Debris flow

French Alps

Dévoluy and Ecrins massifs

Plateau de Bure area (between 44°36’ N and 44°49’ N).
Northern part of the Massif des Ecrins (45°00’ N, 6°30’ E)

1900-2400 m a.s.l for the Plateau de Bure
1600-2400 m a.s.l for the Ecrins massif

~1950-2000

There is no significant variation from the annual mean precipitations taken since 1980, for all weather stations studied.

A small increase in the amounts of rainfall in spring, summer and autumn since 1961 has been observed at MB station. Winter is the only season which shows a small decrease in precipitation for this period. However, these variations between 1960-1980 and 1980-2000 are not significant (Mann–Whitney test), whatever the meteorological station.

The evolution between 1964 and 2000 of the ten highest daily totals rainfall occurring each year between 15/06 and 15/10 at StC has shown no significant difference above 20 mm/d. Above 30 mm the number of rain-days observed have increased significantly since the 1980s for the thresholds of 30 and 40 mm. At LS 13 events between 30 and 40 mm/d occurred between 1970 and 1985, as opposed to 29 after 1985. For the more intense events, the difference was less marked and not significant.

For StC and LS stations, a clear increase of the mean annual temperatures has been observed. With ten-year mean temperatures, there is a decrease of 0.3°C for the period 1960-1970, followed by an increase of 0.4°C between 1970 and 2000. In spring the temperatures have increased significantly since the beginning of the 1980s in StC and LS stations. In summer, the mean increase is around 0.9°C. The autumnal increase (0.6°C) is only significant in StC and LS stations. For winter, a general significant increase (0.7°C) concerning the periods 1960-1980 and 1980-2000 for all the stations.

The number of freezing days has significantly decrease (around 14%) at La Salette station since the beginning of the 1980s. Such a trend is also observed at Le Monétier (12%) and at Saint Christophe (14%).

In the Dévoluy area, data from La Salette (LS) meteorological station (1770 m asl), which provided temperature and precipitation data going back to 1965, have been analysed. For the Massif des Ecrins, Le Monétier-les-Bains (MB 1490 m asl) and Saint Christophe en Oisans (StC 1570 m asl) stations, whose data go back to 1961 and 1964 respectively, have been choosen.

The statistical model has shown that, between 1961 and 2000, the probability of the occurrence of debris flows was due to the cumulated number of freezing days since the last event and the number of rainfall events greater than 30 mm/day between 15/06 and 15/10.

A significant decrease in the number of debris flows has been observed since the mid-seventies in the Dévoluy area. 84 debris flows were triggered between 1950 and 1975 and only 48 since 1975. Over ten-year periods, the total number of debris flows was not statistically different between 1950 and 2000.

102 debris flows were triggered during 1953-1976 period against 89 for the 1977-2000 period, for the Massif des Ecrins, but this evolution is not statistically significant. On the other hand a significant decrease in the number of debris flows less than 400 m in length can be observed between the same periods (but no significant variations above 400 m in length). There was a high activity during the 1970s but the number of debris flows is not significantly different from the other decades. Moreover, 1987 is not characterised by a great number of debris flows even though the number of rainfall days greater than 30 mm was the highest over the 1960-2000 period.

The mean return period (time between two events) has decreased for debris flows with 100-300 m lengths between 1953-1975 and 1976-2000 periods for the Massif des Ecrins. Clear significant variation has not been observed for the other lengths. This return period increases with length of debris flow and is correlated with the mean elevation of debris flow triggering zones. Between 1953-1975 and 1976-2000 the distribution of triggering zones has shifted towards higher elevation zones. The triggering zones lower than 2200 m represent less than 40% for the last period against more than 65% for the 1953-1975 period. Minimum altitudes have increased by more than 100 m between 1952 and 2000. Triggering zones lower than 1800 m asl have been inactive since 1976. The relationship with the increasing temperatures (decrease of freezing days) seems evident.

In the Dévoluy area, aerial photographs from 1948 to 2000 have been analysed and field observations have been made every year from 1995 onwards. These two approaches have been used to date debris flows. Old documents and dendrochronology (scars and wood reaction) have also been used complementarily.

For the Massif des Ecrins, the same methods have been used. For each dated debris flow, its length was measured in the field and the minimum altitude of the starting zone was estimated from photo interpretation and field observations.

The analysis of variations in debris flow occurrence in the Dévoluy area is based on 128 events which have occurred since 1948. For the Massif des Ecrins, data analysis was based on 191 debris flows which occurred between 1953 and 2000. Considering that a relationship exists between the length of debris flows and their frequency, the data have been classified by their length. The mean return periods and the variations in minimum altitude of the debris flow triggering zones have been studied between 1952 and 2000 for all sites of the Massif des Ecrins.

(4) - Remarques générales