Challenges and Strategies in Urban Adaptation to Climate Risks: The Case of St. Mark's Square (Venice, Italy)

In recent decades, Venice has experienced increasing frequency of extreme high-tide events, driven by a combination of land subsidence, eustatic sea-level rise, and changes in climatic forces. To mitigate flood risk, an adaptation strategy was implemented in 2019 at the three inlets (Lido, Malamocco, and Chioggia) connecting the Venice Lagoon with the Adriatic Sea: the Mo.S.E. (Modulo Sperimentale Elettromeccanico) storm-surge barrier system. The barriers are activated when the sea level is expected to exceed 1.10 m a.P.S. (“above Punta della Salute”), being the 1.10 value a compromise between effective flood protection and the economic impacts due to the stop of maritime traffic during barrier closure.

Nevertheless, by the time this level is reached, about 12% of Venice is already flooded, including the iconic St. Mark’s Square (ground elevations between 0.60 and 1.10 m a.P.S.). During flooding episodes, surface runoff from the Square is conveyed into the historic drainage network (gàtoli). When high tide conditions also occur, the runoff interacts with the backwater effect within the gàtoli as saltwater rises from the Lagoon. As flooding progresses, water ponds in front of St. Mark's Basilica (0.6 m a.P.S.) and gradually expands to the rest of the Square.

Following the extreme event of 2019, a first intervention (installation of glass barriers to protect the Basilica) was implemented in 2022. However, a more comprehensive series of targeted interventions has been planned since 2020. This holistic approach extends beyond the Basilica, encompassing the entire Square to preserve the integrity of the whole area. During high tide events, the Square will thus be isolated from the Lagoon when the water level exceeds 0.7 m a.P.S. While water entering the gàtoli system through precipitation runoff, lagoon wave overtopping, and subsurface infiltration, is actively managed by the new flood defence system, efficiently conveying and discharging water outside the Square. The multi-faceted strategy includes the permanent sealing of minor gàtoli-lagoon connections and the controlled operation of major ones, as well as localized elevation of the Square's pavement. Additionally, floating breakwaters are installed to limit wave overtopping discharge, while the gàtoli tunnels are restored to enhance their conveyance capacity. A pumping station, whose strategic position allows easy maintenance procedures, ensures the required outlet discharge.

In this study, the hydraulic processes affecting St. Mark’s Square are analysed using InfoWorks ICM (Integrated Catchment Model), which couples a 1D model of the drainage network with a 2D representation of flooding dynamics over the Square. Water inputs (rainfall, groundwater infiltration, and lagoon wave overtopping) are quantified using a combination of theoretical and experimental approaches, and the overall discharge capacity of the Square is evaluated to assess the current performance of the drainage system. Subsequently, the study investigates the effects of various restoration and adaptation scenarios to assess their effectiveness in mitigating flooding under different forcing conditions.