An underground storage reservoir and two transport sewers shall protect Vienna‘s city district of Simmering from future stormwater events. The project has a value of 30 million euro.
The largest contemporary sewer project on the continent was kicked off in mid-September. This expression of magnitude is truly justified as the huge rainwater reservoir and the two transport sewers with a combined capacity of more than 34 million litres are being constructed directly underneath the public sports ground of Vienna‘s 11th city district. These installations, in combination with the existing sewer system, shall protect Simmering from future stormwater events. Environment City Councillor Ulli Sima explains the motives behind the project. “This project seeks to address the apparent impact of climate change, which brings along capricious weather conditions and torrential rainfalls,” she says.
On August 13th, 2010, a precipitation of 40 l/m2 was measured over a period of ten minutes only. This amount of rainfall corresponds to more than 7% of the annual precipitation and may be compared to tropical rainforest conditions. The district was struck by similar albeit less dramatic events also in previous years.
No ordinary flood protection project
No sewer system in the world is able to take up such water masses. A sewer needs to be dimensioned to transport the wastewater under normal weather conditions and is therefore no flood protection system. Vienna Sewers is responding to the described trend with a whole catalogue of measures for its sewers.
Rainwater is intermediately stored in the new facility and then channelled to “ebswien hauptkläranlage” where it is treated. The installations are the last building blocks of a storage chain within and around the 11th city district which has a combined holding capacity of 86 million litres of rainwater.
The storage reservoir, which is 90 m long, 45 m wide, 7 m deep and holds 28.5 million litres of water, is being built underneath one of the district’s topographical lows (the sports ground in Haidestrasse). High-performance pumps assure that the reservoir is quickly emptied via the sewer system to provide new storage space after a stormwater event.
The two new transport sewers that feed into the storage reservoir are two metres in diameter and 1.9 kilometres in length, providing additional space for six million litres of water. The two transport sewers are largely installed by means of no-dig methods. Construction works started in October and completion is expected in autumn 2016. The costs of the entire sto-rage system will amount to 30 million euros. In the course of the sewer upgrade, the old sports ground of SC Mautner Markhof is also being renovated.
Flooding protection reaches beyond district boundaries
The flood protection measures at Simmering affect an area of 20 km2 and extend across three of Vienna’s city districts. All three districts have a share in the Liesingtal, from where large amounts of combined sewage flow to the city’s wastewater treatment plant at Simmering during stormwater events.
“The engineers of Vienna Sewers therefore opted for retention measures at the source,” says Andreas Ilmer, Director of Vienna Sewers. “They decided to use the historical sewer plant at Blumental in the 23rd Vienna city district. Its two circular tanks take up eight million litres, the former sedimentation tanks more than twelve million litres. During rainfalls they can be filled up with 3,000 l/sec., thus taking a considerable water load off the sewer system of Simmering. The tanks are connected to the existing Liesingtal sewage collector through the Liesingtal relief sewer in the area of Klederinger Brücke. The relief sewer is 5.3 km long und 2.4 m in diameter along this section. The stormwater sewers of the 10th and 11th city districts are connected to the Liesingtal relief sewer via flood control structures.”
In case of a stormwater event, the Liesingtal relief sewer is split up into a cascade of storage reservoirs by ten flood control structures; this activates a total capacity of 17 million litres. During normal rainfalls, the storage chain is primarily used to capture surge water from the stormwater sewers. After the rainfalls, this water is transported to ebswien hauptkläranlage where it is cleaned. During extreme flood events, the flood control structures are closed and the full vo-lume of the Liesingtal relief sewer becomes available to take up the combined sewage coming from the area around Blumental.
“All measures together assure that the water flow from the surrounding areas to Simmering is retained for approximately two hours. Despite all these efforts and forward-looking measures, however, the system does not offer 100% protection. The damage caused by the centenary flood of August 13th, 2010 could not have been entirely prevented,” Ilmer stresses.
House owners are also required to contribute by checking the structural integrity and proper functioning of their domestic sewers, the expert explains. Vienna Sewers sends out an annual information circular to all house owners.
Over the past two years, Vienna Sewers has undertaken further key measures to optimise the flooding protection system in Simmering: the wastewater pumping station at Kaiserebersdorf was upgraded by an additional spiral pump; the relief capacity of the rainwater pumping station at Kaiserebersdorf was also extended, and in critical situations this water volume can be released into the Danube Canal, even when the latter is carrying flood water.