Sunday, September 5th, 2021
After the flood drama in New York
This protects cities from flooding
From Lara Wernig
Heavy rain can hit Berlin or Frankfurt am Main just as badly as it did recently in New York. In Germany, too, large cities are not adequately protected against extreme precipitation. There are already concepts for more security. However, the implementation would be a generational task.
New York is sinking in floods. Storm low “Ida” brought the heaviest rainfall that the northeast of the USA has ever seen. More than 40 people died in the New York area. Pictures of the disaster show flooded streets, floating cars and full subways. Within a very short time, the masses of water turned New York’s basement apartments, cellars and subway shafts into life-threatening traps.
“Large cities are currently not designed for such extreme rainfall,” explains Professor Christian Kuhlicke ntv.de. He is head of the working group “Environmental Risks and Extreme Events” at the Helmholtz Center for Environmental Research (UFZ) in Leipzig. The cause of the flooding in cities is often the overloaded sewer system. As in the current case of New York, this is not a river flood. Instead, the sewer system does not manage to get the water out of the city.
The scenario of an overloaded sewer system could hit German cities as well as New York. Of course, it always depends on the specific topography of the respective city, emphasizes Kuhlicke, but the dangers for people remain the same. As has been seen in the past few days, floating manhole covers, subways, cellars, underground garages and underpasses of bridges are particularly dangerous.
How can cities best protect their citizens from such disasters? Kuhlicke has a clear answer to this: “Warn early.” That is the most important thing. In this way you can ensure that people are not in basements or underground garages.
This is how the concept of the “sponge city” works
In addition to a warning system, Kuhlicke also recommends the concept of the “sponge city”. The aim is to increase the so-called sponge ability of a city. Or to put it more simply: The goal is to retain more water on surfaces so that it does not even flow into the sewer system and into the streets. Rainwater should therefore be temporarily stored where it falls. A large part can evaporate through “green elements” such as hollows, green roofs and facades and seep away on site, which in turn greatly reduces runoff. The tree trenches, for example, are a combination of trees and infiltration. In your “Five principles for climate-safe municipalities and cities” the UFZ describes the idea as follows: “Especially for extreme rainfall, additional storage spaces and green infrastructures must be designed so that they are also prepared as emergency waterways in the event of an emergency.”
The problem is that there are long-term approaches like that of the Sponge City, but implementation is anything but easy. “This is a generational task,” said Kuhlicke. It will take a long time to achieve this ability to spong. And the question remains whether that would ever be sufficient for such major events as in New York.
Another challenge: Cities should use more green spaces and create more storage space during implementation. However, many of these storable areas are in private hands, says Kuhlicke. Or they are public parks. However, private owners cannot simply be ordered to make their land available. “In such densely populated cities as New York or Berlin, you simply reach your limits. Here you have to learn to live with the dangers.”