Blue-green infrastructure 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.

Challenges addressed

The strategies address climate change adaptation, reducing urban flooding as extreme rainfall increases. They also tackle water pollution from sewer overflows and biodiversity loss, and some, such as green roofs and rain gardens, support health and wellbeing.

Costs of NbS

The study compares five blue-green strategies and one grey alternative for the 1.33 km² Grefsen catchment, each sized for 60-minute storms recurring every 5, 20 or 100 years under today’s climate. Costs over 30 years (2023 prices, 4% discount rate) differ widely. Wadis are the cheapest blue-green option at about EUR 13,000–46,000, infiltration crates cost about EUR 0.13–0.45 million, rain gardens with rain barrels and wadis about EUR 0.5–1.0 million and green roofs EUR 0.5–1.9 million. A water square costs about EUR 1.6 million whatever the storm size. The grey alternative, a separate sewer system, costs EUR 8,000–29,000.

The strategies store rainwater and let it soak in, preventing flood damage and reducing sewer overflows that otherwise need treatment. Their storage potential across Grefsen differs widely: about 15,700 m³ for a water square, 7,500 m³ for wadis, 2,200 m³ for green roofs and 360 m³ for rain gardens and rain barrels combined, against 2,800 m³ for a separate sewer system. Green options also make Oslo greener, raise house prices and, for rain barrels, save fresh water. Over 30 years, depending on the storm size designed for, benefits are worth about EUR 0.9–2.0 million for rain gardens with rain barrels and wadis, EUR 0.4–1.6 million for green roofs, EUR 0.17–0.8 million for wadis, EUR 0.09–0.5 million for infiltration crates and EUR 0.04–0.4 million for the water square, against EUR 0.07–0.4 million for the separate sewer system (2023 prices).

Benefit–cost ratio

Wadis give by far the best return, followed by the grey separate sewer system and then rain gardens with rain barrels and wadis. Green roofs and the water square do not pay off at any storm size, and infiltration crates only when sized for 100-year storms. All strategies perform better when designed for heavier storms.

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.

None available

Study overview

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.