Pigniu reservoir: the first-ever removal of deposited sediments via flushing

The Pigniu reservoir of Kraftwerke Ilanz AG was temporarily completely emptied. The reason for this was that in order to ensure smooth electricity production and long-term dam security, the sedimentation had to be removed from the inlet and the bottom outlet.

After years of planning, months of preparations and weeks of readiness, Kraftwerke Ilanz AG received the green light from the Graubünden Amt für Umwelt (Office for the Environment) to flush the Pigniu reservoir. They suddenly had to step on the gas, as in early June 2024, the conditions were finally perfect to start flushing this reservoir above Ilanz at the ecologically optimal time. The current outflows of the various watercourses in the Vorderrhein catchment area above Ilanz were checked several times a day via an online dashboard in the preceding weeks. In addition, outflow forecasts were updated twice a day and used to determine the right time for the flushing.

“We called in around 50 employees who ensured the three-shift operation during flushing,” reports Rudolf Büchi, Head of Kraftwerksgruppe Vorderrhein at Axpo. For the approximately week-long flushing process, he moved his workplace to the reservoir’s operations building and thus had a front-row seat with a view of the dam and reservoir. In just a short while, the view became far from an everyday sight. “The water flowed right out of the Pigniu reservoir in under 12 hours.” After all, Kraftwerke Ilanz AG had been keeping the water level at around a third of the usual height for weeks. This is the minimum operating level for power plant operation and was maintained in order to be able to empty the reservoir as quickly as possible once the required hydrological conditions arose. The required conditions included, in particular, high outflows at the Vorderrhein tributary to dilute the turbidity and protect the ecologically crucial Vorderrhein. The conditions were set out in a flushing permit from the canton of Graubünden. The water of the Pigniu reservoir, which usually shimmers under the sun, was therefore soon replaced by a grey desert of mud. 

Unpredictable substrate

Around 70,000 m3 of sediment had to be removed from the reservoir. This was partly achieved via the Schmuerbach stream, which is normally dammed in the reservoir. It was able to flow through unhindered during the emptying process and took plenty of sediment with it into the valley. This natural process was also helped along by four excavators, which shovelled the sediment towards the Schmuerbach day and night – not an easy job on the muddy and unpredictable substrate. In fact, simply transporting the excavators up to the reservoir on the single-lane, often bumpy and very narrow mountain route was precision work and quite the challenge in and of itself.

Was it all worth it? “Absolutely,” says Rudolf Büchi with conviction. “We were able to remove around 150,000 m3 of sediment from the reservoir – twice as much as expected.” The flushing process was necessary because sedimentation had built up in the bottom outlet area (see box), which would have put the functioning of this key component of any dam facility at risk in the long term. Since sediment kept getting into the works water system (turbine), the minimum operating level had to be raised, leading to a reduced reservoir volume for production and a loss of flexibility.

Owing to this successful first-ever flushing since the dam facility was commissioned in 1992, the Pigniu dam facility is now once again ready for long-term, safe operation. It will likely be decades before the sediment reaches a critical level again. Kraftwerke Ilanz AG has invested several million Swiss francs into the flushing process (engineering, builders, water losses, compensation payments, ecological monitoring), which also makes this an investment into the long-term security of the electricity supply. Refilling began again after about a week, and two weeks later the water from the Pigniu reservoir was once again ready for use to generate electricity at the Ilanz power plant.

Extensive ecological monitoring

Before deciding on flushing, Kraftwerke Ilanz AG examined various options for removing the sediment from the reservoir and from the bottom outlet area in particular. “Flushing turned out to be the only technically feasible solution which would allow all objectives to be achieved. If implemented correctly, the turbidity in the Vorderrhein could also be reduced to an acceptable level. That’s why we coordinated extensively with the corresponding environmental protection agencies in the canton of Graubünden and the environmental protection organisations,” says Joëlle Hirschi from the Environment department at Axpo. The Swiss Federal Office of Energy, cantonal authorities, surrounding communities, the cantonal fishing association, the WWF and ProNatura were all involved.

Thanks to the comprehensive preparatory work conducted by Axpo’s hydrologists – including estimating the maximum possible outflow in the Schmuerbach stream without damaging structures such as bridges or paths, removing deadwood that could block bridges, etc., and the early preparation of measuring points for the three-shift monitoring of the waters below the Pigniu reservoir – it was possible to carry out the flushing process efficiently with a short lead time of only 24 hours.

“The key ecological objective of the entire flushing process was to keep the turbidity caused by the sediments as low as possible in the Vorderrhein tributary,” explains Hirschi. The turbidity was minimised, particularly in the protection-reliant Vorderrhein, by adding clean water from other power plants into the Vorderrhein catchment area as well as through the naturally increased outflows. In order to review this, water samples were taken at four locations during the flushing process and turbidity levels were examined – every fifteen minutes and, during the most critical phases in the three-shift operation, for 24 hours a day. “The extensive monitoring will show whether the process caused any damage to aquatic organisms,” says the scientist. Ecological monitoring is carried out before and after flushing, with studies performed on the water morphology, fish and insects living in the stream bed (macrozoobenthos). This enables an estimate of the impact of flushing on the water ecology. Incidentally, the Axpo employees transferred the turbidity measurements directly into a specially implemented app to ensure that all involved, such as the cantonal Amt für Jagd und Fischerei (Office for Hunting and Fishing), could have a direct insight into the values ​​at all times.