Réf. Chiarle et al. 2007 - A

Référence bibliographique complète
CHIARLE M., IANNOTTI S., MORTARA G., DELINE P. Recent debris flow occurrences associated with glaciers in the Alps. Global and Planetary Change, 2007, Vol. 56, p. 123-136.

Abstract: Debris flows from glacier forefields, triggered by heavy rain or glacial outbursts, or damming of streams by ice avalanches, pose hazards in Alpine valleys (e.g. the south side of Mount Blanc). Glacier-related debris flows are, in part, a consequence of general glacier retreat and the corresponding exposure of large quantities of unconsolidated, unvegetated, and sometimes ice-cored glacial sediments. This paper documents glacier-related debris flows at 17 sites in the Italian, French, and Swiss Alps, with a focus on the Italian northwest sector. For each case data are provided which describe the glacier and the instability. Three types of events have been recognized, based on antecedent meteorological conditions. Type 1 (9 documented debris flows) is triggered by intense and prolonged rainfall, causing water saturation of sediments and consequent failure of large sediment volumes (up to 800 000 m3). Type 2 (2 debris flows) is triggered by short rainstorms which may destabilize the glacier drainage system, with debris flow volumes up to 100 000 m3. Type 3 (6 debris flows) occurs during dry weather by glacial lake outbursts or ground/buried ice melting, with debris flow volumes up to 150 000 m3. A data base of historic cases is needed in order to advance process understanding and modelling, and thus improve hazard assessment.

Mots-clés
Debris flow, glacial outburst floods, climate change, European Alps.

Organismes / Contact
CNR-IRPI, Strada delle Cacce, 73-10135 Torino, Italy. marta.chiarle@irpi.cnr.it , Tel.: +39 011 3977261.
Laboratoire EDYTEM, CNRS-Université de Savoie, 73376 Le Bouget-du-Lac, France.

(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
    Debris flows  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Italy, Switzerland, France Alps 17 sites   ~2000-3000 m a.s.l. for starting zones 1950-2003

(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
 
Observations
Most debris flows initiated at high elevation, ranging from 2000 to 3000 m a.s.l. Starting zones correspond to the uppermost occurrence of large quantities of loose debris along proglacial stream channels. In some cases, these sediments consist of thick, steeply-inclined fluvioglacial deposits and till, often located at the foot of a steep rock face below glacier fronts. In other cases, debris flows initiated with the failure of Little Ice Age end moraines. In two cases, processes initiated from lateral moraines.

Source zones are up to 600 m long and 200 m wide; erosion depths range from meters to decameters. The mobilized volumes range from several thousand cubic meters to one million cubic meters, and the deposit area is generally less than 0.5 km2. The duration of the debris flows is commonly 1-3 h. Travel distances range from 1 to 6 km. Mobility, expressed as the ratio of the vertical travel distance to the length of travel (H/L) is a function of the specific geomorphologic context and amount of water input involved in the debris flow. The lowest value of this parameter is 0.13, the highest is 0.52. Most debris flows occurred during the months of July, August, and September.

The events can be subdivided into three groups, based on antecedent meteorological conditions:

The first group comprises events that occurred during heavy and prolonged rainfall (9 documented debris flows). Failure probably was triggered by saturation of the debris cover due to infiltration of rainwater. The presence of buried glacier ice within the debris, documented for 3 events, may have facilitated failure and/or affected the geometry of the detachment zone (Zimmermann and Haeberli, 1992). In each case, rain water played a primary role in triggering debris flow, as testified by the concomitant occurrence of other instability processes in surrounding areas, out of the glacial basin.

The second group includes two events triggered by a brief local rainstorm that caused no other failures in the surrounding areas. The debris flows may have been caused by rapid water input to the glacier bed from rainfall. Rain alone may not have triggered the debris flows, but may have induced high water pressures that linked water pockets in the glaciers (Walder and Driedger, 1995). It is noteworthy that these streams are repeatedly subject to debris flows.

The third group includes debris flows triggered by the sudden emptying of ice-marginal lakes (3 events) or englacial water pockets (3 events). In these cases, the debris flows occur in dry weather, and in only one case was there abundant rainfall in the weeks preceding the event that may have destabilized the glacial deposits involved in the failure. In some cases, high air temperatures likely contributed large amounts of melt water, potentially modifying the dynamic of the glaciers and destabilizing surrounding sediments.

Debris flows of types 2 and 3 occurred during the first half of the summer, between late June and July, at the onset of snowmelt (Haeberli, 1983). In contrast, group 1 debris flows occurred during the second half of summer, between late July and September, the time of heavy rainfalls in the Alps. Another important difference is the magnitude of the events. Debris flows of group 1 have the highest magnitude (800 000 m3 at Mulinet), whereas debris flows of groups 2 and 3 are generally tens of thousands of cubic meters. Debris flows triggered by glacial outbursts, however, are not always smaller than rainfall-induced events.

Comparing the present data set with extensive historical investigations of debris flows from glacier forefields in the Italian Alps (Dutto and Mortara, 1992), debris flows seem to be increasing in frequency at the margins of glaciers. This fact can be explained by the increased availability of loose sediment for transport in debris flows and, in some cases, by the formation of moraine-dammed lakes, as a consequence of twentieth century glacier retreat. Debris flows from glacier forefields are among the largest in the Italian Alps.
Modélisations
 
Hypothèses
All studied debris flows started at the uppermost occurrence of large accumulations of loose debris along proglacial stream channels. These sediments form undifferentiated till or lateral/end moraine deposited during the Little Ice Age, or fluvioglacial apron of the same age. In the considered geographic context, thus, climate change can influence debris flow occurrence by glacier retreat and consequent exposure of large quantities of unconsolidated and unvegetated glacial sediments, that can be easily mobilized by glacial floods. The role of ground ice melting in sediment failure and debris flow initiation is hardly assessed. The presence of ice cores can influence the stability of the moraines, where important ice masses may be preserved beneath moraine cover, as glaciers retreat.

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
The present paper provides data and analyses on 16 events (Mulinet, Ormeleura, Monte Giove, Pelmo, Grandes Jorasses, Western Montasio, Bodmer, Prà Sec, Rochefort, Belvedere, Chauvet, Weingarten, Freney, Frébouge and Dolent glaciers) which occurred in the European Alps in the last 25 years. Additionally, the Sissone debris flow which occurred in 1950 and is the largest debris flow documented in Italian glacierized areas is discussed. The study area includes the Italian, French, and Swiss Alps, with a focus on the northwestern Italian sector. Only events that mobilized at least thousands of cubic meters of debris are considered. For each case, several simple parameters that describe the glacier, and the instability have been provided. Data were also collected on elevation and triggering mechanism, duration of the debris flow and ensuing damage. Analysis was completed using aerial photographs, field observation, and a review of the literature.

Reliability of the data is variable. Some data have been obtained by accurate photogrammetric measurements or by field surveys carried out by the authors. In other cases, values were estimated, obtained from technical/scientific documents or derived from event maps from which the data quality could not be assessed. Thus, generally speaking, data have to be considered with due caution.

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

Dutto, F., Mortara, G., 1992. Rischi connessi con la dinamica glaciale nelle Alpi Italiane. Geografia Fisica e Dinamica Quaternaria 15, 85–99.

Haeberli, W., 1983. Frequency and characteristics of glaciers floods in the Swiss Alps. Annals of Glaciology 4, 85–90.

Walder, J.S., Driedger, C.L., 1995. Frequent outburst floods from the South Tahoma Glacier, Mount Rainier, U.S.A.: relation to debris flows, meteorological origin and implications for subglacial hydrology. Journal of Glaciology 41, 1–10.

Zimmermann, M., Haeberli, W., 1992. Climatic change and debris flow activity in high mountain areas—a case study in the Swiss Alps. Catena. Supplement 22, 59–72.