珠江河口咸潮上溯和调水抑咸的动力响应机制

The Pearl River Estuary is known as a typical multi-branch water system with complicated hydrodynamics. In recent years, saltwater intrusion becomes one of the most important causes of freshwater shortage. During the low river discharge season, high-salinity water intrudes abnormally into rivers from its estuary and then jeopardizes the freshwater. Measured data and relative researches indicate that its dynamic mechanism of saltwater intrusion in dry season differs from other estuaries. In order to explore the mechanism of saltwater intrusion and the responsive dynamics of saltwater suppression to reservoir regulation, a three dimensional baroclinic numerical model with large scale and high-resolution unstructured grids has been firstly developed to simulate the hydrodynamic process of saltwater intrusion in the Pearl River Estuary. In this model, the numerical solution of diffusion terms and the input module of wind field are properly optimized to improve the computational precision of salt mixing and transporting. Secondly, based on the view that the mixing status of salt and fresh water are dynamically changing, the half-month discipline and its dynamic mechanism, which substantially results in long time and sustained saltwater intrusion in the Pearl River Estuary, are analyzed according to numerical results and measured data. Since the river discharge plays a key role in seawater intrusion, increasing the freshwater discharge of upstream rivers by reservoir regulation is recently taken as a commonly used countermeasure against saltwater intrusion in the Pearl River Estuary. Thus, the responsive dynamics of saltwater suppression to reservoir regulation are further studied. Finally, for the purpose of freshwater saving, the optimal scheme with a period of half-month for reservoir regulation is proposed according to the typical background of salt-water intrusion. The optimal scheme has the advantage of costing least amount of freshwater for the same result of saltwater suppression. This will be helpful for the freshwater supply of cities.

珠江河口是典型的多汊复杂河口，水动力极为复杂，其咸潮上溯规律具有与其它河口不同的特点。为了解析珠江河口咸潮上溯和调水抑咸的动力响应机制，本研究拟构建一个适用于珠江河口复杂水系的大范围、高分辨率、三维斜压数值模式，并对其对流扩散项的数值算法和风场输入模式进行优化与改进，提高盐淡水混合、输移动力过程的模拟精度，实现珠江河口咸潮上溯动力过程准确和高效的模拟。其次，采用数值模拟和理论与实测资料分析相结合的方法，从不同盐淡水混合状态交替转换的角度出发，对真正导致珠江河口咸潮长时间持续上溯的半月周期变化规律及其动力机制进行深入研究，揭示其上溯机理。最后，就咸潮上溯与上游径流过程变化的响应关系进行探讨，基于典型水文情势，以半月潮为周期，提出相同抑咸效果调水总量最少的最佳“调水时机”和“调水量”的确定原则与方法，从而达到节约淡水资源和为珠江三角洲城市群和港、澳地区供水安全提供切实可行的技术依据的目标。

本世纪以来，珠江河口咸潮上溯频次增多、时间延长、程度加剧，所需抑咸流量亦有增加趋势；另外，由于水系复杂和调水线路长，抑咸调度水量的利用率不高，枯季咸潮上溯仍然是沿岸城市供水安全的主要威胁。针对珠江河口复杂水系，本项目首先构建了珠江三角洲网河区-河口-近海整体高分辨率三维斜压数值模型，对其对流扩散项的数值算法、并行计算架构和风场输入模块进行优化与改进，并创新性的提出了盐度初始场垂向插值-自适应调整（VISA）算法，有效提高了盐淡水混合、输移动力过程的模拟精度和计算效率，实现珠江河口咸潮上溯动力过程准确而高效的模拟。在此基础上，采用数值模拟和理论与实测资料分析相结合的方法，分别开展了径流、潮汐、风、拦门沙、河床下切等对珠江河口咸潮上溯的影响机理研究；以磨刀门水道为典型，从宏观规律分析和微观动力机制解析两个层面，对珠江河口咸潮上溯的半月周期变化规律进行深入研究，解析了真正导致其上游河道水体含氯度长时间持续超标的动力机制，为河口咸潮上溯和调水抑咸的动力响应机制研究提供了理论基础。依据咸潮上溯的半月周期变化规律及其动力特性，设计不同的调水抑咸人造“洪峰”流量过程，采用上述数值模型开展数值模拟试验，从时间和空间两个维度，就珠江河口咸潮上溯与调水量和调水时机的响应敏感性进行了定量分析，得到咸潮上溯与抑咸“洪峰”流量的定量响应关系，为抑咸水量调度方案评价和优化提供了技术支撑。最后，综合上述研究成果，提出了半月典型水文条件下的珠江河口抑咸水量优化调度策略，可为节约淡水资源和保障珠江三角洲城市群及港、澳地区的供水安全提供技术支撑。目前，本项目发表学术论文7篇，其中SCI或EI收录4篇，出版专著1部，获国家发明专利授权1项，获大禹水利科学技术奖一等奖1项，珠江水利委员会科学技术奖特等奖1项；部分研究成果已经得到相应水行政主管部门的认可，有望在珠江流域抑咸水量调度中得到推广应用。