Ahr valley

Text by Rainer Bell, Wolfgang Kron, Benni Thiebes and Annegret Thieken (link to full text below, German only)

The July 2021 flood disaster in Germany

Various regions in Europe were hit by extreme rainfall from July 12 to 19, 2021 generated by a quasi-stationary atmospheric low pressure system named “Bernd”. The mainly affected areas were two federal states in western Germany and adjacent regions in Belgium. The July flood was the costliest natural disaster in Germany in recent history, with losses in the order of 33 billion euros (USD 40 billion). At least 189 people died, more than in any other flood in Germany in the past 50 years. Some narrow valleys in the Eifel mountain range were overwhelmed by torrential waters that arose within a span of just a few hours and reached levels never seen before. Extreme destruction due to the flooding, floating debris, bank erosion and deposition occurred. Local traffic infrastructure, power, gas, and water supply as well as the telecommunication networks were damaged and disrupted.

Whole catchments of the affected regions in RLP and NRW witnessed high rainfall depths. The rain hit an area with high soil moisture and largely exhausted retention capacity of the ground. Widespread surface runoff and sometimes even sheet flow was the consequence. The water was channeled in the often very narrow valleys of the rivers Ahr, Erft, Rur, Kyll, Prüm, Wupper, Ruhr and their tributaries. Additionally, local flash floods happened without a larger watercourse being involved at all.

24-hr precipitation in the disaster area of western Germany from 2021/7/14, 5:50 UTC to 2021/7/15, 5:50 UTC (processed by R. Bell from DWD RADOLAN data (combined radar and station-based data)) © Rainer Bell/GIUB

Streams and rivers overtopped their banks almost everywhere. Massive erosion, scouring and undercutting of hillslopes, roads, railways, and buildings took place and trees fell. At many places, water stages were considerably increased after clogging of bridges. At Altenahr gage the water level in the Ahr river exceeded the stage of 9 m, more than 8 m higher than the normal value at mean flow of 7 m3/s and more than 5 m higher than the officially estimated 100-year flood level at 241 m3/s. As much as 10 m were actually observed in Altenahr, but this may be biased by the local hydraulic situation (e.g., sediment deposits or water backup).

Although heavy and disastrous rainfall had been forecast by the weather services a few days ahead, the early warning process and evacuation did not work well. Warnings from the issuing agency did not reach many of the intended recipients in a timely manner and automated early warning systems were either unavailable or did not function properly. Hence, in some places, evacuation measures came too late or not at all. Additionally, many of those that received warning notifications underestimated the severity of the approaching event. Altogether, the low pressure system “Bernd” brought exceptional rainfalls in large parts of western, central and eastern Europe that led to floods and losses in the United Kingdom (esp. London), France, Switzerland, the Benelux countries, the Czech Republic, Austria, Italy, Poland, Slovakia, Hungary, the northern Balkan countries, Romania and Bulgaria, but – except for Belgium – the consequences were far less than in RLP and NRW.

Full text (German) via:

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

Our research contribution

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Landslide in Müsch

Text by Till Wenzel, Rainer Bell, Michael Dietze, Lothar Schrott, Alexander Beer, Anika Braun, and Tomas Fernandez-Steeger

Besides the damage in the floodplains, multiple shallow landslides were triggered along the Ahr embankments. Furthermore, the flood caused undercutting of several old landslide bodies. One such landslide in Devonian Schist bedrock is located at a narrow, bended stretch of the Ahr, near the town of Müsch. As part of his master thesis supervised by Prof. L. Schrott and Dr. Rainer Bell, Till Wenzel is gaining an in-depth understanding of the landslide causes and its transient activity using a multi-method approach: landslide mapping, analysis of pre- and post-event airborne laser scanning (ALS) data, electrical resistivity tomography (ERT), seismic refraction tomography (SRT), passive seismic monitoring, geotechnical analysis and interviews with local inhabitants.

The combination of geophysical and remote sensing methods enables a profound insight into the mechanisms and present processes of the Müsch landslide. Based on this, we will be able to assess the probability for a reactivation of the whole landslide body, which could trigger cascading hazards affecting a much larger region. An improved monitoring concept will be developed which can be adopted to similar structures in the Ahr valley and beyond.