弱潮河口羽状射流对拦门沙形态塑造的影响机制

Plume jet, a typical hydrodynamic factor, plays an important role on the evolution of river mouth bar. Especially for the fluvial-dominant and micro-tidal estuary, the effect of plume jet is much more remarkable. However, the shaping mechanism of river mouth bar has not been well explored considering the effect of plume jet with micro-tide. Based on the long-term hydrologic data and high resolution remote-sensing data, this study will systematically analyze the impact of plume jet on the evolution of river mouth bar by three steps, including physical process, dynamic characteristic and shaping mechanism. Firstly, the process of changing morphology of river mouth bar corresponding to long-term hydrological variables will be studied. What more, with typical flood of different magnitude, the changes of the river mouth bar will be compared before and after the flood event. Secondly, the characteristic of plume jet will be quantificationally described as several dynamic indicators and the characteristic of plume jet will be modulated with multiple fluvial and tidal conditions. Then the hydrodynamic mechanism of plume jet with micro-tidal effect will be explored. Finally, combining with the analysis of plume jet stability, the correlation among the coefficient of plume jet stability, the dimensionless momentum of plume jet and the sediment indicators will be detected. The shaping mechanism of the morphology of river mouth bar body and its distributary channels will be explored through the sediment depositional and transporting pattern. This study will provide theoretical supports for the prediction of river mouth bar evolution and the management of river mouth bar.

羽状射流作为拦门沙发育的典型动力因子，对拦门沙的形态塑造有至关重要的影响。尤其对于强径弱潮河口，射流作用尤为显著。而考虑弱潮动力耦合作用下羽状射流对拦门沙的塑造机制却未得到深入揭示。因此本项目拟以典型的弱潮河口——磨刀门为例，基于长时间的水沙资料和高精度的遥感和地形数据，从塑造过程、动力机制和塑造机理三个层面，层层递进对羽状射流作用下的拦门沙形态演变展开系统研究。分析水沙季节和年际变化规律对拦门沙长期塑造过程，确定不同量级洪水作用下羽状射流的分布特征，对比洪水过程前后拦门沙的变化程度；量化羽状射流动力特性指标，模拟不同径潮组合下羽状射流流场分布形态，明确弱潮动力耦合下羽状射流的动力机制；结合不同动力组合的射流稳定性分析，建立射流稳定系数、无量纲射流动量和泥沙指标的相关关系，从泥沙沉积和输运模式的角度揭示拦门沙体及两侧汊道的形态发育机理，为弱潮河口拦门沙演变趋势预测和拦门沙整治提供理论支撑。

弱潮河口水流呈现羽状射流动力特性，对河口地区拦门沙发育及形态演变具有重要影响，而弱潮动力耦合作用下羽状射流动力结构对拦门沙的塑造机制却未得到深入揭示。本研究结合河口实测地形及分层水流资料、基于FVCOM的河口三维数学模型、断面水槽模型等手段，从羽状射流对拦门沙的塑造过程，羽状射流动力结构对径潮动力的响应规律，羽状射流对拦门沙形态塑造的影响机理等方面开展研究。.主要研究发现如下：（1）利用经验正交函数分解拦门沙演变模态并辨析各个模态的贡献度及驱动因子，其中第一模态（占比46.7%）时间函数与拦门沙体积具有显著的相关性，上游来沙量变化则与第三模态呈明显的正相关（占比13.4%）；（2）建立了强径流下泄形成的羽状射流与拦门沙顶点变动的相关关系，通过拟合拦门沙形态参数与径流因子，推算得出拦门沙位移的定量表达式，有利于预测拦门沙发育位置及研究演变趋势；（3）基于FVCOM建立河口三维数学模型，选取浮力频率和Richardson数表征羽状射流动力特征，揭示了羽状射流平面扩散形态和垂向混合特性对不同径潮作用的响应规律；（4）基于羽状射流的层化混合作用分析射流稳定性，层化作用较强时羽状射流动力结构较为稳定，且对悬浮泥沙的垂向扩散有一定的抑制作用，有利于维持稳定的拦门沙形态。.本研究揭示了弱潮河口羽状射流对拦门沙形态塑造的影响机理，有利于提升河口动力地貌相互作用的研究认识，且对典型弱潮河口磨刀门拦门沙演变趋势预测、河口出海航道布置等实际问题提供了理论参考和研究支撑。