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

This study presents Eulerian data from Winyah Bay, SC, USA collected during the passage of a tidal plume. The data captured the evolution and structure of the plume and include high-resolution velocity and temperature time series, supplemented by temperature – salinity profiles from a MicroCTD profiler. The observations identified a pre-existing plume extending to 4 m, with a water density of 1023.6 kg m−3, laying above denser ambient waters. Upon arrival, the newly discharged tidal plume introduced a fresher layer (1020.7 kg m−3) extending to 2.6 m, 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. In the across-front direction a return flow developed under the plume that extended throughout the water column, resembling an estuarine-like circulation pattern. 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. Within the gravity current, despite the high turbulent kinetic energy dissipation rates in certain regions, shear-induced mixing was minimal. These findings were reflected in the density diffusivity estimates near the surface that varied from 10−6 m2 s−1 prior to the arrival of the front, increasing to 10−5 m2 s−1 very near the front and diminishing to 10−10 m2 s−1 within the plume despite the high velocity shear observed there. The limited mixing within the plume despite the high shear observed can be explained using the concept of a layer-interface structure, where layers of high turbulence and low stratification alternate with regions of low turbulence and higher stratification. Although not resolved by these observations, this structure has been observed in numerical simulations of mixing. The development of the counter-flow under the plume suggests that tide and/or wind induced straining may play an important role in enhancing stratification in plumes like the one studied here.