冬季长时间持续强北风产生沿岸凯尔文波和河口艾克曼输运对长江河口盐水入侵的影响

There happened an extreme severe saltwater intrusion event in the Changjiang Estuary in February 2014, which is never been seen in recent 50 years, and seriously threaten the utilization of freshwater resources and guarantee of freshwater safety in Shanghai. The study of the project would have an important scientific meaning for enriching the estuarine and coastal theory, as well as a significant application for utilization of the freshwater resources in the Changjiang Estuary. The observation data indicated that the severe saltwater intrusion came from the North Channel, the strong north wind lasted for 16 days during the event, and the water level at the tide station increased sharply. The preliminary studies suggested that the long term continuous strong north wind is the major dynamic cause of this severe saltwater intrusion event. However, the processes of coastal Kelvin wave southward propagating along the Jiangsu-Changjiang Estuary-Zhejiang coast and Ekman transport to the Changjiang Estuary induced by the strong north wind, as well as their effects on the saltwater intrusion remain to be further studied. Considered the various dynamic factors in which the wind field is simulated by the WRF, this project will apply the advanced three dimension numerical model of saltwater intrusion to reproduce the processes of the severe saltwater intrusion happened in February 2014, reveal the dynamic processes of coastal Kelvin wave and estuarine Ekman transport and their impact on the saltwater intrusion. By set up numerical experiments, the dynamic processes and mechanisms of impact of strong north wind duration and different tidal pattern with the strong north wind on the saltwater intrusion will be revealed.

2014年2月长江河口发生了近50年未见的极端严重的盐水入侵事件，对河口淡水资源利用和上海供水安全造成了严重威胁。开展本项目研究对丰富河口海岸学理论具有重要科学意义，同时对长江河口淡水资源的利用具有重要的应用意义。观测资料表明该次严重的盐水入侵来自北港的正面入侵，期间强北风持续16天之久，潮位站增水极为显著。初步的研究表明长时间持续的强北风是产生这次极端严重盐水入侵的主要动力成因，但强北风产生的沿苏北-长江口-浙江海岸向南传播的凯尔文波和向长江河口的艾克曼输运的过程，及其对盐水入侵的详细影响过程还待研究。本项目应用先进的盐水入侵三维数值模式，考虑多种动力因子，其中风场由WRF模式计算，再现2014年2月长江河口严重盐水入侵过程，揭示沿岸凯尔文波、河口艾克曼输运动力过程及其对盐水入侵的影响。设置数值试验，揭示强北风持续时间、不同潮型下强北风对盐水入侵影响的动力过程和机制。

2014年2月在径流量正常情况下长江河口发生了极端严重的盐水入侵，与以往在低径流量下发生严重盐水入侵不同。这次盐水入侵导致青草沙水库23天不宜取水，对上海的水资源安全产生了严重的威胁。观测资料表明在这个极端严重盐水入侵期间，出现持续和强劲的北风，同时发生显著的水位上升。本项目采用现场观测资料分析、三维数值模拟和理论分析的方法，研究这次盐水入侵事件的动力过程和机制。实施了2017年2月北支、2018年2月南槽的多船同步定点观测。观测资料用于模式的率定和验证。研究结果表明这次极端严重的盐水入侵是由持续强北风产生沿岸增水和在北港巨量海水净向陆输运，形成北港流进南港流出的水平环流造成的。这个北港向陆净水体输运压倒了向海流动径流，输运大量高盐水进入北港。沿岸增水及其传播也是加剧盐水入侵的原因。回答了北支盐水倒灌在其中的作用（几无影响）、多强的风（10 m/s以上）和多长时间强风（8天）才能产生极其严重的盐水入侵事件。在气候态北风情况下，北港入海淡水与海水混合形成冲淡水，深水航道北导堤阻挡了它向南输运，使盐水较低的水体聚集在北港口门处，导致北港盐水入侵减弱。在持续强北风情况下，从苏北沿岸向南输运到北港口门处的高盐水被导堤阻挡，导致水位上升，大量高盐水受艾克曼输运作用进入北港，产生严重的盐水入侵。因此，深水航道北导堤对北港盐水入侵的影响在平常北风和持续强北风下截然相反。另外，本项目针对北支超浅河道，提出用随水深变化的谢才-曼宁公式代替底边界层对数律的底摩擦拖曳系数，显著提升了北支盐水倒灌的计算精度；给出了针对陈行水库库容仅供水7天的临界径流量；揭示了长江河口近10年地形变化和滩涂圈围工程对盐水入侵的影响；计算典型水文年下扁担沙最长连续不宜取水天数，提出是未来潜在水源地的建议。本项目研究成果丰富了河口盐水入侵理论，为长江口水源地取水安全提供科技支撑。