Observations and Modeling of a Buoyant Plume Exiting Into a Tidal Cross‐Flow and Exhibiting Along‐Front Instabilities

Abstract

Flows in and out of rivers and estuaries often generate dynamic frontal features due to localized sharp gradients in the currents, density, or bathymetry. Fronts often coincide with a narrow region of strong flow convergence, resulting in significant downwelling velocities and an increased surface roughness that make the fronts visible in remote sensing images. Within these energetic frontal zones and the shear layer at the density interface, shear instabilities are also common (Geyer et al., 2010;Horner-Devine et al., 2015;Smyth et al., 2001). These flow structures can enhance mixing and entrain bubbles and suspended particulate matter, affecting the lifecycle of the plume, acoustic sensor performance, and underwater vehicle operation.Remote sensing can be effective at identifying and tracking density fronts and frequently shows the existence of along-front spatial structure (

Lateral Border of a Small River Plume: Salinity Structure, Instabilities and Mass Transport

“…The obtained results on the different physical backgrounds of near-field and farfield instabilities are in a good agreement with the recent study of Simpson et al [41]. They also report the difference in sizes and vorticity patterns of lobe-and-cleft structures with and without velocity shear at the plume-sea interface, albeit that their numerical modeling limitedly addresses variability of sizes of these instabilities.…”

“…However, the depth of the vertical plume-sea interface within this stripe (1-1.5 m) was the same as in the surrounding plume. Simpson et al [41] also reported even greater increase in salinity in the plume in the vicinity of the lobe-and-cleft plume border from 5 to 13 (black line in Figure 8b in [41]).…”

“…In situ salinity and velocity measurements by CTD and ADCP profilers give the main information about the structure and dynamics of the plume-sea interface. However, recent development of quadcopters provided fundamentally new opportunities to observe and measure processes at the lateral plume border, which are visible by aerial remote sensing [2,41]. Usage of autonomous underwater vehicles [42,43] and drifters [27] also showed a certain progress in measurements at the bottom plume border.…”

“…The main straightforward characteristics, which are used to study instabilities in river plumes, include, first, the spatial and temporal characteristics of these instabilities (wavelength, amplitude, motion speed, vorticity, residual time, etc.) and, second, the characteristics of gradients at the plume-sea interface, which govern formation of instabilities (density gradient and velocity shear) [2,29,30,32,33,41]. A large set of characteristics, which describe the role of instabilities in turbulent mixing (eddy viscosity, vertical diffusivity, eddy kinetic energy, buoyancy dissipation, etc.…”

The Structure and Dynamics of an Estuarine Tidal Intrusion Front

“…Simpson et al. (2022) also observed discrepancies between the cross‐front scales of density and velocity in the tidal plume front of the Connecticut River, which they attributed at least in part to the challenge of resolving sharp velocity gradients. The difference in across‐front (convergent) velocity averaged 0.7 ± 0.1 m/s, whereas the difference in along‐front (sheared) velocity averaged 0.6 ± 0.1 m/s.…”