the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
More than heavy rain turning into fast-flowing water – a landscape perspective on the 2021 Eifel floods
- 1GFZ German Research Centre for Geosciences, Section 4.6 Geomorphology, Telegrafenberg F427, D-14473 Potsdam, Germany
- 2University of Bonn, Department of Geography, Meckenheimer Allee 166, D-53115 Bonn, Germany
- 3University of Potsdam, Institute of Environmental Science and Geography, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany
- 4GFZ German Research Centre for Geosciences, Section 2.6 Seismic Hazard and Risk Dynamics, Telegrafenberg F427, D-14473 Potsdam, Germany
- 5University of Tübingen, Department of Geosciences, Schnarrenbergstr. 94-96, D-72076 Tübingen
- 6University of Vechta, Department II - Applied Physical Geography, Universitätsstraße 5, D-49377 Vechta
- 7Freie Universität Berlin, Institute of Meteorology, Carl-Heinrich-Becker Weg 6-10, D-12165 Berlin
- 1GFZ German Research Centre for Geosciences, Section 4.6 Geomorphology, Telegrafenberg F427, D-14473 Potsdam, Germany
- 2University of Bonn, Department of Geography, Meckenheimer Allee 166, D-53115 Bonn, Germany
- 3University of Potsdam, Institute of Environmental Science and Geography, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany
- 4GFZ German Research Centre for Geosciences, Section 2.6 Seismic Hazard and Risk Dynamics, Telegrafenberg F427, D-14473 Potsdam, Germany
- 5University of Tübingen, Department of Geosciences, Schnarrenbergstr. 94-96, D-72076 Tübingen
- 6University of Vechta, Department II - Applied Physical Geography, Universitätsstraße 5, D-49377 Vechta
- 7Freie Universität Berlin, Institute of Meteorology, Carl-Heinrich-Becker Weg 6-10, D-12165 Berlin
Abstract. Rapidly evolving floods are rare but powerful drivers of landscape reorganisation that have severe and long lasting impacts on both the functions of a landscape’s subsystems and the affected society. The July 2021 flood that particularly hit several river catchments of the Eifel region in West Germany and Belgium was a drastic example. While media and scientists highlighted the meteorological and hydrological aspects of this flood, it was not just the rising water levels in the main valleys that posed a hazard, caused damage, and drove environmental reorganisation. Instead, the concurrent coupling of landscape elements and the wood, sediment and debris carried by the fast-flowing water made this flood so devastating and difficult to predict. Because more intense floods are able to interact with more landscape components, they at times reveal rare non-linear feedbacks, which may be hidden during smaller events due to their high thresholds of initiation. Here, we briefly review the boundary conditions of the 14–15 July 2021 flood and discuss the emerging features that made this event different from previous floods. We identify hillslope processes, aspects of debris mobilisation, the legacy of sustained human land use, and emerging process connections and feedbacks as critical non-hydrological dimensions of the flood. With this landscape scale perspective, we develop requirements for improved future event anticipation, mitigation and fundamental system understanding.
Michael Dietze et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-7', Anonymous Referee #1, 28 Feb 2022
This is a laudable article providing new and important background on the geomorphology and environmental conditions that lead to the damages. It is very informative, well written and helpful to national and international scientists. The style is rather descriptive and narrative, but common in certain fields of geomorphology and geography. Maybe a methodology section could help others to understand, how the data and information was retrieved; when, where, by whom etc. As another positive aspect, the text is quite easy to understand for non-specialists, too. Very informative analysis using aerial imagery of the gravel pit in Blessem, too.
Smaller comments:
Line 17 and other areas (not just the Ahr)
Line 18: is it a hazard or rather already a "hazard event" or "process"?
Line 56 explain a bit more, what "is difficult to manage"
Line 78 large parts of text seem to be based on the author's field observations and expert experience and knowledge, it seems. It would be helpful for readers to make this more explicit in some sections, such as 78 - 86. Maybe add something such as "we have witnessed at field observations in the Ahr area after the floods" or similar. Some claims without sources are a bit risky, as in Line 86.Following text: the style is quite descriptive and narrative, as it is common in geomorphology, so I do not criticise it. But at certain claims, some more support could be added, when available. For instance, line 141-142, could you add some more detail such as (oral. com with affected citizens in VILLAGE, DATE...)?
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RC2: 'Comment on egusphere-2022-7', Anonymous Referee #2, 09 Mar 2022
This is an important, though preliminary, assessment of the non-hydrologic, “landscape”, aspects of the July 2015 extreme precipitation event in the Ahrtal and Rhein-Erft region in Germany. The work provides an important complement to purely meteorological and hydrological studies. The investigation is preliminary in the sense that it based on exemplary observations and does not attempt a full systematic recording of landscape effects, but rather tries to identify and describe a few “typical” examples illustrating a couple of process features. In this spirit the publication provides a valuable entry point for more systematic investigations and is certainly a valuable contribution worth publishing.
However, the authors, even in their preliminary approach, might provide some thoughts and further information on at lost two respects:
- Lines 286-289: “To overcome this systematic shortcoming, other systems need to be implemented, systems that are able to collect distributed multivariate data at high temporal resolution and that are not endangered by hostile flood conditions. Instead of just the main channel, such high quality flood related process information should also be available for headwater regions, where the 2021 flood gained its momentum and non-linearity.” – While this is without doubt a valid conclusion, the authors should offer at least some preliminary ideas on if and how they believe this could be achieved in terms of feasible methodological and technical approaches. What system might be envisaged? What type of parameters might be recorded? Where do we have technical/organizational solutions? How might these be integrated into next generation models and risk management? – Not a systematic exploration (which would be beyond the scope of the paper), but a couple of ideas.
- Line 195: In terms of past events the authors just mention the 2006 and 2013 events, whose intensities have considerably below the 2021 event. The authors should at least give a reason why they don´t make use the information on the larger events in the Ahrtal in 2010 and particularly 1804 which have been closer to 2021 with respect to intensity, and for which there is quite some documentation. The question on if and how to use, or not to use, this type of historical information (even if the corresponding report of course don´t reach today´s standards and the environmental and infrastructural was different), should be discussed at least shortly.
In addition, a detail comments concerns statement in lines 18-20. I wonder if nothing beyond “media report” can be cited with respect e.g. to the statement that “hydraulic models underpredicted the actual flood wave...”. The authors might try to list some more solid refeerences for this important statement than just media reports.
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RC3: 'Comment on egusphere-2022-7', Anonymous Referee #3, 23 Mar 2022
The manuscript focuses on the “boundary conditions” that lead to damages in the July 2021 flood in the Eifel region. The paper is very interesting, well written and well organized. I have just a minor comment. In my opinion, in order to better appreciate the figures, in figure 1 the hillshade and slope map must be divided from the 3-day precipitation map, the intensity-duration-frequency curve must stand alone in a single figure and, in the figures 2 and 4, it is necessary to add a map with the locations of the pictures with star signatures that must be deleted from figure 1. For these reasons I suggest to modify the figures as follows:
Figure 1. Case study and meteorological measurements. a) two of the most affected river systems, Erft and Ahr (line width indicating stream order), on top of a hillshade and slope map (red colours). Inset shows the location of the map within Germany. b) 3-day precipitation accumulated for 12–15 July 2021 from RADOLAN data (CDC, 2022).
Figure 2. Intensity-duration-frequency (IDF) curve for the weather station Weilerswist-Lommersum. Observations of 14 July precipitation are added in black based on different measurement intervals. Coloured lines depict different non-exceedance probabilities, respective shadings indicate 90 % confidence intervals.
Figure 3. Landscape features emerging from the flood. a) Locations of the pictures with star signatures. b) Focussed discharge along the hillslope causing deep and fast flow and thus efficient drainage in the background. However, the provided water is not routed downslope in the foreground but ponded by infrastructure, and released at selected spots with increased erosive stream power. c) Deposition area of the debris flow shown in (d), injecting massive debris into the main channel (Trierbach), temporally blocking the stream and causing severe reorganisation of the hydraulic geometry. d) Lateral deposits of the debris flow at the end of the valley confined section. Inset shows upstream knickpoint formed by overspill and erosion of clogged drainage pipe (50 cm diameter). e) Old slope instability (yellow line) above a 20 m high engineered terrace with industrial infrastructure on it. The terrace just east of the town of Antweiler had been undercut by the Ahr river during the flood.
Figure 4. Debris mobilisation features, Ahr valley near Müsch (cf. Fig. 1a) Aerial image (BBK-DLR, 2022) taken one day after the flood. The light green outline depicts the tree limit before the flood. Blues lines illustrate the pre-flood course of the Ahr river. b) View from the green star in (a) towards the eroded right bank, which had activated a 16 m high rockslide (persons for scale). Note flood impact mark on a remaining tree at 5 m above current water level.
Figure 5. Effects of large woody debris. a) Locations of the pictures with star signatures. b) Pair of clogged bridges near Altenahr, bypassed along the left and right bank. Note the bipartition of the collected debris with woody material caught by the downstream road bridge and anthropogenic debris collected later by the upstream railway bridge. Note two remaining standing trees in the river depicting the width of the Ahr river before the flood. Aerial image by (BBK-DLR, 2022). c) Huhnenbach near Aremberg about 2 km from its source (see Fig. 1). Note clogging by woody debris at riparian trees and the resulting ejection of coarse bed material out of the channel. d) Another clogging of the Huhnenbach some 20 m upstream of (c), with both ejected coarse debris and deposition of fine sediments in front of the obstacle.
Figure 6. Aerial image (BBK-DLR, 2022) of the town of Blessem. a) Situation shortly after the flood event, with annotated features. The top right inset (b) shows conditions before the flood. The break in slope along the margin of a gravel pit (red dashed line) had started to erode towards the town by fluvial erosion (yellow line) that formed three individual clusters. The erosion was fuelled by overbank discharge of the Erft river, evading the town of Blessem and moving down the main street as well water flowing over the field west of the town margin, following the line of steepest descent.Water flow directions are indicated by blue arrows where visible from aerial imagery. The four numbered blue triangles depict sites of increased water input towards the pit.
Michael Dietze et al.
Michael Dietze et al.
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