CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS: THE CASE STUDY OF THE RITIGRABEN TORRENT...
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Transcript of CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS: THE CASE STUDY OF THE RITIGRABEN TORRENT...
CLIMATIC CHANGE AND DEBRIS FLOWS
IN HIGH MOUNTAIN REGIONS:
THE CASE STUDY OF THE RITIGRABEN TORRENT
(SWISS ALPS)
Reporter: 簡志安 Number: 79842020
Introduction
Terminology
Site description
Catastrophic Event
Statistical Analysis
Conclusion
Outline
Introduce (1/3)
Rise in temperature caused by climate change
Beniston et al., 1994
Jones and Wigley , 1990
Beniston and Rebetez , 1996
Golbal warming impacts to the research watershed
Haeberli et al , 1993
permafrost reactions to atmospheric warming :
1. Active layer thickening with thaw settlement in supersaturated
materials
2. Disturbance of temperature distribution at depth
3. basal melting of permafrost with thaw settlement in
supersaturated materials
The frenquency of occurrence of these debris flows has
increased since late 1980s.
Introduce (2/3)
The factor which determine the occurrence of
debris flow
Lewin and Warburton , 1994
Zimmermann and Haeberli , 1992
Haeberli et al . ,1993 ,1994
Introduce (3/3)
Terminology(1/1)
Debris flows rapid mass movement of granular solids,
water and air
A mass movement that involves water-charged, predominantly coarse-grained inorganic and organic material flowing rapidly down a steep, confined, pre-existing channel
Site description (1/2)
• Pennine Alps
• South-western Switzerland
• On the slope of a large
intra-alpine valley,Matteral
Site description (2/2)
Approximate 2000m length
From 1050m to 3100m altitude
Slope between 15° and 35°
Catchment area is 1.4km2
1.Rock glacier
2.Present flow path of debris flow
3.Alluvial fan
The debris flow data from 1922~1994 years.
The frequence of occurrence of these debris flow has
increased since 1980s.
Catastrophic Event (1/2)
Catastrophic Events in research watershed
• Two roads, railway, bridge
• Tourist resorts and other
infrasructure
• 20 hectares of farmland
• The volume of material was
estimates at 60000~90000(m3)
Catastrophic Events (2/2)
Statistical Analysis(1/6)
The results of this analysis have shown that the coincidence between triggering of debris flows and extreme precipitation events
Highest than 4σ(standard deviation) threshold.
A three-day period.
Other good relationships 3.5σ/4 、 3σ/5 day period (but 4σ/3 is the best)
Statistical Analysis(4/6)
No debris flows were triggered when rainfall amount below this
threshold.
But there were exist two events out of the ordinary 1979’s,1994’s.
Two of the events rainfall amount above threshold, but there
were no debris flows occurred .
1979’s : It seem to be the lack of sufficient sediments for a debris flow occur,these may had been removed from the torrent system by the previous flow in 1977’s.
1994’s :Debris flows occurred in 1991’s,1993’s at two year interval only ,so there is not enough materials to occur debris flow.
Statistical Analysis(6/6)
Temperature factor:
Higher temperatures play a key role in the availability of sediments through glacier retreat and its impact on permafrost (Haeberli,1993)
Borehole observations indicate an increase of permafrost temperatures during the last decadeat a rate of 0.1 K/year
The study area revealed intense regressive erosion inside these scars due to the degradation of permafrost exposed to the open air (a retreat of about 1 meter was observed in some places)
The snowmelt led to increase soil contains water
Higher temperature play a key role in the availability of sediments through glacier retreat and its impact on permafrost.
Alps and in Gr¨achen in particular, temperatures have risen during the 20th century,and particularly during the last 10–15 years, with far greater amplitudes than those observed at the global level
There is a statistically significant positive correlation between severe storms and minimum temperature. (Dessens ,1995)
Conclusion (1/2)
Conclusion (2/2)
Active layer thickening with thaw settlement in supersaturated
material cause the sediment yield increased.
The necessary conditions for triggering debris flow are :
1.Rainfall must exceed 4σfor the accumulated precipitation on
three-day period.
2.It is essential that there be a sufficient amount of
sedimentary material available to feed the debris flow.