'Our plan worked': How Vienna prepared itself for a 5,000-year flood

"Vienna's flood defence system is designed to cope with a flood discharge of 14,000 cubic metres per second – that's equivalent to a 5,000-year flood," he says. A flood of that size last occurred in 1501, he adds. During the flood over the weekend, around 10,000 cubic metres per second flowed through Vienna's waterways, "significantly below the system's 14,000-cubic-metre capacity," he adds. "Without this system, there would have been widespread flooding."

One cornerstone of this flood defence system is an artificial island, the Danube Island, and a flood control channel, the New Danube. Both were built in the 1970s, in reaction to a powerful flood in 1954 that had overwhelmed existing defences. The New Danube is usually closed by weirs, creating a kind of lake. "The weirs are opened before the flood arrives, and for three, four days, the channel takes in flowing water," relieving Vienna's main river, the Danube, Blöschl says. 

The system was put to a dramatic test in 2013, when the Upper Danube Basin experienced one of the largest floods in the past two centuries. The Danube's flood discharge in Vienna reached around 11,000 cubic metres per second, but major damage was avoided due to the city's flood defence system, according to a report by the city. "Not a single household was threatened in Vienna, compared with 400,000 households across Austria," the report stated.

That does not mean the system wholly contains big floods, however. During the big flood last weekend, the Wienfluss, a smaller river in Vienna, swelled and spilled onto the tracks of an underground train, and public transport has been interrupted as a result.

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