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Why researchers fish stones from mountain torrents with nets

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A catch net approximately 50 centimetres wide runs along a rail in the burbling mountain torrent Avançon de Nant in Canton Vaud. It is not, however, there to catch fish, but rather to trap bedload, i.e. sediment transported by the water. In addition, measuring plates equipped with geophones are installed on the streambed to measure the vibrations of the rumbling stones – this makes it possible to estimate the amount of bedload. But which geophone signal indicates a particular quantity of sediment?

This is what Tobias Nicollier, a PhD student at WSL, wants to find out. He is developing a computing method that will, in future, link each geophone signal automatically to a certain amount of bedload. For this, he needs comparative data, which he can obtain either with the help of such catch nets, or with permanently installed bedload trap baskets. “It is only with these two methods that we have been able to determine the amount of bedload at high discharges,” says Tobias. To obtain the necessary data, he carried out various net measurements in the summer of 2019 in the torrents Avançon de Nant, Albula (Grisons) and the Erlenbach (Schwyz).

Long-term bedload monitoring in steep streams


If bedload is deposited in the wrong place, it can contribute to the damage caused by flooding. It is also important for hydropower plants to know how quickly reservoirs fill up with sediment. Eighty years have, however, passed since the bedload in Swiss rivers was last extensively measured.

In parallel, Tobias is carrying out experiments in artificial streambeds in order to obtain an even more accurate picture of the movements of the transported stones. He reconstructs the soil structure there and the flow behaviour of the torrents as accurately as possible and films the rock particles through plexiglass walls. “The aim is to understand the natural sediment budget better.” This is also important for successful river revitalisation, which also requires natural bedload transport, for ex­ample for forming gravel banks. In Switzerland, around 4000 kilometres of river courses have still to be re­vitalised.
(Beate Kittl, Diagonal 2/19)