Référence
bibliographique complète 
JOMELLI V., D. BRUNSTEIN, C. CHOCHILLON, P. PECH. Hillslope
debris flows frequency since the beginning of the 20th century in the Massif
des Ecrins (French Alps). Debris flows hazard mitigation: machanics, prediction
and assessment, Rickenmann & Chen (eds), 2003, Millpress, Rotterdam,
p 127137. 
Motsclés 
Debris flows, frequency, dating methods, logistic regression, climatic and geomorphological parameters 
Organismes / Contact 
CNRS Laboratoire de Géographie Physique, UMR 8591, Meudon Bellevue Université Paris 1, Panthéon Sorbonne 
(1)
 Paramètre(s) atmosphérique(s) modifié(s) 
(2)
 Elément(s) du milieu impacté(s) 
(3)
 Type(s) d'aléa impacté(s) 
(3)
 Soustype(s) d'aléa 
Torrential events  Debris flow 
Pays
/ Zone 
Massif
/ Secteur 
Site(s) d'étude 
Exposition 
Altitude 
Période(s)
d'observation 
French Alps 
Massif des Ecrins (45° N 6°30’ E) 
6 valleys on the eastern part of the massif 
18002400 m asl 
19002000 
(1)
 Modifications des paramètres atmosphériques 

Reconstitutions


Observations 

Modélisations 

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

(2)
 Impacts 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)
 Impacts du changement climatique sur l'aléa 

Reconstitutions


Observations 
The debris flows frequency evolution is not linked to climate change. 
Modélisations 
For the first model, the independent variables with the best fit are the surface of the rock face and the altitude of the triggering zone. The higher the elevation and the larger the surface, the higher is the predicted probability. 
Hypothèses 
Paramètre de l'aléa 
Sensibilité du paramètres de l'aléa à des paramètres climatiques 
Informations complémentaires (données utilisées, méthode, scénarios, etc.) 
Debris flows frequency  First model: surface, inclination, height and exposition of the rock face, and the altitude of the triggering zone Second model: mean annual and seasonal amount of precipitation in the year the debris flow was triggered, number of days between 15/06 and 15/10 with more than 20 and 30mm rainfall, annual and seasonal average temperature, minimal temperature during winter or spring and the total number of freezing days per year since the last event

The concept of frequency has been regarded on the one hand as the number of cases established over a given period of time and on the other hand as the number of events which occurred with a special length over a given period time. Debris flows were selected from 6 valleys located in a 28km2 area. These valleys consist of broad steep granite walls overhanging slope deposits situated between 1800 and 2400 m as. Precipitation and temperature data from the meteorological station of Monetier les Bains (1490m asl), which is the closest to the selected sites, have been studied. 142 debris flow deposits have been analysed and dated by combining several approaches: analysis of aerial photographs, old documents, dendrochronology, lichenometric measurements (Mc Carroll procedure) for debris flows older than 1953 and field observations since 1995. Data have been grouped by periods of 20 years (highest error estimation of dating methods). The length of each dated debris flow has also been measured in the field and the altitude of the starting zone estimated. A logistic regression was used to create 2 models: one aimed at explaining the frequency variability by geomorphical parameters and the other by climatic parameters. 
(4)  Remarques générales 
(5)
 Syntèses et préconisations 