RAPID - Plume Dynamics under Increased Sediment Discharge following Floods

University Of South Carolina At Columbia, Columbia SC

Investigators

Abstract

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.