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The ongoing climate change is also changing the water balance in our cities. Too much or too little water can lead to major damage. Extreme weather events such as heavy rainfall and periods of heat and drought will affect our lives now and in the future.
Sustainable, near-natural handling of rainwater is crucial for a liveable and resilient city of the future. Bonn therefore wants to develop into a water-sensitive city based on the principle of the sponge city!
The natural water cycle is to be strengthened by no longer draining rainwater into the sewage system as quickly as possible, but (temporarily) storing it, evaporating it or allowing it to seep away. This offers ecological and economic advantages. The soil water balance is strengthened with positive effects on plant growth and the urban climate. In addition, runoff peaks can be curbed as a contribution to flood prevention.
The path to the blue-green sponge city
The transformation to a sponge city requires the cooperation of various stakeholders at different levels, a variety of measures, financial resources and time.
The city of Bonn has embarked on this journey and developed a sponge city concept specific to Bonn, which was adopted in April 2024. In terms of content, a strategic target image and a spatial model are presented on the basis of a basic assessment and positioning. This is followed by a map of windows of opportunity and the start of a growing project map with "good practice" examples in the spirit of the sponge city. A comprehensive catalog of measures (including analytical, structural-spatial, procedural and communicative measures) prepares the implementation of sponge city measures and also serves to implement requirements in relevant administrative processes. In addition, model projects are outlined for five locations in Bonn, which are to be further developed and implemented in the ongoing process. Finally, a communication concept with an online platform as a central element is presented.
There are many possibilities that do not have to be costly. Even small changes add up to positive effects, for example:
Using rainwater to water the garden
In the simplest case, you can collect the rainwater running off the roof in a rain barrel and use it to water the garden. The soft rainwater is better tolerated by many plants and drinking water can be saved. The rain barrel should be covered to prevent overflowing and a plague of midges. An alternative to the rain barrel is an underground cistern into which the roof water is pre-filtered and stored. The overflow for excess water can be connected to the sewage system or, if the site is suitable, discharged into the ground above ground. Underground infiltration into a gravel trench in the working area of the cistern is also possible.
In most cases, you will require a permit under water law for the infiltration of collected rainwater.
Surface infiltration of collected rainwater from smaller roof areas up to a maximum size of 30 m², e.g. carports or patio roofs, via the living soil zone may be possible without a permit, depending on the examination of the individual case by the lower environmental authority.
By unsealing and greening paved areas on your property, you can make a valuable contribution to sustainable rainwater management. By replacing the soil and then planting perennials and shrubs or sowing flowering areas, the water can seep away on site. Partial unsealing or permeable path surfaces also reduce rainwater runoff.
Greening of buildings
Extensive green roofs already store between 20 and 40 l/m² of water in their green roof structure, intensive green roofs even store between 50 and 100 l/m². So-called "retention roofs" offer an even higher storage volume. Green roofs are therefore used for water retention, evaporative cooling and delayed rainwater runoff.
Greening support program
The City of Bonn promotes unsealing and greening as well as the greening of buildings and, in combination, rainwater retention systems (cisterns, rain barrels).
