Extensive loss of salt marshes in back-barrier tidal embayments is
undergoing worldwide as a consequence of land-use changes, wave-driven
lateral marsh erosion, and relative sea-level rise compounded 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
depth-averaged numerical hydrodynamic model to investigate the feedback
between salt-marsh erosion and hydrodynamic changes in the Venice
Lagoon, a large microtidal back-barrier system in northeastern Italy.
Numerical simulations are 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 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 marshes, partially prevented by restoration
projects and manmade protection of salt-marsh margins against wave
erosion, which have been put in place over the past few decades, limited
the detrimental effects of marsh loss on the lagoon hydrodynamics, while
not substantially changing the risk of flooding in urban lagoon
settlements. Compared to previous studies, our analyses suggest that the
hydrodynamic response of back-barrier systems to salt-marsh erosion is
extremely site-specific, depending closely on the morphological
characteristics of the embayment as well as on the external climatic
forcings.