Réf. Stoffel & al. 2005a - A

Référence bibliographique complète
STOFFEL M., LIÈVRE I., CONUS D., GRICHTING M. A., RAETZO H. GÄRTNER H. W. & MONBARON M. 400 Years of Debris-Flow Activity and Triggering Weather Conditions: Ritigraben, Valais, Switzerland. Arctic, Antarctic, and Alpine Research, 2005, Vol. 37, No. 3, 387–395.

Abstract: Three major rainfall events have caused considerable damage in the Valais region (Swiss Alps) since 1987. Substantial debris flows originating from periglacial environments were recorded during the August 1987 and September 1993 rainfall events, whereas no debris flows occurred in October 2000. This paper aims at putting these large-area events and the apparent increase in debris-flow frequency into a wider context by reconstructing the past debris-flow activity in the Ritigraben torrent (Mattertal, Valais) with dendrogeomorphological methods. Results further show that the apparently above-average concentration of events since 1987 was mainly caused by insufficient and short archival data. In fact, debris flows occurred even more frequently in the nineteenth century than they do today. The spatial distribution of injured trees in particular years further indicates that significant events, like the one in 1993, always occurred in the torrent.

Mots-clés
Debris flow, frequency, dendrochronology, flooding, Ritigraben torrent

Organismes / Contact
Department of Geosciences, Geography, University of Fribourg (IGUF), Pérolles, CH-1700 Fribourg, Switzerland. markus.stoffel@unifr.ch.
Swiss Federal Office for Water and Geology (FOWG), Ländtestrasse 20, CH-2503 Bienne, Switzerland.
Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, 8903 Birmensdorf, 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 River floods Debris flows

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Swiss Valais

Ritigraben torrent (46°11’N, 7°49’E)

West
1500-1800m (study area)
2600-3200 (source area)
1922-2002 (archives and direct observations)
1605-2002 (reconstruction)

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

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

 


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
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
Reconstitutions

Reconstructed Ritigraben event years and flooding data in the selected rivers show significant similarities. Comparisons indicate that 68% of reconstructed debris flow event years at Ritigraben correspond with flooding in at least one of the other catchments. This correspondence is especially true for the late 19th and the 20th century, during which only one debris flow appears nowhere else. The indices of similarity between the different data sets indicate that Ritigraben data best match with data on flooding in the Vispa (indsi=0.33) and the Saltina Rivers (indsi=0.31). In contrast, similarities of archival data for the Mattervispa and the Saaservispa Rivers appear to be less evident (largely insufficient flooding data rather than low similarities).
The probability of debris flow occurrence in the Ritigraben is highest when there is simultaneous flooding in the Saltina or the Vispa Rivers. In the opposite way, debris flow occurrence in the Ritigraben is most frequent when flooding simultaneously occurs in the Alpine Rhone River.

In fact, reconstructed Ritigraben data show that phases with accentuated activity and shorter recurrence intervals than today existed in the past, namely after 1827 and until the late 19th century. This period of short recurrence intervals of debris flows in the Ritigraben coincides with a phase of high flooding activity in major Swiss rivers. After 1835 (and until the mid-1890s), considerable above-average precipitation sums in summer and fall are noted for the Swiss Alps. Furthermore, increased debris flow activity in the Ritigraben goes together with the last “Little Ice Age” advance of the Gorner Glacier, which reached its maximum extent about 1859. Similarly, the period of less frequent debris flow activity after 1922 is also replicated in the major Swiss rivers, where low flooding frequencies were recorded between 1927 and 1975.

The analysis of the spatial distribution of sites affected by flooding shows that the 3 types of events that have been observed during the past 25 years also commonly occurred in the past.
Ritigraben debris flows in June, July, and (exceptionally) August appear to have been initiated by type I event.
Similar to the timing of the recent type II events, data indicate that throughout the past 4 centuries this meteorological situation exclusively occurred in August and September.
Type III events occurred mostly in October or May. These results confirm the seasonal timing of debris flow activity, indicating that events are restricted to the period between June and September.

Long-term fluctuations of late summer and fall precipitation sums not only influence the flooding frequency in the Swiss and Ticino Alps, but they apparently drive the debris flow activity of the Ritigraben torrent. Correlations between global warming and modifications in the number or the size of debris flow events, as hypothesized by Haeberli and Beniston (1998), cannot, so far, be confirmed in the study area.

Owing to the elevated position of the source area (2600-3200 m a.s.l.), Ritigraben debris flows only occur between June and September. Over the past 4 centuries, events were most frequent in August (43%) and September (25%). This concentration of events is especially true for the 1948-2002 period, where all but one event occurred in August or September. The exception was not initiated by heavy rain but by rapidly melting snow. Between the early 19th century and 1947, however, all events occurred in June, July (both 26%), or August (48%), and not a single event can be found in September. This subtle shift at the middle of the 20th century from June, July, and August debris flow activity to August and September activity can also be observed in the 17th and 18th centuries, when all attributable events took place in September.

Observations
 
Modélisations
 
Hypothèses
Owing to the high elevation of the source area and the presence of permafrost, the absence of debris flows during flooding events in late autumn appears to be a common phenomenon. The subtle shift of debris flow occurrence from June, July, and August (1800-1947) to August and September (1948-2002) is supposed to be an effect of the changing climate at high-elevation sites.

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Debris flow frequency
Precipitation
The frequency of past debris flow activity has been obtained from dendrogeomorphological analysis and allowed reconstruction of 53 events between 1605 and 1994. Within the period covered by archival data (1922-2002), 3 additional event years could be identified. The reconstructed frequency may not be exhaustive and has to be regarded as a minimum frequency.

Reconstructed events were then compared to archival data on flooding in neighbouring rivers, and the distribution of sites affected by flooding was analyzed. Only events occuring between June and October were analysed and flooding caused by glacier-lake outbursts was disregarded.

3 types of events were distinguished:
• Type I events, debris flows in the Ritigraben are triggered by local or regional thunderstorms. Debris flows may also occur in the adjacent torrents, but no flooding is caused in the major rivers of the Valais and Ticino Alps.
• Type II and type III events affect large areas, causing damage on both sides of the southern crest of the Alps (Valais, Ticino). During type II events, abundant flooding is accompanied by debris-flow activity in the Ritigraben torrent.
• Type III events apparently cause no debris-flow activity in the Ritigraben torrent and other debris-flow systems having a periglacial origin.

Indices of similarity and conditional probabilities were computed in order to compare debris flow event years in the Ritigraben torrent with flooding in the other systems. The indices of similarity (indsi) express the degree of coincidence between two different data sets with values varying between 0 (no similarity) and 1 (identical sets).

(4) - Remarques générales

Reconstructed debris-flow data for the Ritigraben fit very well with flooding data in neighboring catchments. The importance of the late-twentieth-century events has to be thoroughly revised. Events comparable to that in 1993 have previously existed in the region and do not represent a new phenomenon. Even the occurrence of several events within a short period of time can be observed in the mid–nineteenth century (i.e., six substantial events between 1834 and 1868).


(5) - Syntèses et préconisations
 

Références citées (extraits) :

Gers, E., 1998: Dendrogeomorphologische Frequenzanalyse von Murgangereignissen am Beispiel des Wildibaches im Mattertal, Wallis, Schweiz. Diploma thesis, University of Bonn, Bonn, Germany, 86 pp.

Lièvre, I., 2002: Détermination de la fréquence de laves torrentielles sur le torrent du Ritigraben (Valais, Suisse). Diploma thesis, University of Fribourg, Fribourg, Switzerland, 61 pp.

Stoffel, M., Gärtner, H., Lièvre, I., and Monbaron, M., 2003: Comparison of reconstructed debris flow event years (Ritigraben, Switzerland) and existing flooding data in neighboring rivers. In Proceedings, 3rd International Conference on Debris Flow Hazard Mitigation 2003. Rotterdam: Millpress, 243–253.