At the conference, the example of the Oxford neighbourhood in the German city of Münster showed how an entire district with elements of a sponge city can be recreated when the various disciplines pull together in the planning process. Housing for 10,000 people is being built there on the site of a former military barracks. The water management concept of a new neighbourhood is based on an integral approach in which water management, urban planning and drainage planning have worked together very closely right from the beginning. The objective is a near-natural water balance, i.e. the right mix of evaporation, infiltration and drainage of precipitation.

In the view of Eawag specialist Jörg Rieckermann, this major project is exemplary because the new approaches to urban water management were incorporated into the planning process at an early stage. “If it doesn’t happen from the beginning,” he stresses, “it is too late.” At the Aqua Urbanica, it was agreed that the key success factor for the sponge city is interdisciplinary cooperation.

What the conference also showed: The positive effects of the concept on the water balance can be proven. Long-term monitoring of retention green roofs in Hamburg, for example, showed that up to 80 percent of precipitation was retained on the roofs and evaporated. In contrast to conventional green roofs, so-called blue-green roofs are provided with an additional retention layer. Other benefits proven in Hamburg: Permanently storing rainwater on the roof is good for biodiversity. And: The cooling effect of evaporation on the surroundings is increased and can also be maintained during dry periods. Finally retention roof greening is also economically interesting. Although the construction costs are significantly higher, if the rainwater is retained on the roof, it does not need as many installations to dispose of it on the ground.

Closing gaps in knowledge

Integral drainage planning is a complex undertaking in which planners depend on modelling and corresponding data that are as close to reality as possible - this was another focus of the conference. Lauren Cook, for example, who conducts research on blue-green infrastructures at Eawag, presented a stochastic rainfall generator that should be able to provide a better basis for planning in the future, for example to take into account the influence of climate change. This should enable practitioners to assess whether their urban drainage systems will be able to cope with future developments.

Research is also needed in other respects for the implementation of the sponge city concept. In the view of several speakers, for example, far too little is known about the consequences when micropollutants seep away with the rainwater. And the distribution and intensity or rain within a city must also be better recorded. But completely different research projects are also being discussed. “We want to know which levers we have to use so that implementation is most successful,” says May Maurer, leader of the group “Water Infrastructures” and Professor of Urban Water Management Systems at the ETH Zurich. Should one start with zoning? Or first with the legal provisions? Which departments in a municipality need to work better together? Such questions, it is thought at Eawag, which is strongly influenced by natural scientists, could perhaps best be answered by social or political scientists. The topic of blue-green infrastructures demands interdisciplinarity not only in implementation, but also in science.