The Artificial Fluvial Diversions in the Mainland of the Lagoon of Venice during the XVI and XVII Centuries inferred by historical cartography analysis

During the 15th c., the Lagoon of Venice was part of a large coastal belt of lagoons and swamps, up to 20 km wide, bordering the western side of the northern Adriatic coast. The area of interest, including the whole Lagoon of Venice, extends from Adige River to the Livenza River to the North. Lagoons’ extent was regulated by the presence of river deltas and lagoon inlets, being controlled in their evolution by river sedimentary loads and tidal dynamics. The lagoon inlets were menaced by progressive decreasing of water depth due to sand deposition caused by the near fluvial deltas, forming outside the lagoon (mostly Piave and Adige rivers); for that reason, the Repubblic of Venice decided to undertake an epic struggle against rivers, diverting them outside the Lagoon or turning them far away; the projects were only partially carried out, mostly during 16th and 17th c., changing the morphology of the alluvial plain and the coastal margin. The ancient hydrographical and geomorphological framework was reconstructed for the whole area (ca. than 2000 km2). More than 7000 maps were observed; 356 of them were analysed, while the most representatives were georeferenced and their main feature were redrawn. The reconstruction made use of the full cartographic data set, comparing information with historical documents and geological, geomorphological and geo-chronological data. Two case studies are presented (Piave and Brenta diversions) to illustrate actions and consequences on the geomorphology of the plain. The outcome maps give precious information on the diversion project, their effectiveness, the geomorphological changes both related to human interventions and recent climatic changes.