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Landscape reconstruction and major flood events of the River Main (Hesse, Germany) in the environs of the Roman fort at Groβkrotzenburg

Obrocki, L., Becker, T., Mückenberger, K., Finkler, C., Fischer, P., Willershäuser, T., Vött, A.
Quaternary international 2020 v.538 pp. 94-109
alluvial plains, bedrock, direct push technology, electrical resistance, floodplains, geophysics, historical records, landscapes, meteorological parameters, riparian areas, river regulation, rivers, runoff, sediments, stream channels, tomography, topography, water flow, Central European region, Germany
Detailed geoarchaeological studies were conducted at the Groβkrotzenburg floodplain (Southern Hesse, Germany) in order to reconstruct the fluvial palaeogeography and search for Roman harbour installations. Another objective was to reconstruct the local flood history based on stratigraphic and geochronological data. Prospection based on electrical resistivity tomography, vibracoring and direct push electrical conductivity logging were carried out to detect and analyze subsurface stratigraphies. Altogether, 6 sediment cores, up to 7 m long, were drilled along transects along and perpendicular to the present course of the River Main. Based on sedimentological and geochemical data, we classified four sedimentary units. Vibracore data helped to detect the pre-Holocene bedrock topography and to differentiate between sedimentary facies. We present geophysical, sedimentary and geochronological evidence of a river channel right in front of the Roman fort that was probably active during Roman times. Moreover, we found another bridge pier candidate in the prolongation of a well documented Roman bridge. Our findings further indicate that the northern riverbank has shifted towards south since Roman times. However, neither sedimentary nor archaeological evidence for harbour installations could be brought to light. Nevertheless, we assume that conditions favorable of a river harbour, namely permanent water flow and sufficient water depth, were guaranteed in front of the fort where an undercut slope was probably already existing during Roman times.Several sequences of coarse-grained, sandy to gravelly river deposits were found associated with major flood events that affected the site at Groβkrotzenburg during late Medieval and modern times. At that time, central Germany was repeatedly influenced by extreme meteorological conditions and associated extreme runoff events. Based on radiocarbon data we were able to define two periods of major flooding, namely between 1435 and 1484 cal AD and 1646–1792 cal AD. These periods are contemporaneous with periods of increased flood frequency in Central Europe, derived from historical sources, namely 1430–1460 AD and 1730–1790 AD. Based on sedimentary and geo-chronostratigraphic evidence, major flood events detected at the Groβkrotzenburg alluvial plain are most probably related with the AD 1480, AD 1573 and AD 1780 events known from historical records. Our results document that few historical flood events controlled the overall development of the fluvial landscape at Groβkrotzenburg at least since Roman times until the late 19th century, namely by eroding pre-existing fluvial deposits, accumulating thick sequences of event deposits and narrowing the riverbed. Since the late 19th century, river regulation measures severely changed the hydrological conditions of the River Main.