Morphological Consequences of the Nodal Modulation on a Tidal Lagoon

Abstract

Despite its relatively small amplitude, the nodal tide can play a relevant role in several environmental processes, spanning from ecology to morphodynamics; in particular, field studies and numerical investigations indicate the nodal tide as the dominant factor in long-term and large-scale net sediment transport in some estuaries. Nevertheless, while some important results have been obtained about its morphological effect on estuary channels, the response of tidal flats and salt marshes is still to be investigated. In the present work, a two-dimensional (2D), vertically averaged, long-term morphodynamic model based on the concepts of intertidal dispersion and transport concentration has been applied to the case of a schematic lagoon forced by a nodally-modulated tidal range. The effect of such a periodic oscillation has been studied in terms of sediment fluxes, suspended sediment concentration, and morphology. The results of simulations, carried out starting from different equilibrium conditions in which a nodal perturbation was introduced in the tidal signal, were postprocessed via Fourier analysis to detect the characteristics of the nodal perturbation within the domain and to draw some considerations about the spatial differentiation, in terms of both amplitude and phase, of the morphodynamic response throughout the domain. Although taking into account the nodal contribution has little influence in a long-term (several decades to centuries) analysis, as shown in the current study, such a small oscillation in tidal range (approximately 4%) can locally result in bathymetric oscillations of relevant relative amplitude.