高浊度河口近底高浓度泥沙形成机制与数值模拟研究

This study is aimed to reveal the physical mechanisms of dynamic formation of near-bed high-concentrated mud suspension (HCMS) in the high-turbidity estuaries, with the focus on the physical interactions among hydrodynamics, low-concentrated sediment suspension in the upper water column, high-concentrated sediment suspension within the near-bed column, and deposited sediment at the seabed. The parameterizations of these physical processes of the formation, propagation, resuspension, consolidation and breakdown of HCMS are then re-visited and summarized based on previous studies and field observation. An integrated cohesive sediment modeling system is then developed to simulate the formation, transport and transformation of HCMS, which includes multiple physical interactions of hydrodynamics, low- and high-concentrated suspension, and seabed. A state-of-art tripod system is deployed in the high-turbidity Changjiang Estuary to investigate the near-bed high-concentrated sediment dynamics. With the survey data, the modeling system is then validated to make reasonable simulation of formation of the near-bed HCMS around the turbidity maximum zone of the Changjiang Estuary. The spatial and temporal variations of HCMS under different dynamic conditions, such as riverine source, tide, wave and atmospheric forcing will be simulated and studied to reveal the mechanism of the HCMS transport.

本研究拟聚焦于高浊度河口特别最大浑浊带区域的近底高浓度泥沙过程，研究其形成过程中的水动力、上层低浓度悬沙、近底高浓度泥沙、沉积物之间相互作用的物理机制，从而确定近底高浓度泥沙的形成机制，并对其物理过程进行合理的参数化表达，在此基础上建立多种物理过程耦合并能够合理考虑近底高浓度泥沙的产生、输运，以及其与上层低浓度悬沙层和河口底床相互作用的动力学数值模型，并选择具有代表性的长江河口，采用近底观测系统对河口最大浑浊带区域的近底高浓度泥沙过程进行高分辨率观测，在分析其泥沙动力学机制的同时率定验证参数化方案和数值模型系统，模拟研究长江口最大浑浊带区域近底高浓度泥沙形成过程，以及其空间分布及在不同动力条件下的变化特征和输运机制。

河口最大浑浊带区域的近底高浓度泥沙过程对泥沙输运、沉积动力过程影响显著，其观测和模拟都具有较强的挑战性。本项目选择具有代表性的长江河口，从2018-2021年在长江口及邻近区域开展多个航次的现场调查，采用高分辨率近底三脚架观测系统采集长江口北槽、北支、南槽等不同动力环境下的近底高浓度泥沙过程。观测结果表明：长江口北槽区域近底高浓度泥沙在南侧约20g/L，而在主槽北侧其浓度可达47g/L的量级，且所有高浓度泥沙主要形成于涨潮-落潮转换期。长江口区域洪季河流输入的淡水与口外的高盐水在最大浑浊带形成的显著垂向层化限制了垂向混合，同时，在考虑层化的潮汐混合作用在涨潮-落潮期间也显著减小，从而造成近底层泥沙难以通过潮汐混合达到上层水体。同时，近底层泥沙浓度的逐步增加也在一定程度上增加了垂向层化。结果发现无论主槽南北两侧，在小潮期间都形成了显著的高浓度泥沙，其与小潮期间层化达到最强而潮汐混合作用达到最弱直接相关。基于此动力机制，开发建立近底高浓度泥沙数值模型，模拟分析了常态天气和极端天气（台风）条件下长江口最大浑浊带区域近底高浓度泥沙形成、输运和消亡的完整过程，以及相关的特征参数，如维持时间、峰值时间、厚度、输运距离等，并量化了水体-高浓度悬浮层、悬浮层-底床界面上的沉降、悬扬、固结、侵蚀等物理过程的贡献。