The Paleo-Serchio River: history of floods between Lucca and Pisa during the Roman period

Abstract

The reconstruction of flood frequency beyond the Instrumental Era is challenging and mostly based on historical sources, but it rarely covers more than the last 1000 years when abundant documentation is preserved. To investigate the long-term trends in flooding and obtain insight into current climatic changes it is necessary to extend these data to a larger number of rivers beyond the Instrumental Era and available period of historical documentation. In this paper we reconstruct the paleoflood record for the Roman Period of the Serchio River (Auser in antiquity, located in Northern Tuscany, Central Italy) using geoarcheological data. The complex hydrological evolution of the river and the development of the important cities of Lucca and Pisa on the river bank allowed an important collection of data, showing a prominent peak in flood activity during the 1st century  ce, which seems to correspond to an increase in regional rainfall interpreted from speleothem proxies. A secondary peak is present in the 6th century  ce, which corresponds locally with an increase in precipitation recorded by speleothems. The phases of increased flooding, when compared with present-day synoptical meteorological conditions, probably developed during a period of negative North Atlantic Oscillation (NAO) Index, and it is partially supported by comparison with paleoproxies for NAO. These findings confirm that an extensive collection of geoarcheological data, supported by geological and geomorphological investigation, represents a powerful tool to be integrated with historical data for the reconstruction of floods. The concomitance of local paleohydrological proxies can help in disentangling the origin of the signal from other causes.