Evaluation of the High-Energy-Flood of mid-July 2021 as a Morphologic Driver in the Ahr Valley

Abstract

Background: This study explores morphologic changes in the Ahr River, Germany, caused by the high-energy flood in mid-July 2021. Infrastructure systems influence river morphology, but causal relationships are challenging to establish. The study aims to assess the flood's significance as a morphological driver using the Lateral Mobility Index (LMI), as well as the impact of infrastructure on morphodynamics in terms of formation of mud deposits and stagnant water pools, considering three focus areas of about 1 to 5 km river length. Results: The LMI is derived from orthophotos. For the 2021 high-energy event, an increased LMI suggests an increasing change in river course and morphology. Post-flood, the LMI in focus areas 1 (furthest upstream, LMI of 1.5) and 3 (at the Rhine confluence, LMI of 3.2) surpasses the historic LMI by a factor of about 3. Focus area 3 exhibits the highest activity and focus area 2 (LMI between 0.25 and 0.70) the lowest. The area of mud deposits and stagnant water pools increased immediately after the flood, especially in focus area 3. Over the following months, initial post-flood mud deposits washed away but also rebuilt to some degree later on. Conclusions: The LMI enabled to quantitatively identify a rare and significant morphological disruption in the Ahr River’s lateral development in three focus areas, and thus, serves as additional proof that the flood of mid-July 2021 was a high-energy event impacting local river geomorphology. Infrastructure impacted the formation of mud deposits and stagnant water zones during and after the flood, with additional influence from factors such as driftwood and valley width. The data suggested a phase of resuspension of sediments after the flood, likely due to cleanup and ad-hoc river management, highlighting the need for continued river monitoring.