With a few exceptions, the last few years in Bonn have been characterized by severe drought, especially in the summer months. The Helmholtz Centre for Environmental Research's Drought Monitor Germany shows extreme drought intensity for the Cologne/Bonn region, especially in the years since 2018 (with the exception of 2021), reaching up to two meters into the ground. This drought damages urban greenery and thus also impairs its cooling function on hot days. In addition to trees that provide shade, vegetation in particular makes a noticeable contribution to reducing heat stress through evaporation. However, this only works well if the soil and vegetation are sufficiently supplied with water.
The concept for the sponge city was created on the basis of a decision by the Environment, Climate and Local Agenda Committee and will be discussed by the committee on April 16, 2024. The plan forms one of the three pillars of water-sensitive urban development - alongside heavy rainfall prevention and flood protection.
Sustainable use of water as a natural resource
The overriding goal of the sponge city is the sustainable use of water as a natural resource. Rainwater is to be retained "like a sponge" and released directly or indirectly via evaporation. This is achieved, for example, with the help of underground cisterns or infiltration trenches as well as green roofs and facades or flowering meadows and evaporation beds.
Among other things, the City plans to disconnect rainwater runoff from the sewer system at its own depots and use the rainwater for irrigation or as industrial water. Another concrete measure in the spirit of the sponge city, which the city is already implementing, is the planting of new street trees. This is because the small-scale unsealing of tree beds also serves to store water. In addition to its own activities on municipal properties and infrastructure, the City also wants to incorporate the sponge city principle into urban development, for example by adapting standards and guidelines for construction projects. It is also important to motivate owners to become active on private land.
Helmut Wiesner, Head of the Environment Department, explains: "On the way to becoming the climate-resilient and liveable city of tomorrow, we are at the beginning of an urban transformation that presents us with challenges and at the same time offers great opportunities: In upcoming construction and renovation projects, it is important to seize the opportunities and consider the principle of the sponge city right from the planning stage. Citizens can also make their own targeted contribution in the private sector by unsealing and planting greenery, greening buildings or installing cisterns."
In the concept, the city describes various model projects that outline examples of how building blocks of the sponge city can be integrated into ongoing planning projects. For example, as part of the redesign of Rheingasse, areas are to be unsealed and more green space created. In the Rheinaue, a water playground is planned on the site of the former Quasi playhouse, where both rainwater and water will be used optimally as part of the sponge city concept.
Sponge city principle is part of the integrated climate adaptation concept
Since August 2023, a climate adaptation concept funded by the Federal Ministry for the Environment has been developed as a systematic superstructure for the City of Bonn's climate adaptation activities. The sponge city principle is part of this.
In order to effectively develop the entire urban area in a climate-adapted way, the involvement of citizens is of great importance. Until May 5, 2024, Bonn residents have the opportunity to contribute their ideas and suggestions for climate adaptation online on the municipal participation platform bonn-macht-mit.de (opens in a new tab). On Friday, April 19, from 3 p.m. to 7 p.m., an in-person event will also be offered. Registration for the event is possible until April 10 at bonn-macht-mit.de (opens in a new tab).