长江口青草沙水库前沿河床演变与失稳风险研究

位于长江口南北港分流口的青草沙水库是上海市最大的城市供水水源地,开展该水库前沿河床稳定性和失稳风险研究具有重要的理论意义和工程指导价值。基于此,利用长江口南北港分流口2000年至2013年期间实测的高精度地形资料,分析青草沙水库前沿河床近期地貌变化特征,为水库库堤安全预警提供相关理论支撑。结果表明:青草沙水库前沿河床冲淤变化在建库前后有明显差异,建库前总体表现为河槽冲刷、沙洲淤积,呈现准冲-淤振荡的性质;建库后变为幅度逐渐减弱的持续冲刷;沿河床河槽形态由U型向U型与V型河槽叠加的复式河槽转变。青草沙水库前沿沙体沿落潮主流方向下移,成形沙体呈先增长、再减少,最后趋于平衡的态势,隐形沙体大体呈减少趋势。青草沙水库库堤前沿近600 m位置是河床失稳的重点风险区域,尤其是水库库堤中上部河床处于不稳定状态,如前沿边坡所在河槽进一步逼近水库前沿,则河床面临进一步冲刷的可能。

Research on evolution and instability risk of the frontal river bed along the Qingcaosha Reservoir, Changjiang Estuary

The Qingcaosha Reservoir (QCSR), located in the South and North Channel bifurcation, is the largest urban water supply source in Shanghai. It is of great significance for theory and practices to conduct the stability assessment of the frontal river bed in the QCSR. Thereafter, based on high resolution topography and sediment data measured during 2000-2013, the geomorphology characteristics of the frontal river bed of QCSR and associated stability of the river bed were analyzed to provide theoretical support for safety pre-warning of QCSR. The results showed that: 1) The variations in erosion and accretion of the river bed present clearly differences before and after QCSR operation. Before the reservoir operation, the river bed suffered channel erosion and shoal deposition with oscillations, which depended on the diversion of flow and sediment of the South and North Channel bifurcation. After the reservoir operation, the changes in the frontal river bed had been shifted from the fluctuations of erosion/accretion to the uninterrupted erosion with weak scouring trends. Moreover, the corresponding channel configuration appeared changes from the U-shaped to the combined U-shaped with V-shaped formation. 2) The frontal sand bodies along QCSR moved downward to follow the mainly ebb-tidal current direction. During the period of movement, the developmental sand bodies firstly expanded, and then shrinked, to the end with tendency to reach equilibrium, while the clocked sand body occurred reduced trends. 3) It is the key risked region that the location of about 600 m distance to the frontal area of the reservoir is seriously unstable, especially to the upper part of the river bed along QCSR. It can be expected that there is further scouring possibility for the river bed if the channel located at the front river bed slope will be moved approximately to the dyke of QCSR.