On the nature of the frontal zone of the Choctawhatchee Bay plume in the Gulf of Mexico

Abstract

River plumes often feature turbulent processes in the frontal zone and interfacial region at base of the plume, which ultimately impact spreading and mixing rates with the ambient coastal ocean. The degree to which these processes govern overall plume mixing is yet to be quantified with microstructure observations. A field campaign was conducted in a river plume in the northeast Gulf of Mexico in December 2013, in order to assess mixing processes that could potentially impact transport and dispersion of surface material near coastal regions. Current velocity, density, and Turbulent Kinetic Energy Values, ε, were obtained using an Acoustic Doppler Current Profiler (ADCP), a Conductivity Temperature Depth (CTD) profiler, a Vertical Microstructure Profiler (VMP), and two Acoustic Doppler Velocimeters (ADVs). The frontal region contained ε values on the order of 10−5 m2 s−3, which were markedly larger than in the ambient water beneath (O 10−9 m2 s−3). An energetic wake of moderate ε values (O 10−6 m2 s−3) was observed trailing the frontal edge. The interfacial region of an interior section of the plume featured opposing horizontal velocities and a ε value on the order of 10−6 m2 s−3. A simplified mixing budget was used under significant assumptions to compare contributions from wind, tides, and frontal regions of the plume. The results from this order of magnitude analysis indicated that frontal processes (59%) dominated in overall mixing. This emphasizes the importance of adequate parameterization of river plume frontal processes in coastal predictive models.

Intratidal and Fortnightly Variability of Vertical Mixing in a Macrotidal Estuary: The Gironde

“…The microstructure measurements provided direct estimates of TKE dissipation, which were used to quantify vertical mixing by using vertical eddy viscosity as a proxy. Microstructure measurements are becoming more common and have been used to link physical and biological processes (Houghton et al, 2018;Prez-Santos et al, 2014), to study mixing in river plumes (Huguenard et al, 2016;Klicher & Nash, 2010;Luketina & Imberger, 2001) and to quantify mixing in partially stratified estuaries (Huguenard et al, 2015).…”

The Generation of Overtides in Flow Around a Headland in a Low Inflow Estuary

“…The statistical significance of coherence estimates improves with increasing degrees of freedom, which justifies the usage of three FFT segments. Though the Goodman noise removal algorithm was initially used in a horizontal application on autonomous underwater vehicles, it has been recently used in vertical applications (Bluteau et al, ; Fer et al, ; Ferron et al, ; Huguenard et al, ). Values of ϵ were achieved by integrating the velocity shear spectrum, ψ , in wave number space following the method of Lueck et al () under the assumption that turbulence is homogenous and isotropic: where u denotes horizontal velocities, ν is the kinematic viscosity, and k is the wave number.…”

“…With this in mind, vertical eddy viscosity can be defined from measured dissipation data as (Huguenard et al, ; Kay & Jay, ) where ϵ is the TKE dissipation rate and m ef is the mixing efficiency, assumed to be a constant 1.05. This assumption is reasonable due to the weakly stratified nature of the estuary.…”

Coastal protection by a small scale river plume against oil spills in the Northern Gulf of Mexico

“…The Choctawhatchee river plume is a small-scale river plume (Huguenard et al, 2016). River plumes are driven by density gradients which are a result of fresh river water.…”

“…In order to reduce the damage of oil spills, first responders should be pointed to those patches of oil that are most harmful to coastal ecosystems and have greatest socio-economic impact, that is, the patches of oil that wash ashore (Smith et al, 2010;Morris et al, 2013;Huguenard et al, 2016). Understanding where these patches of oil come from requires a thorough understanding of the circulation on the inner shelf, which is the zone where the turbulent surface and bottom boundary layer overlap (Lentz and Fewings, 2012) and reaches from the surfzone to approximately 30 m water depth (Kennish, 2000), which is >20 km offshore for the biggest portion of the coast in the NGoMex.…”