Use of proxy-documentary and instrumental data to assess the risk factors leading to sea flooding in Venice

Due to its geographical position, and because it was built at sea level, Venice is extremely sensitive to climate changes and, in the future, risks being submerged as a consequence of the expansion of oceanic water in response to global warming. A key problem is the increasing frequency of storm surges with the sea flooding the city (locally named acqua alta) that has reached an unsustainable level. After careful examination of both proxy documentary and instrumental data, it has been possible to reconstruct the extreme weather conditions and natural hazards, which have taken place in the last two millennia, with particular reference to surges. A key problem with proxy data from written sources is that periods with missing data may be misinterpreted as ‘normal’, or that inaccurate reporting may lead to duplication or even multiplication of events. For this reason, the accuracy of the proxy data has been checked by comparing it with other known events. An accurate series of the flood surges has been obtained combining instrumental observations (1872–2000) with documentary data for the previous period. The period of minimum solar activity of Spörer (1416–1534) was anomalous for surges. Not only have meteorologically perturbed periods been identified, but also the effects of the accelerated subsidence of the city and of certain works undertaken in the lagoon that varied the exchanges between the sea and the lagoon. The impact of subsidence and of the modified dynamical exchanges of the lagoon on the change in flooding frequency has been established. During the instrumental period, the tide gauge measured a 31-cm rise. The frequency of surges has dramatically increased since the 1960s, reaching about two flooding events per year, the greatest value since 792 AD.