The flood of the 6th century ad in the Tagliamento River (NE Italy): geomorphological and geoarchaeological evidence of the Medieval Diluvium

In the Mediterranenan basin, strong geomorphic processes occurred in several areas during the early Middle Age. In some alluvial plains a fast and strong activity phase was triggered and this led many rivers to avulse and/or to form widespread alluvial depositional units. According to some scholars, the severe flooding episode described by the Lombard historian Paolo Diacono as the aquae diluvium of 589 AD, corresponds to one of the most significant phases occurred in Northern Italy, even if its importance and areal extent is debated (Cremonini et al., 2013). Major changes are clearly testified in the alluvial systems of NE Italy and a wealth of information is especially documented in the alluvial megafan of Tagliamento River, both in the stratigraphic and in the archaeological records. Between the 5th and 9th century, the Tagliamento River experienced its last strong avulsive phase, with the abandonment of the path followed since the beginning of the 1st millennium BC (Lugugnana branch) and the activation of the branches of Concordia Sagittaria and Latisana; these two directions are 15 km a part one from the other but, at the end of this period, only the branch of Latisana survived. The temporary activation of the branch of Concordia is clearly documented by the alluvial unit that sealed large sectors of the Roman city of Concordia Sagittaria with a thickness of 1-5 m of silty sands, burying also the floor of the palaeo-Christian basilica, which is dated to the first part of the 6th century (Fontana, 2006; Fontana et al., 2014). In Cordovado, 10 km upstream of Concordia, a recent excavation exposed part of a gravelly braided palaeochannel with several embedded trunks, up to 5-meters long. Their radiocarbon dating measured a calibrated age between the second half of 6th century and the first part of the 7th, well matching the time of the hydraulic disaster described by Paolo Diacono (Frassine et al., 2014). Along the present direction of Tagliamento, the river formed a fluvial ridge reaching up to 4 m over the surrounding floodplain and sealed the Roman landscape. This elevated landform rapidly became an attracting feature for the early Medieval villages and, in Latisana, its first occupation is dated at least since the 11th century. The depositional period coinciding with the avulsive phase was also characterized by the structuration of the present cuspate delta of Tagliamento, consisting of 2 wings where now the tourist cities of Lignano Sabbiadoro and Bibione are present (Fontana, 2006). Since early studies, many Authors considered this Early Medieval period both for its archaeological and geomorphic importance, trying to sort out the natural from the human-induced component. Several papers stressed the importance of the coincidence between an important natural event/events and the lack of territorial management, particularly when compared to the strong anthropogenic control occurred in the centuries before. New stratigraphic and geomorphologic data from the megafan of Tagliamento support the assessment of the magnitude of the natural component (mainly driven by climate) of the alluvial processes occurred in the early Middle Ages. In particular, the chronological information allows to constrain one of the main flooding phases between the second half of the 6th and the first part of the 7th century. This evidence confirms the importance of so-called “Diluvium” described by Paolo Diacono but, in the Tagliamento River, the alluvial period lasted until the 10th-11th century.