Tidal Flats Morphodynamics: A new Conceptual Model to Predict Their Evolution over a Medium-Long Period

Abstract

The morphological evolution of tidal flats has been widely investigated in recent years as it represents a very important topic which is highly related to the climate-driven environmental changes. The period over which geomorphological changes can be noted is a multi-year up to pluri-decennial time scale, defined as medium-long period. This work presents a new conceptual model which is able to predict and estimate a limit depth between an erosion condition and a no-erosion condition for tidal flats. The domains of applicability are shallow and confined basins, where tidal flats are characterized by near-horizontal topography, as occurs inside lagoons. The theoretical approach provides a general equation which relates the limit depth of tidal flats to current velocity and critical erosion shear stress. The procedure, followed through to its development, takes into account the important role of the bottom friction dissipation in wind wave generation process for shallow water. The relationship between tidal flat depth, current velocity and critical shear stress is provided in three different configurations, depending on the direction of the wave motion compared to the current. The limit depth compared to the measured depth can suggest if tidal flats tend or not towards an erosion state over a medium-long period. In this sense, the conceptual model provides a relevant contribution to the comprehension of morphodynamics of these important environments. This approach has been validated with its application to a real context and the results are provided in the paper.

An Integrated Approach to Study the Morphodynamics of the Lignano Tidal Inlet

“…This factor can vary The results of the performed simulations showed that the sediment transport dynamics in the Lignano nearshore is predominately influenced by high intensity and low frequent wind events instead of more frequent and lower intensity ones. The same happens inside the Marano and Grado lagoon and similar contexts [20,34], for which waves generated by wind speeds greater than 10 m/s play the dominant role in sediment transport mechanisms. Following these considerations and the duration curve of the higher wind velocities depicted in Figure 9b The duration of each wind class, over an average year, has been estimated on the above-defined curve and the summary is proposed in Table 1.…”

“…According to this scheme, the energy regime that governs the coastal area in front of the Marano and Grado lagoon can be defined through the mean tidal range and the mean wave height. The mean tidal range, given by the difference between the mean high water level (MHWL) and the mean low water level (MLWL) is about 0.76 m as results from the analysis carried out by Petti et al [20,34]. The mean wave height close to the Lignano tidal inlet is about 0.6 m as deduced from a 30-year-long wave simulation carried out by Benatazzo et al [35] who reproduced the present average wave climate of the Adriatic Sea.…”

“…On the other hand, the Bora wind generates a lowering of the level in the portion of the sea in front of the Marano and Grado lagoon due to the drag effect on the sea surface. These level differences, which have been reconstructed from the measured data, have proved to be decisive for transport mechanisms in the ebb tidal delta of Lignano, differently from the back-barrier basin which is mainly governed by the tidal astronomical component alone [20,34].…”

“…For this reason and as an initial simple approach, a sinusoidal function with a single harmonic component has been chosen, having an amplitude equal to ± 0.40 m. This value corresponds to the average astronomical tidal range in the northern Adriatic Sea, as reported by Dorigo [51] and confirmed by a zero-crossing analysis, carried out over 24 years of recording of the Grado tide gauge [20,34]. The resulting wind and tide input conditions are shown in Figure 10.…”

Wave Forecasting in Shallow Water: A New Set of Growth Curves Depending on Bed Roughness

“…They are really important phenomena in the sediment resuspension mechanism of coastal lagoons, sheltered estuarine basins, and shallow lakes [3][4][5][6][7][8][9][10]. In these contexts, where depth limits both current velocities and the relative bottom shear stresses, locally generated waves trigger the main morphological processes [11][12][13][14]. Extended literature considers the importance of wave-induced morphodynamic changes [15][16][17] and their impact on environment also from an ecological and socio-economic point of view [18][19][20].…”