Flooding in St. Marco’s Square: Challenges and Strategies for Preservation

The Venice Lagoon connects to the Adriatic Sea through three inlets (Lido, Malamocco, and Chioggia) that enable daily tidal exchanges vital for maintaining its ecosystem. To mitigate the increasing frequency of extreme high tide events, driven by land subsidence, eustatic sea-level rise, and climate change, the Mo.S.E. (“Modulo Sperimentale Elettromeccanico”) storm-surge barrier system was implemented at these inlets in 2019. The barriers are activated when water levels exceed the safety threshold of +1.10 m s.P.S., a trade-off between flood protection and the economic implications of halting the maritime traffic during closure. Nevertheless, by the time this water level is reached, about 12% of Venice is already flooded, including its lowest point (+0.6 m s.P.S.): the iconic St. Mark’s Square. During flooding events, saline water infiltrates St Mark's Basilica, leading to progressive structural deterioration over time. To limit inundation of the square and preserve the integrity of this cultural heritage site, a series of targeted interventions have been planned and some of them have already being implemented. These include the installation of localized protective barriers around the Basilica, restoration of the historic gàtoli drainage tunnels, selective closure of lagoon-connected inlets and outlets, elevation of the square’s pavement and reinforcement of lagoon-facing walls most exposed to wave action. In this work, the hydraulic processes affecting St. Mark’s Square were analyzed using both the Storm Water Management Model (SWMM) and the InfoWorks Integrated Catchment Model. Water inputs requiring drainage (rainfall, groundwater infiltration, tidal rise, and sea overtopping) were quantified, and the square’s overall discharge capacity was evaluated to determine the current efficiency of the drainage system. Subsequently, the impact of various restoration scenarios was assessed. The findings provide an integrated understanding of system performance, enabling the identification of critical points within the drainage network and informing the design of a hydraulic pumping station for water removal. Furthermore, the comparative analysis of the two hydraulic models highlights their respective advantages and limitations.