Stormwater means water draining from rooftops or roads, snow melting waters or leaching waters led to purification plants or running in ditches or pipelines into streams, rivers, lakes or the sea. In the natural state, most stormwaters and snow melting waters get absorbed by the ground or used by plants. The rest percolates through purifying soil layers to groundwater basin or the nearest waterway.

Stormwaters end up either in sewage treatment plants or waterways, where:

  • Heat and impurities degrade water quality in receiving bodies and impoverish ecosystems.
  • Receiving waters become eutrophic and their hygienic condition deteriorates.
  • Banks and river beds erode.

Kaupunkivesistöt kuntoon is a project launched by the cities of Espoo and Helsinki to promote recovery of city waters in 2017–2019. The project seeks to improve water quality in Otsolahti in Espoo and Maunulanpuro in Helsinki by purifying stormwater through natural means before it enters waterways. Otsolahti, located in Tapiola, is a shallow, almost enclosed bay, which over the years has become increasingly shallow and eutrophic.

Application for exemption from joining stormwater system

The Espoo stormwater system covers almost the entire densely built-up area covered by the town plan. In the more loosely constructed areas in the western and northern parts of the city, there is no continuous stormwater system.

According to the Act on Water Services (119/2001), a property located inside a water supply plant’s area of operation has to be connected to the water, sewage and stormwater system of the said plant. The Espoo Environment Department may, however, based on an application and on the grounds stipulated in Section 17c of the Act on Water Services, exempt a property from joining the stormwater system as per section 17b of the Act.

Stormwaters and construction

The increasingly dense urban centres result in more paved areas, which in turn adds to stormwater quantities. Stormwater related problems will increase, unless we are able to counter them with efficient, functional solutions. Construction-based erosion increases the nutrient and solid matter load in water bodies, which can be fatal, especially to small stream and river ecosystems.

Stormwaters and floods

Stormwater quality varies greatly depending on the rainfall and the season. Climate change adds to the problems faced by water supply and sewerage – while floods and downpours are becoming more intense and heavier, droughts are also getting longer.