Storm-surge barriers induce loss of geomorphic diversity in shallow tidal embayments

Flood regulation systems and storm-surge barriers are increasingly being employed to safeguard coastal cities worldwide from the threats of flooding linked to rising sea levels. Prominent examples include London, the Netherlands, New Orleans, St. Petersburg, and Venice. While these barriers effectively control the propagation of surges and tides, they also induce changes in sediment transport, consequently altering the morphological evolution of estuarine systems near the protected urban areas. However, the precise impact of flood regulation on the morphodynamic evolution of tidal environments remains an unanswered question.Our study delves into the effects of recently activated storm-surge barriers, designed to protect Venice (Italy) from flooding, on the morphological evolution of the Venice Lagoon. This investigation combines numerical modeling with field data.The artificial reduction of water levels influences the interaction between tides and waves, leading to increased sediment resuspension on tidal flats. However, the deposition of resuspended sediments on salt marshes is importantly reduced by the diminished flooding intensity and duration resulting from artificially-lowered water levels. This situation could potentially compromise the marsh's ability to adapt to rising sea levels via mineral sediment deposition. At the same time, eroded sediments tend to accumulate within channels, thereby impeding water exchange and escalating dredging costs.Over longer (i.e., seasonal) timescales, we propose that while barrier closures decrease net sediment export to the open sea, they do not necessarily preserve the characteristic lagoonal morphology. Instead, this may hinder salt marsh vertical accretion, promote tidal flat deepening, and lead to channel infilling. Consequently, the operation of flood barriers could trigger a significant loss of tidal landforms, adversely impacting the preservation of shallow tidal environments and the valuable ecosystem services they provide.