多时间尺度下的杭州湾沉积、地貌动力系统转换过程与机制

The Hangzhou Bay is a macro-tidal embayment located in the East China Sea, immediately south of the Changjiang Estuary. It is a funnel-shaped, wide and shallow estuary, covering an area of approximately 8500 km2. Due to the large sediment input from Changjiang estuary, the Hangzhou Bay has been accreting, with an annually-averaged deposition rate of 11.5mm/yr. In recent years, especially since 2003 when the Three Georges was constructed, the sediment load from Changjiang River has been drastically declined, from more than 450 million tons/yr in 1980s and 1990s to less than 150 million tons/yr after 2003. On the other hand, a large amount of tidal flats along the Changjiang estuary and Hangzhou Bay have been reclaimed since 1980s, which fixed a mass of sediment and changed the configuration of Hangzhou Bay to a certain extent. Hence sediment transport and morphological evolution in Hangzhou Bay have been influenced profoundly by the decrease of sediment source and land reclamation since 1980s. Subsequently, obvious variations happened in sediment dynamics and geomorphology system of Hangzhou Bay. The outer Hangzhou Bay has been rapidly eroded after 2003 while it was deposited before 2003. The suspended sediment concentration (SSC) in Hangzhou Bay has also gone through distinct changes in last decades. For example the SSC at northern mouth has decreased by about 20-30% in both summer and winter but the SSC at southern mouth has increased by more than 70% in winter while decreased by about 20% in summer. In this application, the synchronous hydrological data during the Zhejiang Coastal Zone Survey from 1981 to 1983, and 2014-2015 would be collected, and supplementary surfacial sediment sampling and bathymetrical survey would be carried out. Both of the field observation included winter and summer data, and the observed stations were at the same locations. The variations of sediment dynamics systems in Hangzhou Bay at multi-timescales, e.g. flood and ebb tides, spring and neap tides, summer and winter, and interdecadal variations would be analyzed and compared quantitatively. The indexes would include the SSC, the macroscopical routes of sediment transport, residual sediment transport, and sediment fluxes cross critical sections like the Luchaogang section at the adjacent area of Hangzhou Bay and Changjiang Estuary, the bay-mouth section and the Jinshan section, etc. Historical remote sensing data would be collected and compared to study the temporal and spatial surficial SSC distribution in Hangzhou Bay in last decades. It would also study the variations of seabed deposition/erosion patterns before and after the rapid sediment source decline and land reclamation based on historical bathymetries and charts. A three-dimensional numerical model cooperating tidal currents, waves, sediment transport and morphological evolution equations and a long-term idealized morphodynamic model would be set up to study the impacts of the decrease of sediment load from Changjiang River and land reclamation along Changjiang Estuary and Hangzhou Bay. Then the physical mechanisms underlying the sediment dynamics and geomorphological system variations would be revealed. The research findings would have an important significance to the estuarine regulation, the protection and safely operation of the major hydraulic engineering in Hangzhou Bay, and also facilitate the cross development among sediment dynamics, estuary geography and other subjects.

杭州湾沉积物主要来自长江入海泥沙一部分扩散南下后在长江口冲淡水次级锋面作用下进入湾内。上世纪80年代以来，杭州湾沉积、地貌动力系统受到了长江入海泥沙量锐减和高强度滩涂围垦工程的双重影响。本项目计划在收集历史和近期较大规模同步水文测验资料和地形资料的基础上，开展补充测验，采用海洋沉积动力学、数值模型和遥感分析相结合的方法，定量研究上世纪80年代以来杭州湾及邻近区域多时间尺度（时相、季节、年代际）下的悬沙浓度、水沙输移路径、关键界面沉积物通量、余流输移强度等沉积动力特性的分布与变幅；采用不同时期水下地形资料计算海床淤积、侵蚀状况及冲淤速率的变化；揭示杭州湾沉积、地貌动力系统过程转换的物理机制，探讨杭州湾沉积物输运、海床地貌对物源变化、围垦的响应。研究成果对河口综合治理、湾内重大涉水工程的安全运营维护提供技术支撑，对河口海岸学、海洋沉积动力学等学科发展、深化陆海相互作用理论亦一定有促进作用。

杭州湾沉积物主要来自长江入海泥沙在潮流波浪作用下向南扩散进入湾内。上世纪80年代以来，杭州湾沉积、地貌动力系统受到了长江入海泥沙量锐减和当地人类活动的双重影响。本项目在收集历史和近期较大规模同步水文测验资料和地形资料的基础上，开展补充测验，采用海洋沉积动力学、数值模型和遥感分析相结合的方法，研究了杭州湾多时间尺度下的沉积、地貌动力系统过程转换与机制。主要研究内容和重要发现有：（1）上世纪80年代以来，长江口外和杭州湾口区域悬沙浓度在冬季分别下降了43%和12%，其机理是长江口潮滩面积减少，冬季风浪作用下可以再悬浮的沉积物有所减少；但夏季悬沙浓度变化不明显，这是由于长江口外水下三角洲在一定程度上对长江入海泥沙减少起动了缓冲作用。（2）2003年以来杭州湾下部（金山以下）由过去的淤积状态转为冲刷状态，但上部（金山以上）淤积速率1959-2003年的2.3 cm/a增加至13.2 cm/a，原因是当地人类活动的影响超过了长江入海泥沙量减少的影响，加剧了泥沙淤积；（3）基于粘性沉积物输运方程建立了杭州湾河口系统的理想化动力地貌数值模型，复演了湾内主要地貌单元，通过敏感性数值试验证实了科氏力是河口“右旋”型地貌发育的控制因素；（4）建立了覆盖长江口和杭州湾的三维潮流泥沙耦合数学模型，发现近年来乍浦以上潮流流速和悬沙浓度分别减小33%和20%，南岸滩涂前沿由于滩涂淤涨外推，流速和含沙量增大30%左右。本项目发表学术论文12篇，其中包括《Journal of Geophysical Research》、《Marine Geology》等国际著名刊物。研究成果对河口海岸学、海洋沉积动力学等学科发展，深化陆海相互作用理论具有一定促进作用，对河口综合治理、湾内重大涉水工程的安全运营维护亦有实践应用价值。