From Pleistocene gravels to Anthroposphere flooding: The July 2021 flood at Erftstadt-Blessem in a long-term context

Severe flooding in July 2021 exposed Pleistocene Rhine River gravels (upper terrace) overlain by Erft River floodplain deposits near Erftstadt-Blessem, Germany. Though now inaccessible, the site provided an opportunity to investigate fluvial sculpting caused by natural fluvial processes and human interference. Preliminary cosmogenic burial dating places the Rhine gravels to 900-1100 ka, covered by <35 ka clayey sediments of a buried channel (optically stimulated luminescence, OSL, from quartz). Further OSL ages of ~1 ka in a sandy lens and ~0.4 ka in reworked floodplain loess represent early, anthropogenically-induced sediment redistribution.

Historical maps depict a pre-19^th century Erft meander close to our site and the buried channel. Despite subsequent anthropogenic channelisation, the 2021 flood caused backward erosion that mirrored this earlier routing. Striking geomorphological features (theatre-shaped gully heads, pipes) formed, potentially resembling plunge pool and seepage erosion (sapping), as typically occurring where discharge meets sudden elevation changes and lithological contrasts are sharp, such as found at our site.

To evaluate the role of such exotic erosional processes in landform formation, we conducted structural and morphometric analyses using Structure-from-Motion Multi-View Stereo (SfM-MVS) photogrammetry. Several gullies display features characteristic of sapping-dominated landscapes. At least one pipe could have predated the 2021 flood, as indicated by internal manganese incrustations; fallout radionuclide analyses are underway to test soil particle transport through these pipes.

This multi-method study aims to enhance our understanding of Quaternary fluvial dynamics and anthropogenic impacts in the Rhine-Erft region, and how both contributed to the specific post-flood erosional patterns found at Erftstadt-Blessem.