Climate resilient cities through blue-green infrastructures

Due to climate change, the frequency of heavy rainfall events with sometimes devastating floods has increased in recent decades, as have droughts. It is high time to prepare cities for the effects of climate change.

Urban hydrological cycles not designed to extreme weather situations  

Until now, urban districts have been highly compressed, rainwater on roofs, streets and squares cannot seep away. Instead, the majority of it flows into the sewerage system. Being designed first and foremost for wastewater, though, this system is quickly overloaded by heavy rain. Consequently, roads and basements become flooded. In dry spells too, introducing rainwater into the sewerage system proves inefficient.

If rainwater is held wherever it falls, it can evaporate there and create cooling effects. This is made possible by green spaces, green roofs or facades – we also refer to these as green infrastructures. Also reservoir areas, large meadows for example, where 20 to 30 centimeters of water can collect after a heavy downpour. And cisterns: large underground water reservoirs that store rainwater during dry spells – and then repeatedly serve as a source of drinking water in dry regions.

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Fraunhofer IGB analyzes urban water infrastructures, proposes measures and addresses the question of how blue-green technologies can be managed in an equally robust and efficient manner.

The “Leipzig Blue-Green” project, sponsored by the German Federal Ministry of Education and Research (BMBF), is developing such blue-green structures. The plan over the next few years is to use the former open-air railway station in Eutritzsch as the site for 2100 new apartments, a school campus and an industrial building — including green roofs, green areas, cisterns and so on. At the core of the project is the robust and efficient control of the blue-green technologies.

In the project, Fraunhofer IGB is primarily addressing the question of how the blue-green technologies can be controlled in an equally robust and efficient manner. The intelligent control combines certain data, such as the fill level of cisterns, water quality or even data from soil moisture sensors, with externally available data like the weather forecast. If the weather forecast announces heavy rain for the region, reservoirs such as cisterns are emptied automatically to make room for the new rain. Also the blind drains in a retention green roof, which the Fraunhofer researchers are currently constructing on a carport of the center for environmental research in a pilot test along with the company OptiGrün. Not only can plants grow on this roof, water can also be stored temporarily in integrated blind drains. Software has been controlling these blind roof drains since March 2021 and the project results are being integrated into the plans at the old open-air railway station in Eutritzsch in parallel.

Optimizing the hydrological cycle is also an area of focus in the “Street of the future” project currently underway at Fraunhofer IGB and the Fraunhofer Institute for Industrial Engineering IAO. In one of two associated living laboratories in Ludwigsburg, the team has built and commissioned a cistern with a capacity of 50 cubic meters below the road – the rainwater that drips from roofs and cars collects in this cistern.

In a further step, the researchers are now investigating the water quality: Is it sufficient, for example, to irrigate green spaces or fill water basins? The results of the investigations should help to better understand the processes and allow them to be scaled up to larger areas.

Modeling for use of alternative water resources

To investigate whether and under what circumstances rainwater collection on streets can be suitable for purposes such as irrigation, the IGB uses air quality sensors and analyzes possible correlations with water quality parameters of street runoff.

The rainwater cistern built in the project in Ludwigsburg's Dragonergässle is continuously monitored digitally for its filling level.

Due to the high level of pollution in the street runoff, the collected rainwater is primarily used for sewer flushing. With the help of laboratory investigations of rainwater and street runoff quality as well as air quality sensors, scientific research is being conducted to determine the extent to which predictions can be made about the water quality of surface runoff. These could then serve to promote a broader use of collected rainwater, such as the irrigation of green spaces, and thus contribute to greater resource efficiency in streetscapes and neighborhoods.

Of course, the effects of climate change are not limited to just Europe – and neither are the Fraunhofer approaches to tackling climate change. Cities such as Kochi in India and Saltillo in Mexico are being severely impacted by climate change.

In the project called “Morgenstadt Global Smart Cities Initiative”, researchers from Fraunhofer IAO, Fraunhofer IGB, Fraunhofer ISI and Fraunhofer IBP are therefore devising precise courses of action. The project is being financed by the International Climate Protection Initiative (IKI) of the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU).

The work involves three essential steps: The first one is to analyze the interactions within the city. What are the problems? Second, the researchers will be discussing co-creation — working with all partners and associates, they will work out and prioritize the first project ideas. And in the third and final step, they will set the ideas in motion.

Kochi, India

Kochi, one of the most important ports on India’s west coast with a population of 600,000, has first and foremost water problems. The last monsoons caused severe flooding, claiming many lives and disrupting public life. For the remaining months of the year, though, the city faces serious water shortages. The solutions to these two problems are intertwined: If we save some of the water the monsoon season brings, it will be available in the dry spells. Roof gardens appear to be a good solution for Kochi. Another efficient way of discharging the bodies of water would be to use the open channels that were created during the colonization period but are now clogged with garbage. The recommendations shall be realized on a small district – as a blueprint for other districts.

Saltillo, Mexico

Defined by the automotive industry, this prosperous city lies in the Chihuahuan desert and is by nature very arid. Due to the effects of climate change, however, water scarcity is a growing problem. Water efficiency is therefore becoming an important subject, especially for industry. Together with partners and associates, the researchers will decide in summer 2021 which projects are to be realized.  

After implementation, it is possible to assess whether the approach is producing the desired results and replicate or scale it up in other cities after optimization.