Molnar Peter

Fluvial system dynamics in steep headwater basins. A study on the Illgraben catchment (VS)

Project Number: CH-4838
Project Type: Research_Project
Project Duration: 09/06/2007 - 10/31/2008
Funding Source: other ,
Leading Institution: Institut für Umweltingenieurwissenschaften, ETH Zürich
Project Leader: Prof. Peter Molnar
Hydrologie und Wasserwirtschaft
Institut für Umweltingenieurwissenschaften (IfU)
ETH Zürich
HIL D 23.1
Stefano-Franscini-Platz 5
8093 Zürich
Phone: +41 (0) 44 633 29 58 ; +41 (0) 44 633 30 75
FAX: +41 (0) 44 633 10 61
e-Mail: molnar(at)ifu.baug.ethz.ch
https://hyd.ifu.ethz.ch/

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


Research Areas:
Landscape

Disciplines:
environmental sciences

Keywords:
Illgraben, hydrology, sediment, debris flow

Abstract:
The Illgraben catchment in southwest Swiss alps is a steep mountain catchment with high debris flow activity. Due to large sediment transport the Illgraben offers a good opportunity for debris flow research. For investigation of initiation mechanisms of debris flows detailed information about the catchment are needed. In this thesis morphologic and hydrologic basin characteristics are investigated to provide basic information about an active torrent catchment and its behaviour during rainfall events. The analysed Illgraben catchment is situated in upper Wallis, southwest Swiss alps, south of Leuk. The Illgraben with its 9.3 km2 basin is a tributary of the Rhone river. Its high sediment delivery activity including several debris flow events per year generated a large debris flow fan at the Rhone valley bottom.
In sediment delivery the concept of connectivity is of high interest. Connectivity in water and sediment transport is an essential factor determining sediment delivery for debris flow initiation. The spatial distribution and connection of areas involved in sediment processes defines how disturbances such as debris flows or landslides can propagate throughout the system. In coupled systems active transfer of mass and energy between hillslopes and channels occurs. For example a landslide on a hill can reach the channel and transform to a debris flow that reaches the outlet. So the whole system from the outlet up to the instable hillslope is connected. As basis for spatial analysis digital elevation models (DEM) with a 2.5 and a 25 m grid resolution are used. On the basis of an orthophoto land use pattern in the basin are identified. The concept of connectivity is studied in the catchment. By analysing slope and water flow patterns throughout the catchment, areas that are subject to erosion or soil instabilities were defined.
Slope as a major factor for sediment transport is used to determine connected areas in the catchment. The involvement of a number of sediment transport processes leads to a complex interaction of these processes that are characterized by various factors such as slope, upslope contributing area or land use. General formulations approaching processes specific mechanisms were used to study the spatial distribution of these processes.
The spatial analysis does not only result information about sediment delivery processes and transport but also provides support for building a hydrological model of the Illgraben catchment. According to flow path patterns a simple approach for spatial discretisation of the catchment is developed. Topographic information and land use pattern are prepared for input to the hydrological model.
For rainfall, a distinct altitude dependence with increasing rainfall amount with altitude is observed. For modelling purpose rainfall is distributed over the whole catchment according to local elevation. In the Illgraben, rainfall is often directly coupled to a reaction in sediment transport processes. This makes it interesting to study not only sediment transport but to model water flows within the catchment to get a better insight into the reaction mechanisms of sediment transport to rainfall events.
The hydrological model structure is based on findings from Terrain Analysis and rainfall and runoff data analysis. 40% of the active basin area consist of rock faces leading to a fast runoff response. Despite the large rock areas the Illgraben has a low runoff coefficient ranging from 2 to 29%. There exist some indications that the large differences in runoff coefficients are provoked through snowmelt. In consequence the catchments runoff coefficient in snow free state is around 5%.
There is a clear evidence for changing water supply to debris flow initiation throughout the year. In the beginning of debris flow season in May the upper part of the catchment is still covered by snow. Rainfall on snow enhances melting and results in huge water input in addition to rainfall. As the amount of snow in the catchment is reduced the influence of snowmelt gets smaller. From mid July on the catchments own reaction to rainfall shows up with very low runoff coefficients. According to runoff coefficient the catchment has a very high storage capacity.
Modelling results reveal that the backmost part of the catchment with large rock areas is producing the largest contribution to channel runoff. Large talus slopes in this area at the foot of rock faces constitute a high potential for debris flow initiation. Along the Illgraben channel the rock faces contribute further large volumes of water.
An open question remains concerning the mechanisms of debris flow initiation in the catchment. Further research could focus on the role of snow cover for debris flow initiation in the beginning of debris flow season and on the effective initiation mechanisms occurring at different locations in the Illgraben catchment.

Leading questions:
  • Which areas are important for sediment delivery?

  • In which areas large amounts of water are concentrated during rainfall events (through surface runoff / subsurface runoff)?

  • Which areas are more susceptible to erosion through overland flow? Where are the areas especially susceptible to wetness induced instability and landslides (large subsurface flows)?

  • In which areas there is a potential for debris flow initiation?

  • How reacts the catchment to different intensive short duration or long during rainfall events in terms of reaction time, runoff volume, runoff coefficient and hydrograph shape?

  • What is the amount of runoff at different locations in the channel?

  • What is the importance of storage capacities within the catchment?

    Publications:
    Nydegger C., 2008. Fluvial system dynamics in steep headwater basins. Master Thesis. Hydrology and water resources management, Institute of environmental engineering, ETH Zürich


    Last update: 12/29/16
    Source of data: ProClim- Research InfoSystem (1993-2024)
    Update the data of project: CH-4838

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