长江三峡水库坝下游荆江段断面形态冲刷调整成因分析

长江三峡水库建成后，下游河段将发生长时间累积性的冲刷。由于边界条件的不同，不同河型河槽断面形态的冲刷调整特点存在差异。本文基于2003—2018年长江三峡水库坝下游荆江段实测河道断面形态资料，选取典型顺直河型、弯曲河型和分汊河型的典型断面，计算断面宽、平均水深和宽深比等断面形态特征参数，分析不同河型断面形态冲刷调整的异同，并探讨其成因。主要发现：1）不同河型的断面形态调整存在一致性和差异性。不同河型的平滩河槽断面总体向窄深化方向发展；顺直河型、分汊河型的枯水河槽断面宽深比总体减小，断面亦向窄深化方向发展；而弯曲河型的枯水河槽断面宽深比以增大居多，即断面总体宽浅化。2）平滩河槽和枯水河槽断面宽深比调整差异主要受水流动力轴线摆动范围控制。顺直河型、分汊河型水流动力轴线摆动范围覆盖平滩河槽与枯水河槽，即冲刷集中于平滩河槽和枯水河槽，两者均呈现窄深化特征；弯曲河型水流动力轴线摆动范围位于平滩河槽内，却超出枯水河槽，汛期摆到凸岸边滩上并冲刷凸岸边滩，枯水河槽显著展宽，凹岸深槽淤积，断面向宽浅化方向发展，且初期凸岸边滩宽度越大，宽深比变化率越大。3）蓄水后水流动力横向摆动范围缩小且水流冲刷动力集中，宽浅河段受水流冲刷下切的影响更显著，加上宽浅河段的河床组成相对较细，抗冲性弱，导致蓄水前宽深比越大的断面，蓄水后宽深比变化幅度也越大。

Analysis on the Causes of Erosion Adjustment of Cross Sections Geometry in the Jingjiang Reach Downstream Three Gorges Reservoir of the Yangtze River

After dam impoundment, the downstream reach will suffer from cumulative erosion for a long period of time. Due to different boundary conditions, the erosion adjustment characteristics of cross sections in different river patterns are different. Based on the measured date of cross sections in the Jingjiang Reach of the Yangtze River which is downstream of the Three Gorges Reservoir (TGR) from 2003 to 2018, in this paper we selected the typical cross sections of straight reaches, bifurcated reaches and meandering reaches, calculated the cross section characteristic parameters such as width, average depth and width-to-depth ratio, analyzed the similarities and differences of erosion adjustment of cross sections from different river patterns, and discussed its causes. The main findings include: 1) There are similarities and differences in morphological adjustments of cross sections in different river patterns. Under bankfull water level, the cross sections of different river patterns generally develop in the direction of narrowing and deepening. The width-to-depth ratio of straight and bifurcated cross sections under low water level is generally reducing, the cross sections develop in the direction of narrowing and deepening, while the width-to-depth ratio of meandering cross sections under low water level is mostly increasing, i.e., the cross sections develop at the direction of widening and shallowing. 2) The difference in the adjustment of the width-to-depth ratio between the bankfull channel and the low water channel is mainly controlled by the difference in the swinging range of the hydrodynamic axis. The swinging range of the hydrodynamic axis of the straight and bifurcated reaches cover both bankfull channel and low water channel, thus, the erosion is concentrated in the bankfull channel and low water channel, and they both show the characteristics of narrowing and deepening. As for the meandering reaches, the swinging range of the hydrodynamic axis lies in the bankfull channel, but exceeds the range of low water channel. During the flood season, it swings to the convex bank and washes the convex bank beach, thus the low water channel widens significantly, and the deep pool in the concave bank is deposited, the cross section develops in the direction of widening and shallowing. It is also found that the greater the width of the initial convex bank beach, the greater the rate of change of the width-to-depth ratio. 3) The lateral swinging range of the hydrodynamic axis reduced after impoundment, leading to a more concentrated scouring power, and this effect is more significant in wide and shallow reaches during the erosion process. In addition, the relative fine bed materials in wide and shallow reaches lead to low anti-scouring capacity, and thus a greater amplitude of the change in width-to-depth ratio after impoundment in cross sections that have larger width-to-depth ratio before impoundment.