Cost-benefit analysis of blue-green infrastructure strategies for stormwater management in Grefsen, Oslo, Norway

Grefsen is a peri-urban residential district in northern Oslo covering 1.33 km² with an average elevation of 183 metres above sea level and some sloping terrain. The area receives an average annual rainfall of 763 mm and has mixed combined and separate sewage systems. Combined sewer overflows occur several times yearly, particularly during summer months, with a notable event in 2017 lasting over five hours. These overflows discharge into the AK52 overflow point on the Akerselva River, impacting downstream water quality in both the river and Oslo Fjord which are used for bathing purposes.

To assess the economic effectiveness of different blue-green infrastructure investment strategies for stormwater management in Grefsen, Oslo, by comparing costs and benefits across multiple rainfall event protection levels (M5, M20, M100) under both current and future climate conditions, providing decision makers with information to select appropriate strategies and flood protection levels for creating more resilient urban water systems.

• Investment costs varied significantly by strategy and protection level, ranging from 0.11 million NOK for M5 separate sewage system to 43.15 million NOK for M100 green roofs under future climate conditions. 
• Wadis represented the lowest capital requirement at 0.17 to 1.03 million NOK, while water squares required fixed investment of 21.45 million NOK regardless of protection level due to single-location implementation. 
• Operation and maintenance costs ranged from negligible for separate sewage systems to 2.68 to 9.59 million NOK for green roofs over 30 years. 
• Return on investment achieved within 4-5 years for wadis, 5-7 years for separate sewage systems, and 10-14 years for green-blue combination strategies. 
• Sensitivity analyses confirmed robustness of results for wadis and separate sewage systems even when costs or benefits doubled.

Over the 30-year analysis period, wadis generated net benefits ranging from 0.52 to 10.89 million NOK depending on protection level, with prevented flood damage being the primary direct benefit. The combined green-blue strategy produced the highest absolute net benefits at up to 32.78 million NOK under future climate scenarios. 

Co-benefits included improved aesthetics and greener urban spaces, increased property values for green roof installations, freshwater savings from rainwater collection, and prevented sewage water treatment costs. All strategies performed better when dimensioned for 100-year rainfall events compared to 5 or 20-year events, with benefit-cost ratios 2-3 times higher for extreme event protection.

Stormwater Management Model (SWMM) used to simulate discharge volumes for 60-minute rainfall events with 5, 20 and 100-year return periods, identifying overflow volumes exceeding 600 litres per second at the AK52 point. Cost estimates derived from literature research and expert judgment verified with Oslo municipality stakeholders, including capital expenditure, operation and maintenance costs, and economic lifetimes for each measure type. Investment phasing defined through consultation: five-year implementation for wadis, green roofs, raingardens and rain barrels (private landowner programs); one-year for infiltration crates and water squares; ten-year for separate sewage pipe installation during regular maintenance. Social cost-benefit analysis applied 4% Norwegian government discount rate over 30-year reference period with residual values accounted for.

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The Grefsen peri-urban catchment

The Grefsen peri-urban catchment in northern Oslo, Norway, faces increasing challenges from climate change-driven extreme rainfall events that overwhelm conventional combined sewer systems, causing sewage overflows into the Akerselva River and urban flooding that damages infrastructure and deteriorates water quality. In response, researchers conducted a comprehensive social cost-benefit analysis comparing six stormwater management strategies for a 1.33 square kilometre area with approximately 6,500 inhabitants.

The analysis evaluated five blue-green infrastructure (BGI) strategies—wadis, green roofs, raingardens with rain barrels and wadis, infiltration crates, and water squares—against one grey infrastructure option: a separate sewage system. Each strategy was dimensioned for three protection levels representing 60-minute rainfall events occurring once every 5, 20, and 100 years (M5, M20, M100), under both current climate conditions and future scenarios using the RCP 8.5 high-emission pathway.

The study incorporated direct benefits including prevented flood damage and avoided sewage water treatment costs, alongside co-benefits such as increased aesthetic value, elevated house prices, fresh water savings from rainwater barrels, and qualitative improvements to river biodiversity and bathing water quality. Investment and maintenance costs were calculated over a 30-year time horizon with a 4% discount rate following Norwegian government guidelines.

Results demonstrated that wadis delivered the highest benefit-cost ratios ranging from 12.0 to 17.3 across all protection levels, followed by separate sewage systems at 7.7 to 15.1, and the combined green-blue strategy at 1.6 to 2.3. Strategies designed for less frequent but more intensive rainfall events consistently outperformed those sized for more common events, suggesting that dimensioning for extreme events yields superior socio-economic returns despite higher upfront investment requirements.