Strategies and evaluation criteria for retention areas in reconstruction processes – new perspectives and open research questions based on the case study Ahr Valley

In summer 2021 parts of Western Germany, especially North-Rhine Westphalia and Rhineland Palatinate was hit severely by devastating floods resulting from  the cyclone "Bernd" with almost stationary and high precipitation of 115 mm rain in 72 h (Kreienkamp et al. 2021). The resulting damage of the flood event was the highest since decades in Germany with a high number of damaged buildings and infrastructure and around 189 fatalities (DKKV 2022). The Ahr-Valley in the federal state of Rhineland-Palatinate was the most severely affected region.

After one and a half years, the region and affected municipalities are still in the process of reconstruction and facing enormous challenges. Anyway, in order to better cope with future events the responsible planning authorities strive to develop strategies for flood prevention and flood risk reduction. In this regard, the identification of areas for effective water-retention measures and the designation of open corridors for runoff with reduced damage potential plays a crucial role. Possible measures range from adaptations in land use and cultivation patterns or nature based solutions to technical retention systems. Due to this variety of possible measures, which can have different resource requirements and different legal or financial dependencies, a systematic evaluation of potential measures is necessary.

Against this background, this study aims at answering the questions on how to systematize and prioritize measures for potential water-retention areas and lowering flood damage potential while considering constraints and challenges during the reconstruction process after a major flood. In addition to common planning criteria, aspects of land allocation need to be taken into account and different levels of feasibility of a certain measure need to be considered. Therefore, we analyze and systematize the multitude of possible retention measures. Based on this research and on expert discussions with relevant actors in the Ahr-Valley we present and discuss a set of criteria for evaluation.

The results of this study can help to inform political decision makers and planning authorities for land use and urban planning and support decisions on allocation of areas. Furthermore, the paper discusses which measures and strategies should be prepared long before a flood disaster for successful preparedness and improvement of flood prevention.

References

DKKV (Hrsg.,2022): Die Flutkatastrophe im Juli 2021. Ein Jahr danach: Aufarbeitung und erste Lehren für die Zukunft.DKKV-Schriftenreihe Nr. 62, Bonn

Kreienkamp F, Philip SY, Tradowsky JS, Kew SF, Lorenz P, Arrighi J, Belleflamme A, Bettmann T, Caluwaerts S, Chan SC (2021) Rapid attribution of heavy rainfall events leading to the severe flooding in Western Europe during July 2021