In an initial study, the researchers modelled how combined sewer overflows might change in the future for six different climate scenarios in the Zurich municipality of Fehraltorf. “Eawag operates an Urban Water Observatory for wastewater research in Fehraltorf together with ETH Zurich – the sewage network is equipped with over 300 sensors,” explains Giovan Battista Cavadini, PhD student in Lauren Cook’s research group. “Using measurement data that goes back eight years, we were able to calibrate our model and achieve high reliability for future scenarios of combined sewer overflows.” The modelling showed that the volume of combined sewer overflows could increase by about 150 to 250 per cent by the year 2085, depending on the climate scenario. “In another simulation, we were able to demonstrate that the sewage system would be less overloaded if as many green roofs, infiltration basins, or permeable surfaces as possible were present in the catchment area,” says Cavadini. The strongest effect was seen with green roofs, but only if the runoff was directed to a permeable surface.

Combined Blue-Green Infrastructures Could Prevent Volume Increase

Since in practice, it is uncommon to use just one BGI element, the researchers modelled in a second study how 15 different BGI combinations would affect combined sewer overflows in a future climate. In addition to green roofs, bioretention cells, and porous pavements, retention ponds were also considered.

The best results were achieved by combinations of bioretention cells with either porous pavements, retention ponds, or both. These combinations prevented an increase in both the volume and frequency of combined sewer overflows in each of the four climate models examined – even if only a quarter of the potential area was equipped with these elements. The other BGI combinations studied were at least able to mitigate a volume increase.

“Our modelling underscores how important diverse Blue-Green Infrastructure is to prepare wastewater management for climate change,” says Cavadini, co-author of the study. “Each BGI element has its strengths: bioretention cells are good for prolonged, steady rain because they can absorb water over long periods; during intense storms, ponds are helpful as they can quickly accommodate large volumes of water.”

“Our modelling underscores how important diverse Blue-Green Infrastructure is to prepare wastewater management for climate change.”

Giovan Battista Cavadini

More Cost-Effective Than Grey Infrastructure

The researchers were interested not only in the benefits but also in the costs of combined BGI. Considering installation, operational and maintenance costs, they calculated how expensive it would be in a future climate to prevent one cubic meter of combined sewer overflow. One-third of the BGI combinations studied – including those that completely prevented an increase in combined sewer overflows – are more cost-effective than a conventional storage tank. “And that’s not even taking into account the fact that Blue-Green Infrastructure is multifunctional,” Lauren Cook adds. For the same price, it can not only absorb rainwater, but also mitigate heat and promote urban biodiversity.

Modelling as a Decision-Making Tool in Urban Drainage

Although the modelling results are only applicable to Fehraltorf and can only partially be generalised to the entire country, the researchers have laid an important foundation for planning with their methodology. “Our method can serve as a decision-making tool in practice to estimate how much and what types of Blue-Green Infrastructure will be needed in a specific catchment area to reduce combined sewer overflows in the future,” says Cavadini.

Cook adds: “Our research highlights the importance of using climate scenarios rather than current climate data in urban drainage planning. In a future climate with more heavy rainfall, the effect and cost-efficiency of Blue-Green Infrastructure will be much greater than today. It is high time we rethink urban drainage and make more extensive use of Blue-Green Infrastructure.“. Otherwise, more combined wastewater could flow into the water bodies in the future – with negative consequences not only for aquatic ecosystems but in the worst-case scenario also for people taking a bath and for the drinking water quality.
 

Cover picture: Ponds, as here in Opfikon (ZH), can hold large quantities of water in a short time. This will be important in the future if heavy rainfall becomes more frequent and heavier as a result of climate change (Photo: Eawag, Max Maurer).
 

• Eawag
• ETH Zurich
• Swiss National Supercomputing Centre
• University of Exeter
• University of Queensland
• City University of Hong Kong
• Swiss National Science Foundation