Experimental study on the process of neck cutoff and channel adjustment in a highly sinuous meander under constant discharges
Experimental study on the process of neck cutoff and channel adjustment in a highly sinuous meander under constant discharges
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DOI：
10.1016/j.geomorph.2018.11.002
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发表时间：
2019-02
期刊：
影响因子：
3.9
通讯作者：
Zhiwei Li;Xinyu Wu;P. Gao
中科院分区：
文献类型：
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作者：
Zhiwei Li;Xinyu Wu;P. Gao
河颈截断是限制蜿蜒河流，特别是高度蜿蜒河流演化的一个重要过程。然而，这个过程在实验室水槽中极其难以复制。在这里，我们通过将长江中游七公岭弯道的现有平面图（中心线长13公里，河道宽1.2公里，颈宽0.55公里）按1/2500比例缩小，在实验室水槽中再现了这一过程，并具有几何相似性。在不同恒定输入流量的五次试验中，通过流量计和点计测量了水力参数（水深、表面速度和坡度）、岸线变化和河床地形；安装在水槽顶部的六个高架摄像机捕捉到岸线移动和颈部切断过程。通过分析河颈截流过程、截流河道的发育以及老河道截流的调整，我们发现：（1）河颈上、下游河段周围的河岸侵蚀缩短了河颈距离，导致河颈两侧水头梯度增大，导致短时间内河颈截流的发生； (ii) 颈部截断后，新截断河道的宽度迅速增加，因为局部坡度的增加在截断河道中产生了更高的单位水流功率； (iii) 与溜槽截流相比，颈部截流是一个渐进的过程，因此显着增加了河岸侵蚀和上游和下游河道的河道拓宽。这些结果表明，为防止未来突然发生自然断颈，气功岭弯道周围的护岸可能是必要的。
Neck cutoff is an essential process limiting evolution of meandering rivers, in particular, the highly sinuous ones. Yet this process is extremely difficult to replicate in laboratory flumes. Here we reproduced this process in a laboratory flume by reducing at the 1/2500 scale the current planform of the Qigongling Bend (centerline length 13 km, channel width 1.2 km, and neck width 0.55 km) in the middle Yangtze River with geometric similarity. In five runs with different constant input discharges, hydraulic parameters (water depth, surface velocity, and slope), bank line changes, and riverbed topography were measured by flow meter and point gauges; and bank line migration and a neck cutoff process were captured by six overhead cameras mounted atop the flume. By analyzing the neck cutoff process, development of the cutoff channel, and adjustment of the old channel to cutoff, we found that (i) bank erosion around the upstream and downstream channel segments of the neck reduced its distance, subsequently increased water head gradient on both sides of the neck inducing the occurrence of neck cutoff in a short time period; (ii) the width of the new cutoff channel increased quickly after neck cutoff because the increased local slope generated a higher unit stream power in the cutoff channel; and (iii) neck cutoff significantly increased bank erosion and channel widening in upstream and downstream channels as it is a gradual process compared with chute cutoff. These results suggest that bank revetment around the Qigongling Bend might be necessary to prevent the abrupt occurrence of natural neck cutoff in the future.