05/05/2025, 12:00
| Media release
Olten – The Cantonal Council of Solothurn issued the building permit for the planned restoration of the Winznau weir near Olten at its meeting on 5 May 2025. This fulfils the requirements for the start of construction, which is expected to take place in late summer 2025. Two years after the announcement, the more than 100-year-old weir at Alpiq’s Gösgen hydropower plant can now be definitively preserved and sustainably restored. Despite a valid demolition permit, Alpiq Hydro Aare AG decided in favour of preserving the historic weir and was awarded the Solothurn Heritage Prize in 2024 for this project change.
This building permit now enables Alpiq Hydro Aare AG, as owner and operator of the Gösgen hydropower plant, to restore the Winznau weir in the sustainable manner that it planned to do so. The historically valuable weir superstructure will remain in its current form. Alpiq had conducted an analysis in 2023 to determine if it could be preserved and redesigned the project so that the structure will retain its traditional character. Feedback from local residents and the involvement of the communes helped put a fully harmonised project in place.
The restoration works on the weir are scheduled to start in late summer 2025. The main building work is expected to be finished in 2027. This includes the weir power plant, the fish migration facilities, the new slow-moving bridge, the stop log storage and the kolk protection in the Aare below the weir. Each of the five weir openings will then be restored in annual stages. The new bicycle bridge is expected to open at the end of 2026, which will then mean the old footbridge can be restored. The project has been specially adapted to ensure that cyclists and pedestrians can still cross the Aare with as few restrictions as possible during the construction work.
The initial schedule forecasts that preparation work for the start of construction will begin on site over the next few months. The first visible step will be the temporary demolition of the weir staircase on the Olten side in order to build the new, quieter small-scale hydropower plant with the improved fish pass below.
Visualisation of the Winznau weir with the channel to the Gösgen hydropower plant after completion of the sustainable restoration.
Restoration increases flood protection and improves the Aare crossing
Thomas Fürst, Managing Director of Alpiq Hydro Aare AG, believes that the building permit granted by the canton of Solothurn is an important milestone: “We are so pleased that the decision to sustainably restore the weir has been so well received and that we can improve the weir by adding a wide bridge over the Aare that fits the historical character of the Winznau weir.”
The restoration of the Winznau weir, which was built between 1914 and 1917, is linked to the new concession for the Gösgen hydropower plant, which has been running since 2020. Among other things, this stipulates that the weir must fulfil higher requirements in terms of earthquake safety and high-water discharge. However, the upcoming restoration also entails further modernisation as the new weir power plant will lead to a significantly improved fish pass. Finally, the new and wider bridge will make crossing the Aare on foot or by bike easier and safer in future.
The new concession for the Gösgen hydropower plant
After more than 100 years of operation, the new concession for the Gösgen hydropower plant provides for the continued operation of the power plant without any conceptual changes. The power plant facilities will be adapted to current ecological requirements, although most of the 30 compensation measures in total have already been implemented (increasing the residual water discharge of the old Aare, improving fish migration at the weir and machine house, implementing ecological compensation and replacement measures) and the structures will be upgraded to the state of the art (restoration of the Winznau weir with a new weir power plant, upgrading the dams on the head channel). The total investment amounts to around CHF 63 million and will make a significant contribution to ensuring that what is still the largest run-of-river power plant on the Aare can continue to reliably generate renewable electricity for at least 70 years.