周期性潮波运动对复杂河网分流过程的影响机制

The flow division in the complicated river networks has always received a lot of attention because it is highly related to the material flux path. However, most of the previous studies on the tidal river networks ignore the impacts of the tide which is one of the dominant dynamic factors in the coast on flow division. This study will focus on the Pearl River network which is one of the most complicated river networks in the world. Firstly, the physical characteristics and hydrodynamic process of cross section at bifurcations along the delta from the apex of the river network will be analyzed. The hydraulic geometry parameters which can reflect the tide prism influence will be employed to understand the integrity trend and the regional feature of the flow division with the bifurcation order by the impacts of the different periodic tides. Secondly, the established two dimensional hydrodynamic model covering the river networks and its estuary will also be used. By this way, the extent of the tide impacts on the flow division will be quantified and the nonlinear effect of the different constituent will be decomposed. The impacts of the different periodic tide on the water level gradient will be discussed to decompose the contribution of the interaction of the runoff, tide as well as runoff and tide. Finally, the typical river network bifurcation system will be selected to carry out field measurement. The dominant factor of the flow distribution in the complicated river network system will be determined from the momentum balance equation. The temporal and spatial variability in different tidal periods will be explained. The study will particularly focus on the effects of the bottom friction in the complicated river network on the flow division under tide propagation to understand the mechanism of the tide impacts on the flow division in the complicated river network.

复杂河网分流过程由于关系到下游物质通量的输移路径而广受关注。但在感潮河段，前人的研究往往忽略了周期性潮波运动这一重要动力因素对分流过程的影响，因此本课题拟以珠江三角洲河网为研究对象，从河网顶点往下分级，研究汊口处断面物理特征参数和动力过程沿程变化规律，通过建立可剥离纳潮量的水力几何形态关系，阐明不同周期潮波运动下河网分流过程随汊口级数变化的整体性态势和区域性特征；通过构建覆盖珠江三角洲河网与口外海滨的整体平面二维动力模型，量化周期性潮波运动对复杂河网分流过程的影响程度，分解不同分潮及其组合的非线性效应，探讨不同周期潮波运动对水位梯度的影响，从而剥离径、潮及径潮相互作用对河网流量分配的贡献度；选取封闭汊口系统开展现场观测，从动量平衡方程出发，确定方程中不同作用项对汊口系统流量分配的控制作用，研究汊道底部摩阻在潮波传播过程中对分流过程的作用，从而揭示周期性潮波运动对复杂河网分流过程的影响机制。

汊口分流一直是河流和海岸动力学关心的重要科学问题。尤其是河网的分流过程由于关系到下游物质通量的输移路径而广受关注。但前人的研究，往往忽略了周期性潮波运动这一重要动力因素对分流过程的影响。因此，本项目以珠江河网为研究对象，通过构建覆盖珠江三角洲河网与口外海滨的整体平面二维动力模型，从河网顶点往下分级，研究汊口处断面物理特征参数和动力过程沿程变化规律，研究发现：河道水深、径流量和最大潮汐流量振幅呈沿程递减趋势；而河道宽度、面积和宽深比则呈下降-平稳-增加的沿程变化趋势。但剧烈的人类活动导致河道地形及整体的水力几何形态改变。根据潮波影响的不同，整个珠江河网划分为潮控区和径控区两个部分。基于洪枯季一个大潮周期内的两组走航ADCP数据，利用潮流分离技术等方法，对珠江河口分汊顶点处的动力特性展开研究并对比了洪枯季的动力差异对比了洪枯季的动力差异。通过对珠江口分流不均匀系数的描述，研究珠江三角洲顶点分流过程在洪、枯季于各汊口处的细节变化，重点分析各分汊口处平均水位过程与汊口分流变化过程的相关关系，探讨潮汐动力强弱对分流状态均匀程度的影响。利用因素分解法，通过模拟三种不同上下游边界条件组合情况，来定量剥离径流、潮流及径潮相互作用对珠江河网内潮汐汊口分流过程的影响。研究发现潮波对汊口分流的作用与汊口距三角洲顶点的距离成正比，即距离越远，潮波对河网内潮汐汊口分流的作用越强。在潮控区，潮波对部分汊口的分流作用甚至大于径流的作用。总体而言，潮波对珠江河网汊口分流起到了抑制其分流不均匀的作用。从动量平衡方程出发，对摩阻项进行分解，其分解结果分别为径流作用项，径潮相互作用项和潮汐作用项。多元线性回归分析模型利用摩阻项来预测河网的水位分布，发现估计值和观测值有较好的一致性。研究发现摩阻效应与流速的非线性关系，决定了河网内水位的分布，从而是影响河网分流过程的机理原因。