Stormwater refers to precipitation in the form of rain and snowmelt.   As the water runs off over lawns and impervious surfaces, such as roads and rooftops, 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 flooding risks and channel erosion that degrades aquatic habitat and limits recreational uses of water bodies.

The adverse impacts of stormwater runoff in urban areas are primarily a result of the sudden and dramatic effect of increased impervious cover on the local hydrologic balance.  Under natural conditions, most of the rainwater infiltrates through organic rich soils and is returned to the atmosphere in the form of evapotranspiration.  When buildings and roads are constructed, the land is stripped and soils are compacted by heavy machinery.  Natural cover is replaced by impervious surfaces resulting in less evapotranspiration and substantially higher surface runoff.

The practice of stormwater management attempts to mitigate the undesirable impacts of urbanization on watercourses and associated infrastructure by:

(i) preserving the natural hydrologic balance in newly developing areas and re-establishing it, wherever possible, in already developed areas;
(ii) protecting and enhancing quality of stormwater discharged to lakes and streams; and
(iii) reducing the volume and frequency of combined sewer overflows in older urban areas.

The most effective means of achieving these objectives is by adopting a water balance approach to stormwater management, which involves implementing practices that mimic pre-development hydrologic conditions.

A large number of different stormwater management practices have been devised to address the impacts of urbanization.  These practices are typically classified into source, conveyance and end-of-pipe controls, and are best implemented in series as part of a ‘treatment train’.