In a natural Virginia woodland or meadow, very little rainfall runs off. During development, natural vegetation is usually removed and replaced with hard surfaces such as roads, buildings and parking areas. This land surface change decreases infiltration, groundwater recharge and evapotranspiration, and it increases runoff.
Stormwater runoff is water flowing overland into surface waters or that which is channeled into natural or man-made conveyance systems during and after rainfall or during snowmelt. Unmanaged stormwater can cause erosion and flooding. It can also carry excess nutrients, sediment and other contaminants into our waters. Properly managed stormwater protects our lands from erosion, properties from flooding, waters from pollutants, and ensures our general health, safety and welfare.
- After the Storm: A Citizen's Guide to Understanding Stormwater (PDF) from the United States Environmental Protection Agency
- Taking Care of Stormwater: A BayScapes Guide (PDF) from the Alliance for the Chesapeake Bay
Traditional definitions of storm water have usually characterized it as nonpoint source runoff. However, most urban and industrial storm water is discharged through conveyances, such as separate storm sewers, ditches, channels or other conveyances which are considered point sources under the Clean Water Act, and subject to regulation through the National Pollutant Discharge Elimination System (NPDES) permit program.
During construction, a permit may be required for erosion and sediment control. A stormwater permit may be required to discharge stormwater from a construction activity. Such a permit may also be required to discharge stormwater through a stormwater conveyance system owned or operated by a government entity.
Quantity of Stormwater Runoff
Compared with impervious surfaces, such as pavement or rooftops, pervious surfaces, such as meadows and woodlands, absorb and filter rainfall and reduce runoff. When meadows and woodlands are developed, the increase in impervious surfaces increases the amount of runoff that occurs when it rains. This increase in runoff can overwhelm waterways, causing erosion, localized flooding and property damage.
Quality of Stormwater Runoff
Pervious and impervious surfaces in urban areas collect pollutants, such as automobile oil, grease, sediment, bacteria from animal waste, excess nutrients and pesticides, and deposits from airborne pollutants. Stormwater runoff with high concentrations of these pollutants may enter nearby drinking water supplies and waterways when it rains.
For more information:
- Code of Virginia: Virginia Stormwater Act (PDF)
- Virginia Administrative Code: Virginia Stormwater Management Program Permit Regulations (PDF)