Abstract

Loss of salt marshes in back-barrier tidal embayments has been widely documented worldwide as a consequence of land-use changes, wave-driven lateral erosion of marsh margins, and relative sea-level rise compound by mineral sediment starvation. However, how salt-marsh loss affects the hydrodynamics of back-barrier systems and feeds back into their morphodynamic evolution is still poorly understood. Here we use a custom-built, depth-averaged hydrodynamic model to investigate the mutual feedbacks between salt-marsh erosion and hydrodynamic changes in the Venice Lagoon, a large microtidal back-barrier system facing the Adriatic Sea in north-eastern Italy. Numerical simulations were carried out for past morphological configurations of the lagoon dating back up to 1887, as well as for hypothetical scenarios involving additional marsh erosion relative to the present-day conditions. We demonstrate that the progressive loss of salt marshes significantly impacted the Venice Lagoon hydrodynamics, both directly and indirectly, by amplifying high-tide water levels, promoting the formation of higher and more powerful wind waves, and critically affecting tidal asymmetries across the lagoon. We also argue that further losses of salt-marsh area would likely have detrimental effects on the lagoon ecomorphodynamic evolution, though with negligible impacts in terms of increased flooding risk in lagoonal urban settlements. Compared to previous studies, our analyses suggest that the hydrodynamic response of back-barrier systems to salt-marsh erosion is extremely site-specific, as it depends closely on the morphological characteristics of the embayment as well as on the external climatic forcings.