Designing "sponge cities" is becoming a new trend

In July 2011, Copenhagen experienced what has gone down in the city's history as a "cloudburst" - a downpour that dropped 135 mm of rain in less than two hours. The consequences were dramatic: flooded basements, paralysed transport, overflowing sewers and massive power cuts. Deaths were avoided, but for the Danish capital it was a wake-up call.

Based on this experience, the authorities decided to invest €1.3bn in flood defence. The city has already built underground tunnels capable of holding thousands of cubic metres of water, but an even more ambitious step has been to turn streets, parks and squares into elements of a drainage system. In Karens Minde Park, a winding 'river' has been created that collects up to 15,000 m³ of water during the rains and remains a green public space during the dry season. Today, there are already more than twenty such 'sponge parks' in Copenhagen.

The idea of a "sponge city" is based on the principle of mimicking natural hydrological processes: water is retained, absorbed by the soil and filtered by vegetation. This reduces the load on the sewage system, reduces the risk of flooding and at the same time makes the city greener and more comfortable to live in.

But how do you measure a city's 'absorption capacity'? Data from the Copernicus Land Monitoring Service (CLMS) programme can help. They can be used to map development, tree cover density and water bodies. For example, Urban Atlas shows the distribution of land use types, while the Imperviousness layer captures artificially sealed surfaces such as tarmac and concrete. Such data allows planners to see where there is potential for green space.

Copenhagen's example has inspired other countries. The Chinese city of Wuhan, located in the Yangtze basin and prone to severe flooding, has invested more than €3bn in 'sponge' projects since 2015. Wetlands have been restored, permeable pavements, forested areas and blue-green corridors have been created. The economic impact has been impressive: in 2016 alone, 'sponge infrastructure' prevented around €150 million worth of flood damage. Moreover, the costs were €600 million lower than traditional concrete systems. In addition to the financial benefits, Wuhan has enjoyed reduced temperature stress in summer, improved air quality and increased property values.

The global dimension of this concept also relies on CLMS satellite data. Products such as Water Bodies or Tree Cover Density allow changes in water bodies and forest areas to be tracked on a planet-wide scale. This makes it possible not only to design new "sponge" zones, but also to assess how far existing solutions actually reduce risks and increase resilience.