Kraftwerke Sarganserland AG contracted WALO to remove 45,000 tonnes of sediment from the Mapragg compensation reservoir – using a system that WALO developed itself, reaching a depth of 50 m and discharging it into the Rhine in a controlled manner.
The assignment: dredging 45,000 tonnes of sediment
The artificial lake of Mapragg acts as a compensation reservoir. It means water can be pumped back into Gigerwald reservoir or allowed to flow onwards into Sarelli power plant. Since operations started in 1977, the reservoir has continuously built up sediment, such as sand, gravel, silt and driftwood. According to Matthias Kohler, Plant Manager at Kraftwerke Sarganserland AG, the draining of Gigerwald reservoir in the autumn of 2024 also caused large amounts of material to wash up in Mapragg reservoir. The deposits reached the crest elevation of the bottom outlet. If this gets blocked, the reservoir’s drainage can no longer be controlled. This eventuality must be prevented at all costs.
Kraftwerke Sarganserland AG (KSL), a partner of Axpo Power AG, tasked WALO with dredging sediment. The objective is to remove 45,000 tonnes of material by the end of August 2026.
Unique feature: floating dredger with high-capacity material flow
The beating heart of the system is a floating platform that is positioned in the reservoir. A clamshell bucket lifts up to ten tonnes of material from the lakebed at a time. The system excavates at a depth of up to 50 metres. From an engineering perspective, 70 metres would be possible, which is a great advantage compared to a suction dredger.
The excavated material is released onto a coarse screen, where fine material passes through the filter of the rotary drum, while objects like wood and stone are collected into skips and disposed of separately. The liquefied fine material is pumped to the central facility in Vadura by the floating dredger and flows to Sarelli power plant in Bad Ragaz via the existing pressure line. From there, it is discharged into the Rhine.
WALO has continued to refine this system since it was first deployed in 2018. “Material flow has been the biggest area of progress. In other words, we successfully improved the silt filtering capabilities so that the silt-water mixture remains homogeneous when in movement, is prevented from settling and can therefore be pumped out as efficiently as possible,” explains Bernhard Zingg, Head of Machine Development at WALO.
The improved water intake, including new nozzles, prevents clay-rich mud from settling and clogging the system. This means the system runs almost without interruption – its capacity has almost doubled since 2018.
A turbidity sensor at the power plant monitors the amount of solid matter in the water in real-time. If the level exceeds the threshold set by the cantonal authorities, the dredging is stopped. This means the entire material excavation process can be measured, documented and controlled.
Practical implementation: humans, machines and an expert team
The floating machine that is visible from the outside is actually the result of a close-knit collaboration between many different departments.
“The most intense part is definitely the assembly, which takes around a month. It’s very much a collaborative effort: Machine Development, the people from Hydraulic Engineering, the Electronics department, metalworkers and the workshop in Goldach, the WALO haulage crew who install the dredger on site, plus the folks who reposition the lines – the latter even involves staff from the power plant. The teamwork with the power plant staff is top notch,” explains Axel Christ, Civil Engineer at WALO.
When the dredger is actually in operation, two people are deployed per shift. One is a machine operator who controls the clamshell while constantly monitoring the cables, depth and positioning. The other ensures that the material flows through the filter in an evenly distributed manner and that the pontoon moves in sync with the driftwood skips, all while keeping an eye on the 20,000-volt floating electricity cable. The reservoir’s water level fluctuates by up to 13 metres every day. The depth of the dredging is determined by GPS and the length of the cables. The system is not anchored into a fixed position, but is instead secured by three cables. Everything is in constant motion and the two workers are responsible for keeping it that way.
Other factors come into play too, such as older, compacted sediment that requires more water pressure. Negative electricity prices may briefly interrupt the operation of the turbine at Sarelli power plant. The water level of the Rhine in Diepoldsau determines the amount of silt that can be introduced. And the Fishing Act (Fischereigesetz) limits the time window: dredging is only allowed from May to August.
The outcome: bottom outlet unobstructed once again
The blocking of a reservoir's bottom outlet jeopardises the operation of the entire power plant network. The sediment dredging at Mapragg reservoir goes to show how to solve this problem: by deploying hydraulics and measurement processes, and by using a system that can be adapted to the given reservoir.
“WALO’s floating dredger is custom-built for the specific conditions at Mapragg reservoir – that’s why it’s so efficient,” says Matthias Kohler, Plant Manager at Kraftwerke Sarganserland AG.
Heavy precipitation and thawing permafrost are causing increasing amounts of material in storage and compensation reservoirs. Demand for sediment management in Switzerland’s reservoirs is set to increase as a result. WALO is ready to rise to the challenge – with a modular system that dredges deeper than any alternative and a team that knows what they’re doing.
