河流回水对黄河尾闾段决口的影响

Since the Yellow River returned to the Bohai Sea in 1855, the river avulsion and resultant channel diversion occurred frequently in the tail, developing the modern Yellow River delta system. There have been few previous studies on the behaviour of river backwater and its effect on channel evolution and river avulsions. Further, the present situation of suspended riverbed and frequent channel diversion together with local base level rise, pose a huge challenge to the management of the Yellow River mouth, which is directly related to human life and property, healthy operation of Shengli Oil Filed, and wetland conservation on the Yellow River delta. Based on an quasi 2-D numerical model with coupled river backwater and offshore plume, together with field surveys, this project is to study water level, flow velocity, deposition rate, and channel-fill time in the river mouth area during different characteristic discharge events and avulsion characteristic time scale of the Yellow River tail. The aims of this project are to, 1) detect main changes of channel dynamics and water level of the Yellow River mouth during flood events, 2) determine spatial locations of upstream boundary of the Yellow River backwater during wet season, normal season, dry season, and flood events, 3) reveal effects of river backwater, offshore plume spreading and relative sea level rise on the channel dynamics, and 4) explore the mechanisms of river avulsion in the Yellow River tail. The successful implementation of the project not only can improve our understanding of river backwater and offshore plume hydrodynamics as well as delta evolution of a large coastal river, but also is helpful for the management of rivers entering a receiving basin.

黄河为典型平原海岸河流，其在尾闾段的频繁摆动为大型河流近现代史上所罕见，使黄河河口区成为研究河流回水与决口的优良天然实验室。针对目前国内外鲜有黄河口回水动力学及其对河流决口影响的文献报道，本项目拟基于耦合河流回水和离岸羽状流的准二维数值模型，结合现场测量，研究在不同特征流量事件与尾闾段决口特征时间尺度上黄河河口区的水位、流速、沉积速率与河道充填耗时的变化，明确回水区上游边界随着黄河丰水、平水、枯水和洪水的空间变动规律，揭示回水、羽状流和相对海平面上升对黄河河口区河道冲淤的影响，探索黄河尾闾段决口的机制。该项目的成功实施，不但可加深理解大型平原海岸河流的回水动力学行为及河控三角洲的演化机制，而且为（入海、入湖与入水库）河流的治理提供重要的科学依据。

黄河是典型的平原海岸河流，其尾闾段频繁摆动，使其成为研究河流回水与改道的优良实验室。本项目建立了耦合河流回水与离岸羽流扩散的黄河三角洲河道准二维数学模型，模拟了在流量事件与尾闾段改道不同时间尺度下的黄河河道沉积动力学过程。模拟结果显示，在黄河入海低流量期间，在三角洲河道内出现回水现象，发生黄河回水动力学行为，具体表现为：河流水面纵剖面呈上凹状，河道水流流速减弱，发生沉积作用，河床高程增加；在黄河高流量（洪水）期间，发生drawdown行为或称之为洪水地貌动力学行为，具体表现为：河流水面纵剖面呈上凸状，在入海口附近河流流速急剧增加，河床遭受冲刷，河床高程降低。黄河回水与洪水的地貌动力学行为的叠加效应，导致回水区的上游端点附近淤积速率最大，这实际上也是三角洲河流容易发生改道的地点。在黄河三角洲上，实测的河流改道长度与模拟的最大回水长度基本一致，也说明河流在回水区上游端点附近容易改道，这种认识支持尾闾段河流改道的最近新发展的理论。回水区的长度随流量发生变化，回水区的上游边界，随着黄河流量减小/增大，具有顺河道往上游/下游迁移的空间变化规律。模拟结果显示，离岸羽状流控制着河口沙坝的形成，海平面上升增加了三角洲河道的沉积速率，从而缩短了河道填充时间。改道地点具有随海岸线进积有向下游方向幕式迁移的现象。回答了有关黄河改道的四个核心问题，即改道地点、改道周期、控制因素和形成地貌。具体地，黄河一般围绕两个相对固定的地点改道，一个改道地点位于郑州花园口附近，改道受河床坡度控制，改道周期约607年，形成冲积扇地貌；另一个改道地点位于黄河尾闾段的河流回水区的上游边界, 改道受回水控制，自然改道周期约7年，形成三角洲地貌。有关黄河回水与改道的认识，为理解河流改道地点，改道频率，海岸线粗糙度，三角洲形成与地貌演化及其相互作用提供了新思路。