E3S Web of Conferences 415, 01025 (2023)
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Quantifying lateral bedrock erosion caused during a
hyperconcentrated flow in a narrow alpine limestone gorge
Verena Stammberger1*, and Michael Krautblatter1
1Technical University of Munich, Chair of Landslide Research, 80333 Munich, GermanyAbstract. Here, we show results of an unprecedented LiDAR dataset quantitatively determining the lateralbedrock erosion of a narrow limestone gorge during an extreme hyperconcentrated flow. The comparison oftwo point clouds prior and post to the June 15th hyperconcentrated flow event provide information about themassive breakout of particles and abrasion of the channel walls. With a multiscale model to model cloudcomparison analysis, we can show that particles from 0.0001 m3 and 3.5 m3 were eroded from the subverticallimestone gorge walls. A total of 20.9 m3 of massive bedrock was eroded in the observable part of the channel
with 90 % of the particles being smaller than 0.15 m3. We delimited two main erosion processes during thehyperconcentrated flow event: shearing of particles that reach into the flow and particles with predefined
failure surfaces, and abrasion along the whole channel, detectable by LiDAR if the changes are > 3 cm. This
study provides quantitative evidence for massive rock erosion processes in alpine gorges that could also
control rock gorge formation and evolution over Holocene/Lateglacial time scales.

1 Introduction
An increase of annual heavy precipitation events [1] aswell as climate-related extremes such as floods [2] islikely in near-future developments in climate change.
This is directly linked to the occurrence of debris flows
and hyperconcentrated flows of which both can posesevere risk to human life [3]. Due to the massive forcesthat can act during hyperconcentrated flows, they havehigh potential of causing geomorphic changes along theflow path [4]. Several studies showed the erosion andentrainment of loose sediment [5-8] which can cause amassive increase of the mobilised flow volume. Also avast amount of studies discusses fluvial channel incision
by a saltating bed causing abrasion, plucking orcavitation [9-14]. But only very sparse data is available
describing the erosive power of debris flows on bedrock
channels either by field observations [15,16] orlaboratory experiments [17]. Here, we present
unprecedented data that quantitatively and qualitatively
describes the geomorphic changes due to lateral erosion
in a narrow limestone gorge caused by a
hyperconcentrated flow event.

highest point. Due to the steep slopes of the catchment(inclination of ø 110%) a rapid accumulation of thesurface runoff caused a massive mobilization of
secondary sediment storages which were thentransported to the upper entrance of the gorge.
Throughout the narrow ravine, massive dispersive
forces reshaped the channel walls by particle erosion,shearing, abrasion and shifting of boulders up to 20 m3.
The gorge as well as the above catchment consist of Mid
Wetterstein limestone [18].

2 Study area and flow event
On the 13th of June 2020, a local extreme precipitation
event of 50 to 60 mm/h (DWD, Radar measurements)
caused a hyperconcentrated flow event in theHöllentalklamm, a gorge situated in the Wettersteinmountains, Germany (see Fig. 1). The channel lies
between 1032 and 1062 m a.s.l. and drains a catchmentof 10 km2 with the Zugspitze (2968 m a.s.l.) marking theFig. 1. Location of the study site in southern Bavaria (reddot) and 3D-view of the catchment and position of the gorge.
Orthophoto: www.geodaten.bayern.de.

* Corresponding author: verena.stammberger@tum.de© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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3 Methods

corresponding clouds onto a surface perpendicular to therock face and calculating the difference. For further
analysis of the erosion volumes along the channel, we
divided the pointclouds of the gorge in 10 m-sections
and summed up the eroded particle volumes for eachsection.
Simultaneously to the LiDAR measurements, werecorded the state of the gorge after thehyperconcentrated flow event with a 360° camera. Someearlier photographs of the gorge during previous
rockfall monitoring campaigns supported thecomparison. Visual colour changes of the bedrocksurface were identified to determine a minimum flowheight hHCF throughout the entire flow path of thechannel (see Fig. 2). Using the pointclouds wedetermindes the altitude of the channel bed elevation(dashed lines in Fig. 2).

We conducted a terrestrial LiDAR campaign using aRIEGL VZ-400 laser scanner to record the changes
caused by the hyperconcentrated flow event. The
reference data was acquired on the 28th of May, initiallyfor a biannual rockfall monitoring of the gorge. The
second scanning campaign was shortly after the event(19th and 25th of June). In total, we recorded 84 single
scans positioned along the approximately 900 m longgorge. The angular step was set between 0.04 and 0.06°
with an angle measurement resolution below 0.0005°.
Due to the complex environment with overhanging
walls and a windy channel, some data gaps result fromocclusion, thus are unavoidable. The point cloudalignment was applied in RiSCAN Pro 2.9 with aniterative closest point (ICP) algorithm [19-21]. By
minimizing the difference of corresponding planes in
two subsequent scans, a co-registration with standard
deviation below 1 cm was achieved. We reduced thepoint densities to 2.5 cm in XYZ direction to prevent
any bias due to a heterogeneity of the clouds. Togeoreference the resulting point clouds of the wholegorge, we used an airborne lidar dataset recorded in2006.
We applied the Multiscale Model to Model Cloud
Comparison (M3C2) to estimate the geomorphic
changes [22] in this morphologically complex gorge.
This algorithm allows to calculate changesperpendicular to the rock face and eliminate highartificial values close to data gaps. We analysed thesides of the bedrock channel to quantify the lateralerosion. Due to the water discharge in the channel, wewere not able to calculate the vertical incision at the bed.
The detected changes are classified in either statisticallysignificant (change > level of detection) or not
statistically significant and rejected. Thereafter, wevalidated all statistically significant changes manually toeliminate further errors. We then calculated the area andvolume by triangulating the isolated point clouds of thesingle particles/coherent erosion areas (hereafter bothreferred to as ‘particles’). By projecting the4 Results
We were able to determine the minimum flow height of
the hyperconcentrated flow throughout the wholechannel of the mountain gorge by analysing thephotographs pre- and post-event at more than 50
positions. The mean flow height along the 900 m longgorge revealed 2.8 m and a calculated maximum flowheight of 12.3 m. However, these values are influenced
by a steep drop of the channel creating a waterfall. Forthe remaining part of the channel, the maximum flowheight is 7.2 m and the minimum height is 0.5 m.
Using the M3C2 change detection analysis wedetected 232 particles that were eroded during the June
2020 hyperconcentrated flow event. The smallest
detected particles volume equals 0.0001 m3, the largest
is a block of 3.30 m3. 90 % of the changes are below
0.15 m3 (see Fig. 3) but account for only 25 % of theeroded volume. A total volume of around 20.6 m3 oflimestone bedrock was eroded laterally from the channelsides. The corresponding eroded area is 137 m2. This
yields a mean erosion depth of 15 cm for the areas
indicating significant change.

Fig. 3. Histogram of the detected eroded bedrock particle
volumes along the channel sides (outer figure) and histogramof the smallest 90 % of detected eroded particles (innerfigure)Fig. 2. Visual determination of the flow height of thehyperconcentrated flow throughout the gorge. Affected
bedrock areas show a brighter colour due to abrasion.

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particles with a geometry that is prone to shearing during
the flow (nose-like) are broken out of the channel walls.
Likely the same happens with particles with predefinedloose joints. The driving (shearing) forces succeed theholding forces of the rock or rock-bridges. This process
might be driven by plucking but even more so by theincreased yield strength of the flow that forces thebreakout of particles in the rockwalls. ii) massive
abrasion of the bedrock by the suspended particles
within the flow describes the second mechanism. Basedon photographs we visually recorded the results of theabrasion process along the channel. In places withmassive abrasion of more than 3 cm depth we were able
to detect it in the change detection analysis. Thesechanges are characterized by a smooth erosion surface
rather than a spikey one. Around the edges, the detectedchanges between the pointclouds are usually thesmallest and in the centre it shows the maximum erosion
depth.
Subdividing the channel in 10 m sections can give
first information about the spatial occurrence of theeroded volumes. The first 350 m are characterized bysmaller changes with less variation and only a few
sections without any detected change. Most of thechanges are located in the middle part of the gorge(Fig. 4). The middle part between 350 and 700 m is thepart with the most eroded volume and great variation ofthe erosion volume per section. In the third section there
is a large fraction with no erosion and again very highchanges.

Fig. 4. Detected erosion volumes in the defined 10 msections (left axis) and gorge channel bed elevation (rightaxis)

Dividing the gorge in 10 m-parts along the channel axis
resulted in a total of 89 subparallel sections. The total
erosion volume of each section is the sum of the eroded
particle volumes lying in between the dividing profiles.
In 27 of the 89 sections (30 %), we did not detect any
erosion with the M3C2 analysis. The mean erosion
volume per section in which erosion was detected is0.33 m3. In Fig. 4 the erosion volumes per section areplotted at the position along the channel bed and theelevation of the bed.

5 Discussion
The field investigations and photographic records of thewhole gorge clearly showed that the hyperconcentrated
flow event caused extreme changes to the limestone
gorge. Formerly weathered bedrock surfaces on thesides of the channel show a distinct colour change due
to a constant abrasion of the uppermost layer. As weidentified this border of abraded and weathered rocksurface, we were able to define the minimum flowheight of the hyperconcentrated flow along the wholechannel. This height is not necessarily the absolute flowheight of the event, a layer of water flow with lesssuspended sediment above the heavily charged bottom
layer is possible. This hypothesis is further supported bysome sediment deposits above the defined minimum
flow height.
The maximum calculated flow height is influenced
by a drop within in the channel creating a waterfall.
Nevertheless, we detected erosion caused by abrasion insome distance to the drop. This reveals the highdischarge during the event that led to a hose-effect andan increased casting distance. As expected, the flowheight in the remaining channel is heavily dependent on
the width of the cross sections.
The M3C2 analysis showed a total lateral bedrock
erosion volume of 20.6 m3 from 232 particles
throughout the gorge. This only accounts for the erosion
from the rock walls and does not quantify erosion fromthe channel bed nor any entrainment of loose sediment.
The detected changes can be explained with two
different mechanisms that are manually distinguished bytheir pre- and post-event geometry. i) we assume that

6 Conclusions
We were able to record the geomorphic change andmassive rockwall erosion in an alpine limestone gorge
caused by a hyperconcentrated flow event in June 2020.
An unprecedented LiDAR dataset quantitatively shows
the lateral bedrock erosion along the channel walls.
Additionally, we were able to detect visual changes tothe bedrock channel walls and recorded them withphotographs. In conclusion, we delimited two
mechanisms of particle or bedrock erosion during a
hyperconcentrated flow event:i) Abrasion process: along the whole channel, detectedvisually for changes < 3 cm erosion depth and detectedwith the LiDAR data for changes > 3 cmii) Shearing process: shearing force acts on areas with ageometry that reaches into the flow (nose-like) and on
particles with predefined failure surfaces.
This study shows and provides quantitative insights
to rapid erosion processes in alpine gorges. Extreme
events like the observed hyperconcentrated flow or
debris flows might account for a larger proportion of
bedrock erosion in channels than previously thought.
Over longer timescales, they could even act to control
rock gorge formation and evolution.

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