Impervious surfaces

Impervious surfaces are mainly artificial structures—such as pavements (roads, sidewalks, driveways and parking lots) that are covered by impenetrable materials such as asphalt, concrete, brick, stone—and rooftops. Soils compacted by urban development are also highly impervious. Homer and others (2007) indicate that about 76 percent of the conterminous United States is classified as having less than 1 percent impervious cover, 11 percent as having an impervious cover of 1 to 10 percent, 4 percent of the nation as having an estimated impervious cover of 11 to 20 percent, 4.4 percent of the nation as having an estimated impervious cover of 21 to 40 percent, and about 4.4 percent of the nation as having an estimated impervious cover greater than 40 percent

Impervious surfaces are an environmental concern because, with their construction, a chain of events is initiated that modifies urban air and water resources:

The total coverage by impervious surfaces in an area, such as a municipality or a watershed is usually expressed as a percentage of the total land area. The coverage increases with rising urbanization. In rural areas, impervious cover may only be one or two percent. In residential areas, coverage increases from about 10 percent in low-density subdivisions to over 50 percent in multi-family communities. In industrial and commercial areas, coverage rises above 70 percent. In regional shopping centers and dense urban areas, it is over 90 percent. In the contiguous 48 states of the USA, urban impervious cover adds up to 43,000 square miles (110,000 km²) — an area nearly the size of the State of Ohio. Continuing development adds another quarter of a million acres (1,000 km²) each year. Typically two-thirds of the cover is pavements and one-third is building roofs.

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