Temporal and spatial distribution of bank retreat in the Jingjiang reach of the Yangtze River after the Three Gorges Project operation and its influence on channel adjustment

Abstract

三峡工程运用后长江中游荆江河段河床持续冲刷，局部河段崩岸频发，影响河道内悬沙输移与河床形态调整.本研究采用实测长程河道地形及固定断面地形资料，确定了2002-2018年荆江河段的主要崩岸区域，估算了崩岸土体的泥沙总量，进而定量分析了河岸崩退对河床调整的影响.计算结果表明：荆江段累计河岸崩退体积约为2.0亿m³，约占该河段总冲刷量的17.5%；腊林洲及北门口河段累计崩岸体积与累计水流冲刷强度呈较好的幂函数关系（R²=0.95，0.97）；荆江段崩岸土体泥沙总量为2.7亿t，其中枝城-监利河段崩岸土体总量为1.67亿t，约占该河段沿程泥沙补给总量的30.7%，因此崩岸土体对悬沙沿程恢复有重要贡献；河岸持续崩退过程中，崩岸河段内的断面形态趋于宽浅，护岸工程修建后则明显趋于窄深，因此河道崩岸过程在一定程度上减缓了河床的冲深下切程度.;After the operation of the Three Gorges Project, the Jingjiang reach underwent remarkable channel degradation, and bank retreat occurred frequently in local sub-reaches, which affected the transport of suspended sediment and the adjustments of channel geometry. The measured fluvial topographic maps and cross-sectional profiles during the period of 2002-2018 were adopted to determine the main zones and volumes of bank erosion, and the effect of bank retreat was then analyzed quantitatively on the channel adjustments of the Jingjiang reach. Calculated results show that:the cumulative volume of bank erosion in the Jingjiang reach was about 2.0×10⁸ m³ during the period of 2002-2018, which accounted for 17.5% of the cumulative channel scour volume (11.4×10⁸ m³). Empirical relationships were developed between cumulative bank erosion volume and fluvial erosion intensity during flood seasons in the Lalinzhou and Beimenkou sub-reaches, with the determination coefficients of 0.95 and 0.97 respectively. In addition, the total sediment amount from bank erosion in the Jingjiang reach was around 2.7×10⁸ t, and the total amount in the reach between Zhicheng and Jianli was 1.67×10⁸ t, accounting for about 30.7% of the total amount of sediment supplied from the channel. Therefore, bank retreat played an important role in the suspended sediment transport, especially for the recovery of fine sand along the channel. Finally, cross-sectional profiles tended to be wide and shallow during the stage of continuous bank retreat, while they became narrow and deep after the construction of bank revetment engineering. Therefore, bank retreat can reduce the undercutting of the riverbed to a certain extent in the Jingjiang reach.