Geomorphological evolution of the coastal system of Bibione and geoarchaeological aspects of the area of Mutteron dei Frati and Val Grande

The research was carried out in the framework of the project investigating the Roman villa of Mutteron dei Frati and the connected amphibious area of Val Grande in Bibione, that are part of the coastal system of the Tagliamento River (NE Italy). This zone offers a significant record of the interplay between natural processes and human activity since ancient times and this work combines geomorphological mapping, elaboration of LiDAR-derived DEMs, sedimentological and stratigraphic analysis, archaeological excavations and historical cartography to reconstruct landscape transformations over the last millennia. The Tagliamento River underwent multiple avulsions along the Holocene that led to the activation of different directions and, during the Roman period, a main channel belt, defined in the historical sources as Tiliaventum Maius, used to have the mouth several kilometers west of Bibione. In that phase a minor branch, the Tiliaventum Minus, was splitting from it near Lugugnana and had its mouth slightly north of Val Grande. The dunes of Mutteron dei Frati, which reach up to 11 m above sea level, represent the highest coastal ridges in the northern Adriatic and formed between 4000 and 3000 years ago. These dunes marked a relatively stable coastline until Late Antiquity and the Roman villa was constructed on the slope of the inner one, approximately 250-350 m inland from the sea, protected by multiple sand ridges. The villa was at a short distance from a wide tidal inlet connecting the sea with the protected lagoon and the stratigraphic evidence suggests the presence of some harbor structures near the archaeological site. A dramatic geomorphological reorganization occurred during the early Middle Ages, since the 6th century AD, when the Tagliamento River underwent a major avulsion, abandoning the Tiliaventum Maius channel and activating its present direction. This phase was initially characterized by widespread coastal erosion, but then triggered the formation of the present delta, that led to the definitive closure of the formed tidal inlet near Val Grande. A new significant phase of occupation in the area of Mutteron dei Frati dune system occurred in the 16th-17th century, as documented by the Venetian maps, that depict cultivated fields and artificial embankments in Val Grande. At the end of the 17th century the area was likely a brackish environment and was transformed in a fishery. The modern coastal configuration, characterized by the cuspate delta of Bibione and Lignano, developed through alternating phases of progradation and erosion from the Medieval period to the 20th century, but now, especially the area of the river mouth, is prone to major erosive processes.