CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN

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 l Introduction l Terminology l Site description l Catastrophic Event l Statistical Analysis l Conclusion

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

Introduce (2/3) Golbal warming impacts to the research watershed l 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 l The frenquency of occurrence of these debris flows has increased since late 1980 s.

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

Terminology(1/1) l Debris flows rapid mass movement of granular solids, water and air l. A mass movement that involves watercharged, predominantly coarse-grained inorganic and organic material flowing rapidly down a steep, confined, preexisting channel

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

Site description (2/2) l Approximate 2000 m length l. From 1050 m to 3100 m altitude l. Slope between 15° and 35° l Catchment area is 1. 4 km 2 1. Rock glacier 2. Present flow path of debris flow 3. Alluvial fan

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

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(m 3)

Statistical Analysis(1/6) l The results of this analysis have shown that the coincidence between triggering of debris flows and extreme precipitation events l Highest than 4σ(standard deviation) threshold. l A three-day period. l Other good relationships 3. 5σ/4 、3σ/5 day period (but 4σ/3 is the best)

The triggering of debris flow threshold

Statistical Analysis(4/6) l. No debris flows were triggered when rainfall amount below this threshold. l. 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: l Higher temperatures play a key role in the availability of sediments through glacier retreat and its impact on permafrost (Haeberli, 1993) l Borehole observations indicate an increase of permafrost temperatures during the last decadeat a rate of 0. 1 K/year l 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) l. The snowmelt led to increase soil contains water

Evolution of minimum temperatures at study area

Evolution of maxmum temperatures at study area

Conclusion (1/2) l Higher temperature play a key role in the availability of sediments through glacier retreat and its impact on permafrost. l Alps and in Gr¨achen in particular, temperatures have risen during the 20 th century, and particularly during the last 10– 15 years, with far greater amplitudes than those observed at the global level l There is a statistically significant positive correlation between severe storms and minimum temperature. (Dessens , 1995)

Conclusion (2/2) l Active layer thickening with thaw settlement in supersaturated material cause the sediment yield increased. l 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.

Thanks for attention