Hazard mapping in Mattertal VS: Data acquisition and numerical modeling of debris flows
The
degradation of the permafrost in alpine environments causes a reduction of the stability
of alpine slopes. As a consequence there is more loose material available in
the active permafrost layer. This loose material is transported by erosion into
the torrent and intense rainfall may lead to catastrophic events. Villages and
roads in the alpine valleys are faced with a permanently changing hazard
situation.
Instable slopes in the Mattertal
Recent
observations show that thawing permafrost and collapsing fronts of fast-moving
rock glaciers lead to instable slopes in the Mattertal. An large amount of
debris gets displaced and as a consequence the slope and the torrent geometry
change quickly and significantly.
Figure 1: Degradation of Permafrost results in a larger amount
of debris.
In
the Mattertal a variety of research activities of different groups are up and
running. Following a mandate of the Canton of Wallis, the WSL is leading an
applied research project to model the debris flow events and to monitoring the
torrents. RAMMS, a numerical mass movement model developed by WSL, is playing
an important role. The findings concerning the zones at risk are a basis for
planning of protection measures.
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Figure 2: Numerical simulation of debris flow events in RAMMS,
maximum flow depth (left) and velocity (right).
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Figure 3: Visualisation of the simulation results in Google
Earthtm.
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Base data
Digital Terrain Models (DTM) are the basis of
the RAMMS runout simulation for mass movements (avalanches, debris flow, rock
fall) and for the interpretation of changes of the terrain surface. Until today
no high resolution terrain models are available for areas at risk over 2000
meters above sea level. Therefore the WSL is testing different methods and the
resulting terrain data to evaluate their use in hazard analysis.
The module “debris flow” in RAMMS is calibrated
using event data. For this purpose an automatic debris flow monitoring station
has been installed in the Dorfbach Randa. The monitoring system consist of two
main stations which measure flow velocity and flow high. Additionally two
cameras record the event to characterize it afterwards. The monitoring station
delivers detailed data to document the events, whereas historical events are reconstructed
as good as possible. Together with an attentive analysis of the catchment area
it is possible to derive various scenarios, which are used for the hazard assessment.
The event data are also used to improve and optimize RAMMS. The next step for
RAMMS is to improve the modeling of the entrainment and bulking processes. The
base data are partly delivered from new measurement methods, such as laser, and
repeated measurement of a cross-section of a certain torrent segment.
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Figure. 4: Automatic debris flow monitoring station in
the Dorfbach Randa (left) and a cutout of a high resolution digital terrain
model generated by Airborne Laserscanning.
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Numerical modeling
and hazard mapping
The results of the calculated scenarios in RAMMS
are used to support the hazard mapping. The resulting flow intensities and the predicted
breakout points and flow path provide important knowledge for use in defining the
hazard zones. These information are, next to terrain analysis, analysis of
geomorphic field data (silent witnesses) and the chronicle of past events, synthesized
to create a hazard map which forms a basis for risk management and the
identification of deficient zones and the design of mitigation measures.
Project Mandate:
- Communities of Randa and St. Niklaus
- Forest and Landscape Service
of the Canton of Wallis (Monitoring and numerical simulations)
Kontakt
