Mapping past and recent landscape modifications in the Lagoon of Venice through geophysical surveys and historical maps

The Lagoon of Venice (Italy), the biggest lagoon in the Mediterranean, surrounds the historical city of Venice. Its fate is closely intertwined with that of the city, which is itself endangered today by sea level rise. Major modifications are ongoing to protect the city from flooding with the building of mobile barriers at the lagoon inlets. Other plans exist for future dredging of a large navigation channel close to the city center. In this context, a large geophysical survey explored the lagoon sub-bottom. In this study we reconstructed past and recent landscapes extending from the period before the creation of the lagoon up to present day. Crossing our data with environmental records provided by the city's historical archives, allowed several phases of the lagoon's evolution to be distinguished. Using radiocarbon dating and acoustical sub-bottom reconstruction, we mapped a dense network of palaeochannels corresponding to different hydrologic conditions. One of these palaeochannels related to the alluvial channel that crossed the Venice subsoil. Overall, the results of this study show that the number of channels decreased substantially over the centuries. Explanations for this reduction include natural causes, such as the increase of the mean sea level and natural subsidence, as well as human activities, such as artificial diversion of rivers and modifications of the inlets. Comparison with historical maps showed that this tendency for fewer channels increased dramatically in the last century.