Sponge cities. Of trees and water.

In sponge cities, as much rainwater as possible is stored and used locally instead of simply channeling and draining it away.

This has three important advantages, among others:

• By using rainwater, groundwater can be saved, for example for watering trees
• During heavy rains, the city can absorb more water, reducing the risk of flooding
• A sponge city with many trees can also Regulate heat.

Sounds great. But how does it work exactly?

Sponges store water

In a sponge city, sponges are installed in as many places as possible in the form of water reservoirs. Here are some examples:

• Green spaces
• unsealed infiltration areas
• rain-permeable paving
• Green roofs and facades
• Rain storage tanks and cisterns
• Troughs that collect water. On a small scale also on tree disks

These modules allow water to seep into many different locations or be used, for example, to water trees.

The interaction of various measures such as green roofs and facades, permeable paving and water storage makes a sponge city particularly efficient.

Examples and inspiration for green cities can be found in this article. 

Trees as green heroes

Urban trees play a central role in the sponge city concept because they are natural water reservoirs. They and their tree disks absorb rainwater and slowly release it into the environment. This not only reduces the risk of flooding, but also helps regulate the urban climate and mitigate the effects of heat waves.

By planting more trees, taking good care of them and enhancing their role in urban planning, we can create cities that are not only resilient and sustainable, but also more livable.

Role models for sponge cities

One of the first European sponge cities was Copenhagen. After a flood in the summer of 2011, over 300 urban development measures were implemented there to store rain and prevent flooding.

But German cities also have the goal of becoming sponge cities. The Berlin Senate, for example, founded the Rainwater AgencyThere you will find a wide range of information on this topic, including funding opportunities.

Another example is the Hamburg green roof strategyHamburg is the first German city to launch a comprehensive green roof strategy. Its aim is to green at least 70 percent of both new buildings and flat or gently sloping roofs that are suitable for renovation.

Another example is the Watering can heroes, who collect rainwater in tanks in Essen and other cities in the region and then pass it on to the trees on a voluntary basis. People with a watering sponsorship and a suitable piece of land have a water tank set up at their home. The water is then collected by a rain thief and later distributed to the trees. The watering can heroes write the following about this on their website:

"The aim is to collect rainwater in the city and to provide a city-wide network of 'water sources' (1.000 litre tanks including rain thieves and lockable taps) for watering. As a space donor, you receive your tank free of charge. It becomes your property and supports your personal watering commitment."  

Collect rainwater for tree irrigation

Do you have access to a roof? And maybe even a garden? Why not collect rainwater and use it to water trees, for example? This way you can make a small contribution to your city's water supply.

For all people who want to deal with this intensively, we recommend the Technical book on rainwater infiltration, rainwater use by Mehdi Mahabadi, published by Ulmer Verlag.

Brad Lancaster shows in his books and on his Website how to collect rainwater both in the city and in gardens and use it to create fertile biotopes.

New technologies and materials

An architect has developed a material that is intended to make house facades weatherproof. Water collected during rain is intended to cool high-rise buildings in particular. Here is the link to Post . 

Benefits for us humans

Sponge cities offer us humans a noticeable added value. More urban greenery leads to less heat build-up in the summer months, improved air quality and a more pleasant microclimate. Living in a sponge city can also reduce stress, increase well-being and improve the quality of life overall.