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.

Coastal wetlands are intertidal ecosystems based on a delicate balance between hydrodynamic, morphological, and biological processes. Increasing rates of relative sea-level rise, sediment starvation and anthropogenic pressure challenge the existence of wetlands and the ecosystem services they support, extending to water quality enhancement, carbon sequestration, and shoreline protection. Therefore, to preserve coastal wetlands and their ecosystem services, it is of utmost importance to understand sedimentation processes that drive salt-marsh vertical accretion and offset the effects of relative sea-level rise. Tidal flooding propagating via the channel and creek system is considered to be the main mechanism controlling marsh sediment supply. However, storm-induced resuspension associated with enhanced water level can importantly affect the marsh sediment budget, sustaining sedimentation on the marsh surface and signing its topography, which, in turn, affects transport processes. To better understand how tides and storm surges affect spatial and temporal sedimentation patterns in salt marshes, we investigated short-term sedimentation processes through field observation in the salt marshes of the Venice Lagoon, Italy. Sediment accumulation measurements carried out continuously from October 2018 to July 2021 in four different marshes reveal that storm-driven sediment supply accounts on average for 70% of the total yearly sedimentation, despite the brief duration of storm events. On marshes bordering channels, sediment mostly accumulates close to the marsh margin and sedimentation rapidly decreases with the distance from the marsh edge, contributing to form a levee-shaped profile. Conversely, on marshes facing tidal flats, where the action of wind waves is stronger, maximum sedimentation shows an inland displacement, creating a gently sloped, ramped transition at the marsh margin. We conclude that storm surges importantly support marsh sediment accumulation and change the spatial depositional patterns, which largely define the marsh topographic profile.

Conventional engineering measures, such as surge barriers and mobile floodgates, are being adopted in many coastal cities worldwide, threatened by the increasing flooding hazard due to rising sea levels. Famous examples include London, the Netherlands, New Orleans, St. Petersburg and Venice. However, the question of how flood regulation affects the morphodynamic evolution of shallow tidal embayments still lingers. Storm-surge barriers may importantly modify the propagation of tides, surges and wind waves, changing sediment transport and, thus, the morphological evolution of regulated tidal environments, in particular in sediment-starved systems. Combining field data and numerical modelling, we investigate the effect of the Mo.S.E. storm-surge barriers, designed to protect Venice from flooding, on the morphodynamic evolution of the Venice lagoon. Artificial reduction of water levels within the lagoon affects the interaction between tide propagation and wind waves, increasing sediment resuspension on tidal flats. Resuspended sediment hardly accumulates on salt marshes, contributing to their vertical accretion and offsetting the negative effect of relative sea-level rise, owing to the reduction of marsh flooding determined by reduced water levels. Although barrier closures temporarily reduce the sediment export toward the open sea, this does not point to preserve the characteristic lagoonal morphology, hindering salt-marsh accumulation and promoting tidal-flat deepening and channel infilling. We conclude that the operations of flood barriers can promote a significant loss of geomorphological diversity, which will critically impact the ecosystem services provided by the shallow tidal environments they are meant to protect, thus increasing the costs related to their conservation and restoration.