Réf. Jomelli & al. 2003 - A

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 127-137.

Abstract:
The analysis was based on the dating of 142 debris flow deposits by lichenometry, dendrochronology, aerial photographs analysis from 1952 and old documents. Results have shown that it is very difficult to compare the 2 periods of 1900-1950 and 1951-2000 because the number of debris flows over the first half of the century is probably underestimated. Over the second period, the frequency of debris flow has decreased significantly for the 200-399 m class lengths. A decrease in the number of debris flows has also been observed between 1950-1975 and 1976-2000. 2 logistic regression models have been made in order to determine a frequency probability according to geomorphological and climatic parameters. The first model has shown that the frequency probability depends on the debris flow triggering zone altitude and the surface of the rock face, while the second model has shown that the frequency probability depends on rainy events greater than 30 mm/day and also on the cumulated number of freezing days.

Mots-clés

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) - Sous-type(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

 

1800-2400 m asl

1900-2000


(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.

A strong spatial and temporal variability in the number of debris flows since 1900 has been observed without being able to observe a significant tendency. 2 distinct stages appear: 1900-1959 during which the number of events increases and 1960-2000 during which the number decreases. But the 2 periods are not strictly comparable (covering-eroding rate).

For debris flows whose lengths exceed 300m, it seems that there has not been a clear change in the frequency since 1900, even if doubts remain concerning the number of events during the 1900-1950 period.

For smaller sized debris flows i.e. smaller than 300 m if the increase in fequency between 1900 and 1950 may be discussed considering problems linked to sampling, a siginificant decrease in the number of debris flows has been observed between 1960 and 2000 when analyzing the data over twenty-two years periods. If one considers the mean return period of the debris flows with length longer than 400 m in each valley, it decreased from 36 years for 1960-1980 to 15 years for 1980-2000.

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.

For the second model, the independent variables with the best fit are the total number of freezing days since the last event and the number of days with more than 30mm rainfall between 15/06 and 15/10. The more the freezing days are since the last event the more the probability of observing a new triggering increases.  This probability increases according to the rise of the number of rainy days above 30mm/day between 15/06 and 15/10.

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.

76 debris flows which occurred after 1953 were selected and the following independent variables were tested for the first model: the surface, inclination, height and exposition of the rock face, and the altitude of the triggering zone.

For the second model, 39 debris flows which occurred after 1961 were selected and the following independent variables were tested: 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.


(4) - Remarques générales

 

(5) - Syntèses et préconisations