In essence, it is all about infills (infiltration trenches or tree swales) in the subsoil with volcanic rock
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“Sponge Sity“ is a new term in urban planning. What is meant by this is that one no longer builds in such a way that a heavy rainfall merely runs off quickly and, if possible, does not lead to flooding. Rather, the goal is now to retain the water in a controlled manner, i.e., to store it underground and ultimately to drain it into the groundwater. In addition: trees are to act as natural air-conditioning systems.
The reason for this new use of the urban underground is climate change: even in temperate climate zones, heavy rainfall is becoming more frequent. At the same time, the groundwater level is falling, which is a threat to plants and to drinking water supplies.
So actually, the Sponge City is all about locally adapted stormwater management.
We report from the small town of Stein near Nuremberg, Germany: Here, the concept is being implemented in various new development areas using lava rock.
Leading in Europe in this field is Denmark’s capital Copenhagen; already in 2000, China started to build its cities according the new concept.
At the beginning of the rainwater management is, as before, the gully, or better: an inlet manhole with cleaning function.
This is a special device that cleans water coming from the street or a green area from undesirable substances such as oil, abrasion from car tires or simple dirt. In any case, these substances must not get into the subsoil, say the regulations.
Normally, therefore, the gully would be connected to the sewer system. In the Sponge City in Stein, however, the water is led to the street trees after cleaning in the inlet manhole.
Here it meets another central device of the Sponge City concept: the infiltration trench. These are fills of lava rock on which the trees stand and through which they root.
The infiltrators allow the water to penetrate and retain it in two ways: firstly, in the spaces between individual stones, and secondly, in their pores. Volcanic rock is particularly suitable for this purpose because of its high porosity https://www.stone-ideas.com/101998/climate-change-volcanic-infiltration-trenches/.
The concept for this special type of infiltration trench was developed by the Nuremberg engineering firm Gero Siegle. The fillings with lava stones were supplied by the Eifel-based Vulkatec Company.
Lava rocks in two sizes are used: under the tree discs they are small, so that the water does not run through immediately, but can be absorbed by the tree; for the storage trenches the rocks are larger, so that the water can easily penetrate into their many pores.
There it is then virtually on call, for example when the tree roots suck it back during dry spells.
If the reservoir is full and there is another heavy rain, the storage trenches allow their water to escape so that it can seep into the subsoil or run off via the sewage system.
On the outside, a fleece seals off the infiltration trench from the surrounding area, creating a kind of box underground that is filled with rock and permeable.
This is important so that water does not suddenly stand in the cellars of the adjacent residential buildings in the Sponge City.
Mind you, the figurative sponge also absorbs and releases water in the Sponge City. The principle is the same as for an above-ground reservoir – however, there would be no room for a lake or ponds above ground in the new development areas in Stein. This is because these areas are not located on greenfield sites but within built-up areas.
And: The big difference to the reservoir is that in the Sponge City, trees are used as air conditioners: for their photosynthesis, they permanently pump moisture from the ground into the leaves, where it evaporates. This acts as a cooling system and at the same time prevents overly dry air.
Measurements in similar projects have already shown that temperatures in the Sponge City in summer were a few degrees lower than in the surrounding area.
Other components of the Sponge City concept are paving stones instead of asphalt or greened flat roofs.
However, according to the concept implemented in the city of Stein, it is still possible to asphalt footpaths, for example for baby carriages or elderly people – gully and infiltration ditch do not necessarily have to be right next to each other.
But: what happens if waterlogging forms in the soil?
A long-term trial is now to clarify which tree species can cope with the swale construction method. The aim is to select those varieties that, on the one hand, can cope with the challenges of climate change and, on the other hand, can also cope with the special requirements of the infiltration trenches.
City of Stein (German)
Gero Siegle Engineering (German)
Vulkatec (German)
City rainwatermanagement in Sweden:
(03.09.2023, USA: 09.03.2023)
