Mikro­klimaSponge city concept: a clear plus for the urban microclimate

Electricity, gas, water, district heating, sewer and road construction: when the last section of Hattinger Straße in Bochum was reopened to traffic in summer 2023 after a total construction period of a good three years, one of the most extensive local construction projects of recent years came to an end.

The city and municipal utilities had actually tackled the complete package for the renovation of the central connecting axis to the south-west of Bochum - combined with an important contribution to the climate-resilient reorganisation of the urban landscape. Because: ‘Based on our feasibility study, we planned and implemented a special trough-trench system for surface drainage in order to release the rainwater from the road, but also from the neighbouring buildings, in a decentralised and controlled manner into the nearby Marbach,’ explains Alexander Brunne.

A unique project in Bochum to date, which is also visually striking: where old tram tracks used to run, a planted, one to three metre wide trough in the form of planted deep beds now separates the two lanes of Hattinger Straße from each other over a good 390 metres. A rainwater collection basin measuring around 240 cubic metres in total, under which a system of additional storage bodies, known as infiltration trenches, has been installed. What may not sound very spectacular at first, placed high demands on the Essen engineers in terms of planning and implementation: the road drainage and the neighbouring properties and buildings had to be disconnected from the combined sewer and instead a new rainwater system had to be installed in line with a sponge city concept.
 

Nature provides the model: a sponge absorbs a lot of water, stores it and only releases it slowly. And it is precisely this principle that can be used to score points in urban areas, especially in heavily sealed city centres. This is because there is often a lack of green open spaces that fulfil precisely this sponge function and provide additional cooling through evaporation. The result: heat islands - and enormous resource losses for the natural water cycle. Instead of seeping into the groundwater, the rainwater usually flows directly into the combined sewerage system via the sealed surfaces and then to the sewage treatment plant. Until now, this principle has usually been the rule, especially in inner-city road construction.

The Essen planning team wanted to show that there was another way, but had to take into account several local conditions in Bochum: the longitudinal gradient of the road, a very narrow road cross-section of only 20 to a maximum of 30 metres and, above all, the underground line running under the site, which, according to Brunne, ‘once again placed very special demands on the impermeability of the system’. It was clear from the outset that seepage was not an option here. Yet this is exactly how a trench drain actually works, releasing the stored liquid into the deeper soil layers and ultimately into the groundwater. ‘Together with the manufacturer, we converted the type of system so that we can use it as a retention basin. We then channel the water underground towards Marbach via an outlet with the longitudinal gradient and feed it back into the local cycle.’ A ‘simple’ channel connection to the natural stream was not an alternative in terms of water ecology. ‘We can't simply discharge unlimited and unrestricted water. The restriction here was around 75 litres per second.’ This means that a maximum of 75 litres per second can flow out of the infiltration trench - the rest is retained in the storage body. 

In order to create the largest possible storage volume in a small space, the papadakis team planned with so-called plastic rigoles, colloquially known as ‘cola crates’, which are regularly used in city centres. ‘This plastic body forms a support frame and is statically designed so that it can also be covered and driven over. The advantage is that the entire trench can also be activated as a storage area. A trench like this is therefore really efficient.’ A total of four infiltration trench storage cascades with a total storage volume of around 480 cubic metres were installed on Hattinger Strasse. 

However, collecting water and discharging it in a throttled manner was only one part of the task. Another was to clean the surface water. Brunne: ‘We connected the roof runoff from the neighbouring buildings directly to the infiltration trenches, as only clean rainwater is discharged through them. The situation is different with the water from the street, which certainly contains pollutants.’ And this is where the above-ground part of the system comes into play: the system was embedded in a specially constructed trough made of concrete L-bricks, which were lowered significantly every three metres so that the water can flow into the trough. Here it is dammed up and seeps into the infiltration trenches with a time delay through a 30 to 50 centimetre high soil zone. ‘These layers of soil act like a filter and pre-clean the water before it is channelled through the underground system to the stream.’ In turn, overflow caps guarantee that dangerous flooding of the road is ruled out in the event of temporary heavy rainfall, ‘because the water can then flow directly into the infiltration trenches’.

All of this, summarises Brunne, ‘is of course much more complex in terms of planning and execution than the construction of a drainage channel, for example’. However, the benefits in terms of urban microclimate and water ecology are enormous. ‘The fact that various local and regional trades have worked closely together to realise this really extensive road renovation and our system has really helped us move forward. It started with the fact that the Bochum road planners had to know that the water would have to run to the centre of the road in future and not to the edge.’ There was also a close dialogue with the manufacturing companies, such as Funke from Hamm, which supplied the infiltration trenches, in order to implement the modifications. ‘All in all, this is certainly costly and time-consuming, agreements have to be made and compromises have to be found. But it is important that such innovative urban planning projects are realised at all. This is the only way we can gain experience for the future and further optimise the whole thing.’

Text: Redaktionsbüro Schacht11  
Pictures: Stadt Bochum