Hamburg, Germany – As cities worldwide grapple with the escalating impacts of climate change, Hamburg is pioneering an innovative approach to urban planning known as the ‘Sponge City’ concept. This strategy aims to transform the city into a more resilient environment, capable of effectively managing heavy rainfall and mitigating the effects of rising temperatures.
Hamburg’s ‘Sponge City’ Vision: A Proactive Approach to Climate Change
The ‘Sponge City’ principle, which involves storing rainwater more effectively and reducing urban heat, is gaining traction in Hamburg. The city is implementing a range of projects, from green roofs and facades to permeable surfaces and dedicated rainwater reservoirs, to achieve this goal. The initiative is a collaborative effort involving various institutions and stakeholders, spearheaded by Hamburg Wasser and the environmental authority through the RISA (RegenInfraStrukturAnpassung) initiative, established in 2009.
Biberland: Germany’s First Rainwater Playground
A prime example of Hamburg’s innovative spirit is the ‘Biberland’ rainwater playground in Hamburg-Neugraben. Opened 13 years ago, this unique area diverts excess rainwater for recreational use. While on a recent hot summer day, children used groundwater for play, the playground’s design allows for controlled redirection of rainwater, which then flows into a brick streambed and eventually into an adjacent well protection area. Ole Braukmann from Hamburg Wasser highlights the dual benefit of such projects, stating, “The coolest projects for me are those where areas are used twice and suddenly have a completely different function.” He emphasizes the cost-effectiveness of this approach compared to expanding traditional sewage systems.
The Multifaceted Benefits of a ‘Sponge City’
The advantages of the ‘Sponge City’ model extend beyond flood prevention. According to Braukmann, it leads to “cost savings in system expansion, a better microclimate, more groundwater recharge – and we protect our waters and ultimately our buildings.” Hamburg, like many older cities, utilizes combined sewer systems where both wastewater and rainwater are transported in a single pipe. During heavy rainfall, these systems can overflow, discharging untreated water into rivers and canals like the Alster and Elbe, thereby impacting water quality. The ‘Sponge City’ approach helps alleviate this problem by reducing the volume of water entering the sewer system.
Green Infrastructure: From Rooftops to Schoolyards
Sonja Schlipf, a ‘Sponge City’ expert, explains that rainwater storage begins on rooftops and extends to depressions and underground reservoirs. The RISA initiative has already identified over 30 showcase projects. These include green schoolyards, such as the Albrecht-Thaer-Gymnasium in Stellingen, where asphalted areas are being replaced with green spaces to create a more pleasant microclimate for students. Schlipf notes, “It’s great when we can help convert schoolyards to be greener and less asphalted. Through this de-sealing, we get a more pleasant climate in the schoolyards, and the students no longer have to bake in the heat.”
Another notable project is the green facade at DESY, a research center in Bahrenfeld. This large-scale greening project, co-financed by the environmental authority, has significantly cooled the interior temperature of Hall 36, demonstrating an energy-efficient and climate-friendly alternative to air conditioning. DESY also features a massive green roof that serves as an effective rainwater reservoir, with a green water ring connecting various ponds and basins on the premises, providing water for irrigation.
Challenges and the Path Forward
Despite the progress, Hamburg faces challenges. While the city is actively working on de-sealing surfaces to allow rainwater to infiltrate the ground, urban development, including new housing and road construction, continues to contribute to soil sealing. Schlipf underscores the importance of de-sealing wherever possible, referencing the ‘Paving’ competition launched by the environmental authority to encourage property owners to de-seal their land. However, implementation has been slow, highlighting the gap between knowledge and action.
Compared to Copenhagen, which declared itself a ‘Sponge City’ after devastating cloudbursts in 2011, Hamburg still has a journey ahead. Copenhagen’s model involves converting parks and courtyards into large reservoirs while maintaining their original utility, with funding from taxes and household water fees. The Danish capital boasts over 300 such projects.
Lessons from Disasters and Future Outlook
While northern Germany has largely been spared from extreme rainfall events like the Ahr Valley flood five years ago, which caused over 180 deaths and billions in damages, awareness of the need for climate resilience is growing. The ‘Sponge City’ principle alone may not be sufficient to counteract all the effects of climate change. Large, expensive underground reservoirs are not feasible everywhere, as Braukmann points out, due to financial constraints and the existing underground infrastructure of telecommunication lines and subway tunnels. Nevertheless, large retention basins play a crucial role, and Hamburg already has several in place. The current focus is on complementing the
