Responses of the Val d’Arcia small dolomitic glacier (Mount Pelmo, Eastern Alps) to recent climatic changes. Geomorphological and geophysical study

Abstract

On September 14, 1994, due to rapidly changing meteorological conditions an important debris flow on the northern slope of Mt. Pelmo (Dolomites, Eastern Alps), originating in Val d’Arcia, reached the underlying main road (S.S. 251), covering a part of it. In the detachment zone this event caused the outcropping of a small plate of ice, which extended to about a hundred metres in length, which had remained hidden by the slope debris. Following this event a program of geomorphological, meteorological and geophysical research was carried out in the summers of 1996-99 in the Val d’Arcia. The aim was to ascertain the existence, the extension and the thickness of the mass of ice buried under the detritic cover; to locate the presence of permafrost and to find out the correlations with the debris flow, with respect to the geomorphological and meteorological local context. The geomorphological research was based on a detailed mapping, on a 1:10,000 scale, in the area including the whole Dolomitic group of Mt. Pelmo, with a particular interest to the northern slope, where clear evidence of glacial, periglacial and gravitational processes and deposits still active have been recognised. From the study of this deposits it has been possible to reconstruct the würmian glaciation phases from the Late Glacial to nowadays. In particular, the most recent evolutional history of the glacier has been reconstructed by analysing the terminal moraines of the Little Ice Age present along the eastern sector of the Val d’Arcia. This research has brought to light the areas involved by the debris flow of September 1994, the active landslides on the slopes of Mt. Pelmo and the gullies periodically interested by avalanches. The geomorphological research has been complemented by a detailed meteorological analysis of the September 1994 event, carried out by ARPAV, the Avalanche Centre of Arabba (BL). The geophysical prospection carried out in Val d’Arcia, based on a series of GPR profiles and vertical electrical soundings (VES), pointed out the structures underlying the debris cover. The GPR survey was done using a GSSI SIR-2 device, equipped with 100, 400 and 500 MHz monostatic antennas. Three longitudinal and four perpendicular to the valley axis profiles were made, for a total length of more than 2 200 metres, including the parts of profile repeated with different antennas. The results obtained by the GPR survey along the main 1 km long profile, from West to East for the whole width of the glacier, agree with those obtained by VES which also allowed the calibration of the radargrams time scale. The very high resolution given by this technique allowed the recognition of an interface being interpreted as separation surface between the glacier ant the sub-glacial till or the bedrock and the calculation of the debris cover thickness along the GPR profiles directions.