As oil approaches the shorelines and encounters shallow depths, it is affected by bottom processes through upward diffusion of turbulence. It is commonly assumed that the oil does not interact with sediment until the surf zone. However, field measurements have shown that during strong winds, Langmuir supercells (i.e. full-depth Langmuir cells) can form at shallow depths ranging between 10 m and 30 m, bringing sediments upward allowing them to interact with the oil on the water surface and to form oil particle aggregates (OPAs). Some of the OPAs will settle to the bed while others will not. The Langmuir cells can create Stommel retention zones serving to provide a submerged pathway for OPAs to reach the surf zone. We also hypothesize that this premature formation of OPAs causes the settling of oil in the seaward part of the surf zone, which was never considered in oil spill research, which considered that “oil mats” are located within a few hundred meters from the shoreline. The project uses recently developed models by the PIs along with a coastal circulation model coupled with wave and sediment models to provide a holistic description for the physical behavior of oil from the inner shelf through the surf zone. In particular, an OPA model developed by one of the PIs will be integrated with the coastal circulation model within a Lagrangian-Eulerian framework serving to provide estimates of OPA formation throughout these zones.
The expected framework will be equipped with turbulence parameterizations accounting for Langmuir turbulence and associated Langmuir supercells in the surf-shelf transition zone. Highly resolved large-eddy simulations (LES) validated by and in conjunction with acoustic Doppler current profiler (ADCP) measurements at different shallow shelf regions throughout the Gulf of Mexico, will be used to develop the parameterizations of the Langmuir turbulence. Turbulence closures in coastal circulation models currently do not consider the effects of Langmuir turbulence and associated Langmuir cells. Meanwhile, the intense vertical mixing provided by the Langmuir supercells through non-local transport impacting the bottom boundary layer is expected to strongly impact sediment concentration predictions of the coastal circulation model and ultimately the prediction of OPA formation.
Laboratory studies in a Langmuir circulation facility to be built will explore Langmuir supercell dynamics inducing the suspension of sediments and entrainment of oil droplets promoting the formation of OPAs. These studies will (1) identify zones within the Langmuir supercells where mixing between oil droplets and sediments can occur and establish residence times of sediments and oil droplets within these zones, (2) characterize the mixing energy (i.e. the turbulent kinetic energy) within these zones, and (3) measure the coagulation efficiency of interactions between oil and sediment particles. The laboratory studies will serve to provide a case with spatially non-uniform OPA formation by which the OPA model used in the proposed Lagrangian-Eulerian modeling framework may be validated. Overall, these experiments will allow extension of the OPA model to the surf-shelf transition zone in inner shelves where Langmuir supercells are expected to occur.