Stormwater is runoff from rain or snowmelt that does not find its way into the ground or gets lost due to evaporation.
Stormwater can be retained or detained until such time as it can be led off to natural outlets like rivers, lakes or others.
In a detention system, the runoff is temporarily stored until it is allowed to drain slowly into receiving channels. In a retention system, the excess water is retained until it can be used for gardening, irrigation, washing or other purposes. This retention is on a permanent basis and the only losses come from evaporation and absorption. Wet ponds that often occur naturally are prime examples of devices for stormwater retention. You can also build tanks of a more permanent nature that can act as reservoirs for the water and act as a source of water for the community. Storm chambers can act as both retention and detention stormwater management systems. They can be installed underground allowing the surface above to be used in any way desired.
Stormwater retention systems act to control the increase in peak runoff during storms and can be of great help in stormwater management. In these management systems are developed to control the flow of water based on the anticipated runoff during rains based on the expected intensity of rains. These intensities are based on weather statistics maintained over a large period of time and will be designated in inches of rain per hour. The catchment area being served by the system and the rain intensity will indicate the volume of stormwater that needs to be retained, and the period for which it has to be retained until its use or disposal through any outlets, will determine the volume of the stormwater detention device that can be ponds, chambers, large pipes or others.
Captured runoff in stormwater retention systems can be absorbed in surrounding soil that then recharges groundwater sources, or can be pumped back for use in irrigation, gardens or other uses. A lot of the runoff from storms can contain pollutants and you can also use the retention systems to treat and filter the water till it can be considered safe for disposal into natural water sources. Aeration and the use of plants and other biodevices can render the water retained to become safe for diversion into natural water sources. These systems can also be used to reduce flooding.
The need to manage stormwater runoff has led to innovations like permeable pavements that allow the water to slowly infiltrate into the ground and can be used for roadways that have a low intensity of traffic, driveways, parking lots and walkways for pedestrians. Interlocking pavers, pervious concrete, and porous asphalt are types of these pavements that act as stormwater runoff devices.
Urban communities must live in balance with the climate and local environment and stormwater retention has to play a large part in ensuring this. Retention can be enhanced by the use of green roofs, infiltration systems, rainwater harvesting along with traditional pipe and conveyance systems. Any system developed must be appropriate to the climatic conditions prevailing in that region. Cites have designed these systems to cater for a large part of their drinking water supply, thus combining stormwater management with reducing their dependence on other water sources.
Low Impact Development or LID combines technology with the needs of any urban development and can go a long way into making communities that have a reduced impact on water resources, while still protecting them from the ill effects of extreme weather like storms that can lead to flooding and other forms of devastation.