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Impervious Surface Coverage the Emergence of a Key Environmental-Indicator

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Impervious Surface Coverage the Emergence of a Key Environmental-Indicator , Impervious Surface Coverage The Emergence of a Key Environmental-Indicator Chester L. Arnold, Jr. and C. James Gibbons has only recently emerged as an environmental indicator. Natural re• Impervioussource planningland coverusing hasimperviouslong beensurfacecharacte~isticcoverage ofas aurbanframeworkareas, butcan Planners concerned with water re• be a pragmatic and effective way of addressing a host of complex urban source protection in urbanizing areas environmental issues, particularly those related to the health of water re• must deal with the adverse impacts of sources. polluted runoff. Impervious surface coverage is a quantifiable land-use in• Water resource protection at the local level is getting more compli• dicator that correlates closely with cated, largely due to.the recognition of nonpoint source pollution, or pol• these impacts. Once the role and dis• luted runoff, as a major problem. This diffuse form of pollution, now the tribution of impervious coverage are understood, a wide range of strategies nation's leading threat to water quality (Environmental Protection to reduce impervious surfaces and Agency 1994), is derived from contaminants washed off the surface of the their impacts on water resources can land by stormwater runoff, and carried either ooectly or indirectly into be applied to community planning, waterways or groundwater. As programs directed at non point source con• site-level planning and desi&", and land use regulation. These strategies trol cascade down from federal to state to local governments, the techni• complement many current .trends in: cal complexities involved with such control are further complicated by planning, zoning, and landscape de• regulatory and management considerations. sign that go beyond water pollution Stormwater runoff problems are nothing new to local land-use concerns to address the quality of life in a community. decision-makers. However, the principal concern about runoff has always been safety, with the focus on directing and draining water off of paved Arnold is a Water Quality Educator, surfaces as quickly and efficiently as possible. Once off the road and out and Gibbons a Natural Resource Plan• of sight, stormwater has been largely out of mind-dGwnstream conse• ning Educator, at the University of Connecticut Cooperative Extension quences be damned (or dammed). Regulations haye been expanded in re• System. They are currently principals cent years to include consideration of flooding and erosion, yet these in the NEMO Project, which uses geo• factors fall£1.r short of a comprehensive and effective approach to mitigat• graphic information system technol• ogy to educate municipal land-use ing the water quality impacts of development. decision-makers about non point How do planners and other local officials get a handle on protecting source water pollution. their local water resources? While no magic bullet exists to simplify all the complexities involved, an indicator is emerging from the scientific Journal of the American Planning Association, Vol. 62, No.2, Spring literature that appears to have all the earmarks of a useful tool for local , 996. cAmerican Planning planners-the amount of impervious, or impenetrable, surface. This a.r• Association, Chicago, Il. ticle reviews the scientific underpinning, usefulness, and prac~ical appli- APA JOURNAL' SPRING 1?96 /243 CHESTER L. ARNOLD, JR. AND C. JAMES GIBBONS ca.tion of impervious surface coverage as an urban early twentieth century ascendancy of the automobile environmental indicator. over the railways,capped by the mid-century massive construction of the intersta~e highway system, which People, Pavement and Pollution served to both stimulate and facilitate the growth of Impervious surfacescan be defined as any material suburbia. From that point on, imperviousness became that prevents the infiltration of water into the soil. synonymous with human presence-to the point that While roads ~d rooftops are the most prevalent and studies have shown that an are'lS'spopulation density easily identified types of impervious surface, other is correlated wit.h.its percentage of impervious cover types include sidewalks,patios, bedrock outcrops, and (Stankowski 1972). compacted soil. As development alters the natural Impervious surfaces not only indicate urbaniza• landscape, the percentage of the land covered by im• tion, but also are major contributors to the environ• pervious surfaces increases.. mental impacts of urbanization. As the natural Roofs and roads havebeen around for a long time, landscape is paved over, a chain of events is initiated but the ubiquitous and impervious pavement we take that typically ends in degraded water resources. This for granted today is a relativelyrecent phenomenon. A chain begins with alterations in the hydrologic cycle, nationwide road census showed that in 1904, 93 per• the way that water is transpcir~d .and stored. cent of the roads in America were unpaved (South• These changes, depic~ed ~n figure 1, have long worth and Ben-Joseph 1995). This changed with the been understood by geologists and hydrologists. As 38% EVAPO- TRANSPIRATION 1I.lI[lll.llflllllllllJII 21%SlW.LOW 21% DEEP INFILTRATION INFILTRATION - 10-20 % IMPERVIOUS SURFACE PIRATION 35% EVAPO-TRANSPIRATION IIJJnlfllOffJ1lJlfl1fli nnrr 10%SlW.LOW 20% S~UOW 0 /) 0 INFILTRATION 0 Cl S% OEEP INALTRATION 0 Q 00 v0 ~V _ ••..I-i INFILTRATION1S% OEEP o (l 0 0~ 0(l INFILTRATION ° (?O'tJ°0 ~o,o~ 0 ~ 00°0 0 0 00 00 ~ 0 00 o o~ v 0 35-50 % IMPERVIOUS SURFACE 75-100 % IMPERVIOUS SURFACE FIGURE 1. Water cycle changes associated with urbanization Source: Environmental Protection Agency 1993a IAI APA JOURNAL·SPRlNG 1996 , \, I .' ''-~'; . .. ,-.- - .. "." ..- ... -;'..~'. :. IMPERVIOUS SURFACE COVERAGE ,=- impervious coverage increases, the velocity and vol• pollutants tend to adhere to eroded soil particles (En• ume of surface runoff increase, and there is a corre• vironmental Protection Agency 1992, 1993a). sponding decrease in infiltration. The larger volume of The results of polluted runoff are evident in every runoff and the increased efficiency of water convey• corner of the United States. According to the Environ• ance through pipes, gutters, and artificially straight• mental Protection Agency (1994), nonpoint source ened channels result in increased severity of flooding, pollution is now the number one cause of water qual• with storm flows that are greater in volume and peak ity impairment in the United St~tes, accounting for more rapidly than is the case in rural areas (Carter the pollution of about "40% of all waters sur• 1961; Anderson 1968; Leopold 1968; Tourbier and veyed across- the nation. The effects of non point Westmacott 1981). The shift away from infiltration re• source pollution on coastal waters and their living re• duces groundwater recharge, lowering water tables. sources have been of particular concern (U.S. House of This both threatens water supplies and reduces the Representatives 1988; Environmental Protection groundwater contribution to stream flow, which can Agency 1993a). Urban runoff alone ranks as the sec• result in intermittent or. dry stream beds during low ond most common source of water pollution for lakes flow periods (Dunne and-leopold 1978; Harbor 1994). and estuaries nationwide, and the third most common Hydrologic disrup-tion givl:s rise to physical and source for rivers (Environ,mental Protection Agency ecological impacts. Enhanced runoff causes increased 1994). :- erosion from construction sites, downstream areas As point source pollution is increasingly brought and stream banks. The increased volume of water and under control, the true impact of urban non point sediment, combined with the "flashiness" of these source pollution is being recognized. For instance, peak discharges, result in wider and straigpter stream even in an urbanized estuary like Long Island Sound, " channels (Arnold, Boison, and Patton 1982). Loss of where the major environmental problems have been tree cover leads to greater water temperature fluctua• strongly linked to point source discharges from sewage tions, making the water warmer in the summer and treatment plants, an estimated 47% of the pathogen colder in the winter (Galli 1991). There is substantial contamination is from urban runoff (Long Island loss of both streamside (riparian) habitat through ero• Sound Study 1994). sion, and in-stream habitat as the varied natural stream bed of pebbles, rock ledges, and deep pools is covered by auniform blanket of eroded sand and silt Imperviousness as an. (Schueler 1992). Engineered responses to flooding like Environmental Indicator stream diversion, channelization, damming, and pip• Planners wishing to protect their community's wa• ing further destroy stream beds and related habitats ter resources against these threats may not know like ponds and wetlands. Finally, with more intensive where to begin. The site-specific and diffuse nature of land uses comes a corresponding increase in the gener• polluted runoff seems to demand extensive technical ation of pollutants. Increased runoff serves to trans• information on pollutant loadings, hydrologic model• port these polluta!\ts directly into waterways, creating ing, and the effectiveness of various management nonpoint source pollution, or polluted runoff. practices. This informa'tion'is difficult to acquire, not Major categories of..n:onpoint source pollutants in• only because of the cost of such studies, but because clude pathogens (disease-causing microorganisms), nonpoint-source-related
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