珠江蕉门河口分汊格局及跃变机制研究

Bifurcated estuary is one of the typical estuary morphologies under the interplay between river and sea. The distribution pattern of bifurcations can absorb certain degree of disturbances, which are exerted by either abnormal change of natural forcing or human activities. Once disturbance exceeds a tipping point, distribution pattern of bifurcations would respond and shift immediately. Therefore, the distribution pattern of bifurcation and the relevant shifting mechanism are fundamental for understanding morphodynamic evolution of estuary. This project selects the Jiaomen Estuary of the Pearl River as the study area and focuses on its complicated morphodynamic processes. Statistical analyses on historical nautical charts and measured data are the first step to explore the correlation between the division angle and the flow diversion ratio, as well as its morphodynamic features. Based on the results of the first step, a series of flume experiments are designed to investigate the self-adaptive mechanism of the bifurcated estuary and the tipping point for channel shifting between shrinkage and development. On the basis of the knowledge gained from the above two exploratory researches, together with an idealized morphodynamic model, the underlying mechanisms controlling the distribution pattern are studied. This project can deepen the understanding on the morphodynamic mechanism of a bifurcated estuary, and provide theoretical support for the remediation, protection and utilization of estuary, as well as the flood safety and waterway stability.

河口分汊是径潮动力作用下典型的地貌响应方式，河口通过其自适应能力形成河口分汊稳态，分汊控制因子一旦超过稳态自适应阈值，河口分汊格局即发生跃变，因此河口分汊格局及跃变机制是探讨河口动力地貌演变的基础及重要前沿问题。项目针对分汊河口动力地貌过程复杂多变问题，选取珠江蕉门河口为研究靶区，通过对历史海图和实测水文资料的数据统计分析，探讨分汊河口动力参数-形态参数的内在关联，分析河口分汊格局的稳态及其动力地貌特征；以此为基础，结合水槽物理模型实验，探索分汊河口的自适应性及稳态跃变阈值；基于对动力参数-形态参数相关关系及河口自适应性，结合动力地貌数值模拟方法，揭示不同诱因下河口分汊格局跃变过程及其动力地貌机制。项目选题不仅是河口动力地貌研究的重要前沿问题，而且可为基于河口分汊格局稳定的河口治理、保护和开发、行洪安全及航道稳定等经济社会发展的迫切问题提供重要理论支撑。

分汊型河道和分汊型河口分别是径流和径潮动力作用下典型的地貌响应方式，在河流和河口广泛存在。针对分汊型河道的研究较多且成果丰富，但分汊型河口因其空间尺度较大、动力因素复杂、泥沙环境多变等原因，对分汊河口演变机制和演变趋势的研究相对较少。本项目针对分汊河口动力地貌过程复杂多变问题，选取珠江蕉门河口为研究靶区，通过数据分析、物理模型实验和动力地貌数值模拟等相结合的研究手段，开展的主要研究内容包括，分汊河口动力参数-形态参数的内在关联，分汊河口的自适应性及主支汊转换的阈值，分汊河口中长期动力地貌演变的机制等。本项目主要取得的重要成果有：（1）收集珠江蕉门口百余年内的多幅历史海图，构建珠江河口附近区域的DEM数字高程模型，并从动力地貌角度，进行凫洲水道动力参数－形态参数相关关系的分析。研究发现洪水冲缺为凫洲水道从无到有发育的关键因子，人类活动对凫洲水道岸线的固化、蕉门南水道的延伸成为凫洲水道由支汊到主汊转换的主要因子。（2）设计系列定床物模实验，探究凫洲水道落潮分流比随分流角、分汊比、两口门流量比的演变模式。研究发现，在保证流量相似、地形相似、宽度相似的基础上，构建的简化分汊河口模型与真实情况相比，在年均情况和洪水情况均有较好的可信性。类似凫洲水道的“H”型分汊河口，其分流机理与传统的“Y”型分汊河口不同，两上游河道的流量比成为“H”型分汊河口分流比的最重要影响因素，分汊比对主支汊转变的阈值的影响大于分流角的影响。（3）基于开源Delft3D软件，采用概化的河口形态、初始地形以及简化的水沙环境，构建了长周期动力地貌数值模型。研究结果发现，虎门与蕉门之间存在相互作用，虎门河口上游存在蕉门的悬沙输移，虎门上游的狮子洋的沉积受到虎门与蕉门来沙的共同影响。因此单口门三角洲形成演变规律不适用于多口门形成的复合型三角洲，为复合型多河口三角洲的演变发育有重要的指导和借鉴意义。