Coastal flooding prevention measures, such as storm-surge barriers, are being widely adopted globally because of the accelerating rise in sea levels. However, their impacts on the morphodynamics of shallow tidal embayments remain poorly understood. Here, we combine field data and modeling results from the microtidal Venice Lagoon (Italy) to identify short- and long-term consequences of flood regulation on lagoonal landforms. Artificial reduction of water levels enhances wave-induced sediment resuspension from tidal flats, promoting in-channel deposition, at the expense of salt marsh vertical accretion. In Venice, we estimate that the first 15 closures of the recently installed mobile floodgates operated between October 2020 and January 2021 contributed to a 12% reduction in marsh deposition, simultaneously promoting a generalized channel infilling. Therefore, suitable countermeasures need to be taken to offset these processes and prevent significant losses of geomorphic diversity due to repeated floodgate closures, whose frequency will increase as sea levels rise further.

In rural lowland catchments, negligible topographic gradients and possible interactions between overland and channel flows complicate efforts to predict flood formation, propagation, and inundation. In this study, we demonstrate that an approach in which a two‐dimensional shallow water model is coupled with a two‐dimensional model for the saturated flow in the topsoil layer can accurately reproduce floods in such a lowland catchment. The topsoil porous layer is treated as a confined aquifer where water ponds on the ground surface and as an unconfined aquifer elsewhere. The model includes infiltration from the ground surface into the topsoil layer and downward percolation out of the topsoil layer. The equations of both surface and subsurface models are suitably averaged over a representative elementary area to yield a subgrid model for the coupled surface‐subsurface flow. Field data collected in two rural lowland catchments in the North‐East of Italy are used to evaluate the model performance. The good agreement between computed and measured discharge at the catchments' outlet and the agreement between predicted and surveyed spatial pattern of inundated areas indicate that the model effectively reproduces overland flow and efficiently accounts for the surface‐subsurface flow interaction and the relevant subsurface processes.