Modeling wind waves and tidal flows in shallow micro-tidal basins

Observational evidence and mathematical modeling have demonstrated the crucial role of wind waves on sediment resuspension in shallow micro-tidal basins, where tidal fluxes alone are unable to mobilize tidal flat sediments. Carniello et al. (2005) presented a numerical model which combines wind waves with tidal fluxes in a shallow micro-tidal basin. The highly irregular bathymetry typical of these environments characterized by the presence of deep channels, emergent salt marshes and extensive tidal flats suggested the introduction of specific hypotheses while solving the wave action conservation equation to describe wind-wave generation and propagation. In particular, as suggested by field measurements, the wave spectrum in this type of environment is quite often very narrow. Thus Carniello et al. (2005) originally followed a monochromatic approach, further assuming that the direction of wave propagation instantaneously adjusts to the wind direction and a constant wave period both in space and time. In the present contribution, we relax the latter assumption by introducing a variable wave period through a suitable empirical power law which relates wave period to wind speed and flow depth, based on wind-wave data collected in the Venice Lagoon. The same relationship came out to fit quite well also data collected in Lake George (AU) and, more recently, data collected in a system of lagoons at the Virginia Coast Reserve, USA (Mariotti et al., 2010). The noteworthy improvement in the estimation of wave height obtained by considering the local wave period is shown on the basis of the results of a number of simulations carried out for different storm events. The improvement obtained in the wind-wave field description reflects also on the estimation of the bottom shear stress and, therefore, on the description of the processes responsible for the morphological evolution of shallow tidal environments.