Effects of non-uniform sediment grainsize
in the long-term evolution of tidal lagoons
G. Di Silvio, P. Teatini
Dept. of Hydraulics, University of Padova, Padova, Italy
ABSTRACT
Long-term coastal processes usually consist in slight net morphological changes that
result from large positive and negative oscillations occurring 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. 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,
produces residual 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 turbolence 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 space resolution (zero-dimensional and two-dimensional
approach) by considering only one equivalent (uniform) sediment grainsize.
The zero-dimensional procedure, in particular, has been applied to the Lagoon of Venice.
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.
