层化潮汐汊口水沙分配过程与机制研究

The process and the mechanism of water and sediment division at a stratified tidal bifurcation are quite different from those at bifurcations of non-tidal reach. An idealized hydrodynamic and sediment bifurcation model will be firstly established to study the sensitivity of flow and sediment division to the main factors, and to decompose the impacts of runoff, tide, and interaction of run and tide, respectively. The high spatial resolution of velocity, sediment concentration and salinity will be observed by cross section cruises at two tidal bifurcations, one with strong tide and the other with strong runoff, in the Pearl River Networks. The dominant force will be calculated to explain the extent of their impacts based on the momentum equation. Longitudinal baroclinic gradient between cross secitons and lateral baroclinic gradient in cross section will also be analyzed to understand their impacts on the complicated flow structure during stratification. Mechanism decomposition will be dudeced by the 2-D suspended sediment transport model to clarify effects of different forcing factors with the clear physical meanings. The process of bed load at stratified tidal bifurcations will be well understood by the ADCP bottom tracking navigation and multi-beam echosounder. The process of dunes development will be depicted to discuss the effects of lags in dune parameters to variation of discharge on the bed load transport at tidal bifrucations. The spatial and temporal distribution of pattern of bed load will be analyzed to study the impacts of sorting at tidal bifrucations on the bed load division. The feature of sedimentation at tidal junction will be understood by discussion of sediment budget.

层化潮汐汊口在周期性潮波运动的影响下，其水沙分配过程与机制同非感潮河段处汊口有很大不同。本课题通过构建理想潮汐汊口模型，确定汊口处水沙分配对各主要影响因素的敏感度，分解径、潮及径潮相互作用对水沙分配的作用；通过对珠江三角洲强潮与强径两个汊口处断面的走航测量，描述流速、含沙量及盐度断面分布的高精度时空变化特征，根据动量方程分解计算主要驱动力，探讨层化过程中断面间纵向和断面内侧向斜压梯度力对复杂水流结构发育过程的影响，从二维悬沙输运方程出发，推导出悬沙输运机制分解的正演表达，从而阐明层化潮汐汊口处水沙分配的机理原因；利用ADCP的底部追踪功能和多波束的回声测深技术分别获取潮汐汊口附近的推移质输沙过程，揭示沙波的发育过程，分析沙波参数对流量变化的滞后效应对汊口处推移质输沙分配的影响，结合底沙级配的时空分布特征，研究汊口处推移质分配对分选过程的响应，探讨潮汐汊口处的泥沙收支平衡，掌握其沉积特点。

层化潮汐汊口在周期性潮波运动的影响下，其水沙分配过程与机制同非感潮河段处汊口有很大不同。课题首先通过建立理想潮汐汊口模型，在正压与斜压模型中通过有潮，无潮和径潮三种情况下，研究理想河口流量分配对层化过程的响应机理。研究表明随着分汊河道地形差的增加，斜压效应会促使水流向较浅的汊道分流，从而使得汊口分流趋向于均匀，当两汊道水深相差较大时，甚至会出现浅水汊道分流多于深水汊道的现象，这与正压环境下的汊口分流情况相反；河口地区的层化程度对汊口分流也有影响，具体表现为分层程度越高，汊口分流越不均匀，同样地，当两汊道地形相差较多时，浅水汊道分流多于深水汊道；对于分汊河口，斜压梯度力从盐度大的汊道指向盐度小的汊道，从而抑制水流进入深水河道，盐度扩散系数的变化对汊口分流的影响并不是很大，当扩散系数越小时，汊口分流越不均匀。通过对于实际河口的研究表明潮波运动对长江口流量分流起着不可忽视的调节作用，在枯季潮波作用促进了南北支的分流的不均匀，但对南北港的分流不均匀性却产生了抑制作用；在洪季，潮波运动对南北支与南北港都起到了抑制了分流不均匀的作用，使得其流量分配趋势更趋于均匀。基于洪枯季一个大潮潮周期内走航ADCP数据，分析汊口分汊河道的水位流量过程，研究汊口处三维流速时空分布特征；利用潮流分离技术，剥离出动力作用里主要的潮汐信号，探讨潮动力在分汊处的空间分布形态；并对比洪枯季动力特性的差异，从而揭示汊口处洪枯季的动力特征。并近一步利用走航ADCP测量的断面回波强度，结合同步采集的悬沙水样，反演高分辨率的断面悬沙浓度；解释汊口处含沙量的时空分布规律，探讨洪枯季潮周期内各个汊道的悬沙通量变化过程，从悬沙净输移的角度揭示潮汐汊口处泥沙的输运和再分配规律。