Schlunegger Fritz

Debris flow entrainment and sediment transfer processes at the Illgraben catchment, Switzerland

Project Number: CH-4409
Project Type: Dissertation
Project Duration: 06/01/2007 - 06/30/2010 project completed
Funding Source: WSL ,
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

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


Research Areas:
Landscape

Disciplines:
environmental sciences
geomorphology

Keywords:
Sediment transfer processes, erosion and deposition, debris flow, entrainment, aerial images, flood, erosion sensor, channel processes

Abstract:
Debris flows pose a significant natural hazard in mountainous areas [Jakob and Hungr, 2005], and are an important process in modifying landscapes [Stock and Dietrich, 2006]. A debris flow is a fast flowing mixture of poorly-sorted sediment and variable amounts of water and has a large transport capacity [e.g., Loat and Petrascheck, 1997; Major et al., 2005]. Debris flows can transform from an initially small flow to a large hazardous one through entrainment of sediment from the channel bed and banks into the flow, thereby increasing the final debris flow volume by several orders of magnitude [Takahashi, 1981; Rickenmann and Zimmermann, 1993; Gabet and Bookter, 2008]. However, little is known about the governing processes and timing of debris-flow entrainment [Hungr et al., 2005].


The work presented herein investigated debris flow entrainment and the implications on the sediment transfer processes at the Illgraben, Switzerland, and was accomplished within the CCES TRAMM-project (Competence Center of Environmental Sustainability / Triggering of Rapid Mass Movement). At the Illgraben, geomorphic activity is very high and several debris flows occur every year. The sparsely vegetated slopes are composed mainly of quartzites, calcites and limestones, and the debris fan, with a radius of about 2 km, is unusually large for the Alps. Since the year 2000 the Swiss Federal Research Institute WSL has conducted research both in the catchment and on the fan.


Landscape change was measured by photogrammetric analysis of aerial images from Fall 2007, Summer and Fall 2008, and Fall 2009. The debris, often originating from bedrock landslides, was transported stepwise in a complex sequence of erosion, storage, and re-mobilization. The magnitude and nature of sediment transfer processes showed large spatial and temporal variability, and the residence time of the deposits was generally shorter than one year. The observed landslides with volumes of 500 to 4400 m3 did not transform directly into debris flows but stopped in the downslope channel and were an order of magnitude smaller than the debris flows at the Illgraben outlet. While the mechanism of debris flow initiation could not be determined unambiguously, it is clear that the debris flows had to entrain substantial amounts of sediment along the flow path to reach the volumes estimated at the distal end of the fan.


Erosion sensors were developed for this project to determine the timing and amount of sediment entrainment during a debris flow or flood. A sensor column is composed of twenty cylindrical aluminium tubes (50 mm tall) stacked together vertically. These elements are connected by electronic resistors and form a resistance chain. In Spring 2008, five of these 1 m tall erosion sensors were installed vertically and flush with the surface in the riverbed on the lower part of the fan. As one or more elements are entrained by a debris flow or flood, the timing and number of eroded elements can be determined by relating the drop in total resistance to the change in the length of the sensor column and, therefore, depth of erosion. Due to the sensor design, only maximum depth of erosion can be determined and cycles of erosion remain undetected unless the uppermost element of the sensor column is entrained. To determine the time of front arrival at the erosion sensor site, automatically triggered photo cameras took images of the sensor site regularly. Two pore water pressure probes were buried at 1 and 1.2 m in the channel bed nearby the erosion sensors. Flow depth, shear and normal forces measured at the observation station (70 m downstream) were related to the erosion sensor site and considered in the interpretation of the erosion measurements.


The timing and magnitude of channel-bed erosion were measured in 2008 for three debris flows and four floods. During two debris flows erosion was detected within 10 to 20 s after front arrival and before maximum flow depth, shear and normal stress, measured nearby, were reached. At the head of these debris flows, a simultaneous increase and decrease was recorded at both pressure probes. During floods erosion did not coincide with increased flow depths. A sediment layer covered the uppermost elements of the erosion sensors before the third debris flow, and the time of erosion of this layer could therefore not be determined. Terrestrial surveys of the channel bed before and after the debris flows showed that sediment was deposited on top of the sensor columns after erosion was recorded at the head of the flows. This indicates that the bed was reworked to a larger depth than directly visible at the surface after the event. Observations from elsewhere in the channel on the fan support our measurements of the magnitude of debris flow erosion and demonstrate that debris flows can have a large erosive potential, even on the fan.


The findings of the presented work show that the amount of transferred sediment at the Illgraben is large. Furthermore channel reaches can act both as sediment source and sink. Entrainment of sediment is essential for the formation of large, voluminous debris flows, and erosion took place at the head of the flows. The large erosivity, potential for substantial increase in volume following initiation, and influence of the incorporation of entrained material on the flow dynamics should be considered in hazard assessments and debris flow models.

Leading questions:
  • What are the characteristics and patterns of sediment transfer, and where are source and storage areas located?

  • How are sediment mobilization and transfer in the upper catchment related to sediment output estimated at the Illgraben outlet, and what are the implications for understanding entrainment mechanisms of debris flows?

  • How can debris-flow erosion be measured during an event?

  • When does debris-flow erosion take place within an event, and how is the timing and location of erosion within the flow related to flow parameters?

  • Do differences exist between debris-flow and flood erosion?

    PDF: ftp://ftp.wsl.ch/pub/bergerca/Thesis_bergerca/bergerca_final2010_hyperref_red.pdf

    Publications:
    Berger, Catherine. 2010. Debris flow entrainment and sediment transfer processes at the Illgraben catchment, Switzerland. Dissertation, Universität Bern.


    Last update: 12/29/16
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