Green cities reduce the risk of flooding
Are there still green cities? What do thunderstorms and floods have to do with our plant life? If precipitation can spread to fallow land, meadows or green spaces, the risk of rushing torrents and flooded cellars is reduced.
Green cities also exist in Switzerland, see the caption above. However, in many large cities, built-up and sealed areas are steadily increasing due to the rising demand for housing. "In addition, small urban water bodies, such as the Aa River in Münster (North Rhine-Westphalia), are often constricted in narrow channels and inner-city green spaces or alternative areas are lacking. The sewage system cannot hold heavy precipitation and urban flash floods occur," explains Prof. Dr. Helmut Grüning. "Water bodies need space," says the expert in drainage technology from the Department of Energy - Buildings - Environment at Münster University of Applied Sciences.
Extreme events
On average, 800 mm of precipitation falls in one year in this country. "In principle, this is not a problem if it occurs over the course of the year. On the contrary, climatic conditions in Germany are so good that water shortage problems are a rare phenomenon." Normally, two-thirds of precipitation evaporates and one-third seeps away. The aim is to maintain this natural water balance in urban areas as far as possible instead of discharging runoff into the sewage system.
"Plants are of enormous importance here. They are a guarantor for evaporation processes because they absorb water through the roots and evaporate it through the leaves." This simultaneously creates evaporative cooling - a positive effect for hot summer months. "We can do nothing better than to green our buildings." After all, even a green roof or a green house façade helps to create evaporation surfaces. In addition, plants produce oxygen and filter dust and air pollutants.
Do not discharge rainwater into the sewer system
"It is important to me to develop ideas that address the issue of heavy rain and flooding (cf. "A better way to deal with heavy rain events," Environmental Perspectives 2018_04.) counteract." That's why a good concept, especially in new development areas, is drainage via multifunctional areas. These can be sports fields, parks, green spaces or even children's playgrounds that can temporarily absorb water. Open, unsealed surfaces protect the development from the consequences of climate risks because rainwater can seep away and be released again through controlled evaporation.
"Ideally, rainwater should not be fed into the sewage system at all, but should be allowed to seep away and evaporate completely," says Grüning. In mature urban structures, however, that will only be possible to a limited extent. But Grüning is also looking for opportunities for urban greening in tightly built-up streets. That's why he's planning a research project to establish trees that can get by with a minimal amount of space without restricting street-side parking.
"This does require a special irrigation system, because during dry periods the plants need water. The trees provide shade, evaporative cooling and ensure air exchange."
Urban greening, he said, brings many other benefits, especially in terms of expected climate change. "However, urban and transportation planners, as well as politicians and homeowners, need to rethink how they design urban spaces differently to address flooding risk."
Left:
- Prof. Dr. Helmut Grüning
https://www.fh-muenster.de/fb4/ueber-uns/gruening/gruening-helmut.php - Department Energy - Buildings - Environment
www.fh-muenster.de/egu
