After the millennium flood one year ago, the City of Vienna is increasing flood protection on the Wien River. With the completion of renovation work on the weir walls of the Auhof retention basins, the city is now prepared for a flood statistically expected only once every five thousand years, said Gerald Loew, head of the city’s water authority MA 45, in an interview with the Austria Press Agency (APA).
Following the flood, it was decided not only to renovate the weir walls but also to raise them by one meter. “That gives us protection against a five-thousand-year flood,” Loew said. “But there is no such thing as absolute flood safety,” he added. Due to global warming, rainfall events are becoming increasingly intense. The streams in the Vienna Woods also add to the problem, as the soil absorbs little rainwater.
Retention Basins Full For The First Time In 125 Years
Until now, the capital city was protected against a Wien River flood that statistically occurs once every thousand years. This was the case in fall 2024. The flood protection facility in Auhof, built during the imperial era between 1895 and 1899, reached its limits. Had the rain lasted just 20 minutes longer, the Wien River might have flooded the subway system and shut it down for weeks or even months, Loew said, describing the situation on the morning of September 15.
According to Loew, it was the first time in over 125 years since the facility was built that all six retention basins were full—and that happened within two hours. The weir walls were overtopped by up to one meter, Loew explained. On September 15, MA 45 measured a maximum flow rate of 440,000 liters per second, just below capacity. Normally, the rate is only 200 to 500 liters per second.
Permanent Lowering Of Wienerwald Lake Under Consideration
Another unusual aspect of the event was that both the Wien River and the Danube experienced flooding at the same time. This caused a backflow in the Danube Canal. For that reason, the city will now raise the wall near the subway there. In addition, a wall will be built next year to protect residents of Ludwiggasse in Vienna’s Penzing district, which was heavily affected. Another “idea,” Loew said, is to permanently lower Wienerwald Lake, which also belongs to MA 45. However, this is secondary for Vienna’s flood protection.
At the time of the flood, Wienerwald Lake—located between the Lower Austrian municipalities of Purkersdorf, Tullnerbach, and Pressbaum—had already been lowered by three meters for a slope renovation. According to Loew, however, this extra retention volume was used up within 20 minutes. Releasing the lake in case of flooding is not possible, as the lead times are too short. In general, all systems operate passively or automatically “because the characteristics of the Vienna Woods leave no time to react,” Loew explained. Only the emptying of the Auhof retention basins is actively controlled.
Rumors About Auhof Flood Protection Rejected
Loew also addressed rumors suggesting there had been problems in Auhof. These stories arose because many people are unfamiliar with how the flood protection system works, he said. The fact that the first weir remained closed during the flood was not a malfunction but intentional. This is the only way to smooth out the flood wave rushing through the city. “The mayor said, ‘Vienna can handle floods,’ and that is true,” Loew stated.
