PLANNING & PROJECTS
Blue Danube13 November 2003
Stockerau in Austria is a good example of a European city that has effectively protected itself against flooding, says Chris Taylor
As changes in weather patterns bring more rainfall, often in intensive storms, cities along the banks of Europe’s great rivers are having to take steps to protect themselves. The city authorities of Stockerau, on the Danube river 30km upstream of Vienna, Austria, were criticised for the scale of their upgrade to a water removal scheme that more than tripled the capacity that had served the area for a generation. But within several weeks of completion, the new system had proved itself, and two years later, in the face of a 500-year flood, Stockerau suffered no serious flooding.
The Danube is one of Europe's great rivers. From its source in the Black Forest, it takes a journey of over 2750km to the
Black Sea, helping to drain snowmelt from the mountains of Central Europe, as well as providing irrigation for lowland agriculture, and a conduit for wastewater from many of Europe’s important cities.
The Austrian capital Vienna is situated about one third of the way from source to sea. By this stage, the river is already subject to rapid rises and falls in level, as upstream rainfall or melting snow creates dramatic surges. This can lead to deadly floods such as those that swept through Europe in 1999 and again in 2002.
Well accustomed to the variations in level of the Danube, the city authorities have built riverside embankments to keep the stream on course, and out of the town.
But overspill is not the only cause of flooding. There is also the issue of water backing up in smaller tributaries, and the inability to clear rain and other wastewater which normally drains into the river, but which cannot do so when river levels are too high.
This was a critical issue in Stockerau, where some 1000 homes have been built over the past ten to 15 years on meadow land which formerly would not have been permitted for building, as it fell within the nominal 100-year flood area.
Flooding had been kept to a minimum over the past 30 years by a pumped system attached to the town's sewage treatment plant. At normal river levels, Stockerau’s wastewater flows by gravity through the embankment into the river, but at high river levels this route is sealed off as water levels can actually be higher than the plant. In these circumstances, the water is gathered into a draw basin and pumped up and into the river. This had been achieved by four Vogel-Screw pumps of type 30S320 – supplied by Stockerau-based Vogel Pumps – which allow a maximum flow of 2000m3/hr
‘It had worked okay and Stockerau had suffered no serious flooding,’ says Erich Moormann of Vogel. ‘But engineers had been warning that serious flooding it could happen. We are getting so much more rain, and more intensive rain, which affects the river quickly. The same is happening across Europe, so we are also affected by what is happening in Germany and Czechoslovakia.
‘We used to say we would have to work on flood prevention every two or three years, but now it can happen any time.
The 100-year flood could become a regular event; it has been claimed that the 2002 floods were more severe – more in the range of a 500-year flood. New building in the country also makes the problem worse. People knew the land on which houses were built was in the flood area, but no-one believed it would be a problem.’.
Here to stay
All these factors were taken into account in 1998, when the ageing pumps needed attention. New houses had been built, which needed protection. Experts were claiming that increased frequency and severity of flooding on the Danube was a trend that was here to stay; and, not least, the town of Stockerau had grown significantly since the pumps had been installed, and they were approaching their service capacity. They simply could not shift enough water quickly enough to protect the town from flooding.
‘The sewage treatment plant engineers did some excellent work with the old pumps,’ explains Moormann. ‘By stripping all four pumps, they were able to build two good ones by taking the best parts of each and refurbishing them. Then it was a question of what to do about the other two, whether to supply new ones [of the same specifications], or to install much more capacity.
‘Some people thought bigger pumps really would not be needed – but the city authorities of Stockerau were concerned about the flood risk warnings given by experts, and did not want to be responsible for failing to do the right job when they had the chance. They just could not be sure that using the same pumps again would mean the pumping station could move the volume of water that some were predicting,’ he adds.
‘In the end, the cost to install two large pumps to work with the refurbished smaller pair was not so great. It was decided to install large pumps – a decision that saved the town.’
The work was carried out in June 1999, when two of the type 30S320s were replaced by Goulds-Process pumps type 3185 XL 18x18. Each of these pumps has a maximum flow rate of 2340m3/hr at a delivery head of 8.9m and a motor performance of 75kW, and is switched by level sensors, which continuously monitor the water height in the Danube. Because of the change of the pumps, the maximum capacity of the high water pump station was increased from 2000m3/hr to about 7000m3/hr, variable by using the pumps in different combinations, or all together.
The new system did not have to wait long to prove itself. Because of the high water in August of the same year, the pumping station was used for 200hr and played a significant role in ensuring that Stockerau was not affected by the floods. Then in 2002, even higher floods caused deaths and serious damage across much of Europe, but again, Stockerau suffered no serious flooding.
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