MUSTBE

Pilot Site in Spotlight: Söderhamnsporten’s Multi-Purpose Water Retention Area

According to Söderhamn’s plans, Söderhamnsporten will soon be an even busier transport hub as the area is developed further and construction of many new businesses begins. Urbanisation will also affect stormwater management, and here’s how this Swedish municipality on the Baltic Sea region will tackle it.

During the coming years, there will be lots of new construction in the Söderhamnsporten district of the municipality of Söderhamn. Several new commercial buildings are planned. The remarkable development of the district requires new solutions for stormwater management. Photo: Josefin Flink

Urbanisation and density of construction increase the amount of impermeable surfaces, such as pavements and roads. Infiltration of water is reduced, and surface runoff increased. This, combined with climate change related increase in heavy downpours, puts more pressure on management of stormwater.

In the municipality of Söderhamn, Sweden, Södehamnsporten district is a transportation hub that brings together railways, highways, and connections to the town centre. There are plans for further expansion. New streets, parking lots and buildings are planned with a combination of amenity areas like parks and pedestrian ways.

– It will become a new entrance to the town, explains Project Manager, environmental pedagogue Linda Hillberg from Söderhamn municipality.

A large grocery store, Willys, moved from Söderhamn’s centre to Söderhamnsporten in spring 2024. In addition, other stores, restaurants and a hotel are planned to be built in the district. As the amount of the area’s acreage of natural permeable surfaces decreases, a new solution for stormwater management is needed.

This is why the municipality of Söderhamn wanted to have Söderhamnsporten as a pilot site area in the MUSTBE project (Multidimensional stormwater treatment in urban areas for cleaner Baltic Sea). The cross-border project’s main aim is to develop multidimensional nature-based solutions (later in the text NBS) for stormwater treatment in urban areas.

A river called Söderhamnsån passes through the district. Its length is 20 kilometers and river area approximately 100 km². Söderhamnsån eventually flows into the Baltic Sea.
Photo: Nils Kändler

Flooding Control and Toxin Purification Are Needed

Stormwater from Söderhamnsporten’s commercial area has been flowing without treatment into a nearby river called Söderhamnsån. The river eventually flows into the Baltic Sea, and therefore Söderhamnsporten’s stormwater runoff will directly impact the sea water quality. Combined with repeated flooding in recent years, the need for adapted stormwater management is evident.

The MUSTBE NBS for Söderhamnsporten represents multi-objective stormwater solution by not only managing the flow rates but also improving the quality of the stormwater. Furthermore, the solution will also provide improved cooling effect as well as a place for recreation for residents. All in all, the NBS demonstrates feasible utilization of the stormwater at its source which is combined with green infrastructure like bioswales and infiltration beds.

During rainfall, the runoff from parking lots and roof tops of the buildings collects harmful substances. In Söderhamnsporten, the stormwater will flow into the Söderhamnsån and eventually into the Baltic Sea. The MUSTBE project has developed a solution for reducing harmful substances flowing into the sea. Photo: Josefin Flink

Plantation and Boulders for Natural Water Purification

Söderhamnsporten’s NBS will leave sufficient space for nature-based stormwater treatment and, on the other hand, is not restricting the usability of the area. Its efficiency is based on infiltration beds, directing the water through ditches and ensuring sufficient stormwater detention time.

All the stormwater from the catchment area will be led into main collector. It will have a wider and deeper section near the inlet for fostering the sedimentation process. The delay in infiltration helps remove harmful substances from the water. To guarantee sufficient detention time, outflow from the detention pond will be restricted by natural landscaping objects like boulders. Greenery will be planted to improve removal of nutrients from the stormwater.

The water quality was analysed in the pilot sites in the early phase of the project. In Söderhamnporten’s case, the value of concentrations for suspended solids was 174 mg/l. The target value is 105 mg/l. The total nitrogen concentration was 2.3 mg/l, while the target value is 1.4 mg/l. During the MUSTBE project, water quality parameters will be monitored to evaluate the effectiveness of the solution.

The target time for finishing construction of the NBS is during 2025.

Project facts

  • The MUSTBE project is led by Estonian municipality Viimsi.
  • In addition to Viimsi and Söderhamn, other project partners are City of Tallinn (Estonia), city of Pori (Finland), City of Riga (Latvia), Tallinn University of Technology (Estonia), Satakunta University of Applied Sciences (Finland) and Riga Technical University (Latvia).
  • The project has seven pilot sites in total: alongside with Söderhamn’s two pilot sites, there is one in Tallinn, one in Viimsi, one in Riga and two in Pori.
  • MUSTBE, (Multidimensional stormwater treatment in urban areas for cleaner Baltic Sea), is co-funded by the Interreg Central Baltic programme for 1.5.2023-30.4.2026.
  • The project budget is 3 980 476 €.

Writers and editors: Krista Valkonen, Hanna Kajander and Meri-Maaria Salo