Urban Storm Water Runoff
Quantity of Stormwater Runoff
Pervious surfaces, such as meadows and woodlands, absorb and infiltrate rainfall hence generate little runoff. Urban landscape typically covers such areas with impervious surfaces, such as pavement and rooftops. These impervious surfaces generate runoff every time it rains. (A typical city block generates nine times more runoff than a woodland area of the same size!) The quantity of runoff from these areas quickly overwhelms natural channels and streams, often causing channel erosion, localized flooding and property damage.
Quality of Stormwater Runoff
The pervious and impervious surfaces in the urbanizing landscape collect pollutants such as automobile oil, grease, brake pad dust, sediment from construction sites, bacteria from animal waste, excess lawn care fertilizers and pesticides, as well as atmospheric deposition of phosphorus, nitrogen and other airborne pollutants. Rainfall washes these surfaces so the initial flush of runoff can carry high concentrations of these pollutants to nearby drinking water supplies, waterways, beaches and properties. Pollution washed from the land surface by rainfall is called nonpoint source pollution.