黄河下游河床萎缩过程中畸形河湾的形成机理

Formative Cause of Distorted Bends in the Braided Lower Yellow River in the Process of Channel Shrinkage

80年代中期以来 ,进入黄河下游河道的流量大幅度减小 ,河道发生萎缩 ,并且频繁断流 ,使游荡河道的一些河段发育了畸形河湾。游荡段的畸湾是冲积性河湾受到节点限制的情况下 ,由于流量减小导致湾道进行自我调整的过程中出现的一种特殊现象 ,反映改变了的水流条件与原有人为工程条件不相适应而出现的一种剧烈的河势调整和河床形态调整。畸湾的发育使弯曲系数增大 ,河段比降减小 ,故流速减慢 ,因而在同流量下水位壅高 ;畸湾还常常导致横河、斜河现象的出现 ,使防洪面临很大的压力。对于这种现象进行深入研究并进而找到可行的对策 。

Since the mid\|1980s, the runoff entering the lower Yellow River has been reduced to a great degree. Consequently, the channel has been shrinking, and seasonal dry\|ups occur frequently. Influenced by these new conditions, some distorted river bends have developed in braided reaches. The development of distorted bends can be regarded as an abnormal phenomenon in the process of channel adjustment as a response to the reduced runoff under the influence of local bank controls. This reflects the fact that the original man\|made flood\|protecing bank controls can not adapt to the changed flow conditions. Distorted bends lead to a much larger channel sinuosity and smaller gradient, and thus flow velocity declines and water level becomes higher at equal water discharge. In such reaches, flood\|protection often faces a great pressure. So, it is urgently needed to study this phenomenon and find out some countermeasures for flood\|protection engineering practice.