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. 

Picture: The impermeable surface has an impact on stormwater generation and infiltration into the soil. 

a) Water evaporates.

b) There are less surface run-offs in nature, whereas in a built-up urban area more stormwater is generated. 

c) Water infiltrates into the soil.

Stormwater management in a city

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. 

Stormwater and wastewater have separate sewers

The residential areas in the Helsinki Metropolitan Area have a separate sewerage system, except for downtown Helsinki. This means that stormwater and wastewater have their separate sewers.

A property must be connected to a stormwater sewer if such is available in the area. Also the drainage water from building foundations, i.e. underdrain water, is conveyed to the stormwater network. It is forbidden to convey stormwater to the wastewater sewer. If stormwater is conveyed to the wastewater sewer, so much water may suddenly enter the sewer network during heavy rainfall that the wastewater pumping stations cannot pass it any further. The water amount might momentarily grow exponentially compared to the normal situation. This means that the risk of an overflow in the wastewater sewer network increases and untreated, diluted wastewater as such might get into the water body. Stormwater also cools the wastewater and reduces the treatment efficiency at the wastewater treatment plant.

The property can apply for an exemption from the obligatory stormwater sewer connection from the municipal environmental protection authority. Possible grounds for granting an exemption can include for example, the following reasons: 

  • Infiltration of stormwater and other treatment on the property so that the treatment does not cause any harm to the neighbours.
  • Conveying stormwater into a stream or an open trench in a manner that the management of a property’s stormwater does not exploit the HSY stormwater network at all. 

Read more about the stormwater management on a property  »

Read more about stormwater sewerage  »