Rethinking Urban Waterscapes: Sponge City Solutions for Zurich
With climate change, cities are facing more extreme weather events, such as heavy rainfall, heat waves, and droughts1. Cities are particularly vulnerable due to their high proportion of sealed surfaces2. Pavement reduces the infiltration of rainwater and increases heat stress, which contributes to heat islands and increased risks of flooding1,3. The city of Zurich is no exception; an increase in hot days and tropical nights, prolonged dry periods, and heavy rainfall events occasionally overloading the sewage system are forcing the city to rethink its water management4. One approach to deal with these challenges is the implementation of the sponge city concept1.
What is a sponge city? A sponge city is an urban planning concept that aims to minimise surface water runoff. This concept envisions cities reacting similarly to a sponge by enabling local infiltration, storage, and evaporation of water. This not only reduces flooding but also keeps water available during dry periods. Furthermore, it decreases air temperature through evaporation from plants and soil, which in turn can reduce urban heat islands4. A sponge city can be achieved through the expansion of green spaces and planting of trees in urban areas, vegetated roofs and building facades, the unsealing of paved surfaces, and the integration of more urban and open waterbodies1,4.
The city of Zurich is already taking action by integrating sponge-city principles into urban planning and by supporting local projects4. Local examples include the retention basin in Glattpark, which collects drainage from roofs and footpaths, the uncovering and revitalization of streams, such as the Albisrieder Dorfbach, or the creation of infiltration areas and planting of trees to evaporate the stored water at Giessereistrasse5.
Figure 2: Examples of an implementation of the sponge city concept at Glattpark, Zurich. The retention basin collects rainwater from the roofs and footpaths12,13,14How sponge-like is Zurich?
Despite some good examples, heat waves and droughts in recent years highlight the need for further measurements to be implemented1. This is why this blog post examines the current hotspots in Zurich that lack optimal sponge city characteristics by posing the following research question:
Where are areas in Zurich that are subject to low infiltration and retention capacity and heat islands? What are potential measures that could be taken on a local level?
How the analysis worked To identify problematic areas, multiple spatial factors were analysed using geographical information system. The map was mainly created based on heat island effect values and infiltration and retention values. Examining the different land-cover types helped reveal how retention and infiltration vary across the area, allowing us to assess how each area regulates water retention and evaporation. The resulting map is shown below. The heat island effect map as well as the infiltration and retention map can be selected in the interactive map view.
The resulting map visualises hotspots of particularly problematic areas which could benefit most from the implementation of sponge city concepts.
Figure 3: Interactive map with three layers: a) Need for sponge city map9,10, b) Infiltration and retention map9, c) Urban heat island map10. Map a) is taking maps b) and c) into account and highlights zones with the greatest need for sponge city interventions. The layer selection and legend view can be found on the top right.15
The need for sponge city measures is represented by shades of red, with darker areas indicating zones with a higher need for intervention through greening and surface unsealing. Very high-critical areas correspond to locations with strong heat island effects and low infiltration and retention capacities. In general, it is evident that the city centre exhibits more pronounced issues with the heat island effect and low infiltration rates.
Moving towards the outskirts of the agglomeration, the need for implementing the sponge city concept decreases, as the overall relevance of these factors diminishes. In contrast, forested areas and open water bodies clearly have a particularly positive impact on mitigating heat and improving infiltration.
The city centre exhibits more pronounced issues with the heat island effect and low infiltration rates.
Where should something be done?
The most affected areas are located near the city centre and all these areas are characterised by a particularly high proportion of paved, sealed, and built-up surfaces6. Let's take a closer look at some places in the city centre!
Figure 4: "Sihlfeldstrasse" in the city district 416
The south of Zurich’s main train station is densely populated and therefore represents a high-priority area for improving the urban climate through the implementation of the sponge city concept. Due to this importance, the city of Zurich already greened the former workers’ settlement in Zurlinden in 2023. This led to a substantial reduction in surface runoff and also contributed to mitigating heat and promoting biodiversity5.
Figure 5: „Bullingerplatz“ in Zurich after the rearrangement into a meeting area in 201117
For future projects the Sihlfeldstrasse and Bullingerplatz offer further opportunities to reduce paved and heat-stressed areas. Greening and unsealing asphalted surfaces would provide possibilities to lower urban temperatures and enhance water retention and would promote the ambiance of the meeting area. Similar projects have already been highlighted in Zurich’s municipal master plan, demonstrating the city’s commitment to sustainable urban transformation7,8.
Figure 6: Zurich’s historic old town center18
It should be noted that the implementation of sponge city concept is less straightforward for some areas. As the historic city centre is also influenced by high temperatures and low infiltration rates, it shows also urgency to sponge city concept adaptation. However, the implementation of the sponge city concept can be challenging because of the dense urban development and the restrictions associated with heritage-protected buildings. This means that it is more realistic to implement only minor adjustments, such as increased tree planting along walking paths.
The implementation of the sponge city concept can be challenging because of the dense urban development and the restrictions associated with heritage-protected buildings.
Another hotspot is located in Oerlikon. The numerous exhibition and event halls contribute to a increased heat accumulation. The event halls and the adjacent industrial areas create a local heat island effect. The area also exhibits low infiltration and retention values, indicating a high degree of surface sealing and compaction. One potential solution could be the introduction of active green roofs, which would make use of currently bare rooftops to reduce heat and improve water retention. Another approach is to green the numerous parking spaces and asphalted areas or replace them with gravel in order to improve the infiltration and water retention of these areas.
Figure 7: Event hall 622 in Oerlikon surrounded by asphalt19
All in all, this analysis shows that Zurich is densely built-up, with the areas around the main station and in Oerlikon being the most challenging. Targeted greening on the ground and on roofs, as well as surface unsealing, can help reduce heat islands and strengthen the city’s climate resilience. While initial steps have already been taken and further implementation is not always easy, even small interventions can make a visible difference. Looking ahead, applying additional sponge city measures could further transform Zurich into a more sustainable and climate-resilient city.
Even small interventions can make a visible difference.
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