Canal excavation impacts on the hydrodynamics of shallow back-barrier lagoons

Estuaries and lagoons have historically served as sheltered areas for navigation and harbours, fostering settlement and trade. Over centuries, human interventions such as channel dredging and canal excavation have reshaped these environments to accommodate increasingly larger vessels and facilitate harbour access. While these modifications offer immediate benefits for navigation purposes, they alter the delicate hydro-morphodynamic balance of shallow tidal systems, potentially intensifying erosion and vulnerability to sea level rise. Therefore, understanding the side effects and long-term consequences of dredging and excavation is essential for developing informed management strategies for back-barrier lagoons.

Here we examine the effects of canal excavation and dredging on the hydrodynamics of two back-barrier lagoon systems in the northern Adriatic Sea: the Venice and the Marano-Grado Lagoons. In the Venice Lagoon, the Malamocco-Marghera canal, excavated in 1970, is periodically dredged to a minimum depth of -10 m along its 16-km path connecting the Marghera harbour to the open sea through the Malamocco inlet. In the Marano-Grado Lagoon, a 5-km canal completed in 1969, is dredged to -6 m to connect the industrial harbours on the Corno and Ausa rivers to the Porto Buso inlet. We constructed computational grids for the pre- and post-intervention scenarios, as well as for the present-day configurations, based on available bathymetric surveys for both lagoons. Using a 2-D finite element hydrodynamic model, we simulated tidal flows in the considered configurations, setting as boundary conditions a sinusoidal tidal wave with a 0.50 m amplitude and a 12-hour period, typical of the northern Adriatic Sea.

Despite differences in morphology and intervention scale between the two cases, consistent trends emerged. Comparisons of pre- and post-intervention scenarios reveal an increase in the water discharge through the inlet connected to the excavated channel. This increased water exchange leads also to a different subdivision of the sub-basin connected to each inlet. Moreover, the increase in the ebb-phase discharge is more pronounced than that in the flood phase, indicating that channel dredging promotes a shift toward ebb-dominant conditions, with implications for water and sediment dynamics.

These findings highlight the potential long-term consequences of excavation and dredging in shallow tidal systems and emphasize the need for management strategies that reconcile navigational needs with the preservation of the morphological integrity of back-barrier lagoon ecosystems.