RAPID - Plume Dynamics under Increased Sediment Discharge following Floods

2015年10月2日至4日，南卡罗来纳州遭遇创纪录的降雨，造成大规模侵蚀和洪水，堤坝决口，湖泊和池塘泛滥。洪水以一股漂浮的羽状物在沿海海域蔓延。这项快速研究的目的是研究在沉积物流量增加但在其他情况下接近正常海洋条件下大陆架上的羽流动力学。将在源自南卡罗来纳州温亚湾的淡水羽流内对盐度、温度、沉积物浓度和流速进行反复船上调查，皮迪河和瓦卡莫河就是在那里排放的。随着泥沙排放量的减少，这些调查将在几周内重复进行。这些洪水将影响海岸沉积物收支、海滩和内陆架形态，沿海开发商、管理人员和其他利益攸关方应对此负责。观测到的洪水非常“华而不实”，由于污水处理厂的多次故障和其他泄漏，由此产生的淡水脉冲携带的污染物浓度增加。更好地了解淡水在沿海海洋中的排放、扩散和混合，将有助于减少今后发生的类似事件，并最终更好地保护脆弱的沿海生态系统。该项目将为南加州大学海洋科学项目的几名本科生提供所需的实地研究经验。此次活动是一个机会，可以研究淡水排放驱动的沿海浮力羽流动力学的一个新元素：与低咸水扩散和混合之前的重沉积物沉积有关的密度变化。这些密度变化可能会对烟羽动力学产生重要影响：更远的近海海流增强，形成次级凸起，以及涡流驱动的近海质量和示踪剂通量。南卡罗来纳州10月初创纪录的闪电降雨，加上沿海的轻到中等风力强迫条件，使这项研究捕捉到了含沙流量调整的两种状态：(I)由于初始缺乏浮力而使流量迅速混合，以及(Ii)一旦沉积物沉积和流量恢复浮力，羽流“重新通电”。

South Carolina suffered from record rainfalls on 2-4 October 2015 resulting in massive erosion and flooding, dam breaching and overtopping of lakes and ponds. The floodwaters spread in the coastal ocean as a buoyant plume. The objective of this RAPID study is to examine the plume dynamics under increased sediment discharge but otherwise close to normal oceanic conditions on the shelf. Repeated shipboard surveys of salinity, temperature, sediment concentration and flow velocity will be conducted within the freshwater plume originating from Winyah Bay, SC, where the Pee Dee River and the Waccamaw River discharge. The surveys will be repeated over the course of a few weeks as the sediment discharge subsides. These floods will affect coastal sediment budget, beach and inner shelf morphology, and should be accounted for by coastal developers, managers and other stakeholders. The observed flood was extremely "flashy" and the resulting freshwater pulse carried an increased concentration of pollutants due to multiple failures of sewage treatment plants and other leaks. Better understanding of the freshwater discharge dispersal and mixing in the coastal ocean will help to mitigate similar events in the future and ultimately to better protect a fragile coastal ecosystem. This project will provide the required field research experience for several undergraduate students from the USC Marine Science Program.This event is an opportunity to investigate a new element to the dynamics of freshwater discharge-driven coastal buoyant plumes: density variations associated with deposition of heavy sediment load prior to the dispersal and mixing of the low saline water. These density variations might have important consequences for the plume dynamics: enhancement of currents further offshore, formation of secondary bulges and the eddy-driven offshore mass and tracer fluxes. Record-breaking flashy rainfalls in SC in early October in combination with light to moderate wind forcing conditions on the coast allow the study to capture two regimes of the sediment-laden discharge adjustment: (i) rapid mixing of the discharge due to its initial lack of buoyancy, and (ii) "re-energizing" of the plume once sediments are deposited and the discharge regains its buoyancy.