The new Vienna River

This threat was alleviated somewhat around the turn of the century with the construction of the Auhof flood control reservoirs – a series of retention basins and storage reservoirs on the outskirts of Vienna which allowed planners to control the flow of the river through the city centre. An artificial riverbed was also built in the city’s inner districts and these structures were designed to withstand a ‘once-in-a-thousand-year’ flood.

However, things have changed since then and recent calculations have shown that the Auhof reservoirs only provide limited protection against flooding.

This is in part due to changed discharge conditions in the Vienna area and damage to the retention basins caused by ageing. So a system originally built to cope with a ‘once-in-a-thousand-year’ flood can now only handle floodwaters likely to occur ‘once-in-a-hundred-years’. Refurbishing the storage reservoirs to prevent a high water flood from running into the city centre is now an urgent priority.

Vienna River floods: cause and effect

The drainage behaviour of the Vienna River is characterised by a rapid increase in the water level during heavy rainfall and by an equally rapid decrease of high water after rainfall. Outside these periods, the water level of the river is surprisingly low.

The reason for this is the nature of the river’s headwater region -the Vienna Woods. The subsoil in this region is formed by so-called ‘Viennese sandstone’ which is easily saturated in case of rain and thus can only absorb a small quantity of water.

Precipitation flows off immediately and, in a period of heavy rainfall, leads to a rapid increase of the total discharge volume and thus to high water in the brooks of the Vienna Woods, many of which are tributaries of the Vienna River.

This discharge behaviour has severe consequences. At low water, the Vienna River carries 0.2m³/sec but extreme floods may cause the river to swell to a flow of 450m³/sec.

In addition to the high water carried down from the Vienna Woods, there is also the problem of urban stormwater. Sewers in the western districts of Vienna discharge into two ‘Vienna Valley’ interceptor sewers which, even in less severe rainfalls, are filled to capacity and discharge combined wastewater directly into the Vienna River.

Every second, up to 160m³ of rainwater falling over Vienna may be discharged into the Vienna River. In a ‘once-in-a-thousand-year’ flood this would entail an increase in flow from 450m³/sec in the Auhof zone to 635m³/sec at the junction where the Vienna River meets the Danube Canal, creating a potentially dangerous volume of water.

The new flood protection concept

To prevent this, planners have not only focused on the technical side of hydraulic engineering but also on the ecological development potential of the Vienna River.

As a first step, it is planned to ensure maximum use of all available storage capacity. The second phase affects the section of the river crossing the city and provides for the construction of an additional ‘Vienna Valley’ interceptor sewer within the existing riverbed (see cross-sectional diagram on facing page).

Flood control measures

A sufficient retention capacity upstream of the inner-city river section is required as a priority to ensure that extreme Vienna River floods pass through the city without causing damage. Furthermore, the maximum storage volume of the respective reservoirs must be available exactly when it is needed.

Currently, however, retention basins fill prematurely due to an influx of water from local brooks. Moreover the flood peak cannot be reduced as intended because of a lack of appropriate control mechanisms.

It is therefore planned to increase retention capacity by expanding and redesigning the existing Auhof and Mauerbach storage reservoirs and the aim is to finish construction work by the year 2000.

Several rehabilitation projects need to be completed to ensure optimum use of the entire 720,000m³ storage volume:

Inflow weirs must be raised to prevent premature flooding of the reservoirs

Floodgate shutters in the inflow units will be required to control influx into the retention basins

A central processing unit will control the entire retention system

If the velocity of water in the Vienna River exceeds 30m/sec, the floodgate shutters in the inflow unit are closed, and the flood is directed into adjoining diversion channels. Only extreme floods will be able to spill over the inflow weir and fill the five retention basins.

It will also be possible to optimise storage capacity to reduce the volume of water reaching the city in the event of a ‘once-in-a-thousand-year’ flood to a maximum flowrate of 380m³/sec.

The new Vienna River in the city

The second step towards the improvement of the flood situation in the Vienna River lies in the rehabilitation and restructuring of the riverbed in the city area.

Parts of the concrete-lined river walls and base need to be improved but the core of the planned restructuring work will be the so-called ‘Vienna Valley’ interceptor sewer – a closed sewer built within the riverbed to handle all the combined wastewater from the urban catchment area.

With the construction of this interceptor, the Vienna River will no longer be polluted by sewers discharging combined wastewater. Water quality should also be significantly improved allowing the river to act as a natural link between the Vienna Woods and the Danube Canal.

In addition, floods passing through the city will no longer be swollen by discharges of combined wastewater, ensuring a much more balanced water flow. This will allow construction of a footpath and cycling track alongside the Vienna River.

Situated in the left bank in the riverbed, the new interceptor sewer will run from the Brauhausbrcke bridge to the Danube Canal where it will connect with the right main sewer. From there wastewater and combined wastewater will eventually reach the Main STP.

In case of massive floods, water is dammed at vaulted bridges, which can cause waves with a height of up to 2.5m downstream. In some critical areas, these waves could flow over the walls that separate the river from the Vienna Underground – too great a risk to take. For this reason, separating walls in the relevant areas must be raised.

Returning the river to nature

Since the bottom of the Vienna River must be torn up to build the interceptor sewer, city planners have the chance to come up with a more natural design for the riverbed.

Extensive hydraulic engineering is not possible due to the lack of space and a steep embankment, but targeted measures will help restore the river to a near-natural state.

By depositing different natural substrates (stones, gravel and sand) on the concrete river bottom or by replacing the river bottom completely in certain areas, it is possible to create a habitat for a variety of animal and plant life.

Structural elements such as bed reinforcements, islands and groynes will be used to ensure that natural materials will not be swept away by the current.

The Vienna River: catalogue of objectives

Improved flood retention

Construction of a new interceptor sewer

Creation of a ‘green’ water course

Creation of a habitat for animal and plant life

A cleaner Vienna River

Construction of a footpath and cycling track along the water course