How Vienna Prepared for a 5,000-Year Flood

Storm Boris is the latest flood in one of the heaviest periods of rain in Europe in 500 years, but one city has been left largely unscathed, according to BBC. When record-breaking rainfall from Storm Boris hit Vienna on September 15, the impact looked severe, with streets flooded, homes evacuated, and a stream turned into a torrent. In just five days, Vienna and other parts of Austria saw two to five times more rain than the average for September. But despite the torrential rain, the city’s residents suffered relatively little damage, with an estimated 10 people slightly injured and 15 homes evacuated. Overall, the city’s advanced flood management system held back the deluge.

In fact, evidence from previous major floods suggests that some of the protection strategies used by Vienna and Austria in general are proving effective, offering important lessons for other cities dealing with increasingly extreme weather. “Austria has really invested in flood management over the past decades, not least because we experienced two major floods in 2002 and 2013,” says Günter Blöschl, a hydrologist and director of the Center for Water Resources Systems at the Vienna University of Technology, who helps shape Austria’s flood risk management strategy.

In Vienna, a flood defence system developed several decades ago plays a key role in protecting the city, Blöschl explains. Vienna’s flood defence system was designed to handle a flood flow of 14,000 cubic metres per second, equivalent to a flood that occurs once every 5,000 years. A flood of that size last occurred in 1501. During the most recent flood, around 10,000 cubic metres per second of water flowed through Vienna’s waterways, significantly less than the system’s capacity. Without the system, widespread flooding would have occurred.

The pillars of this flood protection system are the artificial island called Danube Island and the New Danube flood control channel. Both were built in the 1970s, after a major flood in 1954 overwhelmed the existing protection systems. The New Danube is often blocked by a dam, creating a lake. The dam is opened before the flood and the channel allows water to flow for three to four days, relieving the Danube from flowing through Vienna, according to Blöschl.

The system was put to a major test in 2013 when the upper Danube basin suffered one of the worst floods in two centuries. The Danube’s floodwaters reached around 11,000 cubic metres per second in Vienna, but Vienna was spared serious damage thanks to the city’s flood defences. Not a single home in Vienna was threatened, compared to 400,000 homes across Austria.

However, that does not mean the system can completely control major floods. In the most recent floods, the Wienfluss, a smaller river in Vienna, overflowed its banks, flooding the subway tracks and disrupting public transport.

Austria has also stepped up its flood defenses. The country invests about $67 million a year in flood protection measures and has seen a reduction in damage. These include regular emergency drills, such as mobile walls to block large volumes of water, and the introduction of more sophisticated and accurate forecasting systems.

The 2002 floods caused $3.6 billion in damage across Austria, according to authorities. While the 2013 floods were also large, they caused much less damage, about $967 million, thanks to flood protection measures. The data also shows the accuracy of the forecasts. After the floods on September 13, 2024, the Austrian weather service found that the actual rainfall matched the predicted levels. “When we looked at the impact of the floods last week, we realized that the flood protection system was worth it. The damage we avoided was much higher than the investment in the system. This is a success story,” Blöschl said.

In Austria, accurate forecasting and flood management drills saved lives during the most recent floods, as well as in Vienna and elsewhere in Australia. Emergency response teams practiced building mobile walls to contain the floods. Accurate forecasting also helped identify where dams were at risk of overflowing and where people needed to evacuate. Research shows that advance preparation is especially important as climate change leads to more intense storms and more frequent flooding in parts of Europe. One reason is that warmer air holds more moisture and energy, fueling powerful storms like Storm Boris. The summer of 2024 was the hottest summer on record in Europe and globally. In fact, record rainfall and widespread flooding are becoming a regular occurrence in Europe. An estimated 1 in 8 Europeans live in areas prone to flooding.