Abstract:The silting of tidal channels is a natural process that affects several shallow lagoons and makes it difficult to navigate, requiring regular maintenance interventions. This phenomenon is the result of the complex non-linear interaction between tidal currents and wave motion. In this work, the morphodynamic evolution of the Marano and Grado lagoon is investigated by means of a two-dimensional horizontal (2DH) morphological-hydrodynamic and a spectral coupled model. An innovative procedure to reproduce the overall bathymetric changes in the medium term and, in particular, the volumes deposited inside channels, is presented. An average year with a sequence of winds and tides acting over that time was reconstructed, carrying out cross correlation techniques and spectral analyses of measured data. The predicted morphological evolution matches the annual dredged volumes in the lagoon critical branches and shows the distribution of erosion and deposition of cohesive sediments according to spatially variable values of critical shear stress.

Lagoons and coastal semi-enclosed basins morphologically evolve depending on local waves, currents, and tidal conditions. In very shallow water depths, typical of tidal flats and mudflats, the bed shear stress due to the wind waves is a key factor governing sediment resuspension. A current line of research focuses on the distribution of wave shear stress with depth, this being a very important aspect related to the dynamic equilibrium of transitional areas. In this work a relevant contribution to this study is provided, by means of the comparison between experimental growth curves which predict the finite depth wave characteristics and the numerical results obtained by means a spectral model. In particular, the dominant role of the bottom friction dissipation is underlined, especially in the presence of irregular and heterogeneous sea beds. The effects of this energy loss on the wave field is investigated, highlighting that both the variability of the wave period and the relative bottom roughness can change the bed shear stress trend substantially.

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The morphological evolution of a tidal inlet is the combined result of tides and wind waves, which interact in a non-linear manner and over very different time-scales. Likewise, the presence of maritime structures built in the vicinity of the tidal inlet, for coastal or port defense or to stabilize the inlet itself, can greatly affect this dynamic equilibrium, changing erosional and depositional patterns of the adjacent shoreline. In this study, the narrowing phenomenon of the Lignano tidal inlet subsequent to the construction of the related port, is examined through an integrated approach in order to propose and verify a possible form of evolution. This approach is the result of the combination of three methods: the historical reconstruction of the shifting of the coastline, an empirical scheme which describes the qualitative morphology of a mixed-energy tidal inlet, and a process-based morphodynamic modeling, which adopts a bi-dimensional depth averaged (2DH) approach. The application of numerical modeling has required the definition of a reduced input set of data representing an average year, in particular for wind and tidal conditions, including the meteorological component. The magnitude and the directions of the simulated dominant sediment transport are coherent with real processes both from a qualitative and a quantitative point of view.

The morphological evolution of a lagoon tidal inlet over a medium-long period is a very important research topic since it can greatly affect both the hydrodynamic balance of the coastal environment and all the several human activities related to its proper functioning. The morphodynamic balance, which is the result mainly of the complex interaction of tidal currents and wind waves, can also be deeply influenced by the presence of maritime structures that are required for sea defence. This is the case of the Lignano inlet, which has undergone a progressive narrowing during last decades. In order to investigate the causes of this process and to evaluate possible solutions for the consequent filling of the port access canal, a morphodynamic-spectral coupled model has been applied to this context. Results are presented and discussed confirming that the numerical modelling can be used as a useful engineering tool for the correct management and the integrated planning of coastal zones.