A systematic screening for potential retention areas in the Ahr valley

The flood event in the Ahr valley in mid-July 2021 surpassed all previous events since the installation of gauging stations on the river (Szymczak et al. 2022) between 1952 and 1991 (Land NRW 2022; LfU 2013a, 2013b). The high-energy flash flood caused severe damage (Kahle et al. 2022; Schüttrumpf et al. 2022), and several gauging stations were either destroyed or malfunctioned (Berkler et al. 2022).As a part of flood prevention efforts, the systematic evaluation of potential retention areas supports the development of recommended measures. Depending on the availability of land and the type of flood event, technical measures can provide different retention volumes. These systems are effective for floods with different annual frequencies. If the flood of mid-July 2021 is used as a benchmark and if retention potentials should be activated for events of HQ100 or higher, then approximately 20-35 million cubic meters of retention space would need to be kept available according to initial estimates. Natural measures, such as floodplain restoration and adapted land use practices, can provide additional retention potential.The Ahr valley is very steep, with narrow forelands shaped by past floods. These conditions limit the potential for technical retention measures such as large dams. Additionally, the largest tributary only accounts for around 12 % of the total catchment area, thus no obvious location for a single universal retention measure exists. Around 55 % of the Ahr catchment is covered by forest, while around 7 % is urban or sealed, and vineyards only make up 1 %. However, improvements in land use practices and retention measures should still be considered. In the Ahr valley, there are approximately 31 square kilometres (~3 % of the catchment) of areas with a surface slope below 10 degrees outside of urban and industrial areas where the feasibility of retention measures can be examined. Promising areas must be balanced with a multitude of conflicting interests, such as nature conservation, regional and urban planning, or the interests of land owners.Berkler SB, Bettmann T, Böhm M, Thomas N, Gerlach N, Hengst A, Henrichs Y, Heppelmann T, Iber C, Johst M, Lehmann H, Stickel S, van der Heijden S, Wallisch S (2022) Bericht: Hochwasser im Juli 2021, Mainz.Kahle M, Kempf M, Martin B, Glaser R (2022) Classifying the 2021 ‘Ahrtal’ flood event using hermeneutic interpretation, natural language processing, and instrumental data analyses. Environ. Res. Commun. 4:51002. https://doi.org/10.1088/2515-7620/ac6657Land NRW (2022) ELWAS-WEB: dl-de/by-2-0 (www.govdata.de/dl-de/by-2-0).LfU (2013a) Messdaten: Pegel Bad Bodendorf / Gewässer: Ahr. Messdaten: Pegel Bad Bodendorf / GewässerLfU (2013b) Messdaten: Pegel Müsch / Gewässer: AhrSchüttrumpf H, Brüll C, Klopries E-M, Lehmkuhl F, Wolf SS (2022) Das Hochwasser 2021 in Nordrhein-Westfalen und Rheinland-Pfalz : Beobachtungen und Erfahrungen. In: Wasserwirtschaft im Klimawandel": digitale 55. Essener Tagung für Wasserwirtschaft, Institut für Siedlungswasserwirtschaft der RWTH Aachen UniversitySzymczak S, Backendorf F, Bott F, Fricke K, Junghänel T, Walawender E (2022) Impacts of Heavy and Persistent Precipitation on Railroad Infrastructure in July 2021: A Case Study from the Ahr Valley, Rhineland-Palatinate, Germany. Atmosphere 13:1118. https://doi.org/10.3390/atmos13071118