Preprints
https://doi.org/10.5194/nhess-2022-225
https://doi.org/10.5194/nhess-2022-225
 
30 Aug 2022
30 Aug 2022
Status: this preprint is currently under review for the journal NHESS.

A multi-disciplinary analysis of the exceptional flood event of July 2021 in central Europe. Part 2: Historical context and relation to climate change

Patrick Ludwig1,2, Florian Ehmele2, Mário J. Franca3, Susanna Mohr1,2, Alberto Caldas-Alvarez2, James E. Daniell1,4, Uwe Ehret1,3, Hendrik Feldmann2, Marie Hundhausen2, Peter Knippertz2, Katharina Küpfer1,2, Michael Kunz1,2, Bernhard Mühr1, Joaquim G. Pinto1,2, Julian Quinting2, Andreas M. Schäfer1,5, Frank Seidel3, and Christina Wisotzky1,6 Patrick Ludwig et al.
  • 1Center for Disaster Management and Risk Reduction Technology (CEDIM), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 3Institute for Water and River Basin Management, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 4Institute of Photogrammetry and Remote Sensing, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 5Geophysical Institute, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 6Institute of Economics, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

Abstract. Heavy precipitation over western Germany and neighboring countries in July 2021 led to widespread floods, with the Ahr and Erft river catchments being particularly affected. Following the event characterization and process analysis in Part 1, here we put the 2021 event in the historical context regarding precipitation and discharge records, and in terms of the temporal transformation of the valley morphology. Furthermore, we evaluated the role of ongoing and future climate change on the modification of rainfall totals and associated flood hazards as well as implications for flood management.

The event was among the five heaviest precipitation events of the past 70 years in Germany. However, considering the large LAERTES-EU regional climate model (RCM) ensemble revealed a substantial underestimation of return values and periods based on extreme value statistics using only observations. An analysis of homogeneous hydrological data of the last 70 years demonstrated that the event discharges exceeded by far the statistical 100-year return values. Nevertheless, the flood peaks at the Ahr River were comparable to the reconstructed major historical events of 1804 and 1910, which were not included in the hazard assessment of flood risk so far. A comparison between the 2021 and past events showed differences in terms of the observed hydro-morphodynamic processes which enhanced the flood risk due to changes in the landscape organization and occupation.

The role of climate change and how the 2021 event would unfold under warmer or colder conditions (within a –2 K to +4 K range) was analyzed based on pseudo-global-warming (PGW) model experiments. These showed that the spatial mean precipitation scales to first order with the theoretical Clausius-Clapeyron (CC) relation predicting a 7 to 9 % increase per degree warming. Using the PGW rainfall simulations as input to a hydrological model of the Ahr river basin revealed a strong and non-linear effect on flood peaks: For the +2 K scenario, the 18 % increase in areal rainfall led to a 39 % increase of the flood peak at gauge Altenahr. The analysis of the high-resolution convection-permitting KIT-KLIWA RCM ensemble confirmed the CC-scaling for moderate spatial mean precipitation but showed a super CC-scaling of up to 10 % for higher intensities. Moreover, also the spatial extent of such precipitation events is expected to increase.

Patrick Ludwig et al.

Status: open (until 23 Oct 2022)

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  • RC1: 'Comment on nhess-2022-225', Anonymous Referee #1, 19 Sep 2022 reply

Patrick Ludwig et al.

Patrick Ludwig et al.

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Short summary
Heavy precipitation in July 2021 led to widespread floods in western Germany and neighboring countries. The event was among the five heaviest precipitation events of the past 70 years in Germany and the river discharges exceeded by far the statistical 100-year return values. Simulations of the event under future climate conditions revealed a strong and non-linear effect on flood peaks: For +2 K global warming, an 18 % increase in rainfall led to a 39 % increase of the flood peak in the Ahr river.
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