Geophysical-geotechnical methodology for assessing the spatial distribution of glacio-lacustrine sediments: The case history of Lake Seracchi

Proglacial lakes are distinctive features of deglaciated landscapes and often act as sediment sinks, collecting solid material from subglacial erosion or washout of deglaciated areas. The solid transport flow, strongly linked to the glaciers and periglacial landforms, may rise due to the rapid changes driven by climate warming, causing deep transformations in the basin hydrology, and even the appearance or disappearance of lakes at a decadal timescale.The goal of this study was to present a geophysical-geotechnical approach that integrates several techniques, to quantify the sediment distribution in a proglacial lake. A geophysical survey is performed with ground-penetrating radar (GPR) installed on a boat, whereas a time-domain reflectometer (TDR) measures the electrical conductivity and permittivity of the lakebed sediments. Unperturbed samples are collected and analyzed to measure the main geotechnical properties of the sediment: grain-size distribution, plastic limit, and liquid limit. Such properties support the interpretation of the GPR data and the detection of spatial variations of the sediment facies.To validate the proposed methodology, field tests were carried out at Lake Seracchi, the largest lake of the Rutor glacier, Italian Alps. It formed around 1880 because of the recent glacier shrinkage, as chronicled by valuable historical documents. Its greyish waters carry a significant amount of suspended sediment recognized as glacial flour, which gradually accumulates on the bottom of the lake.The obtained bathymetry and sediment thickness maps of Lake Seracchi show the strength of the approach: from only a few manual samples, it is possible to extrapolate the geotechnical properties of interest, such as friction angle or hydraulic conductivity, to wider areas, surveyed by the geophysical techniques. This is achieved by investigating the spatial distribution of key geophysical properties linked to the geotechnical properties of interest.