Schlunegger Fritz
Debris-Flow processes in the Illgraben
Project Number: CH-4837
| Project Type: |
Master |
| Project Duration: |
04/01/2007 - 11/30/2007 project completed |
| Funding Source: |
other , |
| Leading Institution: |
Unversität Bern, Geoglogisches Institut |
| Project Leader: |
Prof. Fritz Schlunegger
Exogene Geologie
Institut für Geologie
Universität Bern
Baltzerstrasse 1+3
3012 Bern
Phone: +41 (0) 31 631 87 67 ; +41 (0) 31 684 87 81
e-Mail: schlunegger(at)geo.unibe.ch
http://www.geo.unibe.ch |
Research Areas:
Disciplines:
Keywords:
Debris-flow, Illgraben, Rhône
Abstract:
The Illgraben is known as one of the most active debris flows in the Alps. Many investigations have been carried out in this torrent to better understand the behaviour of debris flows. For this reason many data from different measuring installations, including a force plate in the lowermost part of the channel, are available to determine e.g. the total
sediment flux during a year. Unfortunately these data include just information about the torrent since the year 2000. It was a point of interest to perform an approximate sediment budget for the Illgraben catchment over the last decades in order to see how the Illgraben and therewith probably also other torrents in the Alps have developed in these times. To
achieve this goal, digital photogrammetry has been performed to determine discharge over a time span of forty-five years. To construct two digital elevation models and calculate the mass difference, a segment in the catchment with presumably high erosion rates has been digitized. This area was identified through optically compared images taken at different years. For different reasons that are later elucidated, it was not possible to digitize neither the complete catchment area nor the entire active area. Nevertheless the buildup of a sediment storage in the upper part of the channel could be identified. This implies that more sediment was eroded in the catchment and supplied to the channel than was exported to the trunk stream (i.e. the Rhône). The determination of the petrographic composition of the clasts allowed to allocate the distinct sediment source in the catchment.
Furthermore we found out, that the discharge of the torrent is in the same order of magnitude over the last forty five years compared to the data of the force plate that considers a span of time of four and a half years (2000 – 2004). The ‘active’-area in which we suppose highest sediment yields delivers about 80 000 m3 of mass per year. This volume can be compared to 60 % (72 000 m3) of the measured volume (120 000 m3) of the force plate, because about 60 % could be allocated to this part of the catchment.
On the aerial photographs it is recognizable that the shape and the flow pattern of the Rhône changes where the confluence of the Illgraben is located. For the exploration of potential controls, we analyzed 4 bedload samples of the Rhône for the grain size distribution. Two sites are located above and two beneath the confluence of the Illgraben
with the Rhône. Furthermore a longitudinal stream profile of the Rhône was constructed. The sediment beneath the confluence showed a much coarser distribution and the channel gradient of the Rhône is much steeper there. Above the confluence, oxbow lakes that are aligned with a high sinuosity and that exhibit scroll bars in the inner side of the meander loop imply a meandering flow pattern before the first Rhône-correction was completed between 1863 and 1893. This shows the direct influence of sediment discharge from the Illgraben on the flow pattern, stream gradient and grain size distribution of the Rhône.
Leading questions:
Where are allocated the sediment sources and what are the magnitudes of discharge from the source to the valley?
Publications:
Schnydrig, David. 2007. Debris-Flow processes in the Illgraben. Masterarbeit, Universität Bern.
pdf Mastearbeit
Last update: 7/18/17
Source of data: ProClim- Research InfoSystem (1993-2024)
Update the data of project: CH-4837
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