This information is only available in German!
Aktuelle Informationen für Anwohnerinnen/Anwohner (PDF): Betonarbeiten neue Passerelle (Juli 2026)
The Winznau weir on the River Aare near Olten is being refurbished to comply with guidelines and regulations regarding flood safety, fish migration and minimum flow requirements. Alpiq Hydro Aare AG is renovating the facility, which is over 110 years old, between 2025 and 2031. The project is a sustainable renovation. The historically significant weir superstructure will be retained in its current form, ensuring that the structure retains its distinctive character.
Construction work began in late summer 2025. The main refurbishment works are expected to be completed in 2027. These works will cover the residual-flow power station, the fish migration facilities (fish passage upstream and downstream), the new footbridge for cyclists and pedestrians, the weir beam support and the scour protection in the River Aare downstream of the weir. Subsequently, each of the five weir gates will be refurbished in annual phases.
The new footbridge is scheduled to open around mid-2027. The project has been specifically designed to ensure that cyclists and pedestrians can cross the River Aare with as few restrictions as possible, even during the construction work.
Further information on the renovation of the weir
Current closures and diversions
Current closures and diversions
Aktuelle Umleitung Fuss- und Veloverkehr
This information is only available in German!
Juni und Juli 2026
Aareübergang für Fussgängerinnen und Fussgänger sowie Velofahrende nicht passierbar. Die Umleitung wird signalisiert und erfolgt über die Brücke der Gösgerstrasse (vgl. Infoblatt und Umleitung).
Februar bis Dezember 2026
Umleitung des Fuss- und Veloverkehrs auf Seite Winznau (vgl. Infoblatt)
Information for residents
Information for residents
Progress and schedule
Progress and schedule
We currently anticipate construction to take place in several stages until 2031. Construction is scheduled to begin in late summer 2025.
2025
- Building permit granted in May
- Preparatory work in July
- Start of construction of residual water power plant and fish migration facility at the end of August
- Renovation of scour protection
2026
- Construction of new bridge
- Completion of scour protection renovation
- New construction of dam beam storage
- Start of renovation of the five weir openings on an annual basis (one weir field per year)
2027
- Completion and commissioning of residual water power plant and fish migration facility
Questions and answers
Questions and answers
Why does the weir need to be renovated?
- The renovation of the Winznau weir, built between 1914 and 1917, is linked to the new licence for the Gösgen hydroelectric power plant, which has been in force since 2020. Among other things, this licence stipulates that the weir must meet higher requirements for earthquake safety and flood discharge.
Why can the striking weir superstructure now be retained, contrary to earlier plans?
- According to refined analyses, which have been incorporated into the applicable SIA standards for conservation since 2017, it is realistic to retain the weir structure in its current form. The refined analyses, which better reflect the actual behaviour of structures under earthquake loads, enable sustainable renovation of buildings and, according to experts, provide scientifically sound and clear evidence of the earthquake resistance of the Winznau weir.
What are the most important renovation measures?
- A modern residual water power plant will bring the weir up to the latest technical standards and adapt the power plant to current ecological requirements. This includes, for example, the renewal of the fish migration facility, which will improve fish migration upstream and downstream. A new, wider bridge will make it easier and safer to cross the Aare on foot or by bicycle in future.
Where can I cross the river during the construction period?
- Via a temporary crossing that will remain open at all times during the construction period.
Is there a bathing area with a sandbank below the weir?
- No. Staying in the area of the weir is risky and can be dangerous. This is because a large amount of water can be released over the weir at any time – even in fine summer weather – for example, in the event of a malfunction of the machinery in the power plant in Niedergösgen.
Impressions from the construction site
Images © Finja Basan
History
Who is Alpiq Hydro Aare AG?
Alpiq Hydro Aare AG is a wholly owned subsidiary of Alpiq. The company owns and operates the three run-of-river power plants on the River Aare in the canton of Solothurn.