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.