By Chris Taylor
Jan. 5, 2004 -- As changes in environmental patterns bring more rainfall, often in intensive storms, European cities along the banks of the continent's great rivers are having to take steps to protect themselves.
The city authorities of Stockerau, on the River Danube 30k upstream of Vienna, was criticised for the scale of its upgrade to a water removal scheme that more than tripled the capacity that had served the area for a generation; but within 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 on its 2,750km-plus journey to the Black Sea, it helps drain snowmelt from the mountains of Central Europe, as well as providing irrigation for lowland agriculture, and a conduit for wastewater from many of the continent's important cities.
One such city, the Austrian capital of Vienna, is about one third of the way from source to sea - but to put that into context, the river by that stage has already travelled a distance more than twice the total length of London's Thames. 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 the deadly floods such as those which swept through Europe in 1999, and again in 2002.
Stockerau is a town about 30km northwest - upstream - of Vienna, and the home of Vogel Pumps, the Austrian supplier and installer of ITT Fluid Technology pumps and pumping equipment. Well used to the variations of the level of the Danube, the City authorities have built riverside embankments to keep the stream in its course, and out of the town.
But flooding can occur not just through overspill of the river; there is also the question of water backing up in smaller tributories, and through 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.
It was a critical issue in Stockerau, where some 1,000 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 - where a flood of the severity expected only once a century would cause damage.
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 levels of the Danube, the town's wastewater would flow by gravity through the embankment into the river, but at high river levels this route would be 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, allowing a maximum flow of 2,000 cubic metres per hour.
"It had worked ok, and Stockerau had suffered no serious flooding," claimed Erich Moormann of Vogel. "But the engineers had been warning that it could happen. We are getting so much more rain, and more intensive rain which affects the river quickly. And the same is happening across Europe, so we are affected also 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. And the '100 year flood' could become a regular event - they say the 2002 floods were more severe, more in the range of a '500 year flood'. And the new building makes the problem more severe. People knew the land was in the flood area, but no-one believed it would be a problem.
All these factors were taken into account in 1998, when the ageing pumps needed attention. New houses had been built, which needed protection; the experts were saying 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," says 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 about the same, or to install much more capacity.
"There was a lot of talk, and some people thought bigger pumps really would not be needed, but the City of Stockerau believed the experts who warned about the flood risks, 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 to use the same pumps again would mean the pumping station could move the volume of water that some were predicting.
"And in the end, the cost to install two large pumps to work with the refurbished smaller pair was not so great, so this is what they did - and it was 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 2,340 m3/h at a delivery head of 8.9m at a motor performance of 75 kW, and is switched by level sensors which continuously monitor the water height in the Danube. And because of the change of the pumps, the maximum capacity of the high water pump station was increased from 2,000 cubic metres per hour to about 7,000 m3/h, 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 200 hours 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.
"The City is a good customer for Vogel, and for the Flygt range of pumps which we also supply," says Moormann. "We have also supplied small pumps for the sewage treatment plant. Our big advantage to them is that we are based here, so we can provide almost immediate service as there is no travel time - and they have got used to that level of service. It means we are in a very strong position for an installation where we can be called out for urgent service at short notice, as well as for regular maintenance."
