Molnar Peter
Quantifying and modeling sediment transfer through the Illgraben
Project Number: CH-5286
| Project Type: |
Dissertation |
| Project Duration: |
02/01/2010 - 11/30/2013 project completed |
| Funding Source: |
WSL , |
| 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/ |
Research Areas:
Disciplines:
| |
|---|
| engineering sciences |
| fluid dynamics |
Abstract:
Quantification of sediment yield from mountain basins is an key component in the study of the evolution of mountain ranges and is important to the population living downstream to which sediment can be both a vital resource and hazard. Mountain basins are particularly susceptible to climate change due to their sensitivity to snow and ice melt processes and rapid rainfall- runoff response. Understanding the interactions of earth surface processes with climate is important to be able to model and predict mountain basin sediment yield under a changing climate. In this thesis I aimed to quantify processes of sediment production, transport and yield; identify their interactions, climatic and seismic controls, and finally to model sediment transfer in the Illgraben, a small but highly active mountain basin in the Swiss Alps prone to large slope failures and debris flows.
I quantified sediment production, transfer and yield over a 42-year period from 1963 – 2005, split into 4 sub-periods, from aerial photographs using digital photogrammetry. The hillslopes were eroded at an extremely high mean annual rate of 0.39 ± 0.03 m yr ?1 by slope failures. I extracted an inventory of more than 2000 failures spanning 6 orders of magnitude in volume and analyzed their statistics. The failures follow a characteristic magnitude-frequency distribution with a roll-over and power-law tail. The results support the hypothesis that the shape of the distribution arises from 2 different failure types: shallow slides within the upper weathered layer of the slope that have a restricted depth and therefore volume, and deep- seated bedrock failures that occur along failure surfaces in the slope and have a wide range of volumes. The latter account for more than 98% of sediment supply and limit the relief of the slopes, providing empirical support for the concept of threshold slopes. The results also demonstrate the importance of constraining the scaling exponent between landslide area and volume for different geological settings.
I analyzed climate and seismic data over the study period with the aim of explaining the observed patterns of hillslope erosion (production), channel erosion (transfer) and sediment yield. A marked increase in hillslope erosion rate in the 1980s was most likely related to a significant increase in air temperature and related decrease in snow cover depth and duration. However, analysis of the potential triggers of an individual failure event in the Illgraben illustrates that multiple triggering mechanisms may exist such that slope failure is highly stochastic phenomenon. Channel erosion rate is more clearly related to the frequency of intense summer rainfall events. Hillslope erosion rate exceeded channel erosion rate over the study period, indicative of a downslope-directed coupling relationship in which hillslopes erode independently of channel incision. This implies that hillslope erosion is the first-order control on the high sediment yield in the Illgraben, which averaged 2.3x105 m3 yr ?1 over the study period.
I developed a probabilistic model of sediment transfer in the Illgraben with the overall aim of explaining the observed non-linear and stochastic sediment discharge. The model is based on the concept of a sediment cascade, in which, following erosion sediment goes through multiple cycles of storage and remobilization. Sediment input is drawn from the probability distribution of slope failures and the model is driven by observed climate. The model consists of two sediment storage reservoirs representing hillslopes and channels and a basin-wide water reservoir, through which sediment and water are routed at a daily resolution according to simple but physically meaningful rules. Despite its simplicity, the model reproduces remarkably complex sediment discharge dynamics, which can be explained only by considering jointly the availability of sediment and the triggering potential, quantifying the role of history (system memory) and climate (triggering events) on sediment discharge in the Illgraben. Although the model was developed for the Illgraben, the findings have general implications for fluvial systems that can be schematized into sediment cascades and where the supply of sediment and triggering of events is largely stochastic. The model may be used in future research to investigate uncertainty in erosion rates back-calculated from sediment yield and to predict sediment yield under a changing climate in the Illgraben.
Publications:
Bennet, Georgina Lucy. 2013. Quantifying and modeling sediment transfer through the Illgraben. Dissertation, ETH Zürich.
pdf Dissertation
Last update: 7/18/17
Source of data: ProClim- Research InfoSystem (1993-2024)
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