How Oslo is Building Resilience to More Intense Rainstorms

In response to climate change, Oslo will experience more torrential rain – hence the city must adapt to handle more water. Here are some measures that will make the city better equipped for heavy rainfall, stormwater, and urban flooding.

Oslo has experienced heavy rain almost every year since the early 2000s. These events have caused significant damage to buildings and infrastructure due to stormwater and flooding. The stormwater not only poses a problem on its own, but it also carries pollution from road traffic, including particles, salt, pollutants, and microplastics, which contaminate rivers and the fjord. This is harmful to fish and wildlife and can degrade water quality to the point where swimming is no longer safe.

Oslo is working on both small and large measures to adapt the city to more heavy rainfalls, stormwater, and urban flooding. Many of these measures also make the city greener and more pleasant to live in, says Guro Sørnes Kjerschow, who works on climate change adaptation at the Climate Agency.

– Stormwater knows no boundaries; it flows the only way it can – downwards, says Marie Langsholt Holmqvist from the Agency for Urban Environment. Oslo is shaped like a bowl, causing water to gain speed as it moves towards the city center where the terrain flattens, and the water stops. In the city center, much of the water remains in depressions before flowing into the fjord.

– Therefore, it’s important to look at the big picture, the whole city, when working on adapting the city to more rain, Holmqvist adds. Developments high up in the city can have consequences for areas further down. The city is developed bit by bit, but together and over time, each measure contributes to making Oslo more climate resilient.

Humans are good problem solvers, but often nature has already solved the same problem – often in a smarter way. When facing a challenge, we can use or imitate nature to solve it, such as mitigating floods and stormwater. Typically, in cities, water-absorbing nature has been replaced with impermeable surfaces like roofs, asphalt, and concrete. Oslo is no exception: a city with an increased proportion of impermeable surfaces that leads to more stormwater and faster runoff.

Several solutions to heavy rain, floods, and stormwater involve reintroducing nature and working with it.

The reopening of the Hovinbekken stream has proven to be an effective measure for handling stormwater during extreme weather. Not everyone may be aware that Oslo has ten major rivers and streams. Over the past 150 years, several of them have been piped or built over as the city grew, because they were polluted with sewage and other waste. Major investments in good sewage treatment have made it possible to reopen many streams, as is the case with Hovinbekken.

– Generally, we see that water follows the paths it always has, the old stream beds. During heavy rain, the city’s pipe systems do not have the capacity to handle the water, and it will follow the low points in the terrain where streams or wet areas historically existed. We often see a lot of water on the surface along the historical routes of streams, such as Bislettbekken, Tøyenbekken, and Ilabekken, says Holmqvist.

In Iladalen, stormwater from roofs and asphalt flows into the park: the grassy area forms an elongated structure of both barriers and pools, with vegetation that allows the water to soak into the ground, explains Tharan Fergus from the Agency for Water and Wastewater Services. Along the edges, 40 new fruit trees and 15,000 perennials have been planted. During the extreme weather event «Hans,» this seemingly «modest» structure handled around 280,000 litres of water. Without this solution, this water would have flowed into the sewage system and contributed to overloading the city’s sewer and treatment systems.

The rain gardens in Deichmans gate may look like ordinary flower beds, but they are specially designed to absorb and purify large amounts of polluted stormwater. Not only are they considered effective and space-efficient stormwater measures, but the rain gardens also help strengthen the city’s biodiversity.

In Skådalen in Oslomarka, strategically placed logs hold back large amounts of water during heavy rain. This is a pilot project to delay water from flowing into the city during extreme rain. The logs do not affect the stream under normal conditions, only when there is heavy rain.

Additionally, the nature-based solutions have many positive side effects for society.

We get more value for our money by investing them above ground rather than below, as surface solutions can offer a range of positive side effects for society, such as contributing to clean air, improving water quality in rivers and the fjord, increasing biodiversity, and making society better equipped to handle droughts, heatwaves, and more.

It’s important to cater to all types of rain. Rain gardens and green roofs are both smart and aesthetically pleasing, but they will not be decisive during heavy rain and stormwater flooding. We need both functioning flood pathways and preferably multifunctional surface-based retention solutions.

Yvona Holbein from the Agency for Planning and Building Services points out the importance of holistic planning, and good maps are an essential foundation for that. The municipality has now established a new stormwater guide with guidelines on how stormwater should be managed.