Like numerous torrents in mountainous regions, the Illgraben creek
(canton of Wallis, SW Switzerland) produces almost every year several
debris flows. The total area of the active catchment is only 4.7
km², but large events ranging from 50'000 to 400'000 m³ are common
(Zimmermann 2000). Consequently, the pathway of the main channel
often changes suddenly. One single event can for instance fill the
whole river bed and dig new several-meters-deep channels somewhere
else (Bardou et al. 2003). The quantification of both, the rhythm
and the magnitude of these changes, is very important to assess the
variability of the bed's cross section and long profile. These parameters
are indispensable for numerical modelling, as they should be considered
as initial conditions.
To monitor the channel evolution an Optech ILRIS 3D terrestrial laser
scanner (LIDAR) was used. LIDAR permits to make a complete high precision
3D model of the channel and its surroundings by scanning it from
different view points. The 3D data are treated and interpreted with
the software Polyworks from Innovmetric Software Inc. Sequential
3D models allow for the determination of the variation in the bed's
cross section and long profile. These data will afterwards be used
to quantify the erosion and the deposition in the torrent reaches.
To complete the chronological evolution of the landforms, precise
digital terrain models, obtained by high resolution photogrammetry
based on old aerial photographs, will be used.
A 500 m long section of the Illgraben channel was scanned on 18th
of August 2005 and on 7th of April 2006. These two data sets permit
identifying the changes of the channel that occurred during the winter
season. An upcoming scanning campaign in September 2006 will allow
for the determination of the changes during this summer.
Preliminary results show huge variations in the pathway of the Illgraben
channel, as well as important vertical and lateral erosion of the
river bed. Here we present the results of a river bank on the left
(north-western) flank of the channel (Figure 1). For the August 2005
model the scans from 3 viewpoints were superposed, whereas the April
2006 3D image was obtained by combining 5 separate scans. The bank
was eroded. The bank got eroded essentially on its left part (up
to 6.3 m), where it is hit by the river and the debris flows (Figures
2 and 3). A debris cone has also formed (Figure 3), which suggests
that a part of the bank erosion is due to shallow landslides. They
probably occur when the river erosion creates an undercut slope.
These geometrical data allow for the monitoring of the alluvial dynamics
(i.e. aggradation and degradation) on different time scales and the
influence of debris flows occurrence on these changes. Finally, the
resistance against erosion of the bed's cross section and long profile
will be analysed to assess the variability of these two key parameters.
This information may then be used in debris flow simulation.