Exploring morphodynamic co-benefits of salt-marsh restoration

The Venice Lagoon possesses one of the planet's most fascinating and distinctive ecosystems, owing to its unique geographical location. Situated in the northern reaches of the Adriatic Sea, this natural lagoon has undergone significant human-induced changes over time to accommodate various activities. Originally characterized by shallow waters, resulting from limited erosion from the sea and significant sediment deposition from major rivers such as the Piave, Sile, Brenta, and Bacchiglione, human intervention in past centuries diverted these primary waterways and excavated channels to deepen the lagoon for navigation purposes. In addition, land reclamation efforts were initiated to convert salt marshes (i.e., low-lying vegetated wetlands periodically flooded by tides) into agricultural, residential, or aquacultural land. Salt marshes are a defining feature not only of the Venice Lagoon but also of many low-lying temperate and subtropical coastal regions. Currently, extensive loss of salt marshes is occurring worldwide, and significant efforts are underway to conserve and restore these valuable ecosystems, along with the services they provide to the environment and society, such as blue- carbon sequestration, environmental remediation, shoreline protection, and habitat provision. While it is undeniably valuable to restore salt marshes for ecological and economic reasons, most previous studies have not closely examined the co-benefits of marsh restoration on coastal system morphodynamic evolution. In particular, it remains unclear whether marsh restoration can return the system to the conditions observed before marshes were degraded. The Venice Lagoon provides a unique opportunity to study this issue in detail, given its extensive historical data and state-of-the-art numerical modeling techniques that enable the reconstruction of past, present, and future lagoon morphologies. We can investigate the effects of marsh degradation and the potential impacts of future restoration projects at the scale of the entire tidal basin. In this study, I used numerical modeling techniques to examine the hydrodynamic and sediment transport processes in three different configurations of the Venice Lagoon: the present one and those observed in 1932 and 1901. These two historical configurations serve as references to simulate the effects of marsh restoration projects aimed at restoring the total marsh area that existed at those times. The research I conducted demonstrates that even extensive marsh restoration alone will not suffice in fully returning the Venice Lagoon's hydro-morphodynamics to those observed at the beginning of the last century. This is because not only have salt marshes changed, but the overall lagoon morphology has also evolved, making marsh restoration projects essentially neutral with respect to current hydrodynamic and sediment transport processes within the lagoon. While not diminishing the intrinsic significance of marsh restoration projects, the findings of this study specifically illustrate that marsh restoration may not provide a practical solution to address the long-standing issue of morphological degradation observed in Venice over the past century, also urging caution in considering the restoration of similar shallow-water back-barrier lagoons that are prevalent along coastlines worldwide.