Sediment transfer mapping in a high-alpine catchment using

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Jahrestagung der Schweizerischen Geomorphologischen Gesellschaft
113
Sediment transfer mapping in a high-alpine catchment using
airborne LiDAR
Yves Bühler1 and Christoph Graf 2
WSL Institute for Snow and Avalanche Research SLF, [email protected]
2
WSL Swiss Federal Institute for Forest, Snow and Landscape Research,
[email protected]
1
Abstract
In the high-alpine zones of the Mattertal/VS, gravitational processes are widespread. Changes in the
permafrost zones are reducing the cohesion of high-alpine slopes and produce additional material, which
can be transported to the channels below. Such sediment-filled gullies can act as debris-flow sources.
Frequent topographical surveys are very difficult to accomplish due to the poor accessibility of the
steep terrain endangered by frequent rock fall and debris slide events. We use airborne laser scanning
(LiDAR) from two different dates (summer 2010 and summer 2011) to map sediment transfer over
the entire Grabengufer area and the lower part of the catchment, known for high levels of geomorphic
activity. The fine spatial resolution of the resulting DTM’s (0.5 m) and the high positional precision
(approx. 0.15 m in x,y and z) allow for a detailed analysis of the topographic changes within the one
year time period. We identified areas of erosion and deposition and we quantified as well the relocated
volumes. We find substantial relocation of material especially in the rock glacier area (6760 m3 erosion)
as well as in the gully (23 720 m3 deposition). This information is fundamental for further debris-flow and
permafrost studies in the Mattertal area. The method tested in this investigation has a big potential to be
helpful for subsequent studies in other debris-flow affected regions.
Keywords: LiDAR, DTM, debris flow, erosion, deposition, RAMMS
1Introduction
In recent decades, Alpine permafrost has undergone major changes and the reduced cohesion
of high-alpine slopes due to melting ice produces additional debris, which may be deposited
in channels and gullies below by gravitational forces (Gruber and Haeberli 2009). These
filled gullies can act as sources for extreme debris flows triggered by heavy precipitation or
the outburst of glacial lakes (Zimmermann and Haeberli 1992). Such events threaten people,
buildings and traffic infrastructure in the valleys. This impact has changed significantly
during the last decades (Bloetzer et al. 1998). The development was already recognized and
predicted more than 25 years ago (Haeberli 1985).
Recent observations confirm increasing slope instabilities caused by permafrost melting
due to climate change in diverse areas within the Mattertal valley (Fig. 1.) (Rebetez 1997,
Stoffel and Beniston 2006, IPCC 2007; Delaloye et al. 2008; Lugon and Stoffel 2010).
The transported debris volumes, fixed in place by permafrost beforehand, are considerable
and the resulting changes in topography are accordingly substantial. Numerous fast moving
rock glaciers are situated at the eastern face of the Mattertal valley (Delaloye et al. 2010).
These rock glaciers are coupled with steep channels and have a high potential to feed large
debris flow events (Graf and McArdell 2005). In these channels, gravitational mass moving
processes such as rock falls, shallow landslides or small debris flows transport and relocate
the material. It is deposited temporarily in gullies where it may be remobilized by torrential
sediment transport or entrained by larger debris flows.
The current trend suggests a potentially rising frequency and even magnitude of debrisflow events (Zimmermann et al. 1997; Graf et al. 2011). This influences the exposure to risk
114
Yves Bühler and Christoph Graf
Fig. 1. Overview on the active rock glaciers and the coupled channels at the eastern face of the
Mattertal valley, VS, Switzerland. (Topographic map ©swisstopo (DV033492.2).
115
by loss of the infrastructure in the valley bottom and calls for a reexamination of hazard
assessment and mitigation measures. There is a high probability that other Alpine areas
within the Swiss Alps as well as Alpine areas around the world will face such changes in the
near future. Investigations in the highly active Mattertal area can therefore be an important
opportunity to better understand the gravitational processes and their triggering mechanism
in the context of changing conditions.
Up-to-date remote sensing technology such as airborne laser scanning LiDAR or digital
photogrammetry provide the opportunity to acquire spatially continuous, high resolution
digital terrain models DTM’s of poorly accessible regions such as high-alpine catchments
(Bühler et al. 2012). The derived mass balances of the catchments provide fundamental
information for the development of protection systems for the Mattertal valley (Graf et al.
2011; Graf et al., this volume). Numerical simulation tools such as RAMMS (Christen et
al. 2010) or DAN3D (Hungr 1995; McDougall and Hungr 2004) are getting more and
more important for hazard zoning and mitigation measure planning also within the Mattertal
valley (Deubelbeiss et al. 2011; Deubelbeiss and Graf, this volume). The quality of the
model outputs rely heavily on the quality of the input data including process relationships
(Rickenmann et al. 2006) and digital terrain models DTMs (Bühler et al. 2011). There is
an important motivation to evaluate and improve advanced methodologies for topographic
mapping in high-alpine regions.
2
Airborne LiDAR
Helicopter-based laser scanning data was acquired over the Dorfbach/Grabengufer area twice,
on August 31, 2010 and on July 8, 2011 (Fig. 2). LiDAR technology uses high precision laser
beams to measure millions of positions on the ground. By measuring the time that is needed
2010
2011
both
0km
1km
2km
Fig. 2. Investigation area Dorfbach/Grabengufer overlayed by the coverage of the two data acquisition
2010 and 2011. Topographic map ©swisstopo (DV033492.2).
116
by the laser beam to travel from the sensor to the ground and back the precise distance of the
sensor from the ground can be calculated. Combined with a precise and frequent positioning
of the sensor by Differential Global Navigation Satellite System DGNSS and an Inertial
Measurement Unit IMU, a location precision of approx. +/– 0.10 m (x,y) and +/– 0.15 m (z)
can be achieved (Vosselman and Maas 2010, Skaloud et al. 2005). Because a helicopter is
able to fly slowly, close to the complex terrain and can hold the distance between the sensor
and the ground approximately constant, it is an ideal platform for airborne laser scanning
data acquisitions in high-alpine terrain. In addition, the sensor can be tilted dependent on
the slope angle to optimize the LiDAR footprint, improving the accuracy of the position
measurements significantly. This assures a constant quality of the resulting DTMs over the
entire investigation area.
The extraction of the ground from raw point cloud is done using several techniques: a)
where vegetation is present, we use a routine to extract ground. It is an algorythme based
on triangulation (TIN) which tends to get the lowest point in the point cloud (terrascan
software, www.terrasolid.com). Thus the automatic extraction is checked manually to correct
the errors and to edit the cliff section because the overhangs are not processed by the routine.
b) where there is no vegetation, bare earth, the point cloud is de-densified to remove the
noise or useless points within a level of detail tolerance (for example 10 cm level of detail).
c) keypoints: based on iterative triangulation and altimetric tolerance the points are
removed where they are useless to define the surface, and they are kept where it is needed
(for example, on smooth surface: road, asphalt), points are removed and are kept when
terrain is chaotic. After this steps, the regular grid is extracted by triangulating the surface.
The center of each cell is projected on the triangles to get its height (information provided
by J. Vallet, Helimap).
The very high point density and the optimized footprint of the sensor suggest a good quality
and accuracy of the LiDAR-DEMs. We base this statement on numerous investigations
proofing the high quality of similar LiDAR systems (e.g. Hodgson and Bresnahan 2004;
Aguilar and Mills 2008; Hopkinson et al. 2009)
Figure 3 gives an overview on the data acquisition concept and shows pictures of the
sensor system in action. To assess the transfer of debris we subtracted the digital terrain model
acquired on August 2010 from the DTM acquired on July 2011 (Fig. 2). Error propagation as
performed in Lane et al. 2003 was not investigated.
Fig. 3. Functionality of airborne Laser Scanning system (left) and the sensor system in action operated
by Helimap (right) (Bühler et al. 2012).
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Fig. 4. Photograph (top) and shaded relief (bottom) of the 0.5 m DTM of a subset in the upper part of
the Grabengufer area (image: Ch. Graf, summer 2011)
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The main advantages of helicopter based LiDAR data are its high precision and its
spatially continuous coverage capacity even in very complex terrain. A major disadvantage
on the other hand is its high costs. The coverage of a single high-alpine catchment such as
the Grabengufer/Dorfbach (approx. 2.5 km2) costs around CHF 15 000 and includes approx.
2.5 hours of helicopter flight time as well as the entire data processing (approx. one day). To
cover larger areas different methods such as airborne LiDAR (Vosselman and Maas 2012)
or digital photogrammetry might be more economic (Bühler et al. 2012, Marty 2012). To
cover only subsets of high-alpine catchments, terrestrial laser scanning might be the most
feasible method (Kenner et al. 2011a).
3Results
The difference between the summer 2011 DTM and the summer 2010 DTM allows precise
(< dm) three dimensional quantification of the surface changes within this timeframe covering
the entire investigation area. Sediment flux is clearly visible. Figure 5 gives an overview on
the material dislocation over the entire test site and defines three subsets for more detailed
analysis.
Rock Glacier (A) Figure 6
The biggest amount of surface change within this subset is located in the upper part between
2375 m and 2875 m a. s. l. A large rock fall occurred in the night of September 21/22, 2010 and
destroyed the suspension bridge of the Europa hiking path between Grächen and Zermatt.
Fig. 5. Difference between DTM2011 and DTM2010. High amounts of sediment transport can be
identified in the area of the active rock glacier (A), the upper channel (B) and the lower channel (C).
Aerial imagery ©swisstopo (DV033492.2)
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The traces are clearly visible in the results (c, Fig. 6). We calculated a release volume of
4935 m3 for this event.
We identify a spatially continuous erosion zone close to the front of the active rock glacier
(b, Fig. 6). Here material is pushed over the eastern and western arm into the adjacent channel
below. Between summer 2010 and summer 2011 a total volume of 6760 m3 was eroded here.
At the front of the rock glacier, situated directly above an escarpment, we identify an
isolated package of debris with a volume of 2290 m3 (a, Fig. 7). This package of debris was not
there in August 2010. In the case of a complete failure of this package the traffic infrastructure
and buildings in the main valley could be endangered. Additional loose material in the upper
Fig. 6. Sediment transport in the area of the active rock glacier Grabengufer. a) Debris package at the
front, b) Erosion in the frontal area and c) Hollow of the large rock fall. Topographic map ©swisstopo
(DV033492.2).
120
Fig. 7. Oblique view of the debris package (a) at the front of the rock glacier Grabengufer above an
escarpment (image: Ch. Graf, summer 2011).
part of the channel (section B, Fig. 5) acts as potential start volume of bigger debris flows
reaching the main valley.
The terrain changes at the surface of the rock glacier can be identified also very well. Using
single, well distinguishable rocks, the average speed of the rock glacier can be estimated
(Kenner et al. 2011b).
Upper part of the channel (B) Figure 8
In the channel from the front of the rock glacier at approximately 2375 m a. s. l. until the exit
of a steep gully at approximately 1650 m a. s. l., a total volume of 23 720 m3 was deposited and
a total volume of 6760 m3 eroded (Fig. 8). A trench, located directly below the rock glacier
is continuously filled by freshly mobilized debris. This material can be seen as the maximal
available starting volume for a potential debris-flow event.
We do not observe any freshly built trenches within the observation period of approximately
one year. This indicates that there is not a large amount of freshly material available which
would significantly increase the danger for an extreme debris-flow event. But if the northeastern area of the rock glacier front continues its rising activity, this hazard can further grow.
Lower part of the channel (C) Figure 9
Below the exit of the gully at approximately 1650 m a. s. l. the majority of the terrain changes
are due to anthropogenic activities. Freshly accumulated debris was dug away mechanically
to open space for upcoming torrent events. By constructing an adapted mitigation measure
including a depositional area after cutting the debris-flow breaker in 2010 (Graf et al., this
volume) (a), 10 460 m3 have been removed. Above the road (b) 1635 m3 have been eroded
and 1436 m3 have been deposited.
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Fig. 8. Map of the terrain changes within the upper channel area with well visible freshly filled trenches.
Topographic map ©swisstopo (DV033492.2).
Fig. 9. Lower part of the channel with clearly visible anthropogenic terrain changes (a and b).
Topographic map ©swisstopo (DV033492.2)
122
4Conclusions
In this study we used digital terrain models DTMs derived from helicopter based laser
scanning datasets acquired in summer 2010 and summer 2011 to assess sediment fluxes
in the high-alpine catchment of Dorfbach. The high spatial resolution of 0.5 m enables
detailed mapping of even small terrain changes. Even though the data acquisition is costly,
this methodology achieves very impressive results. We can identify areas of deposition and
erosion even in very steep, inaccessible terrain and we can determine the resultant volumes.
By repeating this kind of data acquisition over the same area, a detailed database of the
mass balance for the debris material transportation can be generated. This information can
be used to better understand debris-flow processes and to improve numerical simulation
tools such as RAMMS::DEBRIS FLOW. If larger areas have to be covered, methods such
as digital photogrammetry might be more economic but they will not achieve the very high
accuracy of LiDAR data. However, dependent on the planned application, such data might
as well be suitable for tasks such as hazard mapping or mitigation measure planning.
Acknowledgements
The authors thank the community of Randa and St. Niklaus, the canton of Valais and the
Federal Office for the Environment FOEN for financial support of the study and Helimap
for the high quality LiDAR data acquisitions as well as the reviewer Stuart Lane for his
helpful comments.
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Accepted 11.09.2012

Sediment transfer mapping in a high-alpine catchment using

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