Foen-Project: Roughness of Alpine Mountain Streams
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The torrent
Steinibach (canton Nidwalden) has transported more than 60.000 m3 of sediment
in the rain storm event 2005. The picture shows the downmost reach after being
dredged. The sediment height from after the event is still visible (gravel on
both embankments). (Manuel Nitsche, WSL)
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In an upper
reach of the Steinibach, slopes are steep and free of vegetation. The brittle
sediments exposed there are an almost inexhaustible source of bed load
material. (Manuel Nitsche, WSL)
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In the
Erlenbach (canton Schwyz) we measure transport rates with acoustic sensors. These
sensors record the signals due to the impact of moving sediment on steel plates.
The plates are installed in a check dam across the stream. (Manuel Nitsche,
WSL)
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Why are we interested
in steep mountain torrents?
Heavy rain storms and associated floods can pose a threat to people and their goods. While the
discharge occurring in steep mountain torrents is small compared to large rivers, bed load
may be transported due to the steep gradient of the river beds. Hence, the torrent is capable of entraining woody debris, gravel and also boulders that may
have remained immobile for decades. If this load hits buildings or other
infrastructure, it can cause major damage. Even though such events are rare,
the risk of extreme bed load transport is always present. The last extreme
event occurred during the heavy storm in Switzerland in 2005. To better manage this risk, we study the
availability of bed load and the transport capacity (i.e. the maximum bed load
volume the stream is capable to transport per unit time) of torrent systems. The
aim of this project is to increase basic knowledge needed for improved future
risk assessment.
Wild, complex and
unpredictable?
If we could predict
the bed load volume transported by a torrent during a rain storm, risk mitigation will
be improved. But the crucial point is this: hydraulics and bed load
availability are dependent on so many factors that it is demanding to compute
and predict sediment transport. There exist frequently used equations to calculate
transport capacity (as a function of shear stress, or bed gradient and discharge)
– but they are accurate for large and less steep rivers only. Since processes
in torrents are different and much more complex in comparison to large rivers, we
cannot just apply those equations to torrents.
What can we improve?
A torrent’s bed has a
geometry that can be different at every meter as one climbs up the valley. This
complex geometry is hard to measure and thus hard to appropriate describe in a
model. Therefore it is critical to derive precise hydraulic conditions to
calculate for example the torrent’s effective transport energy. To meet this
complexity some of the above-mentioned equations for example take the roughness
produced by the pebbly river bed into account. Until now they do not include
the effects of large immobile boulders, woody debris or exposed rock, each of
which may affect the energy available to the stream to transport sediment. We
suspect this “form roughness” to have a major impact on transport capacity. To
consider all those factors to model bed load transport is one major subject of
our project.
Research methods
The project
is mainly realised within a dissertation and benefits from a well-developed research environment, such as the Erlenbach catchment. This well-instrumented Swiss torrent catchment offers an exceptional series of measurements of transport rates and
discharge. In field campaigns and with additional computer-aided analysis of
remote sensing data we will assess the role of form roughness and bed load
availability in influencing transport capacity.
In addition, we use a sediment transport
simulation program, SETRAC, that helps us to test and our approaches. Experiments
can be conducted in our physical lab, which is, among other things, equipped
with flumes and a rain simulator.
Initiator
Federal Office for
the Environment (FOEN), Group for Operational Hydrology
Cooperations
- FOEN, Prof. Manfred
Spreafico
- ETH Zürich, Physics
of Environmental Systems, Prof. James W. Kirchner
- University
of Natural Resources and Applied Life Sciences, Vienna, Institute of Mountain
Risk Engineering, Michael Chiari
- University of Berne, Institute of Geography, Eva Gertsch
Project Staff
- Manuel Nitsche
- James Kirchner
- Dieter Rickenmann
- Alexandre Badoux
- Jens Turowski
- Brian McArdell
Contact
