Badoux Alexandre

Temporal and spatial variation of ersion processes in the Illgraben, an alpine debris flow catchment

Project Number: Parcs Data Center 9904 / 4D CH-4832
Project Type: Master
Project Duration: 09/01/2006 - 06/30/2007 project completed
Funding Source: other ,
Leading Institution: WSL Birmensdorf
Project Leader: Dr. Alexandre Badoux
Mountain Hydrology and Mass Movements Research Unit
WSL
Zürcherstrasse 111
8903 Birmensdorf
Phone: ; +41 (0) 44 739 22 83
FAX: +41 (0) 44 739 22 15
e-Mail: alexandre.badoux(at)wsl.ch
http://www.wsl.ch/fe/gebirgshydrologie/index_DE

related to this project.
for which the project has a relevance.


Research Areas:
Landscape

Disciplines:
environmental sciences

Keywords:
erosion processes, Illgrabren catchment, debris flows

Abstract:
For this thesis, temporal and spatial variations of sediment erosion processes in a steep catchment were analyzed. Erosional processes include both hillslope and gully erosion, and also mass movement processes that are dominated by debris flows. Debris flows are a very destructive form of rapid mass slope movement in mountainous areas and typically arise where the rapid onset of water flow mobilizes unconsolidated debris mantling steep slopes.

The Illgraben catchment (9.5 km2), situated near Susten (Leukerbad) in canton Valais, is characterized by a very high degree of sediment transport activity and shows rapid dynamic landscape changes and evidence of significant erosion events, including frequent large debris flows. In addition to the high degree of geomorphic activity, the relatively easy access and abundant instrumentation make it an ideal place for such a project.

In contrast to the current research on the debris fan, this thesis is directed at the catchment. The overall goal was a better understanding of the erosion processes operating in the Illgraben catchment, their location and in particular the magnitude and timing of surficial erosion on typical geomorphic sub-systems in comparison to well-constrained estimates of the total sediment exported from the catchment.

Analyses of the character and behavior of the geomorphic or landscape systems and how they are geomorphically connected provided a platform to interpret the overall sediment transfer processes operating in the Illgraben catchment. The main focus was determination of sediment yield and temporal changes in the areas of the various systems, which can be used to infer the nature of the coupling relationship among them. Following a recent advance in the literature, a distinction was made between coupled- and decoupled systems, where the decoupled system was divided further in three subsystems (grassland, forest and decoupled erosion). The analysis of the character and sediment transport rates of those systems was done with field work, where hillslope erosion was measured using a standard silt fence technique, aerial- and ortho- photography work combined in a GIS environment, petrography composition of debris flow sediments and their source areas, and climate and precipitation information.

Analyses of aerial photographs from years 1959, 1999 and 2004 showed changes inside of the decoupled system, but the spatial relation between the coupled- and decoupled system stayed almost constant. Within the decoupled system, an increase of forest and decoupled erosion area as well a decrease of grassland was measurable. The increase in the area of the forest subsystem corresponds with the natural afforestation related to landscape use changes that has been observed all over Switzerland. How much of the increase of the decoupled erosion area is related to changes in the landscape use or climate warming (and the associated higher precipitation intensities) could only be speculated. Using climate observations from the Sion station, a trend towards warmer temperature was evident, but a trend towards a higher number of precipitation events with a high intensity could not be discerned using the available daily precipitation sums from stations Sierre, Hérémence and Grimentz.

In the coupled system, only the overall erosion rate for the catchment was determined. The mass discharge rate was calculated from the weight, height, and velocity of debris flows at a force plate situated near the basin outlet. While large temporal fluctuations have been reported in the coupled system (e.g. periods of aggradation and degradation of on the order of a few meters), they were not investigated in this work. The erosion rate in the coupled system was calculated to be three orders of magnitude larger than the values measured in the decoupled system, and therefore the coupled areas contributed more than 99% of the sediment leaving the catchment. The sediment transport rate in the coupled system is strongly related to rainfall intensity (McArdell and Badoux, in Prep).

Variability in the rate of measured sediment transport in the decoupled systems was explored using rainfall intensity and other parameters. The decoupled system surprisingly showed almost no correlation with precipitation as in the coupled system where every precipitation event during the measurement period with an intensity larger than 2 mm/10 min at Pluviometer 3 (one of the automatic precipitation stations in the catchment) was associated with debris flow occurrence. The sediment amount in the silt fence plots depended mainly on the vegetation layer, slope angle, grain size composition and the measurement interval, where the missing correlation with precipitation in the decoupled system could be due to the relatively small precipitation amounts during the measurement period, a lack of surface runoff in the decoupled system related to large soil infiltration capacities, etc.

Additional analyses from a collaborative and parallel project at the Univ. of Bern provide complimentary evidence which supports the work in this thesis, such as the conclusion from the petrographic analysis of the sediment in the coupled system which indicates that more than 60% of the sediment output came from approximately 6% of the entire catchment area. Also, the volume of sediment delivered by the Illgraben to the Rhone River is estimated to be more than 20% of the yearly Rhone sediment budget.

The division into a coupled- and decoupled system as well as the subsequent independent observations clearly showed large differences in landscape connectivity and demonstrated the utility of this approach for interpreting the distribution of geomorphic processes occurring in this catchment and may be applicable to other steep Alpine catchments.

Leading questions:
  • How large were the fractions of forest, grassland and erosion areas compared to the entire catchment area in 2004, 1999 and 1959?
  • What is the slope angle distribution in the different subsystems?
  • Did some recognizable area changes take place and how can they be explained?
  • What are the process rates in the copled and decoupled hillslope subsystems?
  • How large is the yearly sediment output of the catchment?
  • Where is the debris flow initiation area located?

    URL: http://e-collection.library.ethz.ch/eserv/eth:29289/eth-29289-01.pdf

    Publications:
    Gwerder, Corina. 2007. Temporal and spatial variation of erosion processes in the Illgraben, an alpine debris flow catchmen. Diplomarbeit. ETH Zürich.


    Last update: 4/5/22
    Source of data: ProClim- Research InfoSystem (1993-2024)
    Update the data of project: CH-4832

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