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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 〔Homer, C., Dewitz, J., Fry, J., Coan, M., Hossain, N., Larson, C., Herold, N., McKerrow, A., VanDriel, J.N., and Wickham, J., 2007, Completion of the 2001 National land cover database for the conterminous United States: Photogrammetric Engineering and Remote Sensing, v. 73, no. 4, p. 337-341.〕〔Granato, G.E., 2010, ''Overview of Methods Used to Estimate Imperviousness in a Drainage Basin'' Appendix 6 in ''Methods for development of planning-level estimates of stormflow at unmonitored sites in the conterminous United States'': Federal Highway Administration, FHWA-HEP-09-005 ("Available on-line." )〕 ==Environmental effects of impervious surfaces== Impervious surfaces are an environmental concern because, with their construction, a chain of events is initiated that modifies urban air and water resources: * The pavement materials seal the soil surface, eliminating rainwater infiltration and natural groundwater recharge. From a recent article in the Seattle Times: "While urban areas cover only 3 percent of the U.S., it is estimated that their runoff is the primary source of pollution in 13 percent of rivers, 18 percent of lakes and 32 percent of estuaries."〔Cappiello, Dina. ("Report: EPA Failing to stop Sprawl Runoff." ) Seattle Times, 16 Oct. 2008〕 :Some of these pollutants include excess nutrients from fertilizers; pathogens pet waste; gasoline, motor oil, and heavy metals from vehicles; high sediment loads from stream bed erosion and construction sites, and waste such as cigarette butts, 6-pack holders and plastic bags carried by surges of stormwater. In some cities, the flood waters get into combined sewers, causing them to overflow, flushing their raw sewage into streams. Polluted runoff can have many negative effects on fish, animals, plants and people. * Impervious surfaces collect solar heat in their dense mass. When the heat is released, it raises air temperatures, producing urban "heat islands", and increasing energy consumption in buildings. The warm runoff from impervious surfaces reduces dissolved oxygen in stream water, making life difficult in aquatic ecosystems. * Impervious pavements deprive tree roots of aeration, eliminating the "urban forest" and the canopy shade that would otherwise moderate urban climate. Because impervious surfaces displace living vegetation, they reduce ecological productivity, and interrupt atmospheric carbon cycling. 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.〔Schueler, Thomas R. ("The Importance of Imperviousness." ) Reprinted in (''The Practice of Watershed Protection.'' ) 2000. Center for Watershed Protection. Ellicott City, MD.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Impervious surface」の詳細全文を読む スポンサード リンク
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