The new and nowadays often used expression Sponge Citydescribes a complex city construction concept to directly contribute to lowering climate change impacts. A well-planned sponge city prevents flooding as well as overheating of urban areas. Whereas increasingly frequent flooding directly destroys the environment and the homes of its inhabitants, overheated urban areas are having heavy effects on global warming and therefore indirectly lead to even more flooding or droughts of other areas. Measures like rainwater retention, infiltration, evaporation and the use of permeable surfaces are effective elements of Sponge Cities.
The use of or shift to permeable surfaces is considered as the most favorable measure against urban warming and flooding. By avoiding water impermeable surfaces, rainwater can sweep away where it falls or in the immediate surroundings. This prevents the water from accumulating and effectively reduces the volumes that are removed by the sewage system. Ground-sealing surfaces are the main cause for overheated urban areas and flooding.
There surely are several areas, where the use of permeable surfaces is no option to infiltrate water to the ground – for example on roofs, balconies or in high-traffic polluted areas such as petrol stations or airports. In such cases or other areas, where high expanses of impermeable paving are used, rainwater retention is the method of choice. Retention systems can be used as an ‘end-of-pipe’ solution, but are more efficient if installed directly at the location of rainfall to relieve not only the surrounding waters but also the local sewer system. In this way rainwater retention solutions are directly reducing peak flows of any kinds of heavy rain event.
Another solution to drain occurring rainwater is to infiltrate it directly to the ground. This might be achieved through any kinds of permeable floor coverings or surfaces. The advantage is that rainfall directly contributes to the area where nature intended it to sweep away. Therefore, rainwater infiltration somehow reequalises the imbalance of our rainwater cycle, which was negatively impacted by human’s lifestyle for decades. Moreover, infiltration areas tend to cool down the city as the soil or surface will be soaked by water after rainfall. However, it also has its limits: Cities are only able to directly infiltrate clean rainwater to the ground soil. Polluted water needs to be cleaned first before it can be drained away in order to not harm the environment through other channels.
The most famous way for increasing the evaporation performance of a city is the integration of so-called Green Roofs and Green Walls, which simply means to cultivate living plants on roofs and walls. This concept even has several beneficial effects against climate change effects. On the one hand, since plants will cover most of the building’s surface, and plants retain heat much less effectively than stone, glass or other building materials, they instantly prevent the building from heating up in the sun. On the other, plants will lower the overall temperature by evaporation. Another aspect is that plants are using and capturing rainwater. They therefore help to first absorb the falling water before slowly releasing excessive portions to its next destination. A last beautiful effect of green roofs and walls is that they are positively influence air conditions and climate of the city itself.