AN UNSTABLE MOUNTAIN CATCHMENT IN THE NORTHERN ALPS - GEOMORPHOLOGICAL PROCESSES AND ASSESSMENT FOR FUTURE FLOODS AND DEBRIS FLOWS (LEGLISE-ARC 1800, TARENTAISE)

A 4-year-long research programme has adressed the geomorphological fac tors and processes acting in a small catchment in order to assess futu re streamflood and debris-flow hazards (L'Eglise, 5 km(2), 2500-770 m a.s.l.), in Tarentaise, the northern Alps. The nearby watershed (La Ra voire, Bourg St. Maurice) was badly eroded on March 31-April 1, 1981, when mudflows and debris flows swept down the channel and adjacent slo pes and removed about 300 000 m(3) of debris. This study analyses the factors which render the slopes unstable, and the channel able to carr y small to large debris flows. We identify 5 morpho-climatic factors a nd 4 geologic-hydrological factors which may trigger debris flows in a steep mountain catchment. Such severe events are medium- to large-siz ed (about 0.1 to 1 x 10(6) m(3)), and not very frequent in the Tarenta ise valley (4 to 5 streamfloods, but only I or 2 debris flows per cent ury). Conclusions are drawn from comparisons between the geomorphologi cal and hydrological characteristics of similar mountain catchments, a nd debris flows in the northern Alps.