High Resolution Model to Predict Oil Spill Dispersion in Harbour and Coastal Areas

We present a novel, state of the art model, which accounts for the relevant short-term physical processes governing oil spill at sea, (Zanier, Petronio, Roman, et al., 2014). Particles and tars are modelled as Lagrangian phase having its own density and diameter; taking into account the main forces acting on them, namely: buoyancy, drag and Coriolis forces. Oil transported in form of thin-film is treated by means of an improved Nihouls model (Nihoul, 1983/84). The latter considers the main forces (gravity, wind and sea currents stresses) governing oil slick spreading and transport in the first hours after spilling, up to 24h for large spill. Our main improvement to the classical model consists in the introduction of Coriolis effect, avoiding using empirical formulations (Zanier, Petronio, and Armenio, 2015). Finally the relevant short-term (12-24 hours) weathering processes (mainly emulsification and evaporation) are taken into account through established literature models (Mackay, Paterson, et al., 1980 and Mackay, Buist, et al., 1980, respectively). To preserve secondorder accuracy of the overall numerical method, convective terms, in the Eulerian model, are discretized using SMART a third order accurate upwind numerical scheme (Gaskell and Lau, 1998). We validate the model on standard test cases. The underground hydrodynamics is resolved using LES-COAST (IEFLUIDS University of Trieste), a high definition numerical model suited for coastal or harbour areas. LES-COAST model solves the filtered form of three dimensional, non hydrostatic Navier-Stokes equations under Boussinesq approximation and the transport equation for salinity and temperature. It makes use of Large Eddy Simulation approach to parametrize turbulence, the variables are filtered by way of a top-hat filter function represented by the size of the cells. The subgrid-scale fluxes (SGS), which appear after filtering operations, are parametrized by a two-eddy viscosity anisotropic Smagorinsky model, to better adapt to coastal flow in which horizontal length scale is larger than vertical one (Roman, Stipcich, et al., 2010). The subgrid-scale eddy diffusivities of temperature and salinity, Prandtl and Schmidt numbers, are set Prsgs = Scsgs = 0.8, by assuming that Reynolds analogy holds also for both scalars. Complex geometry that characterizes coastal flow is treated by a combination of curvilinear grid and Immersed Boundary Method (IBM) (Roman, Napoli, et al., 2009). Wind action on the free surface is taken into account by means of the formula proposed by Wu (Wu, 1982), in which the wind stress on the sea surface is computed from the wind velocities at 10 m above the surface. A 20% of variance is added to the stress to ease the generation of turbulence and to take into account of wind stress variations in time and space. Moreover near obstacles such as docks, ships and breakwaters, the wind stress is linearly reduced considering the relevant reduction of stress in recirculation regions. On the open boundaries the velocities and scalars quantities are obtained by nesting LES-COAST within Large Circulation Models (Petronio et al., 2013) or are imposed from in-situ measurements. Near the wall velocities are modelled using wall functions (Roman, Armenio, et al., 2009). We apply the coupled oil spill model and hydrodynamical one to simulate hypothetical oil spill events in real case scenarios in Barcelona harbour (North-west Mediterranean Sea, Spain, Galea et al., 2014) and in Panzano bay (North Adriatic Sea, Italy).