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Measurements of buoy and tracer dispersion in coastal areas
J.M. Redondo, M. Bezerra Diez and A. Carrillo
Dept. Fisica Aplicada, Univ. Politecnica de Catalunya
redondo@fa.upc.edu(Abstract received 05/09/2005 for session A)
ABSTRACT
The eddy diffusivities in the ocean exhibit a large variation and show a marked anisotropy, not only horizontal values are much larger than vertical ones but there is a strong dependence on the spatial extent of the tracer dye or pollutant and at larger scales the topology of the basic flow is very important. In the case of oil spills , soluites or buoy tracers, these are strongly influenced by the buoyancy and horizontal diffusion depends on ambient factors such as wave activity, wind and currents. Measurements have been made near the coast for a variety of weather conditions and these values have been compared with cruise measurements and with estimates from satellite observations. There is a strong dependence of horizontal eddy diffusivities with the Wave Reynolds number as well as with the wind stress measured as the friction velocity from wind profiles measured at the coastline. Some of these results have been published in Bezerra et al. (1998). Both effects are important and give several decades of variation of eddy diffusivities measured near the coastline (between 0.0001 and 2 m2s-1). Longshore currents are also important near the coast. Experiments of dye diffusion such as those performed filming the evolution of slicks allow to characterise the ranges of Kx and Ky as a function of the distance to the coast and other environmental factors (Wave height and frequency, wind stress and mean current). A good estimate of the eddy diffusivity comes from a scaling that includes the thickness of the surf zone as well as the depth and the wave period. Measurements in the Mediterranean are almost two orders of magnitude smaller than in the Pacific coast. On a larger scale, and further away from the coast the relevant eddy diffusivities are much larger, because large eddies, that often scale on the Rossby deformation radius. Rd=N/h f, disperse further oil or tracer slicks in the sea surface. Here N is the local Brunt-Vaisalla frequency, f is the Coriolis parameter and h is the relevant depth. A laboratory model of coastal mixing in rotating-stratified flows is usefull to realize the wide parameter space exhibited by real flows. A series of field experimental measurements of the Lagrangian characteristics of the surface currents in and near the Barcelona harbour as well as in other sites (Ebro delta, Recife, Toulon) have been performed during several years in order to include a wide range of different wind and wave conditions. The seasonal influence on the water recirculation and the influence of tidal seiches is apparent when the formation of a local thermocline also forces strong vertical shear. The identification of stagnation points and the role of the friction by the wind have to be taken into account in certain typical conditions. Image analysis of both buoys and tracers help to identify the recirculating areas and to estimate the flushing time in terms of different forcings outside the harbour area. There are regions with parabolic or even smaller time dependence of the relative dispersion while for other conditions Richardson's law or hiperdispersion applies.
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2005 LAPCOD Meeting, Lerici, Italy, June 13-17, 2005