When the reservoir in Taminatal needs dredging.

Background

A reservoir that constantly accrues 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. Whatever the water carries along with it is left behind: sand, gravel, silt and driftwood. As the years pass by, material builds up. There are also more uncommon events – like when Gigerwald reservoir was deliberately drained in the autumn of 2024 – that cause this process to accelerate. The deposits reached the crest elevation of the bottom outlet. If this gets blocked, the reservoir’s drainage can no longer be controlled. This must never be allowed to happen. Kraftwerke Sarganserland AG tasked WALO with solving this problem – for the second time since 2018.

The machine

A system that WALO is constantly working to improve.

The work at Mapragg reservoir presents an enormous set of demands: great depth, fluctuating water levels, a narrow Alpine valley and strict environmental requirements. Rising to the challenge, WALO refined its existing system and built a modular floating dredger that can be adapted to the conditions of the given reservoir.

Its beating heart is a clamshell bucket that can carry up to ten tonnes of material per lift – from a depth of up to 50 metres. From an engineering perspective, 70 metres would be possible. This makes the system unique on the Swiss market and means it can reach deeper than any available suction unit. The excavated material is released onto a coarse screen that initially filters out any large stones or pieces of wood. Once this is done, the filtered material is passed through a rotary drum that eliminates the particles that are too big for the pump. The liquefied fine material is then pumped into the pressure tunnel by the floating dredger and flows to Sarelli power plant via the pressure line. From there, it is discharged into the Rhine.

Ever since the system was first used in 2018, Bernhard Zingg, Head of Machine Development at WALO, has continued making ongoing improvements. “Material flow has been the biggest area of progress. We successfully improved the silt filtering capabilities so that the mixture remains homogeneous when in movement, is prevented from settling and can be pumped out as efficiently as possible,” explains Bernhard Zingg.

With new nozzles, more water and better continuity, the pump runs almost without interruption. Zingg reports that the dredger’s potential capacity has nearly doubled since 2018.

Operating the dredger

Intuition, GPS and three cables.

There are two people per shift. One is a machine operator who controls the clamshell. The other ensures that the material flows through the filter in an evenly distributed manner – a question of keeping a constant eye on floating lines, the pontoon and the 20,000-volt electricity cable. It might sound straightforward, but there’s more to it than you might think. There are numerous aspects that need to be coordinated.

To begin with, the dredger is held in place by three wire cables – an anchor to the rear, and one stabilising line each to the left and right. This keeps the pontoon mobile and manoeuvrable. The platform also moves along with the reservoir’s water level, which fluctuates by up to 13 metres every day. A GPS sensor specifies the altitude, while the length of the cables determines the depth of the dredging. All lines – power, water and silt – must move in synchrony and must not kink. If the cables become too taut, there is a risk this can cause damage – especially the power line, which is afloat in the water while carrying 20,000 volts.

In addition, there are external factors that set the pace: older, compacted sediment requires more water pressure to get loose. Volatile electricity prices may interrupt the operation of the turbine at Sarelli power plant – and silt only flows away when the turbines are running. The water level of the Rhine in Diepoldsau determines the amount of material that can be introduced – in summer, when less water is flowing through, the limit of 150 m3/second is soon reached. And the Fishing Act (Fischereigesetz) sets the time window: dredging is only allowed from May to August.

It wouldn’t be correct to say that “more” is automatically “better”. More specifically, the machine operator constantly needs to monitor changes in the density of the silt and react accordingly: increasing or decreasing the water pressure, or perhaps extracting a bit less material on the next lift. It’s better to excavate a smaller amount of material that the system can process without issue than to max out the load and cause the whole process to grind to a halt. The truth is that, in spite of everything automation has achieved, the relationship between the human being and the machine remains crucial.

“The knack for it isn’t something you can learn from a textbook. At some point, you get to know the way the system feels – when it runs smoothly and when it starts struggling. That’s how you know what it can handle,” explains Markus Holinger, Supervisor at WALO.

Team spirit

When assembly means more than just assembly.

“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 is top notch,” explains Axel Christ, Civil Engineer at WALO.

On board the floating dredger, there’s another unique detail that no technical specification will tell you about: a rubber duck by the name of Ryan. Much like Saint Barbara gives protection when building tunnels as tradition would have it, Ryan the duck watches over Mapragg reservoir – in his own special way. He quite literally emerged from the depths when the system was first deployed back in 2018 and has been a permanent part of the inventory ever since. Every morning, he is fed a symbolic breakfast. You might call it superstitious, but you never know what’s going to happen on board a floating piece of machinery. That’s why the regulations require two people to be on board the dredger at all times for safety reasons.

The four people who work on the system during the week live in the hotel in Bad Ragaz. They only get to see their families at the weekend. This means that working on the dredger requires technical expertise, but above all the ability to get along well with others.

“The people need to be the right fit for each other. Over a period of several weeks, they spend several hours per day in close proximity – in the hotel, on the construction site, aboard the dredger. The interpersonal dynamic has to work,” explains Axel Christ, Civil Engineer at WALO.

Measurements and regulatory compliance

45,000 tonnes – and one vision.

A turbidity sensor at Sarelli 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 stops. This means the entire material excavation process is measurable, documented and controllable. A system that was more or less operating at its limits in 2018 now has almost double the capacity.

Heavy precipitation and thawing permafrost are causing increasing amounts of sediment in Switzerland’s storage and compensation reservoirs. Using a reservoir to generate hydroelectric power requires dredging. WALO is ready to rise to the challenge – with a system that dredges deeper than any alternative and a team that knows what they’re doing.