Stormwater refers to rainwater and melted snow that flows over roads, parking lots, lawn and other sites in urban areas. Stormwater management practices help to minimize the impact of polluted runoff flowing into lakes and streams, and reduce the strain that stormwater places on municipal infrastructure.
Under natural conditions, stormwater is intercepted by vegetation and then absorbed into the ground and filtered and eventually replenishes aquifers or flows into streams and rivers. Later, part of it is returned to the atmosphere in the form of evapotranspiration. In urbanized areas, however, impervious surfaces such as roods and roofs prevent precipitation from naturally soaking into the ground.
Free stormwater management planning tool available from TRCA’s Sustainable Technologies Evaluation Program
Instead, the water runs rapidly into storm drains, municipal sewers and drainage ditches into streams, rivers and lakes and on its way it picks up pesticides, road salts, heavy metals, oils, bacteria, and other harmful pollutants and transports them through municipal sewers into streams, rivers and lakes. The sheer force and volume of polluted runoff causes:
- Increased downstream flooding risksRiver bank and bed erosion.
- Increased turbidity.
- Aquatic Habitat destruction.
- Changes in the stream flow regime.
- Combined sewer overflows.
- Infrastructure damage.
- Contaminated streams, rivers, and coastal water.
In order to mitigate the undesirable impacts of urbanization on watercourses and associated infrastructure, stormwater management practices need to be implemented:
- To preserve the natural hydrologic balance in newly developing areas and re-establish it, wherever possible, in already developed areas.
- To protect and enhance quality of stormwater discharged to lakes and streams.
- To reduce the volume and frequency of combined sewer overflows in older urban areas.