Evaluation of design wave conditions for a floating breakwater to be installed in front of San Marco square (Venice, IT)

The flooding of San Marco square (Venice, IT) during high tide is not entirely prevented by the Mo.S.E. system: in fact, the elevation Marciano quay (the quay in front of S. Marco square) is just at the safeguard threshold guaranteed by the Mo.S.E., the highest accepted water level (“design water level”) corresponding to a null quay freeboard. Hence, the wave action becomes critical for the flooding of the square. Specific mitigation measures involving a combination of sheet piles and floating breakwaters to be placed in front of the quay are under investigation, aiming at reducing the flooding discharge within acceptable limits. The specific aim of this note is to evaluate the design wave conditions in front of the quay, assuming the Mo.S.E. is operating according to the design strategy. In order to achieve this aim, the wind-generated wave statistics in the Venetian lagoon is predicted assuming the “design water level” and using a consolidated spectral wave model (SWAN) based on updated statistics of the winds. The adopted model calibration is specific for enclosed basins. Comparisons of the model result with available measures show a good agreement. The design conditions have significant wave height of order 0.5 m and peak period of 3 s. Although such wave conditions are apparently mild, it is difficult to find mitigation measures that limit possible flooding while respecting cultural heritage and environmental constraints.