EFFECTS OF NON-UNIFORM SEDIMENT GRAINSIZE IN THE LONG-TERM EVOLUTION OF TIDAL LAGOONS

Abstract

Long-term coastal processes [9, 10) usually consist in slight net morphological changes that result from large positive and negative oscillations occuring to a much shorter time-scale. As soon as one is not interested in these short-term variations, one may perform a preliminary time averaging of the basic waterflow and sediment transport equations in order to obtain a much simpler and manageable model for long-term simulations E6]. Long-term mathematical models, in fact, not only require much less computer time, but can run without knowing the detailed time - history of all the boundary conditions (which on the contrary is absolutely needed by short-term mathematical models). Averaging of non-linear equations, on the other hand, produce residqal terms that either may be neglected or should be expressed, in some convenient way, as a function of the averaged quantities. The procedure, indeed, is analogous to the averaging of the Navier-Stokes equations in order to eliminate turbulence pulsations, where the Reynolds stresses should be conveniently expressed in terms of averaged velocity. In the case of long-term morphological models of tidal lagoons, semi-empirical expressions of the residual terms can be found. The relative calibration coefficients may be then identified by comparison with field data and/or with a limited number of simulations carried out on short-term models. In some previous papers, long-term morphological models of a tidal lagoon have been developed with different spaceresolution (zero-dimensional [4] and two-dimensional [5] approach) by considering only one equivalent (uniform) sediment grainsize. The zero-dimensional procedure, in particular, has been applied to the Lagoon of Venice [8]. In the present paper the two-dimensional model is reconsidered and extended to the case of particles with different grainsize, ranging from sand to silt.