Vienna upgrades flood defences to withstand 1-in-5,000-year flood

Vienna has completed work to raise the flood walls at its six Auhof retention basins by up to a metre, upgrading the city's protection from a 1-in-1,000-year flood event to one expected roughly once every 5,000 years, city officials said. The walls were extended along a 1.2km stretch using about 11,000 cubic metres of concrete.

The basins, between the Lainzer Tiergarten and the Westbahn rail line, now hold more than 1.2 million cubic metres of water, roughly 10% more than before, said Gerald Loew, head of the city's MA 45 water authority. The upgrade cost 33.6 million euros, Mayor Michael Ludwig said.

The work followed record flooding in September 2024, when the Vienna River swelled within hours from a trickle to a torrent that swept away trees, road signs and bins. The six basins filled within two hours and came close to their limit, with flow reaching 440,000 litres a second against a built capacity of 460,000 litres a second at the time, Loew said. The city now expects to be prepared for flows of around 500,000 litres a second.

Around 100 people had to be temporarily evacuated from the Ludwiggasse area of Penzing district during the 2024 flooding, with some rescued by fire brigade boat after flood water made their homes unreachable. A new protective wall has since been built in Ludwiggasse, and the city has also reinforced a flood wall near the Danube Canal underground line and strengthened defences along the Liesing stream, combining engineering with renaturation measures.

Austrian law requires municipalities to protect built-up areas against a 1-in-100-year flood. Ludwig said Vienna had already coped well with the 2024 flooding compared with neighbouring areas, some of which criticised the city at the time for not opening the Auhof weirs sooner. Loew rejected that criticism, saying the basins must be kept in reserve for the most extreme events rather than opened at the first sign of rising water.