Published on: 28.10.2019
Information produced by Finnish Environment Institute
What is stormwater?

Rain lashes the streets of the city, and the downpipes of houses are spewing out more water at street corners. The water flows along the street but soon gushes down through gaps in an iron grid. This is the point at which it becomes stormwater.
Stormwater is rain and drainage water removed from yards and streets, particularly in cities where the ground is covered with asphalt or paving.
In earlier times, stormwaters went down the same sewage pipes as wastewater, and this continues to be the case in the old sewage networks of cities. Separate wastewater and stormwater networks are built in new residential areas to reduce the water load of treatment plants and to prevent the flooding of sewers.
Stormwater brings its own problems, as it may pick up contaminants in the streets: de-icing salt, oil, heavy metals and animal waste. Rainstorms are becoming more common and may also test the system to the limit. This is why particular attention has been paid to stormwater management in recent years.
Stormwater accumulation can be reduced at the source: if yards and car parks are left unpaved, rainwater is infiltrated into the ground and does not need to be conveyed away. Plant cover, including green roofs, increases evaporation. Stormwaters can also be retained, giving the solid particles carried by them time to settle at the bottom of a basin, which also removes most harmful substances from the water. Brooks, ponds and stormwater wetlands slow down the water on its journey towards a river or lake.
Image: iStockphoto