Aerial shots show the change in terrain
In order to quantify which sediments the melt water of a glacier can carry away, the researchers analysed a series of aerial images of the Gries Glacier in the canton of Valais. The series comprises annual photographs of the area taken since 1986 when the glacier retreated from the Gries reservoir. On the basis of these images, the scientists created so-called digital terrain models that state a height above sea level for each point. By comparing the annual heights, they were able to determine the volume of material that had been eroded. This reveals that during the initial years relatively little material was washed away – less than 5,000 cubic metres per year. From the mid-1990s, however, this volume increased and reached its maximum between 2011 and 2012 when 20,000 cubic metres was washed away each year – much of this as small particles in the so-called suspended load. After this time, the quantity washed away declines once more and the glacier foreland appears to have been stabilising since this time.
However, the melt water does not only contain sediments from the glacier foreland: even more material is eroded directly beneath the glacier. In the case of the Gries Glacier, weather analyses revealed that 70 % of the sediments originated from beneath the glacier.
In order to obtain a more precise picture of what quantity of material is released during the course of the year and whether there are differences between glaciers, the researchers installed measurement devices beneath glaciers in Valais – the Aletsch Glacier and the Gorner Glacier. After two years, it was observed that the largest glacier in the Alps, the Aletsch Glacier, had eroded around 325,000 cubic metres of rock, while the figure for the smaller Gorner Glacier was approximately 60,000 cubic metres. More precise analyses indicate that it is not only the size of the glacier that is decisive for the amount of sediment, but rather also other factors such as the volume of melt water and the course of the seasons.
The collected data helped the researchers to create a computer-based model of the sediment transportation. This enabled them to simulate what happens during the retreat of the glacier: it was shown that there is an increase in the sediment load but that after a certain time the quantity declines despite additional melt water. This is linked to the availability of sediments that can be transported by the water.