Study Describes Transport Pathways During Estuary and Coastal Waters Exchange
– August 11, 2015
An international science team conducted a tidal-cycle study across a Destin, Florida inlet to better understand currents and the transport of dissolved and suspended materials between an estuary and the coastal ocean.
They observed salinity and water temperature fronts at low and high tide. The high tide front transported oceanic buoyant material into the estuary throughout the water column. In contrast, a plume-like front at low tide moved material seaward near the surface. These plume-like fronts may prevent buoyant material from entering an estuary during most of the tidal cycle. After a tidal cycle, water density gradients in the water column were the main drivers of residual flows. They published their findings in the January 2015 edition of Estuarine, Coastal and Shelf Science: Tidal and nontidal exchange at a subtropical inlet: Destin Inlet, Northwest Florida.
Water exchange between estuaries and the ocean affects pollutant fate from the land and larval transport of many commercially and ecologically important marine species into and out of estuaries. Tides, waves, winds, and freshwater that enters an estuary contribute to this exchange. Prior inlet studies focused on tidal front formation outside inlets or did not account for both flood (rising tide) and ebb (falling tide) conditions and did not fully examine the influence of density gradients on residual exchange flows. For this study, researchers collected data on temperature, salinity, and current velocity along two cross-inlet transects over a 24-hour tidal cycle in December 2013. They combined their measurements with an analytical model to capture flood-ebb variability and residual flows after one tidal cycle.
Average or residual flows consisted of a vertically varying structure with outflow at surface and inflow at depth. Within a tidal cycle, there were rapid salinity changes coincident with end-of-tidal outflow and inflow. In contrast, there was little temperature change in landward and seaward flows, indicating that salinity played a more dynamic role than temperature. Water density closely followed salinity variations, with dense water persisting longer at the seaward station than at the landward station.
Strongest tidal currents were observed near the surface, similar to the friction-influenced behavior of damped waves. Curvature of the shoreline appeared to alter tidal current behavior locally. A theoretical model confirmed the damped wave behavior and potential curvature influence in tidal flows and indicated that residual currents were inherent of a dynamically narrow inlet with relatively weak frictional effects. The team noted that these exchange structures might change when tidal forcing increases or when river discharge decreases.
The study’s lead author Arnoldo Valle-Levinson explained the importance of understanding ocean-estuary water exchange when an oil spill approaches coastal regions: “Water from the estuary is discharged to the coastal ocean during a little more than 6 or 12 hours, depending on the location. Estuarine outflow can thus represent a barrier or protection against shoreward transport by building fronts where materials accumulate along one or several lines or well defined rows, potentially helping cleaning up operations.’” Valle-Levinson said that even though the estuary becomes more vulnerable to materials transport during the inflow part of the tidal cycle, these materials may accumulate in fronts during certain inflow times.
Because both outflow and inflow at the ocean-estuary transition vary considerably in time and in three dimensions, Valle-Levinson said that it is imperative to study these processes to establish vulnerability regions to oil spills and to help cleanup operations.
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) consortium. Other funding included the National Science Foundation (OCE-1332718).
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 https://gulfresearchinitiative.org/.
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