Flow Structure and Mixing Near a Small River Plume Front: Winyah Bay, SC, USA

Abstract. This study presents a comprehensive analysis of Eulerian data collected in the vicinity of the front of a recently discharged river plume from Winyah Bay, SC, USA. The data presented capture the plume structure and evolution and include high-resolution velocity and temperature time series, supplemented by T-S profiles from a MicroCTD profiler. The observations identified a pre-existing plume extending to 4 meters, with a water density of 1,023.6 kg m^-3, laying above denser ambient waters. Upon arrival, the newly discharged plume introduced a fresher layer (1,020.7 kg m^-3) extending to 2.6 meters, gradually thinning due to radial spreading. The plume’s frontal propagation measured at 0.36 m s^-1 with a calculated Froude number of 1.32, indicating gravity current dynamics. Mixing processes were examined using the available overturn potential energy (AOPE) in the water column as described in Smith (2020). The analysis showed that near the bed, bottom boundary layer turbulence is the main mixing mechanism both before and after the passage of the front. In the surface layer, before the arrival of the front, mixing is driven by wind-induced shear and overturning. Despite high turbulent kinetic energy dissipation in certain regions, shear-induced mixing within the gravity current was minimal. These findings were reflected in the density diffusivity estimates near the surface that varied from 10^-6 prior to the arrival of the front, increasing to 10^-5 very near the front and diminishing to 10^-10 within the plume despite the high velocity shear observed there. Evidence of internal waves was observed, particularly in the pre-existing plume, providing further insights into the complex hydrodynamic interactions within river plumes and their role in coastal mixing.