OBSERVER: Designing sponge cities with Copernicus Land
sonia
Thu, 21/08/2025 – 10:34
Cities are racing to adapt to heavier cloudbursts, rising flood risk, and hotter summers. The sponge city approach treats streets, parks, and waterways as living infrastructure which slows, stores, and cleans rain where it falls, improving liveability and biodiversity while easing pressure on drainage systems. The Copernicus Land Monitoring Service (CLMS) provides the consistent, high-resolution information needed to plan and monitor these nature-based solutions, from mapping sealed surfaces to monitoring tree cover and surface water. In this Observer, we show how CLMS supports sponge city design, point to lessons from Copenhagen and Wuhan, and highlight the datasets planners can use to measure and improve urban sponginess.
In July 2011, the city of Copenhagen, Denmark experienced a flooding event which would change it forever. Above the city, a warm air mass coming from the south collided with the cooler air over southern Scandinavia. As warm, wet air rose quickly, it cooled and condensed, forming towering cumulonimbus clouds which opened like floodgates and dropped a staggering 135 mm of rain on Copenhagen in less than 2 hours.
This event, known as a “cloudburst”, left a massive trail of destruction in its wake. The extensive flooding submerged basements, crippled transportation networks, and caused the sewers to back up into the streets. There were power outages across the city and emergency services were absolutely overwhelmed. Though fortunately there were no fatalities, it was a wakeup call for a city which was completely unprepared for such an event.
Investing in the future
The 2011 cloudburst became a catalyst for the city, providing the necessary justification for a €1.3 billion investment in flood preparedness. The first part of the plan is nearly complete. Copenhagen has built a series of seven enormous tunnels beneath the city designed to hold 10,000 cubic metres of water, equal to the volume of four Olympic swimming pools. These tunnels will be necessary to deal with future cloudburst and extreme rainfall events, but to address the more frequent but less severe storms, Copenhagen has taken a different approach, transforming streets, parks, and public spaces into dynamic elements of the stormwater management system. From sunken playgrounds which double as collection basins to tree-lined boulevards which channel water safely downhill, Copenhagen’s urban fabric is being reimagined to manage water where it falls.
This idea has come to be known as the sponge city approach, a model of climate-resilient urban design which integrates green and blue infrastructure into the urban fabric in a way which mimics natural hydrological processes like slowing runoff, absorbing excess rainwater, and filtering pollutants before they reach waterways. By weaving nature into the city, the sponge city approach not only reduces flood risk but also supports biodiversity, improves air quality, and enhances urban liveability.
So far, the city has transformed more than 20 green spaces into sponge parks. Take Karens Minde Park in the Sydhavn neighbourhood for example. This low-lying green space is capable of holding up to 15,000 cubic metres of stormwater, equivalent to about 1.5 of the city’s underground cloudburst tunnels. A 600-metre-long tiled channel winds through the park, guiding rainwater through a series of trickle meadows which help filter out sediment before it flows into a retention basin. Through a simple excavation project, Copenhagen was able to construct a walkable riverbed which becomes a major waterway during heavy rain, protecting the neighbourhood of Sydhavn from flooding while providing a vibrant public space the rest of the year.
Assessing sponginess
These individual examples of successful green infrastructure are important, but city planners and researchers also need tools to assess how well the urban landscape absorbs water overall. One method for measuring a city’s “sponginess” is based on Arup’s Three-Factor Model, which looks at three key aspects: green and blue space coverage, soil permeability, and runoff potential.
The Copernicus Land Monitoring Service provides standardised, free and open, high-resolution datasets which can help evaluate each of these components. For example, the Urban Atlas (UA) product, which provides detailed land cover/land use maps for 788 Functional Urban Areas across Europe, can be used as a first step to map land cover types across a city, offering a snapshot of potential absorption zones.
To get more specific, layers like the High Resolution Layer (HRL) Imperviousness can be added. This product is designed to identify artificially sealed surfaces and how sealing has changed over time, helping planners identify opportunities to “green the grey”.
Even finer detail comes from products such as the Tree Cover Density and HRL Small Woody Features, which provide information about tree cover density and the presence of small, linear, or patchy vegetation features such as hedgerows and tree lines, respectively. This helps urban planners distinguish, for example, between a mown lawn and a dense grove of trees, both of which show as green on a satellite image, but are very different in terms of how much water they can absorb.
By combining these datasets, cities can build a detailed map of their current hydrological situation, highlight areas which need improvement, and track progress as new sponge infrastructure is added.
Wuhan’s Sponge City Initiative
These days, the popularity of sponge cities has gone global. In the last decade, China has been particularly eager to adopt this model of urban design, with Wuhan standing out as one of China’s flagship sponge cities. Located within the Yangtze River basin, Wuhan is very vulnerable to extreme flash flooding events, which have increased in frequency and intensity over recent decades as a consequence of climate change. By improving natural water retention and absorption across the city, Wuhan has become a model of urban resilience with an improved ability to reduce flood risk and adapt to the mounting challenges of climate change.
Wuhan began implementing sponge city principles in 2015, allocating over €3 billion to projects such as wetland restoration, permeable pavements, urban forests, and expanded blue-green corridors. These natural and engineered systems work synergistically to absorb and retain stormwater, reduce surface runoff, and filter pollutants, while also providing crucial co-benefits including biodiversity preservation and improved quality of life.
Economic analyses suggest that investments in sponge city infrastructure in Wuhan ended up being €600 million cheaper than grey infrastructure alternatives, while delivering additional social and ecological benefits. For instance, during the 2016 flood season, Wuhan’s sponge city features are estimated to have prevented approximately €150 million in direct flood damage by absorbing runoff and delaying peak flows in vulnerable neighbourhoods. Moreover, by reducing the load on drainage and wastewater treatment systems, these nature-based solutions contribute to long-term operational savings and decreased maintenance costs. Beyond direct economic benefits, sponge city investments generate wider environmental and social returns. Urban cooling effects from increased vegetation improve liveability and reduce heat-related health risks, while the improved air quality and expanded green space boost public health and property values.
CLMS global portfolio
Monitoring the success and progress of sponge city initiatives requires detailed, accurate, and timely data. Here, CLMS can offer invaluable support through high resolution (both spatial and temporal) data such as the Water Bodies and Land Cover Forest Monitoring (LCFM) product suites. Water Bodies products offer a near real-time overview of surface water extent across the world, capturing seasonal and longer-term variations on a monthly basis at 100m resolution in lakes, rivers, and wetlands. This product can help city planners track how natural water bodies respond to rainfall events, assess floodplain connectivity, and identify areas where water retention capacity can be improved.
Complementing this, the global 10m resolution Tree Cover Density dataset offers detailed insights into the condition and changes of forested areas across the tropics, including in Wuhan. Forests play a fundamental role in urban hydrology by intercepting rainfall, promoting infiltration, and reducing surface runoff. Healthy forest cover acts as a natural sponge, slowing the flow of water and allowing it to percolate into the soil, thereby reducing flood peaks and improving water quality.
The 10m resolution global Land Cover layer can also play a crucial role in tracking various land cover classes, including the important Tree Cover class. While dense forests are rarely found within urban areas, the Tree Cover class is often present and highly relevant. The LCFM product monitors forest dynamics such as deforestation, afforestation, and degradation with high spatial and temporal resolution. This enables planners and environmental managers to assess whether sponge city initiatives, such as reforestation or urban greening projects, are effectively increasing tree cover and improving natural water retention. For example, newly planted urban forests or restored riparian buffers can be identified, and their impact on local hydrology evaluated over time. Conversely, areas experiencing forest loss or degradation can be flagged as potential vulnerabilities where flood risk may increase.
As climate change accelerates and extreme weather events become more frequent, cities around the world are rethinking how they manage water. Copenhagen and Wuhan, two cities on opposite sides of the globe, provide compelling examples of how sponge city principles can deliver long-term economic and ecological benefits. However, making cities more absorbent isn’t just about big infrastructure projects. It’s also about data. With detailed, high-resolution products from CLMS, planners and policymakers can target the right interventions and monitor the impacts of nature-based solutions over time. In this way, satellite data becomes an important ingredient in building greener, more resilient cities. Cities that don’t just survive the next storm but thrive in its aftermath.
Thu, 21/08/2025 – 12:00