GNGTS 2019 - Atti del 38° Convegno Nazionale

652 GNGTS 2019 S essione 3.2 ESTIMATING GLACIER THICKNESS IN A CHANGING CLIMATE: A CASE STUDY FROM THE TERMINAL LOBES OF BELVEDERE GLACIER (NW ITALIAN ALPS) C. Colombero 1 , C. Comina 2 , E. De Toma 2 , D. Franco 1 , A. Godio 1 1 Politecnico di Torino, Dipartimento di Ingegneria dell’Ambiente, del Territorio e delle Infrastrutture (DIATI), Torino, Italy 2 Università degli Studi di Torino, Dipartimento di Scienze della Terra (DST), Torino, Italy Introduction. Alpine glaciers, due to their relatively small size and high mass turnover rates, are extremely sensitive to temperature and precipitation modifications and visible indicators of climate change. Since these ice bodies are a key component of local and regional hydrogeological cycles, their globally observed retreat affects both physical and socioeconomic systems (e.g. loss of freshwater storage, modifications in resource availability for water consumption, irrigation and power generation, glacier-related natural hazards). Variations in glacier volume and mass are consequently of primary interest for the understanding of ongoing modifications and the forecast of possible future scenarios. Traditional glaciological measurements are usually limited to the shallowest part of the glacier and do not extensively cover wide areas of investigations. Remote sensing monitoring can help to track thickness fluctuations from time-lapse reconstructions of the topographic surface of the glacier (Ye et al. , 2006; Kulkarni et al. , 2007). Comprehensive knowledge of the ice bottom depth and morphology is however needed to provide a total volume estimate and reliable mass balance evaluations. Ice thickness estimation is also of primary importance for the modeling of future glacier dynamics, hydrological projections, glacier-related natural hazards and ice core analyses. Despite their relevance, ice bottom depth and morphology are often poorly known, mainly due to inadequate characterization methods and logistical issues. Geophysical methods can overcome these limitations, mitigating the operational efforts and enlarging the depth of investigation and data density of traditional techniques (Maurer and Hauck, 2007; Navarro and Eisen, 2009; Binder et al. , 2009; Merz et al. , 2016; Picotti et al. , 2017). In this study, we present geophysical investigations acquired on the terminal lobes of the debris-covered Belvedere Glacier (Macugnaga, NW Italian Alps) devoted to ice thickness estimation (Colombero et al. , 2019). The glacier thickness reconstruction mainly came from Ground-Penetrating Radar (GPR) data. However, single-station passive seismic measurements and Electrical Resistivity Tomography (ERT) profiles were complementary acquired to attempt a more complete characterization of the ice bottom morphology and properties. Test site. Belvedere Glacier is located NE of the highest peaks of Monte Rosa Massif, in NW Italian Alps (Fig. 1a). The glacier has a debris-covered surface, its area and total length are of approximately 1.4 km 2 and 3 km respectively, spanning in elevation from 2397 m to 1810 m a.s.l., thanks to the debris cover and the favorable solar exposure. The terminal portion of the glacier is bilobate (Fig. 1b). The two lobes are separated by a median morainic relief, hosting the chair lift station and the Belvedere Mountain Hut. Both lobes are currently exhibiting a visible retreat. De Visintini (1961) carried out the first study attempting ice thickness estimation on the glacier by means of P-wave reflection seismic measurements. Explosive sources were adopted, and the active shots were recorded with a 12-channel analog seismograph. Seven local areas of the glacier were surveyed with seismic profiling, from the vicinity of the Zamboni Zappa Mountain Hut, down to the confluence between the two lobes.Aglobal contour map of ice bottom depth was achieved from the interpolation of these measurements, even if the two terminal lobes were not directly investigated during the survey. VAW-ETH (1985) later performed GPR local measurements, with the aim of completing ice bottom characterization in unexplored compartments. A low-frequency GPR instrumentation (USGS Monopulse-radar, with variable central frequency in the range 1–5 MHz) was adopted, with 40 sparse measurements located along 9 transverse profiles covering almost all the glacier length. For each measurement, the time delays between the transmitted EM pulse and the received echoes were recorded. The highest-amplitudes echoes were referred to ice bottom reflections, their depth was estimated