Sponge city: how does it work?
The past few months have shown that heavy rainfall and flooding are also making life increasingly difficult for us. There is a lot that can be done about this – especially with solutions from our company...
Floods in the Saarland, the worst flooding in Swabia and Bavaria in a century: 2024 was anything but dry. Withered fields, forests dying of thirst and dried-up riverbeds – everything we have come to know and fear from the past few very hot years was reversed last year. But that hasn't improved the overall situation. In 2024, farmers are once again complaining of serious crop failures – this time due to excessive moisture. In addition, due to numerous severe storms and flooding, not only is there billions of euros worth of property damage, but there are also many injuries and even a few deaths to mourn. Cities are also struggling with the increasing number of heavy rainfall events...
‘Unfortunately, the situation is that, despite a few flagship projects, our communities are still far from being prepared for these extreme weather events and many have not even begun to prepare for them," says Prof. Carsten Dierkes, an expert in rainwater treatment from Münster. The necessity for this is more than obvious. All too often, the proportion of sealed surfaces is much too high and conventional drainage systems are insufficient to absorb and divert the huge volumes of water that fall in a very short period of time. Innovative solutions and targeted concepts have been available for some time. ‘In future, we just need to think about them immediately and plan for them when we are undertaking construction work,’ says the expert, who also advises us on product development at GRAF.
The key word here is the sponge city principle. The idea behind this is that in future, water will no longer be drained away, but will either seep away in a controlled manner, be stored or reused where it occurs. The city becomes a sponge, so to speak, with many small storage spaces; evaporation also plays a role. The whole thing goes hand in hand with a variety of other measures: with more green spaces and plantings, for example – including on roofs, where there are now so-called retention or blue-green roofs that have a water reservoir built into the vegetation layer – with tree trenches, photovoltaic systems and more.
‘Either way, we need large amounts of rainwater storage in cities for this concept. After all, we not only need storage space, but all the greenery also needs enough water in dry periods,’ explains Dierkes. And that's where the cycle closes again. The Sponge City is not just about heavy rainfall events, but about the entire climate change – including periods of drought, according to the expert.
Large tanks are needed for both larger properties, where containers with a capacity of 100,000 litres or more can be installed, as they are manufactured in our plant in Neuried, and for the many smaller properties, as well as for the planning of urban green spaces: ‘In the end, it's the amount of rainwater storage that matters. This also includes the many homeowners,’ says Dierkes. However, he believes that more political support is needed here, for example in the form of a reduction in bureaucracy or subsidies.
Another key element of the sponge city principle is the use of French drains, i.e. plastic hollow blocks that can be laid in the ground over a large area to fulfil several functions: ‘These elements can be used purely as infiltration systems,’ explains the expert. The water is gradually drained into the groundwater. On the other hand, the buffer storage can also be used only for partial infiltration and additionally for rainwater storage. The big advantage during heavy rainfall events is that large quantities of water can be quickly absorbed by the surfaces provided with the swale systems and covered with water-permeable surfaces. According to Dierkes, the type and dryness of the soil is no longer a factor here. ‘It is often a problem that the few green spaces in cities have dried out so much that all this water cannot be absorbed quickly during heavy rainfall.’ With underground swale fields, there are no such problems and the products from our GRAF portfolio even fulfil a second sustainable function: they are made entirely from our own plastic recycling. Those who allow excess rainwater to seep away in this way can not only make an important contribution, but also save on precipitation fees – and this applies not only to companies, but also to home builders and renovators (after all, rainwater utilisation systems and retention measures can also be retrofitted to existing buildings).
‘Another innovation for even more efficiency and benefits has only just been introduced at the beginning of 2025,’ says Carsten Dierkes, referring to our so-called , a pinch drainage particularly intelligent and powerful throttle that ensures ideal characteristic curves in rainwater storage tanks and thus an optimal outlet volume flow. That may sound complex to non-techies, but it is just one of many small, highly developed pieces of the puzzle that will ultimately lead to the big picture – to a life in our cities that is as carefree as possible with regard to flooding…
