Kraftwerke Sarganserland AG commissioned Walo Bertschinger AG to dredge sediment at a depth of up to 50 metres from the Mapragg reservoir in the Tamina valley near Bad Ragaz.
Mapragg compensating reservoir was put into operation in 1977. Due to continuous, persistent sedimentation, deposits have built up in front of the dam, which have risen as high as the threshold for the bottom outlet.
Among other tasks during the preparation phase from December 2017 to the start of construction on 2 May 2018, the team had to prepare a suitable dredging system which could meet the necessary requirements. Since no such system existed on the construction machinery market, WALO built its very own. The system had to be designed to reach a dredging depth of 50 metres. The excavation was carried out using a dredger measuring an impressive 5 m³. The material then had to be sorted by the system. While fragments of material smaller than 8 mm were pumped towards the dam via a floating pump line and fed into the Rhine by the power plant’s pressure tunnels, coarse material (wood, stone) had to be disposed of over land.
The system’s control, which WALO also built and programmed itself, had to meet some stringent requirements. The dredger had to be operated such that it kept to the required dredging depth at all times, even as the water level fluctuated (by up to 13 m per day). The system’s energy supply came from a connection to the power plant, which provided 20,000 volts. This voltage was then conducted to the system via a floating pipeline and fed into a transformer stationed on top to supply the system components with electricity.