DataSheet_1_Sediment resuspension and transport in the offshore subaqueous Yangtze Delta during winter storms.docx

Storm-induced episodic sediment redistribution in coastal systems can reshape geomorphic bodies, disrupt ecosystems, and cause economic damage. However, cold-wave-storm-induced hydrodynamic changes and residual sediment transport in large, exposed subaqueous deltas, such as the Yangtze Delta, are poorly understood because it is typically expensive and difficult to obtain systematic field data in open coast settings during storm events. We conducted a successful field survey of waves, currents, changes in water depth, and turbidity at a station (time-averaged water depth of 20 m) in the offshore subaqueous Yangtze Delta over 10 days during winter, covering two storms and two fair-weather periods. During the storm events, strong northerly winds drove southward longshore currents (~0.2 m/s) and resulted in increased wave height and sediment resuspension, thereby leading to massive southward sediment transport. In contrast, both southward and northward transports were limited during the fair-weather periods. A better understanding of the storm-induced sediment transport can be obtained by using an approximately half-day lag in sediment transport behind wind force, given the time needed to form waves and longshore drift, the inertia of water motion, and the slow settling velocity of fine-grained sediment. Our results directly support previous findings of southward sediment transport from the Yangtze Delta during winter, which is deposited in the Zhejiang–Fujian mud belt in the inner shelf of the East China Sea. In addition, the southward sediment transport from the Yangtze Delta occurs mainly during episodic storm events, rather than during the winter monsoon, and winter storms dominate over typhoons in driving southward sediment transport from the delta. This study highlights the importance of storms, especially during winter storms, in coastal sediment redistribution, which is of particular significance when considering the projected increase in storm intensity with global warming.

海岸系统中风暴引发的间歇性沉积物再分配，可重塑地貌体、破坏生态系统并造成经济损失。然而，针对大型暴露水下三角洲（subaqueous delta）——如长江三角洲——由寒潮风暴引发的水动力变化与残留沉积物输运过程，目前仍缺乏深入认知，这是因为在风暴事件期间于开放海岸环境中获取系统的野外数据通常成本高昂且难度极大。我们于冬季在长江口外水下三角洲的一个站位（平均水深20米）开展了为期10天的野外调查，获取了波浪、海流、水深变化与浊度（turbidity）数据，调查覆盖了两次风暴事件与两个晴好天气时段。风暴事件期间，强劲的北风驱动形成南向沿岸流（longshore current，流速约0.2 m/s），导致波高增大与沉积物再悬浮（sediment resuspension），进而引发大规模南向沉积物输运。与之相反，晴好天气时段内南向与北向沉积物输运均受到限制。考虑到波浪与沿岸流的形成需要时间、水体运动的惯性以及细颗粒沉积物（fine-grained sediment）的缓慢沉降速度，若将沉积物输运相较于风力的滞后时间设定为约半天，则可更深入地理解风暴引发的沉积物输运过程。我们的研究结果直接佐证了既往研究结论：长江三角洲冬季存在南向沉积物输运，这些沉积物最终沉积于东海内陆架的浙闽泥质带（Zhejiang–Fujian mud belt）。此外，长江三角洲的南向沉积物输运主要发生于间歇性风暴事件期间，而非冬季季风期；且相较于台风，冬季风暴在驱动三角洲南向沉积物输运方面占主导地位。本研究强调了风暴（尤其是冬季风暴）在海岸沉积物再分配过程中的重要性，而在全球变暖背景下风暴强度预计将增强的背景下，这一结论具有尤为重要的意义。