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CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS :

CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS :. THE CASE STUDY OF THE RITIGRABEN TORRENT (SWISS ALPS). Reporter: 簡志安 Number ： 79842020. Outline. Introduction Terminology Site description Catastrophic Event Statistical Analysis

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CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS :

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  1. CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS: THE CASE STUDY OF THE RITIGRABEN TORRENT (SWISS ALPS) Reporter: 簡志安 Number：79842020

  2. Outline • Introduction • Terminology • Site description • Catastrophic Event • Statistical Analysis • Conclusion

  3. Introduce (1/3) • Rise in temperature caused by climate change • Beniston et al., 1994 • Jones and Wigley , 1990 • Beniston and Rebetez , 1996

  4. Introduce (2/3) • 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.

  5. Introduce (3/3) • The factor which determine the occurrence of • debris flow • Lewin and Warburton , 1994 • Zimmermann and Haeberli , 1992 • Haeberli et al . ,1993 ,1994

  6. 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

  7. Site description (1/2) • Pennine Alps • South-western Switzerland • On the slope of a large • intra-alpine valley,Matteral

  8. Site description (2/2) • Approximate 2000m length • From 1050m to 3100m altitude • Slope between 15° and35° • Catchment area is 1.4km2 1.Rock glacier 2.Present flow path of debris flow 3.Alluvial fan

  9. Catastrophic Event (1/2) • The debris flow data from 1922~1994 years. • The frequence of occurrence of these debris flow has • increased since 1980s.

  10. Catastrophic Events (2/2) 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)

  11. 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)

  12. The triggering of debris flow threshold

  13. 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.

  14. 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

  15. Evolution of minimum temperatures at study area

  16. Evolution of maxmum temperatures at study area

  17. Conclusion (1/2) • 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)

  18. 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.

  19. Thanks for attention

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