Réf. Bardou & al 2003 - P

Référence bibliographique complète
BARDOU, E., FOURNIER, F., SARTORI, M. Palaeoflood reconstruction at Illgraben torrent (Switzerland): a current need for event frequency estimation. 2003. In: V.R.Thorndycraft, G. Benito, M. Barriendos and M.C. Llasat (Eds). Palaeofloods, Historical Floods and Climatic Variability: Applications in Flood Risk Assessment, Proceedings of the PHEFRA Workshop, Barcelona, 16-19th October, 2002, p. 53-59.

Mots-clés
Torrential events, reconstuction, geological, anthropological and climatic factors.

Organismes / Contact
WSL, div. Water, Earth & Rocks Movements; antenne ENA-Valais; av. de l'Industrie 45; CH­1950 Sion; Switzerland
UNIL, Section of Earth Sciences, BFSH-2, CH-1015 Lausanne, Switzerland
UNIGE, Section of Earth Sciences; Rue des Maraîchers 13; CH-1205 Genève; Switzerland

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
    Torrential events  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Southern Switzerland Walliser Alps Illgraben Torrent      

(1) - Modifications des paramètres atmosphériques
Reconstructions
During the last millennium, 5 different climatic phases are described (Zryd, 2001):
a warm phase called “the climatic optimum of the year thousand”;
a small refreshing phase during the XIIth century;
a warm phase from 1200 to 1550;
the Little Ice Age from 1550 to 1850;
the current warming phase.
Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Bibliographic review

(2) - Effets du changement climatique sur le milieu naturel
Reconstructions
 
Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

(3) - Effets du changement climatique sur l'aléa
Reconstructions
Major changes occurred at the location analysed in between the middle of the warm phase corresponding to a new and intense exploitation of the territory and the middle of the Little Ice Age. Transitions between periods do not correspond with possible upsurge of flooding on the fan. It is difficult to separate effect due only to the climate or to the impact of anthropologic changes.

Comparing well documented events of the last decades (from 1932 to 2000) to the rainfall in Sierre (a town located 4 km away) no trend in the rainfall-magnitude relation appears. Furthermore the largest recorded event (in 1961) occurs with very low rainfall.

During the XXth century more than 3 events of over 100'000 m3 were reported. During years without high magnitude events it is assumed that 100,000 to 200,000 m3 of sediments are eroded from the catchments. It induces productivity rates from 25,000 to 50,000 m3/km²/a.

Prediction of “magnitude-frequency” characteristics of solid transport events in such torrents is impossible if geological or anthropological events are not taken into account (rock avalanche, fan formation, deforestation, dams, etc.). It cannot be directly related to climate change. Further, even if it is not totally sediment-constrained, this torrent shows poor correlation with rainfall events.
Observations
 
Modélisations
 
Hypothèses
 

Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
Almost every year a few flash floods with high suspended sediment content have been observed in the studied area. The nature of these phenomenon ranges from hyperconcentrated flows to debris flows. The total catchment's area is 10 km² but only 3 km² are intensely involved in sediment supply.

Available datas only cover the last millennium. Four main sources were used to cover the wide range of time to be investigated

Dendrochronology to date flooding terraces in the current overbank channel and on the inactive part of the alluvial fan;
Stratigraphy analysis to date palaeosoils in eroded banks;
Radiocarbon (C14) dating (palaeosoils in eroded banks);
Historical records (additional information on the dynamics of the system).

(4) - Remarques générales
 

(5) - Préconisations et recomandations
This study shows how a careful analysis of the system’s dynamic helps to explain changes of the “magnitude-frequency” relationship through the time. However it only gives a qualitative trend and more work is needed to get to a quantitative analysis. Analysis of all debris flow events could be helpful for “magnitude-frequency” reconstruction of geological events too. The public authorities needed a magnitude-frequency analysis to be carried out. Particularities of these phenomena (transitory, non-newtonian, 3D deposits) did not allow us to use classical hydraulic and hydrological approaches to identify their magnitude and frequency.

Références citées :

Burri, M. 1955. La géologie du Quaternaire aux environs de Sierre. Bulletin de la Société vaudoise de Sciences naturelles. 66/289, pp. 135-154.

Eisbacher, G.H. and Clague, J.J., 1984. Destructive mass movement in high mountains: hazard and management, 84-16. Geological Survey of Canada.

Lichtenhahn, C., 1971. Zwei Betonmauern: die Geschieberückhaltsperre am Illgraben (Wallis). In: F.f.v. Hochwasserbekämpfung (Editor), International Symposium Interpraevent, pp. 451-456.

Zimmermann, M., Mani, P. and Gamma, P., 1997. Murganggefahr und Klimaänderung - ein GIS-basiert Ansatz. vdf, Hochschulverlag AG, Zürich.

Zimmermann, M., 2000. Geomorphologische Analyse des Illgraben, GEO 7, Bern, unpublished report.

Zryd, A., 2001. Les Glaciers, ed. Pillet, Martigny.