Stormwater and its management

Stormwater refers to the surface runoff of rainwater and meltwater formed in built-up areas. Drainage water from foundations is also counted as storm water. Runoff from undeveloped areas is not stormwater, but runoff from natural areas.

The more compactly the area is built and the larger part of the catchment area's surfaces are impervious to water, the more and faster surface runoff, i.e. stormwater, forms. As the urban structure becomes denser and the plot size decreases, an ever larger part of the ground is covered under buildings and asphalt surface. The proportion of the ground surface that absorbs water decreases and, in addition, the number of trees that evaporate water decreases, so surface runoff increases.

In built-up areas, population, traffic and industrial activity cause emissions that reduce the quality of stormwater. Harmful substances end up in stormwater from, among other things, traffic exhaust gases, road wear, corrosion of vehicles and building materials, animal excrement, sewer overflows, and pesticides.

The previously conventional method of directing stormwater away from built-up areas in sewers as efficiently as possible directly into waterways weakens the quality of receiving waterways, causes erosion and the risk of a decrease in groundwater resources, and changes the hydrology of built-up areas. Today, in stormwater planning, even more attention is paid to the local treatment of stormwater, which aims, among other things, to prevent the resulting pollution of waterways, reduce the load on the stormwater drainage network, improve the conditions for groundwater formation, and prevent erosion in urban streams and other drainage channels.