The new weir system in the Bavarian town of Jettenbach is 160 metres wide and incorporates a residual flow hydropower plant, four weir sections and a fish bypass including a hydrodynamic screw. 40,000 m3 of concrete and around 4,000 tonnes of steel were used in the construction. To ensure the weir system has a secure foundation in the water, the groundwater level in the foundation pit was lowered by around nine metres. Bulkheads measuring 23 m long were inserted and DSV pillars with diameters up to 3.4 m were used as foundation elements. A 90-tonne digger was used to remove the existing limestone and concrete blocks.
The first part of the weir, on the right-hand side of the river, was completed in 2019 and 2020. Over the past 12 months PORR has continued the work on the left-hand side.
Overcoming geological challenges – building in the river on tricky ground
The weir system was primarily constructed in the river itself while the water continued to flow. Building work was carried out on one side of the river, while the water coursed by on the other. The main challenge was that when the river flooded, more than 1,000 m3/s of water was flowing directly past the foundation pit lining. As a result, the water level in the river could be up to 8 m higher than the areas being worked on within the bulkhead enclosures. With concrete-filled blocks and various groundwater levels, both the left-hand and right-hand sides of the river presented a major technical challenge. PORR’s experienced team of experts handled this challenge with confidence, completing the project on schedule. By mid-September the concrete and excavation work were completed in full. At the end of September, the foundation pit enclosure for weir section 1 was dismantled as planned, and the last weir section was flooded. This step, just before completion of the construction work, posed a final major challenge – in the space of a week, a total of 130 tonnes of bulkhead had to be removed from the river. To cut the bulkheads, a temporary dam was poured into the Inn to create an enclosure, and the water in that area was lowered using pumps. Then the bulkheads were cut off approximately 1.5 metres below the water level of the surrounding river and chopped into pieces.
Completion on target: final steps before commissioning
Not only was the weir system in the water finalised at the end of September, work on the operating building has also now been completed on schedule. Electrification and completion of the hydraulic steelwork – both of which are being carried out separately on behalf of the VERBUND – will take place in autumn 2021 and winter 2022. Final completion and commissioning of the overall project is scheduled for autumn 2022.
Completion of the Inn Canal Bridge will be another final milestone for PORR in the next few months. The superstructure of the 130 m long road bridge has already been removed. Currently the bridge piers are being raised and a new superstructure will be built by the end of 2021, with the bridge scheduled to be commissioned in March 2022. Finally, in spring 2022 PORR will work on the surrounding areas, rebuilding and asphalting the access roads.
A PORR partnership project
PORR has experience in a wide spectrum of areas. Complex projects, such as rebuilding the Jettenbach weir system, require broad-ranging expertise and good teamwork. The contractor, PORR GmbH & Co. KGaA, Saaldorf-Surheim branch, put together a team from the Salzburg and Upper Austria PORR branches, the Rail and Engineering Construction Department and Specialist Civil Engineering in Vienna. PORR subsidiary, pde Integrale Planung GmbH, also collaborated on some of the detailed planning. With comprehensive expertise and established partnerships, the construction site team were able to handle every challenge and task with confidence.
More power – green, regional energy for 200,000 households
The overall project, the renovation of the Töging-Jettenbach power plant, will provide green energy for approximately 200,000 households. The result will be an increase in output of around 25 per cent. The reconstruction of the weir in Jettenbach will make a significant contribution to this increased energy production and its sustainability, with some of the water being drawn from the River Inn by the weir system. This water will then flow along the 23 km manmade Inn Canal to the Töging power plant, also currently being renovated by PORR, enabling it to produce energy for around 200,000 households. Once the water has been used in the Töging power station, it will be channelled back into the River Inn.
