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Winznau weir: Alpiq is assessing an option to retain the historic structure

Olten – Saving a building through sustainable renovations: as part of its preparatory work in advance of the renovation of the Winznau weir, Alpiq is determining whether the historic weir superstructure can be retained in its current form. According to independent experts, the revised renovation project fulfils the key requirements of seismic safety standards. At the same time, this new option involves less construction and therefore fewer restrictions for local residents while the renovations are in progress.

Now more than 100 years old, the Winznau weir at Alpiq’s Gösgen hydropower plant is in need of renovation following the introduction of more stringent requirements with respect to seismic safety and floodwater discharge. Alongside technical upgrades to the weir system, the plan is, or rather was, focused on the hotly debated demolition of the historic weir superstructure. For many years, the generally accepted viewpoint was that demolition was the only option that would allow the requirements of the Gösgen hydropower plant’s new concession – which was granted in early 2020 – and the more stringent requirements of the Federal Water Retaining Facilities Act to be met. The renovation project was duly planned out and a construction permit was issued in 2020.

Undeniable evidence of the weir’s resistance to earthquakes

In preparation for the actual construction phase, however, the project – which was launched in 2010 – has now been revised to ensure compliance with the latest standards and the current state of knowledge with regard to seismic safety. As a result, its lead partner Alpiq Hydro Aare AG is now assessing the feasibility of significantly changing the project so that the culturally significant weir superstructure can be retained. Although the weir is not a protected monument, experts consider it to be an important site of its time, and believe that it holds significance as an early example of a concrete frame structure in Switzerland.

According to detailed analyses that were added to the SIA standards relating to structural retention in 2017, retaining the weir in its current form is actually now a realistic option. These standards issued by the Swiss Society of Engineers and Architects are compulsory national rules for the construction and planning industry. The detailed analyses, which better depict the actual behaviour of structures in the event of an earthquake, enable buildings to be renovated in a sustainable way and, according to the experts, provide undeniable, clear, scientific evidence of the Winznau weir’s ability to withstand earthquakes.

This deformation-based method of providing such evidence has previously been successfully applied in Switzerland in the case of the weir at the Chancy-Pougny hydropower station on the Rhône near Geneva. Alpiq is therefore confident that the method could also be applied in Winznau. Further in-depth investigations are now ongoing in close cooperation with scientists and government experts.

New weir power plant facilitates the required floodwater discharge

The new option also meets the more stringent requirements with respect to floodwater discharge. The reactivation of the fifth weir opening is being implemented as planned and the weir turbine currently integrated into it is being dismantled. A new weir power plant with an existing construction permit is being built next to the weir so that Alpiq can continue to use the residual water that is fed into the Alte Aare river to generate electricity.

Making it easier to cross the Aare on foot or by bike

For Thomas Fürst, Managing Director of Alpiq Hydro Aare AG, the new option for the project is a pleasing development: “Retaining the superstructure makes the project more sustainable and aligns with the historic character of the Winznau weir and the Gösgen hydropower plant. The change to the project also cuts down on the amount of construction required, and the length of time it will go on for, meaning that noise, traffic volumes and restrictions for the local residents during the construction phase can all be reduced.”

If the change comes to fruition, the Winznau weir will still be renovated, but will remain in its current form with its bridge and steps to the sides. However, one key improvement from the existing project would be carried over. A new, wider bridge will make crossing the Aare on foot or by bike at the Winznau weir considerably easier and safer.

No impact on current work

Detailed documentation of the revised project for submission to the responsible cantonal and federal authorities, and the ongoing schedule for the actual renovation work at the Winznau weir, will be drawn up over the coming weeks and months. According to the revised construction permit, the work is still planned to begin in 2024.

Preparatory work that started in the last year is continuing despite the possible change to the project. Since October 2022, work has been carried out to reinforce the dam, including local counterbalancing and replacement measures. The groundbreaking for the new weir power plant mentioned above is planned for 2024.

The Winznau weir – part of the Gösgen hydropower plant

The Winznau weir near Olten was built between 1914 and 1917. It is part of the power plant at Alpiq’s run-of-river power station at Gösgen, the machine house for which at Niedergösgen was rebuilt between 1996 and 2000 at a cost of around CHF 150 million. The Winznau weir routes the Aare into the 4.8 km long upstream canal to the main power plant. Maintenance and servicing are performed regularly to ensure its reliability. In 2004, a new fish ladder was built at the weir, and the control technology was renovated in 2018. A weir power plant is also used to generate electricity from residual water as a form of renewable energy.

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