What is stormwater?

When it rains or snow melts, it creates stormwater in a built-up environment. Stormwater is rainwater and melt water which flows off from the ground, building roofs and from other similar surfaces.

The more impermeable surface there is in the area, for instance asphalt, the faster and the more stormwater is generated. Stormwater results especially from melting snow in the spring, from heavy rains in the summer and from rains in the autumn.

Not all flowing water is stormwater. In areas which are in a natural state, water flowing into a stream is natural run-off. 

Stormwater runs into stormwater sewers and open trenches towards water bodies, such as streams, lakes or the sea. As a rule, stormwater is not cleaned but it is discharged into the water body untreated.

It is forbidden to discharge stormwater to the wastewater sewer. If it is conveyed to the sewer meant for wastewater, the result might be a sudden excess of water in the sewer network during heavy rainfall. This means that the risk of an overflow in the wastewater sewer network increases. Stormwater also cools the wastewater and reduces the treatment efficiency at the wastewater treatment plant. 

Liability distribution concerning the stormwater management

In the Helsinki Metropolitan Area, the liability distribution concerning the stormwater management is as follows:

  •  The city is responsible for the management of stormwater as a whole. For example, the city prepares a stormwater programme and takes care of the stormwater management in public areas, such as street drainage and flood routes. It also takes care of the city’s stormwater system, such as open canals and detention structures. 
  •  HSY is responsible for constructing the stormwater main drain network and its maintenance. 
  •  The owner or holder of a property is liable for building drainage and management of stormwater generated on the property, for instance by detaining and infiltrating stormwater on the property if possible. Stormwater must be conveyed in a manageable manner to the stormwater system managed by HSY or the city. The owner or holder of the property has an obligation to get connected to the stormwater sewer if the property location is in the stormwater sewerage area. 

Read more about the stormwater management on a property  »

Read more about stormwater sewerage  »