for simulating the accumulated erosion and deposition volume in the LYR.The coefficient of determination R^(2)between the calculated and measured values from 2000 to 2020 is 0.99.Currently,the LYR is undergoing continuous erosion,however the erosion rate is gradually slowing down,and the difference between the equilibrium and calculated values of accumulated erosion and deposition volume gradually decreases,which means riverbed erosion has a tendency towards equilibrium.Additionally,we derive a formula to simulate the spatial distribution of the main channel accumulated erosion volume per unit river length in the LYR based on the non-equilibrium suspended sediment transport equation.The coefficient of determination R^(2)between the fitted values and measured values from 2003 to 2015 is approximately 0.98-0.99,with a relative error of approximately 6.2%.The findings in this research suggest that under the current background of decreasing sediment inflow and continuous erosion in the LYR,it takes approximately 3.0 years for the riverbed to achieve half of the erosion and deposition adjustment and approximately 13.0 years to achieve 95%of the adjustment.Moreover,the spatial distribution of accumulated main channel erosion volume in the LYR tends to become uniform with the continuous development of erosion.These results could provide a valuable reference for analysing the complex spatiotemporal variation process in the LYR.

梯级水库蓄水对三峡水库泥沙淤积滞后响应规律的影响

Delayed response behaviour commonly occurs in conjunction with changes in riverbed scouring and sediment deposition and is a key component in understanding the intrinsic behaviour of reservoir siltation.Due to the complexity of the riverbed siltation process,the variability in the factors that influence siltation and the limitations of available research methods,the understanding of the delayed response behaviour of the sedimentation process in the Three Gorges Reservoir(TGR)is currently merely qualitative,and there is a lack of quantitative in-depth understanding.In addition,the effects of changes in water and sediment conditions on sedimentation in the TGR before and after cascade reservoir impoundment have not been quantified,so further studies are needed to provide a reference for better understanding the intrinsic behaviour of sedimentation in the TGR and the implications for the long-term use of the reservoir.Based on measured water and sediment data from 2003 to 2020 and topographic data from 2003 to 2018,a delayed response model for sedimentation in the TGR is constructed and combined with theoretical derivation to analyse the changes in the delayed response behaviour of the TGR before and after the impoundment of the cascade reservoirs and the associated causes.Then,the influence of changes in water and sediment conditions in previous years on sedimentation in the reservoir area is determined.The results show that(1)the improved delayed response model of sedimentation,which considers variations in external water and sediment conditions,reservoir scheduling,and riverbed adjustment rates,can effectively reflect the sedimentation process in the TGR,especially after the impoundment of the cascade reservoirs.Additionally,the typical section elevation delayed response model can simulate the section elevation adjustment process.(2)After the impoundment of the cascade reservoirs,the decreased variation in incoming water and sediment and more concentrated incoming sediment in the flood season increased the adjustment rate of the riverbed,and the delayed response time of TGR sedimentation was shortened from the previous 5 years to the previous 3 years.The impact of the previous water and sediment conditions is not negligible for the sedimentation process in the TGR,and the cumulative proportion of the previous influence reaches more than 60%.(3)The influence of incoming sediment on the sedimentation process and typical section adjustment process in the reservoir area increased after the impoundment of the cascade reservoirs,and the influence of the water level in front of the dam on sedimentation remained the largest.