The Diluvium aquarum: geologic evidence and geoarchaeological constrains of extreme floods in Northern Italy during early Middle Ages

In the Mediterranean region the period following the collapse of the Roman Empire is described by many authors as an interval characterized by important alluvial processes (e.g. floods, river avulsions, alluvial aggradation in the valleys), strongly contrasting with the general geomorphological stability of Roman Age and Late Antiquity. Some important chronicles of early Middle Ages report the occurrence of very high-magnitude floods, often described by ancient historians as diluvium (deluge), contributing to create a sort of myth around an out-of-scale event or a longer meteo-climatic phase. In the framework of the INQUA project “EX-AQUA: Palaeohydrological Extreme Events, evidence and archives” (1623P), a review of the traces of early Medieval floods occurred in Northern Italy was carried out. The study considered new information and previous available data supplied by geomorphological, stratigraphic and geoarchaeological evidence, ancient documents and written sources. In the investigated area a fast and strong sedimentary phase occurred between 5th and 9th century AD, leading many large Alpine rivers to avulse. In the system of Tagliamento, thanks to recent geoarchaeological excavations and geochronological analyses, a major extreme event is clearly constrained between the second half of the 6th and the first part of the 7th century, when the river avulsed and destroyed the ancient city of Concordia Sagittaria. This episode overlaps with the one reported by the Lombard historian Paolo Diacono, dated to 589 AD, which strongly damaged Verona and, downstream of this city, possibly triggered the avulsion phase of Adige River near the so-called “Rotta della Cucca”. Important fluvial changes affected also the Piave and Livenza rivers, while an avulsion channel of Brenta River started to form in the 6th century AD. Notwithstanding, for some minor alluvial systems the detailed chronology supported by archaeological and radiocarbon chronology allows to detect the existence of earlier flooding units, formed since the 2nd and 3rd century AD. Recent data in the Alpine valley of Adige River, around the city of Trento, point towards the occurrence of some important flood events in the alluvial cones of the major tributary creeks already during the 3rd century AD. However, in the same area the floor of Adige valley experienced a vertical aggradation only since the 4th and 5th century AD, with an enhanced rate of deposition between 6th and 10th century AD. A rather comparable chronology characterizes part of the alluvial cones of the main Apennine streams flowing towards the Po Plain (e.g. near Modena), which aggraded during early Medieval, but this trend started already in the 3rd century AD. This research supports new data for comparing the palaeoflood record of early Middle Ages with palaeoclimatic proxies, with the aim of distinguishing global forcing factors from regional constrains and anthropogenic disturbances.