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Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022
https://doi.org/10.5194/nhess-22-1845-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
More than heavy rain turning into fast-flowing water
– a landscape perspective on the 2021 Eifel floods
Michael Dietze1,2, Rainer Bell3, Ugur Ozturk4,5, Kristen L. Cook5, Christoff Andermann1, Alexander R. Beer6,
Bodo Damm7, Ana Lucia6, Felix S. Fauer8, Katrin M. Nissen8, Tobias Sieg4, and Annegret H. Thieken4
1Section 4.6 Geomorphology, GFZ German Research Centre for Geosciences,
Telegrafenberg F427, 14473 Potsdam, Germany
2Faculty of Geoscience and Geography, University of Göttingen,
Goldschmidtstraße 5, 37077 Göttingen, Germany
3Department of Geography, University of Bonn, Meckenheimer Allee 166, 53115 Bonn, Germany
4Institute of Environmental Science and Geography, University of Potsdam,
Karl-Liebknecht-Str. 24–25, 14476 Potsdam, Germany
5Section 2.6 Seismic Hazard and Risk Dynamics, GFZ German Research Centre for Geosciences,
Telegrafenberg F427, 14473 Potsdam, Germany
6Department of Geosciences, University of Tübingen, Schnarrenbergstr. 94–96, 72076 Tübingen, Germany
7Department II – Applied Physical Geography, University of Vechta, Universitätsstraße 5, 49377 Vechta, Germany
8Institute of Meteorology, Freie Universität Berlin, Carl-Heinrich-Becker-Weg 6–10, 12165 Berlin, Germany
Correspondence: Michael Dietze (mdietze@gfz-potsdam.de)
Received: 16 February 2022 – Discussion started: 18 February 2022
Revised: 24 April 2022 – Accepted: 27 April 2022 – Published: 2 June 2022
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 sub-
systems and the affected society. The July 2021 flood that
particularly hit several river catchments of the Eifel region
in western 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. In-
stead, the concurrent coupling of landscape elements and the
wood, sediment, and debris carried by the fast-flowing wa-
ter 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 differ-
ent from previous floods. We identify hillslope processes, as-
pects 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 im-
proved future event anticipation, mitigation, and fundamental
system understanding.
1 Introduction
The 14–15 July 2021 flood in western Germany as well as
parts of Belgium and the Netherlands revealed the unpre-
paredness of societies, policymakers, and scientists across
many dimensions. The anticipated precipitation amounts had
been communicated several days ahead; hydrological mod-
els and numerous stream gauges were in place; and informa-
tion chains and authority responsibilities were implemented.
Yet the flood, which hit several catchments that originate
in the Eifel (Fig 1), developed into a massive hazard event
that propagated downstream for long distances and persisted
for many hours. Meanwhile, and evident from media reports
focussing on the preceding rain event and the evolution of
Published by Copernicus Publications on behalf of the European Geosciences Union.
1846 M. Dietze et al.: Eifel floods
the flood, mitigation efforts remained insufficient; the ac-
tual flood wave was underpredicted; and emergency activi-
ties failed to prevent a disaster that led to 184 fatalities and
2 missing persons (as of 24 November 2021) in Germany
alone (Thieken et al., 2021). This unexpectedly high toll is
the highest in Germany for the past 6 decades.
The meteorological driver of the crisis was a cyclone
named “Bernd” (Schneider and Gebauer, 2021). It travelled
from the North Atlantic via France towards central Europe.
Over western Germany, its propagation speed was slowed
down by an anticyclone over eastern Europe, causing al-
most stationary precipitation over the Eifel region from 12
to 15 July, releasing 115mm rain in 72 h on average in
for example the Ahr catchment (Junghänel et al., 2021),
with a maximum value of 157 mm on 14 July at the DWD
(Deutscher Wetterdienst) station Köln-Stammheim. The soils
in the entire region were already mostly saturated due to fre-
quent previous rain events, which is expressed by an average
free storage capacity <70 mm within the soils’ top 60 cm as
well as an average soil field capacity of 80%–100 % (DWD-
Agrowetter, 2022). The meteorological situation was prop-
erly forecasted days in advance by several weather predic-
tion models (DWD, 2021; Schneider and Gebauer, 2021).
The discrepancy between the accuracy of the meteorological
forecasts and the shortcomings of flood hazard forecasting
and communication reveals some of the challenges involved
in anticipating the impacts of extreme events.
The next element of the evolutionary trajectory of the cri-
sis was the surface runoff of excess rainfall that was not able
to infiltrate into the ground and hence triggered several non-
linear processes, positive feedbacks, and process connections
(Dietze and Ozturk, 2021). Altogether, these dynamics am-
plified the impact of the flood on the landscape, particularly
in the anthropogenic realm. To understand these dynamics,
it is necessary to first examine the different landscape ele-
ments activated by the event and then to explore their modes
of interplay. The flood hit several European countries. In Ger-
many particularly, it impacted two geomorphically distinct
regions: the Eifel with the Ahr valley to the south and the
Lower Rhine Bay with the Erft catchment to the north. The
Eifel is a typical low mountain range with steep, deeply in-
cised valleys (see Fig. 1 for the slope signature). These val-
leys cut through numerous Paleozoic lithologies with varying
degrees of fracturing, crack orientation, and ground perme-
ability, which impose a pronounced predisposition to grav-
itational mass movement (Damm et al., 2010). In general,
hillslopes are covered by Pleistocene, 1–3 m thick periglacial
cover beds that are largely unstable due to the specific grain-
size distribution, sensitive to water supply, and frequently
incorporated in landslides (Bell, 2007; Damm et al., 2013).
Rockfall-dominated cliffs are developed on steep slopes. In
contrast, the Lower Rhine Bay region including the Erft
catchment is an area of subsidence with smooth topogra-
phy, filled by highly permeable Cenozoic sediments with a
widespread cover of Quaternary loess deposits. There is no
susceptibility to gravitational mass movement in this land-
scape, except for steep landforms that predominantly result
from man-made construction (e.g. road escarpments, waste
dumps, open-pit mining; cf. Fig 1). Both regions hold a long
legacy of human land use.
Although the official data collection on economic flood
impacts in Germany’s federal states of North Rhine-
Westphalia and Rhineland-Palatinate is ongoing; current es-
timates point to EUR 33 billion in damage to private house-
holds, infrastructure, forestry, and agriculture as well as viti-
culture enterprises (Fekete and Sandholz, 2021). In Germany,
the Ahr valley was hit hardest: 62 out of 75 bridges were de-
stroyed, and almost all wineries were heavily affected (BMI,
2021). In Rhineland-Palatinate at least 65 000 people were
directly affected by the event. A total of 135 people lost their
lives; at least 766 people were injured; and 2 are still missing
(Schmid-Johannsen et al., 2021). However, societal impacts
occurred not only during the event itself. Months later, in-
ner cities remain severely damaged, and commercial and gas-
tronomy businesses are still heavily disrupted. Their reopen-
ing depends on the timely and simultaneous restoration of
key infrastructure like electricity, telecommunications, wa-
ter, and sewage. Disruption also affects schools and childcare
facilities in some places, putting another burden on affected
families. Physiological illnesses as well as psychological im-
pacts on people who lost their homes, relatives, and friends
during the flood represent a long-term legacy of the flood
event beyond the areas actually impacted by the flood. For-
mer floods in Germany revealed that a devastating experience
preoccupied affected people for years (Thieken et al., 2016).
Therefore, socio-psychological support is a key to recovery.
Based on empirical field studies during, immediately af-
ter, and over several weeks after the flood, we propose four
critical non-hydraulic dimensions of flood-related processes.
We present examples for each of these dimensions and dis-
cuss consequences of their interaction. These representative
case studies form the basis of our synthesis of requirements
to improve mitigation efforts for future events.
2 Method overview
In this text, we decisively pursue a descriptive approach, fo-
cussing on generalised implications of systematic findings
during field mapping campaigns carried out early after the
flood event. There are several reasons for this approach. The
amount and quality of data during the flood, such as gauge
data, satellite imagery, and on-site instrumental data, are lim-
ited due to abundant cloud cover. Likewise, post-flood re-
mote sensing data (BBK-DLR, 2022) is available for the
main valley sections but not for headwater systems, where
the flood gained its momentum and non-linearity (Dietze and
Ozturk, 2021). Finally, model results that are currently pro-
duced do not capture the non-hydrodynamic dimension of
the flood, while high-resolution 3D terrain information, es-
Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022 https://doi.org/10.5194/nhess-22-1845-2022
M. Dietze et al.: Eifel floods 1847
Figure 1. Map of the affected area with 3d precipitation accumu-
lated for 12–15 July 2021 from RADOLAN data (Radargestützte
Niederschlagsanalyse und -vorhersage; CDC, 2022). Stars indicate
locations of pictures of other figures in this article. Light-blue lines
depict rivers of special interest mentioned in the text. Background
map shows two of the most affected river systems, Erft and Ahr
(line width indicating stream order), on top of a hillshade and slope
map. Inset shows the location of the map within Germany.
sential for quantitative evaluation of erosion and deposition
patterns, is currently still being produced (Bell et al., 2022;
Wenzel et al., 2022).
Nevertheless, we are able to constrain the antecedent con-
ditions; driver mechanisms; and, to some extent, the im-
pacts of the flood event based on instrumental data. Spa-
tially resolved precipitation information (Weigl and Winter-
rath, 2009; CDC, 2022) such as 1 km2gridded hourly data
is stacked as 72 h sums throughout the main event dura-
tion (12–15 July 2021). In addition, instrumental station data
(CDC, 2022) of representative sites were used to characterise
the severity of the rain event. We fitted intensity–duration–
frequency (IDF) curves to historical annual maximum pre-
cipitation measurements. The IDF model was fitted with
the extended consistent quantile estimation method (Fauer
et al., 2021) and evaluated for the DWD station Weilerswist-
Lommersum (Fig. 2).
Flood-affected areas and the surface change due to erosion
and deposition of sediment and large woody debris were in-
spected by analysing aerial RGB (red–green–blue) imagery.
The material was collected 1 d after the flood and allowed
for a comparison of emerging surface features with imagery
from months before the event (BBK-DLR, 2022). However,
the availability and format of the data (web service) only al-
lowed for non-quantified, descriptive studies.
Core information about the flood event is based on exten-
sive and multi-temporal field mapping campaigns through-
out the wider Eifel region, decisively including headwater
regions and their coupling with tributaries of the Ahr River.
Surveys of flood water marks, erosional features along chan-
nels and hillslopes, deposited sediment bodies caused by
ponding and hydrodynamic ejection of bed material, and
large woody debris accumulations were carried out 1 d after
the flood and repeatedly within 2–3 weeks and 2–4 months.
Surveys included the documentation of the type of feature
as denoted above, information on the location and size, and
a description of the geomorphic process regime responsible
for its creation. The material presented in this text is limited
to representative examples because a quantitative regionali-
sation needs to be based on post-event high-resolution terrain
information (see above).
3 Flood dimensions and emerging aspects
3.1 Hydrometeorological dimension
According to Fig. 1 based on radar and station observa-
tions (RADOLAN data set; Weigl and Winterrath, 2009) the
precipitation amount for the 72 h period of 12–15 July at
Weilerswist-Lommersum is similar to the amount observed
in Blessem (Fig. 1). Moreover, the time series at the selected
station is sufficiently long for the calculation of robust IDF
curves (daily precipitation is available since 1905, as well as
hourly precipitation since 2004). The analysis confirms the
unusual extreme character of the precipitation amounts dur-
ing the July event (black lines and crosses in Fig. 2). Sub-
daily as well as daily and multi-day amounts exceeded the
values corresponding to a 500-year return period determined
under the assumption of stationary climate conditions.
3.2 Hillslope dimension
In typical scenarios, hillslope runoff delivers unconcentrated
Hortonian or saturation overland flow to the streams, where
discharge can accumulate and build up deeper and faster
hydrographs. For the July 2021 flood, during post-event
mapping campaigns we witnessed several occasions where
ephemeral headwater drainages had turned into streams, con-
veying overland flow some decimetres deep (Fig. 3a). These
local phenomena allowed overland flow velocity to increase
by 1 to 2 orders of magnitude (Dietze and Ozturk, 2021),
which was further amplified by the effects of infrastructure
(see Sect. 2.3), such as forest tracks with rain ditches running
alongside. While the tracks perpendicular to slope aspect
were extensive runoff collectors (evidenced by systematic
high water marks throughout mapped regions), the ditches ef-
ficiently conveyed collected water to release spots that acted
https://doi.org/10.5194/nhess-22-1845-2022 Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022
1848 M. Dietze et al.: Eifel floods
Figure 2. IDF curve for the weather station Weilerswist-
Lommersum. Observations of 14 July precipitation are added in
black based on different measurement intervals. Coloured lines de-
pict different non-exceedance probabilities; respective shadings in-
dicate 90 % confidence intervals.
as new gully heads (based on freshly developed erosional
features). Once such gullies had formed, it is physically plau-
sible that a positive feedback loop was implemented (Molina
et al., 2009; Anderson and Anderson, 2010), leading to in-
creasingly faster and more erosive discharge towards the val-
ley bottoms. Hence, during the flood, concurrent landscape
reorganisation, namely gullying by a concentration of over-
land flow amplified by infrastructure, most likely played a
key role in changing the drainage efficiency. This spatial
feedback is not accounted for in commonly applied hydraulic
models.
Hillslopes contributed material not only through uncon-
centrated overland flow and focussed fluvial processes but
also via gravitational mass wasting, most importantly in the
form of debris flows and shallow and deep-seated landslides.
During field mapping campaigns immediately after the event,
we documented numerous debris flows emerging from hill-
slopes. These debris flows altered the hillslopes by erosion of
soil and vegetation and severely impacted the channels that
drain the valleys. They injected coarse particles and woody
debris into the channels, which had a series of consequences.
In many cases, the channel thalweg was displaced, or the en-
tire trunk river was relocated within the flood plain; the local
gradient was changed; and in some cases the entire flow was
blocked at least temporarily (Fig. 3b). These abrupt changes
have follow-on effects on channel morphology, for exam-
ple bed armouring due to sediment sizes exceeding transport
capacity, and the excavation of the bed including bedrock
downstream, with enhanced capacity of future bedload trans-
port and potential knickpoint migration upstream (Berger
et al., 2011; Hu et al., 2021). Most of the mapped debris
flow source points were locations where excess water was
able to enter a debris flow channel, ultimately triggering this
mass-wasting process. In most cases there were ditch-lined
roads that collected and delivered the water. In addition to ex-
cess water, the investigated debris flows had at least one loca-
tion in their contributing area that served as a massive source
of mobile material. For example, the debris flow in Fig. 3c
mobilised large amounts of material downstream of a 4–5m
high knickpoint, which formed by the overflow of a manu-
factured forest track crossing the channel. This artificial dam
had a 50 cm drainage tube that quickly got clogged, causing
backfilling and finally overtopping by the flood water. The
high energy release at the resulting waterfall base caused
erosion of the surrounding slope sections and the channel
bed, sourcing rock material and trees into the debris channel
and then into the main channel, the Trierbach, downstream
(Fig. 1). Evidently, there were several previous debris flow
deposits visible in the eroded bank of the Trierbach, indicat-
ing that the entering debris channel had been active several
times in the past.
The landsliding component of hillslope material contribu-
tion had a series of triggers, mechanisms, and time lags to
the precipitation phase. Post-flood mapping revealed numer-
ous shallow landslides that were not related to fluvial un-
dercutting as a trigger but were located on steep concave
slope sections, hence at preferentially wetter slope positions
(Giuseppe et al., 2021). This suggests that their activity was
triggered by excess precipitation before and during the rain-
fall event. These features typically had a spatial extent of a
few to a few tens of metres and were in most cases not con-
nected to a channel. In contrast, there were also numerous
river banks on the outer side of river bends that showed sig-
nificant undercutting and, consequently, slope failures (Oz-
turk et al., 2018). These well-connected landscape elements
were able to deliver sediment and woody debris directly
to the channel (Fig. 4a). Such slope instabilities ranged in
length from a few metres to features that have affected sig-
nificant parts of valley hillslopes. In some cases, entire hill-
slopes with older instabilities were undercut (Fig. 3d) and
might become reactivated subsequently. One such example
of a river meander bend is depicted in Fig. 4b, where a 100m
long and 16 m high rock face was stripped of its debris apron
as the Ahr River level rose by about 5 m above the current
water level (cf. the flood impact scar in Fig. 4b). Subsequent
visits to the site revealed traces of slope movement, such as
extending cracks in a paved road crossing just above the rock
slope. It is unclear how increased soil moisture during the
winter period (Dietze et al., 2020) will affect the transient
activity of this rock slide. Hence, further close monitoring of
the slope instability is required to anticipate its failure and the
potential for subsequent blockage effects on the Ahr River.
3.3 Debris mobilisation
Large woody debris played a critical role in rendering the
flood non-linear and difficult to predict, from small headwa-
Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022 https://doi.org/10.5194/nhess-22-1845-2022
M. Dietze et al.: Eifel floods 1849
Figure 3. Landscape features emerging from the flood. For locations of these pictures, see star signatures in Fig. 1. (a) 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. (b) Deposition
area of the debris flow shown in panel (c), injecting massive debris into the main channel (Trierbach), temporally blocking the stream and
causing severe reorganisation of the hydraulic geometry. (c) 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 (50cm diameter). (d) 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.
ter channels down to the main streams. Tree logs could have
been recruited from forest-covered hillslopes with abundant
dead wood due to the drought years of 2018 to 2020 (van der
Wiel et al., 2021). However, apart from the linear erosive
features described in Sect. 2.2, there was limited field ev-
idence of systematic unconcentrated overland flow on hill-
slopes, and in no occasion did this point to potential flow
depths necessary to entrain logs (Baudrick and Grant, 2000)
and route them through a maze of standing trees. The re-
cruitment of large woody debris from riparian zones, where
lateral erosion impacted former tree habitats (green line sig-
nature in Fig. 3a), is thus much more likely. Aerial imagery
collected along main rivers 1d after the flood (BBK-DLR,
2022) shows substantial removal of trees along the Ahr River
and other floodplains. Nevertheless, a quantitative survey of
the dead-wood delivery capability of the forested hillslopes
and riparian zones still needs to be conducted. Ideally, such
a survey would be based on high-resolution point cloud data,
for example from dedicated airborne laser scan missions.
Understanding the relative importance and pattern of differ-
ent sources of large woody debris is important not only for
restoration efforts in the flood-affected areas but also for mit-
igating future flood hazards (Lucia et al., 2018). In a preven-
tive manner, especially given the likely increase in extreme
events (IPCC, 2021), the general impact of large woody de-
bris on central European landscape dynamics and the suscep-
tibility of different tree species should be investigated.
The subsequent transport of large woody debris through
the river channels had a series of effects (Jochner et al., 2015;
Okamoto et al., 2020). In the main streams, clogging of ob-
stacles, mostly bridges, resulted in temporary ponding and
backwater effects (Fig. 5a). As a consequence, upstream wa-
ter levels rose, and the inundated areas grew until the obstacle
was either bypassed, overtopped, or destroyed. In the former
case, the bypass location experienced increased flow veloc-
ity and thus bed shear stress, resulting in focussed erosion.
The latter effect has the potential to generate a pulse of wa-
ter travelling downstream like an outburst flood. Given the
presence of many such obstacles along the course of the Ahr
valley (62 out of 75 bridges destroyed) and most likely other
streams draining the Eifel, the failure of obstacles most likely
contributed to non-linear, pulsed hydrograph behaviour, as
partly confirmed by affected people and experienced during
earlier floods (Roggenkamp and Herget, 2022). However, the
large spacing, insufficient time resolution (15min sampling
interval), and eventual failure of existing stream gauges pre-
vent us from resolving this conceptualised hydrograph be-
haviour and its resulting inundation and shear stress pulses.
The traces of these clogs are visible both in the main val-
leys (BBK-DLR, 2022) and in headwater regions, where we
were able to map numerous blockages of first-order streams,
either at anthropogenic structures (bridges, water passages,
fences) or at narrows formed by riparian trees (Fig. 5b). In
many reaches, these blockages were formed at a spacing of a
few tens of metres, ponding backwater due to accumulation
of organic fine material (Schalko et al., 2018) and implying
significant effects already at very small contributing areas.
The propagation of non-linear flow effects from small creeks
to and throughout the trunk river of a catchment is a crucial
step to take for successful future flood impact anticipation.
A further important effect of large woody debris, espe-
cially in headwater regions, was the role in ejected coarse
https://doi.org/10.5194/nhess-22-1845-2022 Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022
1850 M. Dietze et al.: Eifel floods
Figure 4. Debris mobilisation features in the Ahr valley near Müsch (cf. Fig. 1). Aerial image (BBK-DLR, 2022) taken 1d 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 panel (a) towards the eroded right bank, which had activated a 16m high rockslide (persons for scale). Note flood impact mark
on a remaining tree at 5 m above current water level. Please note that the date format used in this figure is year-month-day.
inorganic debris from the stream bed (Fig. 4b) onto the flood-
plain. Also fine material was deposited in front of woody de-
bris obstacles (Fig. 5c) and clogged anthropogenic structures,
such as bridges. This readily available fine material is now a
temporary source for increased fluvial sediment flux. Hence,
even low-intensity floods will be able to carry comparably
high concentrations of sediment particles. In contrast, the
ejected coarse bed particles are currently removed from the
fluvial domain until future bank erosion (or human land use
practice) reincorporates them to the channel. Debris also re-
sulted in permanent alterations to stream courses, due to lat-
eral and vertical erosion, and in some steep channel reaches
even led to incision into the underlying weathered bedrock as
mapped out systematically in headwater reaches. This again
resulted in undercutting of the banks and local landslides.
The spatial reorganisation of sediment in the fluvial domain
as well as overall changes in river geometry and bed proper-
ties – from small creeks a few kilometres past the watershed
to major rivers like the Ahr – inevitably caused a transient in
the catchment reaction to future floods.
3.4 Anthropogenic dimension
The flood happened in a cultural landscape with a long
legacy of human land use. Accordingly, there were typical
primary effects of land use, particularly surface features on
flood dynamics such as an increased and accelerated sur-
face runoff on cultivated hillslopes (Bronstert et al., 2020),
some of which are already mentioned above (e.g. Figs. 2a,
4a; clogged structures, shallow landslides due to undercut,
or oversteepened slopes). During our mapping campaign, we
observed systematic changes in fluvial erosion features along
small headwater channels. Forest-covered floodplains with
strong erosional features were connected to virtually unaf-
fected grassland sections, and intact slopes suddenly showed
linearly incised sections without a plausible contributing
area. In most of these cases, however, we were able to iden-
tify artificial subsurface drainage systems, often visible as
fragments of drainage pipes. Not surprisingly, according to
interviews with residents, systematic tile drainage is a com-
mon practice in the area to improve grassland quality or sim-
ply to manage the wastewater of dispersed houses. The con-
sequences during the July flood were either an increased dis-
charge contribution where tile drainage remained intact or in-
jection of excess water into the ground where drainage pipes
eventually filled up with debris and were clogged. In the for-
mer case, the hydrological flashiness of the landscape in-
creased, while in the latter case the result was an elevated sus-
ceptibility of hillslopes to failures and incision. Such failures
may be local effects, but the increased flashiness had an ex-
ternal effect by increasing the rapid build-up of flood waves
in subsequent channels. Since many of the tile drainage and
wastewater systems are several decades old and not necessar-
ily documented, including their effects in runoff models will
be a challenging though necessary future task.
Both regions, the Ahr valley and Lower Rhine Bay, hold a
long legacy of human land use that influence the susceptibil-
ity to gravitational mass movements. According to the Ger-
man landslide database (Damm and Klose, 2015) the pro-
nounced susceptibility to mass movements in the Ahr val-
ley is exemplified by 49 database records from the middle
Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022 https://doi.org/10.5194/nhess-22-1845-2022
M. Dietze et al.: Eifel floods 1851
Figure 5. Effects of large woody debris. For locations on wider map see stars in Fig. 1. 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). (b) Huhnenbach near Aremberg about 2km 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. (c) Another clogging of
the Huhnenbach some 20 m upstream of panel (b), with both ejected coarse debris and deposition of fine sediments in front of the obstacle.
and lower valley sections over about the last 70 years. The
records document a preference for fall processes (73 % of the
events), in general consisting of rockfalls and boulder falls
of low magnitude. Sliding processes amount to 27 %. Inad-
equate land management as a mass-wasting driver or trig-
ger is conservatively estimated to explain almost 20 % of all
cases (Damm and Klose, 2015). In contrast, the Lower Rhine
Bay is virtually not susceptible to gravitational mass wast-
ing, except for artificially oversteepened landforms. Over the
last 130 years, only 26 events were documented (Damm and
Klose, 2015), all of them exclusively located in engineered
landscape parts such as slope cuts, hillside fillings, road em-
bankments, waste dumps, open-pit mining, and river man-
agement activities. Triggers of the few large-magnitude pro-
cesses (collectively linked to open-pit mining sites) were pre-
dominantly the direct intervention related to mining and the
intrusion of external water into the pits, caused by heavy rain-
fall or flooding. Hence, direct links of flood magnitude to
mass-wasting activity are convoluted with land use practice
and thus hard to disentangle, at least for past events.
The unprecedented economic damage of more than
EUR 30 billion was most likely caused not only by the ex-
tent of the affected area (see Sect. 2.1) but also by the ve-
locity of the water flow and the combined impact of water,
wood, and debris on buildings and infrastructure. Field in-
spection revealed inundation depths at buildings of several
metres affecting not only cellars and ground floors but also
the first floor of many houses (e.g. Roggenkamp and Her-
get, 2022, and own mapping efforts). Previous major floods
in the Ahr valley, Germany, namely 2006 and 2013, also
caused inundation damage. However, those previous water
levels rose on average to 0.83 and 0.46m above the ground
level, respectively (Thieken et al., 2022). The Ahr valley
experienced larger events in the more distant past, such as
1804 (Roggenkamp and Herget, 2022). However, the settle-
ment and land use structure was significantly different from
2021, which makes comparisons of comparably large but
temporally more distant events challenging. In addition to
inundation depths, high flow velocities and resulting dam-
age due to impacting debris, undermining of paved surfaces
and souring of foundations were reported frequently for these
valley-confined flood events (Laudan et al., 2017), processes
specifically harmful to critical road infrastructure (Kreibich
et al., 2002). At downstream reaches, water levels became
elevated by deposited debris that reduced accommodation
space. None of these processes and damage mechanisms are
included in common damage models (Laudan et al., 2017).
Likewise, without their inclusion, identification of affected
buildings by automatic flood mapping routines (e.g. CEMS,
2022) remains incomplete and requires tedious and time-
consuming manual data collection to derive a proper damage
estimate. In the Ahr valley, CEMS identified around 5000 af-
fected residential buildings, while an overlay of the recon-
structed water mask and an OpenStreetMap data set identi-
fied about 7000 buildings. This estimate of 7000 buildings is
likely to be too low, as the water masks usually do not map
the rapid surface runoff, which could also lead to structural
damage of buildings. Hence, the development of concepts
and readily applicable tools to reliably estimate the damage
particularly to infrastructure is an important emerging goal.
The high death toll along the Ahr River could however not
just be related to the shortcomings of damage estimates dis-
cussed above. A further effect was the general underestima-
tion of the flood magnitude by locals due to an anticipation
https://doi.org/10.5194/nhess-22-1845-2022 Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022
1852 M. Dietze et al.: Eifel floods
legacy. During the 2016 flood in the Ahr valley, discharge
was estimated to resemble a 100-year event (Demuth et al.,
2022). In 2021, residents tended to recall that past event and
the way they coped with it. Since the 2021 event was con-
siderably higher and accompanied by geomorphic processes,
this ultimately led to an underestimation of its impact. An
online survey that was conducted from August to October
2021 in the affected areas (Thieken et al., 2022) revealed
that based on the warnings just 14.8 % of 856 warned resi-
dents anticipated massive damage and life-threatening situ-
ations (assessed on a Likert scale from 1 to 6). In addition,
public authorities, particularly in the district of Ahrweiler,
evacuated very late in the evening when the water had al-
ready flooded houses. Hence, many residents endured this
threatening flood situation on the roofs of their buildings (or
drowned).
3.5 Interactions and process connections
Landscape elements and process domains are typically
linked by the river network that drains them. Hence, changes
in equilibrium processes such as sediment fluxes into a river,
ponding, or advancing flood waves can be seen as input sig-
nals that are transmitted downstream while becoming modu-
lated in their response. The magnitude and filter function of
this modulation can be so strong that the initial input signal is
no longer discernible; it gets shredded (Jerolmack and Paola,
2010). One example of this concept, namely the change of
the flood’s hydrograph by cascades of clogged bridges, has
been described in Sect. 2.3. Nevertheless, the modulated re-
sponse (here the modulated flood wave) still severely impacts
downstream reaches – and sometimes even upstream reaches.
We follow this concept of signal shredding during landscape
interaction through connection mechanisms in the Erft catch-
ment (Fig. 1).
At a comparably small scale, the town of Blessem on the
Erft River was subject to such an emerging landscape con-
nectivity case. The excess rainwater of Blessem was col-
lected and channelised in pipes below the main road (Frauen-
thaler Straße, then passed to Radmacherstraße). These pipes
ended in a drainage ditch at the western town limit that routed
the water towards the Erft River (Fig. 6b), passing by a gravel
pit that was protected by a rampart a few metres high. Dur-
ing the flood, overbank discharge of the Erft River moved
water across the main streets of Blessem (light-blue arrows
in Fig. 6) and not only injected excess discharge into the
drainage pipes but also ran as overland flow along the streets.
In addition, Erft overbank discharge was routed over a field
west of the town, towards the drainage ditch already carrying
the town’s excess water. Whether that excess water caused
overspilling of the 2 m deep drainage ditch or if the addi-
tional water inflow from the field caused the overspill cannot
be resolved here. Regardless, the ditch overtopped and dis-
charge followed the line of steepest descent into the gravel
pit, whose protection rampart was not fully closed but had a
gap through which the water could enter the pit. As the flow
path gradient changed from less than 1 to about 20◦down
the pit slope, the shear stress increased by 2 orders of magni-
tude (Anderson and Anderson, 2010). This high shear stress,
combined with the high erodibility of the underlying mate-
rial (a thin loess cover on tertiary Rhine gravel), resulted in
extremely rapid erosion and the development of an erosional
margin (yellow line in Fig. 5) that cut backwards by about
250 m within a few hours. The final shape of the erosional
margin shows four discrete tongues of enhanced slope re-
treat (cf. numbered triangles in Fig. 6), which correspond to
the main sources of excess water: the road and drainage pipes
running through the town of Blessem (triangle 1); two over-
flow spots of the Erft River across the field at the edge of
the town (triangles 2 and 3); and ultimately, the river Erft
itself (triangle 4), which routed its whole discharge into the
eroded gravel pit. This led to rapid inundation of the pit, mas-
sive deposition of >450000 m3of material, enhanced lateral
erosion along the (new) outer bank of the diverted river, and
significant depth erosion not only of the new course of the
Erft River but also in its old bed, forming a knickzone and
resulting in a bed elevation lowering of 1.4–1.8m.
Despite initial media reports of a landslide (i.e. a gravi-
tational mass-wasting process) happening near Blessem, all
evidence rather implies that it actually was a process driven
by flowing water. That process in turn was the result of a lo-
cal process connection mechanism: backward erosion of the
gravel pit margin. At the same time, the mechanism was also
controlled by emerging feedbacks, i.e. the reduction in back-
ward erosion by cessation of overbank discharge through
base level lowering of the Erft River as it flowed into the
gravel pit. Hence, what happened in Blessem is an example
of how geomorphic processes first amplified and later coun-
teracted the impact of a hydrological extreme event.
At a larger scale, the Blessem site was almost subject to
another process connection case, which would have deliv-
ered a further significant wave of water. Through the Erft,
Swist, and Schießbach rivers, the town is connected to the
Steinbach reservoir some 30 km upstream (Fig. 1). The 12 m
high earth dam of that reservoir was severely dissected by
sustained crown overspill for several hours during the flood
event. Gullies of 10m width and up to 4 m depth formed
over a length of about 100 m (quantified by UAV-based – un-
manned aerial vehicle – structure-from-motion topographic
data). If the Steinbach reservoir dam had failed, another
1.2×106m3of water would have moved downstream, most
likely refuelling the erosion processes in Blessem with an-
other new flood wave all along the river’s course from the
reservoir.
4 Challenges and future needs
A particular phenomenon of the July 2021 flood was the
widespread activity of mostly small features that ultimately
Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022 https://doi.org/10.5194/nhess-22-1845-2022
M. Dietze et al.: Eifel floods 1853
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 as 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.
added up to unexpectedly large effects in the main valleys of
the Ahr and Erft rivers. There was not one dominating fac-
tor that can explain the event magnitude but rather the inter-
action of many seemingly unrelated effects, a situation that
needs to be considered jointly and conceptualised and imple-
mented in predictive models as well as upcoming mitigation
strategies. Some of these isolated effects were straightfor-
ward to detect and can be implemented in future strategies,
such as insufficiently designed bridges or protective dams.
Other effects are inherently difficult to identify and even
harder to conceptualise and ultimately implement into mod-
els and risk management strategies. Examples of the latter
category are tile drainage systems of unknown extent and ca-
pacity or injected versus ejected volumes of sediment. These
latter effects could, however, be incorporated to the models
via Monte Carlo simulations, e.g. considering the full range
of potential sediment budget, to at least quantify within the
uncertainties bounds of the model estimates.
These emerging effects rendered the flood an extreme
beyond the hydrological scope. Hence, this underlines the
need for a cross-topic consideration of its internal and ex-
ternal drivers, its effects, and its internal feedbacks. This
touches especially on the non-hydrological processes and
their representation in posterior models, future predictions,
and concepts as well as hazard zone definitions, in addition
to the fruitful efforts already emerging from the hydrolog-
ical realm. For example, while it is evidently important to
provide close-range forecasting of potentially inundated ar-
eas, it is as important to develop and implement methods to
forecast potential effects driven by overland flow and stream
discharge. These include outlining potentially unstable hill-
slopes, riverbed changes, cascading effects, and landscape
connectivity effects. Connectivity effects do not necessar-
ily need to be restricted to gravel pits and upstream water
reservoirs, as revealed here. More likely are far-reaching ef-
fects, for example triggered by blocked tunnels, undermined
bridges, valley-damming mass-wasting deposits, and channel
straightening that swiftly initiate long-lasting effects.
Upcoming fundamental research needs to quantify the
severity and modes of landscape reorganisation as well as
constrain the duration of the transient that is now dominat-
ing the functional relationships of the affected regions. Key
elements of these functional relationships are the reorgani-
sation of the hydraulic geometry, the fluvial and sediment
coupling between channels and adjacent hillslopes, and land
use and settlement planning. Approaching all these ques-
tions crucially depends on dedicated, high-resolution, and
continuous empirical data from distributed field instrumen-
tation (also properly operating during future extreme events)
in close junction with metrics of societal activities. The 2021
flood clearly demonstrated a flaw in classic flood sensing ap-
proaches: collection of just a single metric, water level, by
gauges at a few spots along the main stream that overall were
destroyed significantly before peak discharge had arrived. To
https://doi.org/10.5194/nhess-22-1845-2022 Nat. Hazards Earth Syst. Sci., 22, 1845–1856, 2022
1854 M. Dietze et al.: Eifel floods
overcome this systematic shortcoming, other systems need to
be implemented that are able to collect distributed multivari-
ate data at high temporal resolution and that are not endan-
gered by hostile flood conditions. For example, at scales from
global to small catchments, seismic networks from Ekström
and Stark (2013), Cook et al. (2021), and Walter et al. (2017)
have proven to deliver near-real-time information that allow
for the detection, location, and description of catastrophic
mass-wasting events. 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.
Headwater regions are also the areas where proper flood
risk reduction actions can be implemented. The German Fed-
eral Water Act allows the federal states to identify flood gen-
erating areas which are areas that tend to quickly produce
surface runoff. Land management can be regulated in such
statutory areas to prevent further deterioration of the infil-
tration capacities of soils. Currently, only the Free State of
Saxony makes use of this option. Besides land management
planning and engineering solutions, flood risk reduction de-
cisively needs to consider the sensitisation of citizens to over-
come the anticipation bias due to the legacy of experienced
events of lower magnitude of less non-linear effects.
Data availability. All data used in this article are freely available
in the references denoted in the text.
Author contributions. MD and UO conceptualised the article and
collected field data. MD, RB, KLC, CA, ARB, AL, and AHT con-
tributed to the field survey and mapping efforts. BD contributed the
landslide database analysis. FSF, KMN, TS, and AHT performed
and evaluated climatic and hydraulic risk and damage analysis. All
authors contributed equally to the preparation and writing of the
manuscript.
Competing interests. The contact author has declared that neither
they nor their co-authors have any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. This research was supported by the HART
(Hazard and Risk Team) action of Eifel Flood Event 2021 funded
by the Helmholtz Centre Potsdam – GFZ German Research Cen-
tre for Geosciences and the DFG (German Research Founda-
tion) Research Training Group NatRiskChange – Natural Haz-
ards and Risks in a Changing World (grant no. GRK 2043/3).
Katrin M. Nissen, Felix S. Fauer, and Bodo Damm received
funding from the BMBF (German Federal Ministry of Educa-
tion and Research) project ClimXtreme (grant nos. 01LP1903A/K
and 01LP1902H). The publication was supported within the fund-
ing programme “Open-Access-Publikationskosten” of the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation;
project no. 491075472).
Financial support. This research has been supported by the
Helmholtz-Zentrum Potsdam – Deutsches GeoForschungsZentrum
GFZ (HART EifelfloodS), the Deutsche Forschungsgemeinschaft
(grant no. GRK 2043/3), and the Bundesministerium für Bildung
und Forschung (grant nos. 01LP1903A/K and 01LP1902H).
The article processing charges for this open-access
publication were covered by the Helmholtz Centre Potsdam –
GFZ German Research Centre for Geosciences.
Review statement. This paper was edited by Olga Petrucci and re-
viewed by three anonymous referees.
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