Tønsberg, Norway’s oldest city, is a beautiful city nestled in at the entrance to the Oslo Fjord. In recent years, the city has experienced an increased pressure on their sewerage system and needed help to rethink the entire rainwater management.
Together with COWI, Tønsberg Municipality, and Multiconsult, we made a bold plan that not only tackles future problems with rain management but will create a better city.
Tønsberg was originally designed with long, narrow streets – the ‘Allmenninge’ - running from the top point in the city - ‘the Haugar’ - down to the canal. The rainwater would naturally flow down the streets and end in the canal. Over time, the water management was put underground and a large crossroad was built, prioritizing cars and cutting the connection between the city and the canal for pedestrians and bicyclists. For the rainwater to be naturally – and most logically - handled on the surface, we’re re-introducing the old city structure with the ‘Allmenninge’ streets as the city’s veins. They get cobblestone that will continue across the crossroad, forcing traffic to slow down. Through the implementation of surface rainwater management, we’re then able to revitalize the historical city, give the streets back to bicyclists and pedestrians and recreate the connection to the canal.
This new natural structure makes it possible to utilize the waters’ natural flow as a recreational element in the city because we need to build infiltration-, detention- and retention systems to protect the downstream water body from erosive flows. This is a benefit because these systems double as aesthetic and recreational amenities for all ages. Specifically, in Tønsberg, five new urban spaces will be implemented with the rainwater detention and cleaning. The hard basins can become skate ramps, basket courts, open spaces for open-air cinemas, green playgrounds and so much more. These urban spaces will be attractive both in dry and wet periods; during sunny times, a recessed basket court is a place to hang out and enjoy the weather, and during winter times, it can be a skating rink.
Because the water is collected and cleaned on the surface, in general, the surfaced rainwater system will provide opportunities to create a much more blue city – with water playgrounds, basins, water vapor and the like. When the conduit is relieved, contaminated water will no longer be in the canals, and potentially the water will be clean enough to swim in.
Also; Greening through street trees and other landscaping captures more rainwater and thus will be implemented with the surfaced rainwater management. This provides an improved habitat for both humans and wildlife, such as birds and insects. Specifically, in Tønsberg, we will fill basins with water plants, which will clean the rainwater, while green bioswales will delay the rainwater through the steep streets. Bioswales consist of a swale drainage course with gently sloped sides and are filled with vegetation. This new urban nature will attract biodiversity and have a positive impact on humans connecting to nature both visually and physically.
The benefits of handling rainwater on the surface - instead of in drains underground - are countless. The money to save on construction costs compared to conventional sewerage is one thing – and the release of pressure on the sewerage is another - but the amenities the rainwater adds to the city and the urban spaces plays an even greater and more important part.