Rainwater retention on the Kronsberg in Hanover

 

Rainwater is stored temporarily in the sloping avenue open spaces and then runs down to the bottom of the slope in a little stream.

 

We heard a great deal about the Kronsberg – before and after Expo 2000. But when the bulldozers have finally moved away, perhaps it will only be the local residents who find their way up this 43 metre hill. Which would be a pity. Whatever else happened, the Kronsberg development was one of the exhibits off the main World Fair site that was marketed and realized under the motto ‘Man – Nature – Technology’ as a model ecological project. As is well known, this challenge was not met in all fields.

One field in which it was successful, however, was the rainwater manage­ment in this new urban district of 130 hectares. The head of Hanover’s envi­ronment department applied the word ‘revolutionary’ to this approach, which promised that the hydrological condi­tions would not deteriorate despite the sealing which the development made inevitable. The idea was that not a drop of rainwater that fell on roofs, roads and squares would be taken into the sewerage system, but would be retained on the Kronsberg or at its foot, soak away in part and feed a valuable body of water underneath, even in dry periods. This was largely achieved by two green areas, 13 by 30 metres, run­ning parallel with the slope and various bands of parkland arranged one behind the other along the bottom of the slope. These three strips received all the sur­face water from the districts that could not soak away, evaporate or be retained in the private and semi-private areas, and in the district parks.

The retention and soakaway areas on the slope and at its foot are rightly called open spaces. Compulsory ecologi­cal measures are not seen as technical facilities here, but as parks. Water, even when it is intended to flow away slowly, or disappear by infiltration, and thus not a permanent feature, is an exciting and enhancing sight for all age groups in the green areas. In the two ‘slope avenues’

 

The ‘slope avenues’ under construction; the stream is fitted with sheet seal.

 

Regulating devices are built into the retention lips. This means that the outflow can be varied and the retention frequency controlled.

 

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Rainwater retention on the Kronsberg in Hanoverгагату

 

on the southern and northern periph­eries of the development the planners made use of the slope of approximately 5 % with a cascade design. A naturally extended stream bed runs through the two green areas and takes the water to one of nine terraced basins, where it is retained until it soaks away. If there is very heavy rainfall the surplus water flows over the concrete retention lips, drawn on the site as civilization lines, and down into the next basin. The stream produced in this way is piped under intersecting roads, and footpaths run through the bed of the stream on reinforced fords. The water is intended to remain visible for longer in some basins which are an additional 30 centi­metres lower and reinforced with a cohesive substrate. These are mainly in the areas along the foot of the slope and take up the water from the two sloping avenues and the surface water from the base road running parallel with the long side. A wooden regulating device set in the retention lip makes it possible to control the outflow quantity individually.

The Kronsberg definitely deserves a place as a show project in terms of rainwater management for a settlement of this size. It is worth a visit – perhaps especially after the World Fair.

 

At the foot of the slope the retention basins hold back rainwater to slowly release it over time.

 

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Section through retention areas at the foot of the slope

 

Footpath Retention lip

 

Retention basin

 

Railway line

 

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Rainwater retention on the Kronsberg in Hanover

Rainwater retention on the Kronsberg in Hanover

Development plan showing areas built by the year 2000. The sloping avenues and retention areas show up as linear strips of urban parkland.