Study Characterizes River Plume Mixing Processes in Coastal Waters

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Kimberly Huguenard at the University of Maine uses a microstructure profiler to measure turbulence and mixing in coastal waters. Photo provided by Holland Havercamp of the University of Maine.

Scientists assessed the behavior of a Florida river plume to determine how it might influence the transport and dispersion of surface oil near coastal regions. The researchers found that the near-surface measurements of dissipation at the front’s bounding edge were four orders of magnitude larger than the environment beneath. Frontal processes accounted for approximately 60% of the overall mixing of river plume water observed near the coast. An energetic wake trailed the frontal edge, which could potentially increase frontal and plume mixing and could push surface-trapped oil downward. The researchers published their findings in Journal of Geophysical Research: Oceans: On the nature of the frontal zone of the Choctawhatchee Bay plume in the Gulf of Mexico.

River plumes are common transport pathways between estuaries and coastal regions. Mixing inside these plumes controls where estuarine material is deposited into coastal waters. Because the highest social-economic impact of the Deepwater Horizon oil spill was on coastal communities, it is important to represent coastal processes in oil transport model predictions.

This study’s researchers used acoustic and microstructure profiling to collect and analyze the velocity, density, wind speed, and dissipation rates (which help determine small-scale turbulence) of a river plume in Destin, Florida’s Choctawhatchee Bay. The team used a satellite image of the area and synthetic aperture radar in situ measurements to compare with a model simulation of the plume’s frontal footprint.

The researchers observed a turbulent bore head (the leading edge of the plume forms a wave or waves that travel against the current), which detached from the plume and generated instabilities in the trailing wake. The bore head’s detachment is significant because it can expand the frontal zone and evolve into internal waves.  The observed plume footprint was much larger than predicted by the model.

This study demonstrates that energetic frontal zones influence mixing, plume spreading, and internal wave generation. “If ambient coastal currents oppose the direction of the plume, as observed in our study, a very active frontal zone is formed,” explained study author Kimberly Huguenard. “Understanding that this occurs in nature provides scientists with the opportunity to find new ways to include these processes in their models, improving the accuracy oil spill transport predictions.”

This study was the first to quantify the plume’s frontal zone mixing using near-surface microstructure observations rather than simplified parameterizations.

Researchers used the Rockland Scientific Vertical Microstructure Profiler in a new, uprising profiling mode to quantify mixing in the Choctawhatchee Bay river plume. The upriser mode resolves turbulence in the upper meters of the water column, otherwise truncated in the traditional downward deployment. (Filmed by Nathan Laxague and edited by Dave Ortiz-Suslow under the advisement of Brain Haus of the University of Miami and CARTHE)

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi: 10.7266/N73776Q9.

The study’s authors are K.D. Huguenard, D.J. Bogucki, D.G. Ortiz-Suslow, N.J.M. Laxague, J.H. MacMahan, T.M. Ozgokmen, B.K. Haus, A.J.H.M. Reniers, J. Hargrove, A.V. Soloviev, and H. Graber.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) and the University of Miami Rosenstiel School of Marine and Atmospheric Science for their project Monitoring of Oil Spill and Seepage Using Satellite Radars.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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