A multi-methodological approach to investigate the erosion of arable land caused by the July 2021 flood event in Erftstadt-Blessem, Germany

Severe flooding in July 2021 has caused massive erosion of arable land located close to the village of Erftstadt-Blessem, Germany. Backward incision formed local drainage networks, evacuating Quaternary sediments towards a gravel pit located to the north of the village. The fluvially shaped topography was largely preserved after the flooding, providing the opportunity to characterise the mode of erosion and to identify factors that governed the manifestation of processes involved. The presence of somewhat exotic erosive features (e.g. amphitheatre-shaped channel heads, natural pipes) leads us to the hypothesis that groundwater sapping could have played a major role in eroding the arable land and underlying sediments. To test this hypothesis, we use Structure-from-Motion Multi-View Stereo photogrammetry to reconstruct drainage geometries (drone imagery provided by the Kreisverbindungskommando Köln, M. Wiese; supported by T. Gersthofer, ESRI Deutschland GmbH) and for in-situ density measurements of subsurface layers. Subsurface characteristics are further assessed by grain size analyses and X-ray diffractometry. The data are used to compare the post-flood topography of Erftstadt-Blessem to sapping-shaped topography on Earth and Mars. Furthermore, 239+240Pu activities are measured in pipe infills to investigate hypothetical spatial connections to the uppermost soil horizons. Burial dating techniques (cosmogenic 26Al/10Be and optically stimulated luminescence) are used to constrain the age of the eroded sediments. Our overarching goal is to use our data as input for landscape evolution modelling to assess the importance of individual factors (e.g. substrate properties, vegetation cover) on the severity of erosion in Erftstadt-Blessem and comparable settings.