Large eddy simulation model for wind-driven sea circulation in coastal areas

In the present paper a state-of-the-art large eddy simulation model(LES-COAST), suited for the analysis of water circulation and mixing inclosed or semi-closed areas, is presented and applied to the study ofthe hydrodynamic characteristics of the Muggia bay, the industrialharbor of the city of Trieste, Italy. The model solves thenon-hydrostatic, unsteady Navier-Stokes equations, under the Boussinesqapproximation for temperature and salinity buoyancy effects, using anovel, two-eddy viscosity Smagorinsky model for the closure of thesubgrid-scale momentum fluxes. The model employs: a simple and effectivetechnique to take into account wind-stress inhomogeneity related to theblocking effect of emerged structures, which, in turn, can drivelocal-scale, short-term pollutant dispersion; a new nesting procedure toreconstruct instantaneous, turbulent velocity components, temperatureand salinity at the open boundaries of the domain using data coming fromlarge-scale circulation models (LCM). Validation tests have shown thatthe model reproduces field measurement satisfactorily. The analysis ofwater circulation and mixing in the Muggia bay has been carried outunder three typical breeze conditions. Water circulation has been shownto behave as in typical semi-closed basins, with an upper layer movingalong the wind direction (apart from the anti-cyclonic veeringassociated with the Coriolis force) and a bottom layer, thicker andslower than the upper one, moving along the opposite direction. Thestudy has shown that water vertical mixing in the bay is inhibited by alarge level of stable stratification, mainly associated with verticalvariation in salinity and, to a minor extent, with temperature variationalong the water column. More intense mixing, quantified by sub-criticalvalues of the gradient Richardson number, is present in near-coastalregions where upwelling/downwelling phenomena occur. The analysis ofinstantaneous fields has detected the presence of large cross-sectionaleddies spanning the whole water column and contributing to verticalmixing, associated with the presence of sub-surface horizontal turbulentstructures. Analysis of water renewal within the bay shows that, underthe typical breeze regimes considered in the study, the residence timeof water in the bay is of the order of a few days. Finally, verticaleddy viscosity has been calculated and shown to vary by a couple oforders of magnitude along the water column, with larger values near thebottom surface where density stratification is smaller.