Sediment concentration in tidal lagoons. A contribution to long-term morphological modelling

A long-term morphological model of tidal lagoons based on the concept of “transport concentration” has been proposed since a long time. The “transport concentration” is defined as the concentration, which provides the long-term net transport of sediments in a given location of a lagoon, as the sum of a dispersive and a residual advective flux. The intertidal dispersion coefficient and the residual water flow are provided by a more or less detailed tidal hydrodynamic model. The “transport concentration” is computed as the long-term average concentration produced in a given location by currents and waves. In previous contributions, simplified expressions of the transport concentration have been given for the channels and the shoals, as an empirical function of the local depth and the local hydrodynamics, to be calibrated against morphological data. In the present paper, a justification of these empirical expressions is provided, with special attention to transport concentration on shoals. Some morphological implications of the same expressions have been singled out and discussed. The interaction between fetch distribution and surface covered by salt marshes is also briefly discussed.