Réf. Corominas & al 1994 - R: EPOCH

Référence bibliographique complète
COROMINAS J., WEISS E.E.J., VAN STEIJN H., MOYA J. Use of dating techniques to assess landslide frequency, exemplified by case studies from European countries. Temporal occurrence and forecasting of landslides in the European Community (EPOCH Programme) - Final Report. / ed. by. Casale, Fantechi and Flageollet. European Commission, Bruxelles, 1994, 1, p. 71-93.

Landslides, debris flows, dating methods, trends, climatic factors.

Organismes / Contact
T.T.S. Ingenieros de Caminos, Universitat Politèchnica de Catalunya, Spain
Faculty of Geographical Sciences, University of Utrecht, The Netherlands
European Commission - Science Reserch Development

Principaux rapports scientifiques sur lesquels s'est appuyé le rapport
Bibliographic review, see "Références citées".

(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
    Mass movements
Torrential events
Debris flows

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
French Alps   Bachelard valley, south of Barcelonette      

(1) - Modifications des paramètres atmosphériques

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

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

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


(3) - Effets du changement climatique sur l'aléa
Bachelar valley (French Alps):
Since the begining of the 19th century, an alternation of periodes of higher and lower debris flow activity has been observed. The absolute values of the peaks shown cannot be interpreted in terms of varying activity levels, however, because of the impossibility to date all older events. The absence of dated events for the period 1960-1980 is problematic, for it is not possible to conclude that debris-flow activity was absent. Lichens need a relatively long period before their presence becomes macroscopically visible. For Rhizocarpon geographicum this colonization time appears to be some 25 years at the south side of the Alps. Futhermore, only a low number of debris-flow deposits are sufficiently in contact with trees to allow dendrochronological dating. Finally, many debris flows come down unnoticed in this scarcely populated region. Thus, part of the undated flows might have come down during the period 1960-1980. The peak of the years 1980-1987 is mainly caused by the events of 1987, as at many other places in the Alps. Available evidence for the French Alps shows that debris flow are triggered by high-intensity rainstorms occurring during summer and autumn, while snow melt rarely causes debris flows.

Landslide occurrence in the late Quaternary:

Several authors have tried to establish models of the temporal activity of the landslides. Datings made in Switzerland and UK show that the large post-glacial landslides were not produced just after the glacial retreat, but with a delay of up to several thousand years (Schoeneich 1991). These movements could not have been simply produced by basal undermining of the slope by glaciers. It is necessary to find other factors which could explain this delay, such as the presence of deep permafrost and its thaw or simply the time needed for the development of a failure. Schoeneich (1991) stated that in the Swiss Alps, several datings of catastrophic events are grouped between 0 and 500 years A.D. and might correspond to an increase in heavy rainfalls. Starkel (1966) argued that the time of Holocene failures had not been completely random, but had been climatically controlled. He identified three main periods of activity: one in younger Dryas when permafrost was generally vanishing (between 11,000 and 9,000 yr. B.P.), and two more humid and warmer, the first one during the Atlantic period (7,000-5,000 yr.B.P) and the last one during the Sub-Atlantic period (1,500-500 yr. B.P.).

Using additional data, the authors of the present study can not confirm these statements for the whole of Europe. Landslides have taken place in other different periods and they are especially frequent during the last hundreds years. At present, the scarcity of dated landslides does not allow us to get an insight into the actual temporal distribution but all locations. Landslides have occur continuously since the glaciers retreated.

A close relation has been found between heavy rains and triggering of shallow landslides like debris flows, mudflows, slab slides or rock falls (Rat 1984, Gallart & Clotet 1988, Zimmermann & Haeberli 1992, Corominas 1993). Nevertheless it is not always possible to establish a direct relationship from the frequency distribution of landslides, as it was seen in the Barcelonette area in the French Alps (Braam et. al. 1987); although it is assumed that climate should have played a role in combination with factors such a bedrock, slope characteristics among others (Weiss 1988).

With regard to deep-seated landslides, Johnson (1987) also concluded that there are no sufficient data available yet for a proper testing of Starkels hypothesis. The last century exhibits a very active period of landslides. As it has been stated before, most of the shallow movements are due to heavy rain storms but large landslides have often no direct link to them (Séchilienne in the French Alps or Randa-Zermatt in the Swiss Alps). It is not clear that old large landslides can always be in general, associated to changes in climatic regime. Thornes & Brunsden (1977) warned that we can not infer the long term behaviour unless we know the variability of the process involved. Some large landslides may have been triggered by earthquakes, and in fact, most of the large prehistoric rock avalanches that have been recently dated are accepted as caused by earthquakes.

At present, there are not enough data to link landslide activity with past climate conditions. It seems clear that activity in Holocene times is not exclusively due to the retreat of glaciers and that landslides have been taking place since that time. Some shallow landslides can be directly related to rainfall events, whereas most of deep-seated landslides can not. It is not probable that large landslides can always be associated to changes in climatic regime and, earthquakes, mountain upfliting, and stress relief may be found as alternative triggering factors. The role of deforestation in the increasing historical landslide activity is not well known. Because all these variety of conditions, a better understanding of the triggerring factors of old landslides is needed before establishing their eventual climatic relationship.

Paramètres de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Debris flows activity

High-intensity rainfalls

Using dendrochronology and lichenometry, several sites affected by debris flows (estimated volumes displaced per event in the order of 100 - 1000 m3 ) were studied with regard to frequency analysis within the Bachelard valley, south of Barcelonette (Van Steijn, 1991 and Van Asch & Van Steijn, 1991).

Deposits are found at altitudes of 1700 to about 2400 m, while source areas reach to some 2800 m. Flow tracks are present mainly on talus slopes below high and very steep rock walls. In a few cases the debris flows built fans in front of short torrential systems. A rapid alternation of resistent and weak rocks is a characteristic of debris-flow prone parts of this region.

Frequency analysis of debris-flow activity is based on lichenometry (using Rhizocarpon geographicum (s.l.)) and dendrochronology (analysing treering eccentricity patterns for Larix decidua). Within the study area of about 15 km², over 200 tracks were counted. Along a large part of these tracks no trees are present, in which case lichenometry is the only dating technique available. Nevertheless, 38% of the tracks within the study area remains undated.

(4) - Remarques générales
The landslide dynamics (frequency and areal extent) can be deduced from dating successive landslide events. Dendrochronology is specially valuable in frequency-magnitude analysis of shallow movements such as debris flow, in which magnitude is considered as both the volume of flow deposits itself, and the areal extent of these phenomena triggered by a single rainfall event. Nevertheless, series obtained from dating old landslides should normally be very incomplete because only very few individual events can be dated. Landslide reactivation is also difficult to detect. These facts may lead to an underestimation of the activity.

(5) - Préconisations et recomandations
Destinataires et portée du rapport Scientific community
Types de recommandations et / ou préconisations
Adequate knowledge of the landslide context (type of movement, dynamics and morphological features) is needed for the search of datable elements associated to it and for a proper interpretation of the data obtained. A better understanding of the triggerring factors of old landslides is needed before establishing their eventual climatic relationship.

Références citées :

BRAAM R.R., WEISS E.E.J. & BURROUGH P.A. 1987. Spatial and temporal analysis of mass movement using dendrochronology. Catena Vol. 14. pp. 573-584.

COROMINAS J. (1993) in press. Landslide occurrence: a review of the Spanish experience. U.S.- Workshop on Natura1 Disasters. Barcelona.

GALLART F. & CLOTET N. 1988. Some aspects of the geomorphic processes triggered by an extreme rainfall event: the November 1982 flood in the Eastern Pyrenees. Catena. Suppl. Band 13, pp. 75-95.

JOHNSON R.H.. 1987. Dating of ancient, deep-seated landslides in temperated regions. in M.G. Anderson & K.S. Richards (Eds.) Slope stability. John Wiley. pp. 561-600

RAT M.. 1984. Météorologie, hydrogéologie et glissements de terrain. 4th Int. Congress on Landslides. Toronto. Vol. 3, pp. 33-41.

RAT M. 1988. Essai de prévision de la date de rupture d'un grand glissement. II Simposio sobre Taludes y Laderas Inestables. Andorra la Vella. pp. 419-431.

SCHOENEICH P. 1991. La datation des glissements de terrain. in Bell (Ed.) Landslides. 6th. Int. Conference on Landslides. Christchurch. Balkema. pp. 205-212.

STARKEL L. 1966. The palaeogeography of Mid and Eastern Europe during the last cold stage and West European comparisons. Phil. Trans. Royal Soc., London, B280, pp. 351-372.

THORNES J.B. & BRUNS DEN D. (1977). Geomorphology and Time. Methuen and Co. 208 pp.

VAN ASCH TH.W.J. & VAN STEIJN H. 1991. Temporal patterns of mass movements in the French Alps. Catena. Vol. 18. pp. 517-527.

VAN STEIJN H. 1991. Frequency of hillslope debris flows in a part of the French Alps. Bulletin of Geomorphology 19. pp. 83-90.

WEISS E.E.J. 1988. Treering patterns and the frequency and intensity of mass movements. in Ch. Bonnard (ed.) Landslides. 5th. Int. Conference on Landslides. Lausanne. Balkema. pp. 481-483.

ZIMMERMANN M. & HAEBERLI W. 1992. Climatic change and debris flow activity in high mountain areas- A case study in the Swiss Alps. Catena. Suppl. Band 22, 59-72.