Flooding of Piazza San Marco (Venice): Physical Model Tests to Evaluate the Overtopping Discharge

Abstract

This paper aims at evaluating the wave overtopping discharge over the pavement of “Piazza S. Marco” (Venice) in order to select the best option to mitigate the risk of flooding of the Piazza and to protect the monuments and historic buildings, e.g., the “Basilica S. Marco”. In fact, the MO.S.E. (MOdulo Sperimentale Elettromeccanico) system is designed to keep the water level below a certain value, for the safety of the lagoon, but it does not guarantee the defence of the Piazza, where flooding is still possible, being its pavement locally much lower than the maximum expected water level. To completely defend the Piazza, specific additional works are planned to prevent the back-flow through the natural drainage system (now the primary pathway) or by filtration or by overtopping. This paper investigates on the overtopping mechanism, under conditions compatible with a fully operational MO.S.E. system, through 2-D experiments. The pavement of the Piazza is gently sloping towards the masonry quay which, in some parts is formed by 5 descending steps, and in some other parts, is just a vertical wall. Close to the “Marciana” Library, a critical part is present, with a slightly lower crest freeboard. In total, three cross-sections were examined in the 36 m long wave flume of the Padova University. The test programme includes 10 irregular wave attacks and three different water levels. The test results differ considerably from the results of the available formulas, since the investigated cross-sections by far exceed their range of applicability. The presence of the steps affects only the reflection coefficient rather than the overtopping discharges. In general, if the waves incident to the Piazza are higher than 40 cm, which is a possible scenario, some other adaptation works must be considered, such as the pavement rise, temporary barriers or the reduction of the waves impacting the quay through, for instance, floating breakwaters.

Physics-based Bathymetry and Water Quality Retrieval Using PlanetScope Imagery: Impacts of 2020 COVID-19 Lockdown and 2019 Extreme Flood in the Venice Lagoon

“…The hydrodynamics of the lagoon is affected by several factors, such as meteorological conditions (e.g., wind waves), fluvial inflows, and variations in tidal levels [38]. To assess the performance of WASI on the PlanetScope data, two extreme events are considered: (i) the COVID-19 lockdown in spring 2020, and (ii) a sequence of floods occurred in mid-November 2019 -unprecedented since 1969 -which were driven by a combination of extreme high tides, severe storms, and inland water inflows [39]. For the former event, we have analyzed three images acquired before (23 January, 15 and 28 February 2020) and three images captured during (19 March,8 and 13 April 2020) the COVID-19 pandemic lockdown (Figure 1a,b).…”

A Spatial Structure Variable Approach to Characterize Storm Events for Coastal Flood Hazard Assessment

“…Recently, two extreme events occurred along the Venetian littoral in 2018 (Vaia storm described in [30,31]) and in 2019 ( [32,33]). The first event was characterized by an extreme significant wave height of 5.92 m and a sea level of 1.56 m relative to the local tidal reference (named ZMPS, Zero Mareografico Punta Salute), conversely, the 2019 event was characterized by an extreme sea level of 1.82 m ZMPS and a wave height of 3.5 m. Both events caused localized erosion and marine ingression along the Venetian coast.…”

Adaptation Works to Avoid the Flooding of Piazza San Marco (Venice): Physical Model Tests to Evaluate Overtopping Discharge

“…The measured overtopping discharge for Configurations A1 and A2, relative to a cross section with vertical wall and a temporary defense barrier located at different positions, will be hereafter compared to the results presented in Ruol et al (2020) for Configuration A, that is the same cross section, in the absence of any barrier.…”

“…is closed, with reference to the local chart datum (0.23 m below m.s.l.). COASTAL ENGINEERING PROCEEDINGS 2020 Ruol et al (2020), in similar tests carried out in the absence of any mitigation measure, showed that the critical stretch of the quay is placed in proximity of the "Marciana" library. In this area, with the highest water level (1.1 m CD), almost all the tested wave states caused overtopping.…”

“…The expected overtopping discharge was studied in the laboratory by the Authors (Ruol et al, 2020) in the absence of any mitigation measure. The experiments showed that, under waves higher than 50 cm (an event that is expected to occur a few times per year), the overtopping is significant for all the tested water levels.…”