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Aktueller Flyer (PDF): Start der Bauarbeiten - Informationen für Anwohnerinnen/Anwohner (Juni 2025)
The Winznau weir on the Aare near Olten will be renovated over the next few years to comply with guidelines and regulations on flood protection. Alpiq Hydro Aare AG will sustainably renovate the facility between 2025 and 2031.
The new licence for the operation of the Gösgen hydroelectric power plant has been in force since 1 January 2020. It also includes the renovation of the Winznau weir. The conversion and modifications are in line with the new Swiss federal regulations and take into account future requirements for flood and earthquake safety. To meet these requirements, Alpiq is refurbishing the weir, which is over 100 years old. The project envisages a sustainable renovation. The historically valuable weir superstructure will remain in its current form, so that the structure retains its typical character.
Construction started in late summer 2025. The main renovation work is expected to be completed in 2027. It will affect the hydroelectric power plant, the fish migration facilities, the new slow traffic bridge, the dam beam storage facility and the scour protection in the Aare below the weir. Each of the five weir openings will then be renovated in annual stages.
The new bridge is scheduled to open towards the end of 2026, which will then enable the renovation of the old footbridge. The project has been specially adapted to ensure that cyclists and pedestrians can cross the Aare with as little disruption as possible during the construction work.
Further information on the renovation of the weir
Current closures and diversions
Current closures and diversions
26 August: Temporary access to the excavation pit
During the detailed planning of the construction work, it was decided that access to the excavation pit for the residual hydropower plant on the Olten side would now have to be via the riverside path along the Aare dam.
This means that Tannwaldstrasse behind your property in the area of the SBB substation up to the weir must be used in both directions for lorry traffic. This will affect the period from September 2025 until the end of the year.
Access to the property will remain guaranteed. The road will be closed to cyclists and pedestrians. The diversions will be signposted.
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21 August: Temporary closure of the footbridge
Due to construction work, the footbridge over the Aare will be closed from Monday, 25 August 2025 until mid-September.
Diversions are signposted on both sides of the weir.
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
- Construction of new bridge
- Renovation of scour protection
2026
- Completion 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.
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.