,
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 research and engineering are nutrients, toxic contaminants, and debris. Pathogen new and evolving fields. contamination indicates possible health hazards, re• Enter impervious surfaces. When doing sulting in closed beaches and shellfish beds. Over• community-level planning, or where detailed site in• abundance of nutrients such as nitrogen and formation is unavailable, impervious coverage may of• phosphorous can threaten well water supplies, and in ten be the most feasible and cos.t::effective vehicle for surface waters can lead to algal "blooms" that, upon addressing water pollution. Two major factors argue decaying, rob the waters of life-sustaining oxygen. for its potential utility to the local planner. Toxic contaminants like heavy metals and pesticides First, imperviousness is integrative. As such, it can pose threats to the health of aquatic organisms and estimate or predict cumulative water resource impacts their human consumers, and are often persistent in without regard to specific factors, helping to cut the environment. Debris, particularly plastic, can be through much of the intimidating complexity sur• hazardous to animal and human alike, and is an aes• rounding nonpoint source pollution. Although imper• thetic concern. Sediment is also a major non point vious surfaces do not generate pollution, they: (1) are source pollutant, both for its effects on aquatic ecol• a critical contributor to the hydrologic changes that ogy and because of the fact that many of the other degrade waterways; (2) are a major com,ponent of the
APA JOURNAL' SPRING 1996 1245 ·'
. CHESTER L. ARNOLD, JR. AND C. JAMES GIBBONS
intensive land uses that do generate pollution; (3) pre• different criteria-pollutani"loadS, habitat quality, vent natural pollutant processing in the soil by pre• aquatic species diversity and abundance, and other venting percolation; and (4) serve as an efficient factors. In a recent review of these studies, Schueler conveyance system transporting pollutants into the (1994a) concludes that "this research, conducted waterways. It is not surprising, then, that research in many geographic areas, concentrating on many dif• from the past 15years consistently shows a strong cor• ferent variables, and employing widelydifferent meth• relation between the imperviousness of a drainage ba• ods, has yielded a surprisingly similar conclusion• sin and the health of its receivingstream (Klein 1979; stream degradation occurs at relatively low levels of Griffin 1980; Schueler 1987; Todd 1989; Schueler imperviousness (1O-.?O%)"{lOO).Recent studies also 19~2;Booth and Reinfelt 1993;Schueler 1994a). suggest that this thr~shold applies to wetlands health. Figure 2 is a stylized graph of this general rela• Hicks-(1995)found a well-defined inverse relationship tionship, showing stream health decreaSing with in• between freshwater wetland habitat quality and im• creasing impervious coverage of the watershed, or pervious surface area, 'with wetlands suffering impair• drainage basin, of the stream. The horizontal lines ment once the imperviousness of their local drainage mark average threshold values of imperviousness at basin exceeded 10%. Impervious coverage, then, is which degradation first occurs (10%),and at which both a reliable and integrative indicator of the impact degradation becomes so severe as to become almost of development on water resources. unavoidable (30%).These thresholds serve to create The -second fact~r in favor of the use of impervi• three broad categories of stream health, which can be ousness is that it is nfeaSurable.This enhances its util• roughly characterized as "protected" (less than 10%), ity both in plinning and regulatory applications. "impacted" (10%-30%),and "degraded" (over 30%). (Examples follow in a later section.) Depending on the Thresholds are alwayscontroversial and subject to size of the area being considered and the particular change, yet it is important to note that to date, the application being applied, a wide range of tech• threshold of initial degradation in particular seems to niques-with a wide range of price tags-exists for the be remarkably consistent. The scientific literature in• measurement of impervious coverage. cludes studies evaluating stream health using many For site level applications, on-site measurement
50% w ~ ffi 40% o> () C/) •••••••••••••••••••• 111111.1.1 I I I I I I I I I I I I I I 5 30% > ex: w a. ~ 2-0% o IW C/) ex: 10% I,I'S"I"".'.II ••• "'11'11'1"" ""1 W ~ 3: o PROTECTED IMPACTED DEGRADED
STREAM HEALTH
FIGURE 2. Stylized relationship of imperviousness to stream health Modiped from Schueler 1992
2461 APA JOURNAL· SPRING 1996 IMPERVIOUS SURFACE COVERAGE
using surveying equipment (sometimes as basic as a unable to afford GIS, and others·have-been disillu• tape measure) is the most accurate and appropriate sioned at its cost and complexity once they invested in method. On the neighborhood level, "windshield" sur• it. Evolution of the technology, however, is making veys may be appropriate where it is less important to GIS more accessible to local officials every day. have exact numbers. For community- or regional-scale areas, land cover derived from aerial photographs pro• The Components of vides perhaps the best compromise between accuracy Imperviousness and cost. Finally, for applications encompassing even To measute and use impervious coverage as a tool larger areas, remotely-sensed satellite-based land cover for protecting water resources, it i~ necessary to know can be a viable option. At present, impervious esti• how imperviousness is dis'tribured abour the land• mates bas'ed on satellite data must be calculated by scape. On a scale of increasing refinement, impervious applying literature values of imperviousness to satel• coverage can be broken down by land use, by function lite land cover categories. We are currently involved within each land use, and by its relative impact on run• with a remote sensing research project at the Univer• off. Each of these pieces of the puzzle can help to tar• sity of Connecticut that is attempting to devise a get planning and/or regulatory approaches to method for directly estimating imperviousness from reducing impervious coverage. As with measurement satellite images (Civco ann-Arnold 1994). techniques, the extent to"which planners need detailed It is important to note that all of these methods information on these components depends on the of measurement are increasingly being digitized and particular application.- presented in the form of computerized maps in a geo• The percentage oflanclcovered by impervious sur• graphic information system, or GIS. This trend even• faces varies significantly with land use. The most fre• tually will make the information easier to acquire, quently cited estimates come from a report by the Soil. often at lower expense. Many communities have been Conservation Service (1975) (figure. 3). "Strip" type
~ c ~c.Q~=- 100605040 tSIII -~~ 30 20 807010090
~OZr'-lU~E-...l-~-U...l ~;J ~ 1 Z~::I:~-.~1/21/80<~1/31/4 Eo- Residential Lot Size (acres) r'-l-Z< -U ~
FIGURE 3. Average percentage of impervious coverage by land use Source: Soil Conservation Service 1975
APA JOURNAL' SPRING 1996 1247
·r -, CijESTI;:R L. ARNOLD, JR. AND C. JAMES GIBBONS
commercial development tops the chart at around tivelynew to the scientific community, but existing in• 95%coverage,with other business areas and industrial formation supports the commo~ri:·senseassumption development lagging slightly behind. In residential that some land uses are more contaminating than oth• areas, there is a wide range of imperviousness that var• ers; for instance, runoff from gasoline stations con• ies predictably with lot size, going from about 20%in tains extremelyhigh levelsof hydrocarbons and heavy one-acre zoning to as high as 65% in one-eighth-acre metals (Schueler 1994b). zomng. Recent research from Wisconsin goes one major The City of Olympia, Washington, recently con• step further, actually determining the pollutant con• ducted a thorough study of impervious coverage in centrations from specificcategories of impervious sur• their area. For 11 sites measured, they found coverage faces.Using micro-monitOl:ingsamplers that collected values similar to the SCS values, finding four high• the runoff from 12 diffe?ent types of surfaces (e.g., density residential developments (3-7 units/acre) to roofs, streets; parking lots, lawns, driveways) in resi• average 40% impervious, four multifamily develop• dential, commercial, and industrial areas, Bannerman ments (7-30 units/acre) to average 48% impervious, et al. (1993) were able to show distinct differences in and three commercial/industrial sites to average 86% the types and amounts of certain pollutants, de• impervious coverage (City of Olympia 1995) (table 1). pending on the source of the runoff. The study clearly In addition to the relationship between land use identified streets as the impervious surfaces having and the total amount of impervious coverage,studies the highest pollutant loads for most land-use catego• show that all land uses are not equal with regard to ries (table 2).- Roofs, wit~ the exception of the zinc the levels of contaminants present in the runoff. As from industrial roofs;wer(generally low in pollutant noted, pollutant or land-use-specific studies are rela- loads, while parking rot~ had surprisingly moderate
TABLE 1. Site coverage for three land uses in Olympia, Washington
Commercialn/a03532604138660 (7-30053111151748(65%Y1934 units/ 16030654254015 Adapted from City of Olympia 1995" (70%) Multifamilyacre) Average Approximate(3-7(63%)units/acre)n/a Site Coverage, % 4.6.(Road-related2.3.5.Road-relatedOpenSidewalksRoofsLawns/landscapingParking/drivewaysspaceimperviousas a percentage.surfaceofSurface(1-3)total Coverage Type Residential imperviousHighcoverage)Density Total impervious1. Streets surface (1-4)
TABLE 2. Surfaces exhibiting highest levels of runoff~bome pollutants, out of twelve surface types sampled in selected urban areas in Wisconsin
POLLUTANT SURFACE Highest levels Second highest levels Third highest levels
e. coli (pathogens) residential feeder streets residential collector streets residential lawns solids (sediment) industrial collector streets industrial arterial streets residential feeder streets total phosphorous residential lawns industrial collector streets residential feeder streets zinc industrial roofs industrial arterial streets commercial arterial streets cadmium industrial collector streets industrial arterial streets commercial arterial streets copper industrial collector streets industrial arterial streets residential collector streets
Adapted from Schueler 1994d
d APAJOURNAL·SPRING 1996 1.. IMPERVIOUS SURFACE COVERAGE
levels of pollutants. The one unpaved surface moni• creasingly, environmental and natl.!,ral.Xe5ourceprofes• tored, residential lawns, showed high levels of phos• sionals recommend planning based on the phorous, presumably from lawn and garden fertilizers. organization of natural systems (Environmental Pro• As this study is augmented by others over time, reli• tection Agency 1993c). Ecosystems as an organiza• able relationships between pollurant loads and spe• tional unit have been suggested, but the functional cific landscape components will undoubtedly emerge. definition of an ecosystem remains elusive. Impervious cover can be further broken down into A more promising trend has been toward using its functional components. Schueler (1994a) and oth• watersheds as planning units (Environmental Protec• ers point out the two major categories of impervious tion Agency 1993b). A watershed,; .or drainage basin, is surface: rooftops, and the transport system (roads, an area that drains to a c6mmonbody of water, be it 'parking'lots, driveways, sidewalks). In general, the a lake, river, stream, aquifer, or bay. Watersheds have transport system is the dominant component, rein• an advantage in that they can be clearly defined as geo• forcing the concept of an automobile-centric society. graphic units. In addition, the watershed can be used In the Olympia study, for ins rance, the transportation as a system of organization at any number of scales, component ranged from 63% for single-family residen• from a major basin encompassing several states, to a tial development to 70% for commercial development regional basin involving several municipalities, to a (City of Olympia 1995}=(table 1). local sub-basin on the neighborhood level. One last refinement of the impervious component Thinkingjn terms of watersheds is particularly ap• is its relationship in the landscape to surrounding propriate for stormwater'=-m-anagement, which, after areas, in the sense of how much of the rainfall onto a all, is all about drainage. At the University of Connect• given surface is actually conveyed to a stream or icut, we have developed a regionalfcommunity-Ievel storm water collection system. In general, the rooftop planning approach that provides an example of the component, which often drains to a lawn or other per• use of both watersheds and impervious coverage. The meable areas, has less impact than roadways, which Nonpoint Education for Municipal Officials (NEMO) typically channel runoff directly to the stormwater project was initiated in 1991 to assist communities in system. The Olympia study (1994b) calls this factor dealing with the complexities of polluted runoff man• the effectiveness at producing runoff, and estimates im• agement (Arnold et al. 1993). The project, funded by pervious areas in low-density residential develop• the United States Department of Agriculture's Coop• ments to be about 40% effective, while those in erative State Research, Education and Extension Ser• commercial/industrial areas are close to 100% effec• vice, is run by an interdiscipfinary team that includes tive. In theory this concept could be applied to all sur• water quality, natural resource planning, and com• faces-lawns themselves, for ins rance, can have a puter technology expertise. NEMO uses geographic significant coefficient of runoff-but to our knowl• information system (GIS) technology as a tool to edu• edge this level of refinement has not been researched, cate local land-use decision-makers about the links be• nor is it generally needed for most applications. tween their town's land use and its water quality. Natural resource information on waterways and wa• Imperviousness in Planning: A tersheds is combined widr..satellite-derived, land-cover information, and then displayed on colorful maps cre• Framework,.Some Examples ated with the GIS.. - By considering tlle distribution of impervious At the heart ofNEMO is an analysis of impervious cover by land use, fun€tion, and contribution to run• cover. Literature values for the percentage of impervi• off, strategies begin to emerge for the reduction of ous cover are applied to satellite land-cover categories both current and future levels of imperviousness. We to come up with rough estimates for the current level suggest that these strategies can be grouped into three of imperviousness within a town or watershed. These basic categories: community or regional planning; values are averaged and displayed by local drainage ba• neighborhood and site planning, and regulation. Each sin (average area about one square mile) and catego• category presents opportunities to revisit the status quo rized according to the protected/impacted/degraded with an eye to water resource protection. Following are scale of increasing imperVious cover previously de• some general concepts and specific examples of such opportunities. scribed and shown in figure 2. The current values are then contrasted with a zoning-based, build-out analy• sis of imperviousness, again displayed by local sub• Planning at the Community or Regional Level basin (figure 4). The build-out allows town officials a Land-use planning, even at the town level, need look into the possible future of their town, not in con• not be based on traditional political boundaries. In- ventional terms of population or lot coverage, but in
APA JOURNAL· SPRlNG 1996 1249 CnESTER L. ARNOLD, JR. AND C. JAMES GIBBONS
Existing levels based on satellite land cover Projected levels based on zoning build-out analysis N Percent Impervious D Oto 14.9"10 D Local Basins _ 15% to 29.9"10 o 1 Miles + DOpenWater _ Greater than 30%
FIGURE 4. Impervious coverage analysis for Old Saybrook, CT
terms of impervious cover-and by inference, the water trout fishery has 'prompted researchers from health of their local water resources. Grand Valley State University to design a watershed• The results of the impervious surface analysis can based GIS decision support system for local land-use be used to help guide planning emphasis within each authorities (Frye and Denning 1995). The system local basin area. For areas in the lower impervious makes use of a number of hydrologic and land-use fac• zone, emphasis should be placed on preventive mea• tors, including impervious surface estimates and zon• sures that retain existing natural systems, using tech• ing-based build-out analyses. In Montgomery County, niques like open space planning and stream buffers. Maryland, a detailed pl1l.nning study was done to for• For areas that are in, or will be in, the "impacted" (10• mulate a land-use strategy to protect the water re• 30%) zone, prev~ntive planning should be accompa• sources of the Paint Branch stream (Montgomery nied by a foc!,ls on_site design considerations that County MD 1995). The study both measures and proj• reduce runoff and imperviousness. Finally, for areas at ects future impervious surface coverage by subwater• (or climbing into) the "degraded" (over 30%) zone, the shed basin, and uses this information to help guide its focus shifts to remediation through pollutant mitiga• recommendations for protective actions. tion and resource restoration. Each of these efforts contains the elements of im• NEMO is one example of the use of impervi• pervious cover, subbasin-level analysis, and build-out ousness for broad-based community or regional water projections. An even more comprehensive treatment is resource planning. Similar approaches are beginning that undertaken by the City of Olympia, Washington. to spring up around the country. Schueler (1994a) rec• During 1993 and 1994, Olympia conducted their Im• ommends watershed-based zoning that "is based on pervious Surface Reduction Study (ISRS), from which the premise that impervious cover is a superior mea• information is cited repeatedly in this paper. The ISRS sure to gauge the impacts of growth, compared to Final Report (City of Olympia 1995) contains an im• population density, dwelling units or other factors." pressive and comprehensive body of research, policy In Alpine Township, Michigan, concern about the ef• analysis, and build-out scenarios, culminating in 19 fects of urbanization on a formerly productive cold- specific action recommendations. The s~udy concludes
2501 APAJOURNALoSPRING 1996 , . IMPERVIOUS SURFACE COVERAGE
that "a 20% reduction [in future impervious cover] pacted soil. Construction s~uld becsequenced with is a feasible and practical goal for Olympia and will this goal in mind, and it may be-necessary later to not require exceptional changes in the Olympia com• loosen compacted areas and/or cover them with addi• munity." The recommended reduction is equal to ap• tional pervious materials (CrauI1995). proximately 600 fewer acres of impervious coverage From construction, we move to reduction. For vir• by the year 2012. Planners wishing to see an example tually all land uses, one of the best design-related op' of a comprehensive approach to reducing impervi• portunities for reducing imperviousness is through' ousness would do well to read the Olympia ISRS the reduction of road widths. As has been seen, roads report. both constitute a major fractiol) of a community's im• As with other natural resource protection efforts, pervious coverage, and tend- to produce the most community and watershed-level planning approaches pollutant-laden runoff. like these are often the most effective way of achieving The long-established concept of road hierarchies, results. Addressing the issue at this scale provides an which relates road size .to the intensity of use, has overall perspective and rationale for the design and many positive aspects beyond water quality, among regulatory tools described in the following sections. them cost reductions and aesthetic benefits. Yet Site-level considerations are then based not only on Southworth and Ben-Joseph (1995), in a recent article the immediate impa~s of a given development on the on the history of residential street design, found that, local stream or pond, but also on the site's incremen• for a varie_ty of historical and institutional reasons, tal contribution to the pollution (or protection) of a road hierarchies are often 0verlooked by local planners larger-scale water body or aquifer. Review of site de• and commissions. The authors conclude that an over• sign and storm water management plans, for instance, emphasis on traffic control has resulted in a "rigid, can be checked for consistency with goals for the ap• over-engineered approach ... deeply embedded in en• propriate watershed. gineering and design practice." Simple math dictates Providing this broad context has the added benefit that for a given length of subdivision road, reduction of allowing for greater flexibility at the site level. Plan• from a typical 32-foot to a 20-foot width results in a ners can evaluate individual factors like a site's loca• 37.5% reduction in pavement, or over 63,000 square tion within the watershed, its land use, and the feet (about one and one-half acres) per linear mile. The relative priority of the receiving stream as they relate Olympia study estimated that changing the width of to the overall plan, rather than applying a rigid and local access roads from 32 to 20 feet would result in uniform set of requirements to all parcels. an overall 6% reduction in imperviousness for a given development site in their region, that is, six acres less Site-Level Planning street pavement for a typical 100-acre subdivision Site planning is perhaps the least-explored ap• (City of Olympia 1994b). proach to reducing water pollution. Kendig (1980) Road surface reduction is a primary reason why states that "good design begins with an analysis of the clustering is the most pavement-stingy residential de• natural and environmental assets and liabilities of a sign. Large-lot subdivisions, which have long been rec• site," and that these factors should be the determi• ognized as being anti APA JOURNAL-SPRING 1996 1251 .,1 · CHESTER L. ARNOLD, JR. AND C. JAMES GIBBONS 1; o 100 200 :lOC1 CLUSTER SUBDIVISION , I STANDARD SUBDIVISION Site area = 20 acres Site area = 20 acres 1/4 acre lots 1/2 acre lots Density = 1.45 DU/Acre Density = 1.45 DU/Acre Total impervious surface = 10.7%(2.14 acres) Total impervious surface = 17.51.(3.5 acres) Lot impervious surface = 141. Lot impervious surfa~ = 13.7% Designated open space = 10 acres Designated open space = 2.6 acres Road length = 2,127 feet (22' width) Road length = 2,390 feel; (30' width) FIGURE 5. Clustering reduces overall site imperviousness. Source: John Alexopoulos, University of Connecticut feet of office floor space is often almost twice what is age can be attained in ways other than adjusting park• actually needed. Using a generic, medium-sized office ing supply ratios. ShoulL (1995) suggests that parking building as a hypothetical example, he shows that a can be reduced through economic incentives that ef• typical parking supply ratio of 3.8 results in an extra fectively end the subsidy-provided by employer-paid 55,000 square feet of parking lot, compared to using a parking. Employee commuter option programs, man• more factually~basecfratio of 2.5. dated by the Clean Air Act Amendments of 1990 in The City of Olympia found not only parking over• areas of "severe nonattainment" for ozone standards, supply, with vacancy rates of 60-70%, but also devel• hold some promise for reducing parking demand. The opers consistently building parking above minimum Olympia study (City of Olympia 1994d) concluded ratios, with 51%more parking spaces at their 15 survey that sharing, joining, or coordinating parking facilities sites than were required by zoning (City of Olympia can reduce parking significantly. Finally, vertical gar• 1994c). This agrees with our observation that, at least ages (above or below ground) can be encouraged, al• in Connecticut, overbuilding of parking appears to be though this alternative" can be expensive. Many of a recent trend with "big box" retail store developers, these strategies were recently combined in an innova• who typically require at least 5 spaces per 1,000 square tive office park design in Lacey, Washington, where the feet, principally to meet peak demands on weekends new 360,000-square-foot headquarters of the state De• and during the busy period from Thanksgiving to partment of Ecology was designed around a "parking Christmas. diet" that slashed parking spaces from 1500 to 730 Reductions in parking-related impervious cover- (Untermann 1995). 2521 APA JOURNAL' SPRING 1996 ------IMPERVIOUS SURFACE COVERAGE Imperviousness also has a role in design related initiatives. Parking is one example.: Exce~s parking can comitigation of polluted runoff. "Best management be attacked from many angles other than water qual• practices" (BMPs) is the most commonly-used term to ity, including air quality, traffic congestion, promotion describe the wide range of on-site options available to of sprawl, and inefficient use of building lots. A park• manage stormwater runoff. BMPs are often divided ing reduction initiative could be combined with a plan into two major types: those involving structures such to use the recouped paved area either for active as stormwater detention ponds or infiltration stormwater treatment (infiltration basins, detention trenches, and "nonstructural" practices that usually ponds) or for more modest stormwater management involve use of vegetated areas to buffer, direct, and (vegetated strips). Such a strategycould be combined otherwise break up the sea of asphalt. Maintenance with the creation of "veS't pocKet" parks and other measures like road sand sweeping and storm drain green spaces, shown by urban forestry research as hav• cleaning are also included. ing positive sociological and psychological effects on It is not within the scope of this article to give a city dwellers (Gobster 1992; Schroeder and Lewis thorough discussion of these practices; choosing the 1992). correct assemblage is a combination of art and science, Research on the pollutant-processing capability of and involves many cOf!siderations. From the stand• various types of vegetation suggests a slight twist on point of imperviousness, however, BMPs can be parking lot design thar may reap large benefits in wa• viewed in terms of how well they replicate the natural ter quality for urban areas. Parking lots often incorpo• hydrological functioning of the site. This perspective rate landscaped APA JOURNAL-SPRING 1996 1253 CHESTER L. ARNOLD, JR. AND C. JAMES GIBBONS ' .. FIGURE 6, Sunken vegetated parking lot "islands" intercept and treat runoff. Source: John Alexopoulos,Universityof Connecticut growth around existing centers. They concluded that cially effective in achieving local environmental objec• for these communities, the less consumptive pattern tives for stormwater management and groundwater resulted in savings of $28.8 million in local road costs, recharge." Performance zoning has the added effect of $9.1 million in annual water treatment costs, $8.3 mil• encouraging mixed uses, which generally result in less lion in annual sewer treatment costs, as well as an 8.4% impervious coverage and less pollution, by reducing reduction in overall housing costs, and a 6.9% savings roads and vehicle traffic. in annual costs of local public-sector services (Bur• Community-wide applications encompassing chell, Dolphin, and Moskowitz 1995). large areas with varied land use will require sliding scales of impervious coverage limits that depend on The Use of Imperviousness for Regulation the location, size, and tyPe of use. Such standards have Planning approaches at the community and site been in place in some Flor.i~a communities for almost level can be complerrlented with specific applications a decade (American Planning Association Zoning that give regulatory -teeth to planning objectives. To News 1989). More recently, ordinances limiting imper• begin with, planners can revisit their current zoning vious cover have been enacted in Austin and San Anto• and subdivision requirements with an eye to impervi• nio, Texas, driven by concern about pollution of the ousness. For instance, many lot coverage limits, partic• area's major drinking water aquifer (City of Austin ularly for residential uses, refer to rooftops but do not 1992; City of San Antonio 1995). include parking space, sidewalks, and driveway cov• In instances where protection of a particularly im• erage. portant resource is desired, strict limits on impervious Impervious cover lends itself well to zoning that coverage may be imposed. Such is the case in Bruns• uses performance standards. In fact, Kendig (1980) de• wick, Maine, where a "coastal protection" zone was fines performance zoning as that which regulates de• created for areas draining to Maquoit Bay, site of shell• velopment on the basis of four fundamental measures fish beds critically important to the town. The special of land-use intensity, one of which is the impervious zone has certain stringent performance standards, surface ratio. Jaffe (1993), in a critical assessment of among them a maximum impervious-surface lot cov• performance-based zoning, concludes that "Kendig's erage of 5%. This coverage includes "... buildings, recreational and impervious surface ratios are espe- roads, driveways, parking areas, patios, and other simi- 2541 APAJOURNALoSPRING 1996 IMPERVIOUS SURFACE COVERAGE lar surfaces" (Town of Brunswick 1991). In this case, 1995). The Kitsap County, Washington! Comprehen• the very low impervious limit was feasible because the sive Surface and Stormwater Management Program, total area affected was fairly small, the use was largely established in 1994, creates a rate structure based on residential, and the specific pollutant of concern was an "equivalent service unit" equal to the average esti• nitrogen emanating from septic systems, resulting in mated amount of impervious surface area on a single• zoning that called for a minimum lot size of one unit family residential parcel (Kitsap County 1994). per five acres. This "down-zoning" approach, which Such programs not only raise funds for mitigation has also been used in the Buttermilk Bay area in Mas• of adverse impacts, but also, by attaching a cost to im• sachusetts (Horsley and Witten 1991), is practicable perviousness, provide an economic incentive to reduce for small areas with septic-related concerns, but if ap• it. Apparently, this effec~,is beginning to be seen in 'plied OVerlarge areas, can lead in the long run to pro• Florida, where the cost savings associated with lower motion of sprawl. stormwater utility fees have provided the impetus for Strict limits may be appropriate, yet in practice reduction of impervious cover during site redevelop• they can result in the need for complicated exemption ment (Livingston 1995). ' provisions, or even raise the specter of private prop• erty rights takings (Land Use Law and Zoning Digest 1995; Ross 1995; Setde,Washburn, and Wolfe 1995). Integrating Stormwater Control One method for "sof!ening" the concept oflimits is to into Community Planning allow for flexibility on the site level. In this scenario, The strategies descril2.eq above demonstrate that an ordinance setting a limit (or goal) for a site's imper• for the planner, imp~rviousness can provide a useful vious coverage would require more stringent on-site framework for addressing the impacts of urbanization stormwater treatment when the limit is exceeded. This on water resources. But the advantage of this approach type of approach will undoubtedly become more com• goes beyond any specific application. We have found mon as the information base on removal efficiencies that working with a town on water resource protection of various treatment measures expands. Another type often leads to related natural resource issues like open of flexibility comes from applying performance stan• space preservation and forest management. Our re• dards to specific elements of imperviousness within cent experience with NEMO has taught us that fram• the landscape. In their discussion of next steps, the ing water issues largely in terms of imperviousness Olympia study (City Of Olympia 1995) cites the devel• serves to expand the range of these connections. opment of performance-based standards for side• Once water pollution is,linked to impervious cov• walks, parking, and landscaping "to encourage erage and its various components, it has a way of in• innovation and provide flexibility in meeting impervi• sinuating itself into issues currently "on the table" in ous surface reduction goals." town. Road widths and curbing may be subjects of One practical regulatory application of impervi• town debate about cost or neighborhood character. ous coverage is for stormwater utility assessment, an Parking and landscaping requirements for commercial "impact fee" that is growing in use in urban areas of zones may be undergoing reexamination for aesthetic the country as a way of paying for the treatment and reasons. The appropriateness of "big box" retailers control of polluted runoff. Impervious surface has may be a hot topic, with~arguments centered around long been a key determinant in mathematical models traffic congestion and the impact on local merchants. that predict the volume of runoff from a given piece An open space plan may be in the formative stages, or ofland. Storm water utllity assessments have ~aken the the use of stream buffers being questioned. Citizens lead from these models in using imperviousness as a may be interested in naturalistic lan'dscaping, water basis for a utility rate structure that fairly distributes conservation, or volunteer monitoring of local water• the cost of treatment according to a property's contri• ways. These typical local debates, drawn from towns bution to runoff. working with the NEMO Project, now have elements Such systems are now in place in many areas, in• of water quality and impervious surface reduction as cluding Kansas City, Missouri; Kitsap County, Wash• part of the mix. And through tnese debates, the sub• ington; and throughout the state of Florida. This type ject of water quality in the community is extended be• of application requires a community-wide assessment yond land-use-related staff and boards to include of impervious coverage, and a wide range of tech• engineering and public works departments, land niques is being used. In Kansas City, rate structures trusts and other nonprofits, and citizens. are based on digitized high-resolution orthorectified Cross connections of this type are an important aerial photos (Murphy 1995), while in Florida they are key to ensuring the implementation of any planning based on statistical surveys of area lots (Livingston initiative. For the professional planner, they create APAJOURNALoSPRlNG 1996 1255 ••• CHESTER L. ARNOLD, JR. AND C. JAMES GIBBONS opportunities to reinforce complementary planning the issue of non point source pollution in terms of im• concepts from several different angles. Beyond the perviousness, although it may be a bit simplistic, well-established concept of planning and designing appears to be an effective way of enabling local with nature (McHarg 1969), there are many relatively decision-makers to grasp the issue sufficiently to take recent themes in transportation, subdivision design, action. and landscape architecture that go hand-in-glove with Conclusion the reduction of impervious surfaces. Performance zoning is one example. Another is neotraditional resi• Water pollution is getting more complex, while at dential design, which champions styles of develop• the same time the resPQPsibility for water resource .ment patterned after the traditional New England protection is shifting toward local authorities. The use village in order to foster a sense of community (Duany of impervIOus surface coverage as an environmental and Plater-Zyberk 1991). The open space subdivision indicator can assist planners to construct a game plan designs promoted by Arendt (1994b) for land conser• to protect their communitY's natural resources. vation are also a good fit. On another front, residential Imperviousness integrates the impacts of develop• street layouts promoting "traffic calming" for a variety ment on water resources, so it can help to cut through of safety, aesthetic, and sociological benefits (Hoyle much of the complexity. It is measurable, and so ap• 1995; Ben-Joseph 1995) could easily incorporate pave• propriate for a wide range of planning and regulatory ment reduction. Landscape architects are calling for applications.-It is a cross·fu~ting feature that is a fre• more naturalistic schemes that follow the natural con• quently hidden, but_nonetheless substantial, compo• tours and make use of low-maintenance, drought• nent of many current trends in road, neighborhood, resistant plants (Ash 1995). Planners should seize the and landscape design, so it can be used as a reinforcing opportunity to "piggy-back" water quality with these connection between seemingly unrelated planning ini• complementary initiatives, making sure to explicitly tiatives. Finally, the basic tenets of reducing impervi• incorporate the reduction of paved surfaces and their ousness-retaining the natural landscape, minimizing impacts into official policy, plans, and procedures. pavement, promoting infiltration to the soil-are sim• The other advantage of the cross-cutting nature of ple concepts that can b~ understood by a community and its residents. water resource protection in general, and impervi• ousness specifically, is that it seems to make sense to Impervious cover is rarely specifically identified or the average citizen. Reduction of paved areaS is one addressed in community· goals, policies, or regula• tions. It should be. In this article, we have tried to fa• of relatively few planning initiatives that "plays" at all levels, from the suburban driveway to the big box cilitate the use of this indicator by (1) reviewing the parking lot, and even to the Chief Justice of the Su• scientific literature to provide a comfort level with its preme Court, who recognized the link between the appropriateness; (2) creating a framework for its use growth of paved surfaces and increased runoff (in Do• in overall planning, site-level planning, and regula• lan v. City a/TIgard) (Merriam 1995). tion; and (3) providing real-world examples of such ap• From our standpoint as educators, this feature is plications. 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