Morphodynamic Evolution Of The Jianli Reach In The Middle Yangtze River Under Upstream And Downstream Controls

The 94.5 km long Jianli reach is located in the most strikingly meandering section of the Middle Yangtze River.During the past 50 years,remarkable morphodynamic evolution occurred in the Jianli reach of the Middle Yangtze River owing to these combined effects of an artificial cutoff,the upstream operation of the Three Gorges Project(TGP),and the downstream confluence of the Dongting Lake.To better understand the characteristics of morphodynamic changes in the whole study reach,variations in channel geometry and flood-discharge capacity of this reach were quantified by a one-dimensional hydrodynamic model and a reach-averaged method,with the effects of upstream and downstream controls being investigated.These findings can provide a relevant contribution to the current river training of this meandering river reach.Variations in planform geometry indicate that:(i)events of an artificial cutoff and high flows caused the average rate of reach-scale thalweg migration being greater than 35 m/a,but there was a 23%reduction in the mean migration rate after the TGP operation because of the effect of various bank-revetment works;(ii)the process of bankline migration was remarkable in some local regions of the Jianli reach since the TGP operation,with the cumulative bank erosion length of 23.93 km;61%of the riverbank erosion regions were located on the left side,and severe bank retreat processes occurred at convex banks,accounting for 46%of the whole riverbank erosion regions in the Jianli reach;and(iii)a nature cut-off may occur in the Qigongling bend since its sinuosity has been greater than 3.9,approximately two times the sinuosity of the whole channel.Adjustments in bankfull channel geometry indicate that:(i)although there were significant bank erosion processes in local regions,channel geometry adjusted mostly in the aspect of bankfull depth under various river regulation engineering,with the reach-scale bankfull depth increasing by 1.13 m from 2002 to 2017;(ii)the reach-scale bankfull dimensions are closely associated with these accumulated effects of both the altered flow-sediment regime because of upstream dam construction,and the local base-level variation owing to the downstream confluence of the Dongting Lake;and(iii)these bankfull variables can be expressed by power functions of two key parameters,covering the previous five-year average fluvial erosion intensity during flood seasons at Jianli(upstream control),and the corresponding level difference between Jianli and Lianhuatang(downstream control).In addition,the Jianli reach tends to be more stable in the longitudinal channel profile after the TGP operation,with the longitudinal channel slope of 4.32×10^-5 being formed.Variations in flood-discharge capacity of this reach were investigated,covering the changes in reach-scale bankfull discharge(Q_bf),as well as the specified discharge under the warning levels at the water gauge station of Jianli(Q_wn^JL)during the period 2002?2017.Furthermore,these variables were represented by empirical functions of two key hydrodynamic parameters,covering the previous five-year average fluvial erosion intensity during flood seasons at Jianli(upstream boundaryF_5f),and the corresponding difference between the average water stages at Jianli and Lianhuatang(downstream boundary⁽³⁵⁾Z_f).Calculated results indicate that:(i)the flood-discharge capacity of this reach varied greatly in different years after the TGP operation,showing no monotone increasing or decreasing trend,with the average values of Q_{b f} and Q_wn^JL being 32,813 and 34,236 m³/s,respectively;and(ii)the variation in the flood-discharge capacity was mainly influenced by the downstream boundary condition,with the values of Q_{b f} and Q_wn^JL increasing with a larger value of⁽³⁵⁾Z_f.Adjustments in low-water channel geometry indicate that:(i)the reach-scale low-water width changed slightly owing to the restriction of various river regulation engineering,and the corresponding depth increased by 0.49 m from 2002 to 2017,leading to a 8%reduction in the width-depth ratio,with the low-water navigation condition being generally improved;(ii)the width-depth ratio at low water increased in some subreaches such as the sites of Damazhou and Qigongling,because the central-bar and convex point-bar in these regions underwent dramatic degradation;and(iii)variation in the low-water channel geometry was highly influenced by both the upstream and downstream boundary conditions,and the reach-scale low-water channel dimensions can be well expressed by power functions of the previous five-year average fluvial erosion intensity during flood seasons at Jianli(upstream control),and the corresponding level difference between Jianli and Lianhuatang(downstream control).