PROTECTING WATER RESOURCES


WITH HIGHER-DENSITY DEVELOPMENT

Acknowledgements 

The principal author, Lynn Richards, from the U.S. Environmental Protection Agency’s 

Development, Community, and Environment Division, would like to recognize people who 

contributed insights and comments on this document as it was being developed: Chester 

Arnold, University of Connecticut—Non-Point Source Education for Municipal Officials; John 

Bailey, Smart Growth America; Deron Lovaas, Natural Resources Defense Council; Bill 

Matuszeski, formerly with EPA Chesapeake Bay Program; Philip Metzger, EPA Office of Water; 

Rosemary Monahan, EPA Region 1; Betsy Otto, American Rivers; Joe Persky, University of 

Illinois at Chicago; Milt Rhodes, formerly with the North Carolina Department of Environment 

and Natural Resources; and William Shuster, EPA Office of Research and Development. 

Additional recognition is extended to EPA staff from Office of Water (Robert Goo, Jamal Kadri, 

and Stacy Swartwood) as well as staff at EPA’s Development, Community, and Environment 

Division (Geoffrey Anderson, Mary Kay Bailey, and Megan Susman). 

To request additional copies of this report, contact EPA’s National Service Center for 

Environmental Publications at 800-490-9198 or by email at ncepimal@one.net and ask for 

publication number 231-R-06-001. 

To access this report online, visit <www.epa.gov/smartgrowth> or <www.smartgrowth.org>. 

Front cover photos: 

Left: The Snake River flows outside Jackson, Wyoming. Photo courtesy of USDA NRCS. 

Top right: Rosslyn-Ballston Corridor, Arlington County, Virginia. Arlington County 

Department of Community Planning, Housing, and Development received a 2002 

National Award for Smart Growth Achievement in the Overall Excellence category for 

its planning efforts in the Rosslyn-Ballston Corridor. Photo courtesy of Arlington County. 

Middle right: People gather at Pioneer Square in Portland, Oregon. Photo courtesy 

of US EPA. 

Back cover photos: 

Top left: This hillside in Northern California is covered by wildflowers. This open space 

provides habitat to wildlife as well as serving important watershed services. Photo courtesy of USDA NRCS. 

Middle left: A family enjoys open space in central Iowa. Photo courtesy of USDA NRCS. 

Bottom left: A stream flows through western Maryland. Photo courtesy of USDA NRCS. 

Right: This redevelopment site in Arlington, Virginia, which includes stores, apartments 

townhomes, single family homes, parking garages, and a one-acre public park, was formerly a large department store surrounded by surface parking. Photo courtesy of US EPA. 

i 
ii 

Dear Colleague: 

We are excited to share with you the enclosed report, Protecting Water Resources with 

Higher-Density Development. For most of EPA’s 35-year history, policymakers have focused 

on regulatory and technological approaches to reducing pollution. These efforts have met 

with significant success. But, the environmental challenges of the 21st century require new 

solutions, and our approach to environmental protection must become more sophisticated. 

One approach is to partner with communities to provide them with the tools and information necessary to address current environmental challenges. It is our belief that good environmental information is necessary to make sound decisions. This report strives to meet 

that goal by providing fresh information and perspectives. 

Our regions, cities, towns, and neighborhoods are growing. Every day, new buildings or 

houses are proposed, planned, and built. Local governments, working with planners, citizen 

groups, and developers, are thinking about where and how this new development can 

enhance existing neighborhoods and also protect the community’s natural environment. 

They are identifying the characteristics of development that can build vibrant neighborhoods, rich in natural and historic assets, with jobs, housing, and amenities for all types of 

people. They are directing growth to maintain and improve the buildings and infrastructure 

in which they have already invested. 

In addition to enjoying the many benefits of growth, communities are also grappling with 

growth’s challenges, including development’s impact on water resources. In the face of 

increasing challenges from non-point source pollution, local governments are looking for, 

and using, policies, tools, and information that enhance existing neighborhoods and protect 

water resources. This report gives communities a different perspective and set of information 

to address the complex interactions between development and water quality. 

Protecting Water Resources with Higher-Density Development is intended for water quality professionals, communities, local governments, and state and regional planners who are grappling with protecting or enhancing their water resources while accommodating growing 

populations. We hope that you find this report informative as your community strives to 

enjoy the many benefits of growth and development and cleaner water. 

For additional free copies, please send an e-mail to ncepimal@one.net or call (800) 490-9198 

and request EPA publication 231-R-06-001. If you have any questions concerning this study, 

please do not hesitate to contact Lynn Richards at (202) 566-2858. 

Sincerely, 

Ben Grumbles Brian F. Mannix 

Assistant Administrator Associate Administrator 

Office of Water Office of Policy, Economics, and 

Innovation 
PROTECTING WATER RESOURCES


WITH HIGHER-DENSITY DEVELOPMENT


iii 
Contents 

EXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

IMPACTS FROM DEVELOPMENT ON 

WATERSHED FUNCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

CRITICAL LAND USE COMPONENTS FOR 

PROTECTING WATER QUALITY FOR BOTH 

LOW- AND HIGH-DENSITY DEVELOPMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

LOW-DENSITY DEVELOPMENT—CRITIQUING 

CONVENTIONAL WISDOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

TESTING THE ALTERNATIVE: CAN COMPACT 

DEVELOPMENT MINIMIZE REGIONAL WATER 

QUALITY IMPACTS?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

THE MODEL AND DATA INPUTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

SUMMARY OF SCENARIOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

FINDINGS/DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

OTHER RESEARCH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

REFERENCES AND BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

iv 
Exhibits 

1: WATERSHED SERVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2: SUMMARY OF SCENARIOS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

3: TOTAL AVERAGE ANNUAL STORMWATER RUNOFF 

FOR ALL SCENARIOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

4: EACH SCENARIO ACCOMMODATES EIGHT HOUSES . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

5: 10,000-ACRE WATERSHED ACCOMMODATING 

10,000 HOUSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

6: 10,000-ACRE WATERSHED ACCOMMODATING 

DIFFERENT NUMBERS OF HOUSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

7: 80,000 HOUSES ACCOMMODATED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

8: TIME SERIES BUILD-OUT ANALYSIS: 

BUILD-OUT IN 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

9: TIME SERIES BUILD-OUT ANALYSIS: 

BUILD-OUT IN 2020 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

10: TIME SERIES BUILD-OUT ANALYSIS: 

BUILD-OUT IN 2040 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

11: SUMMARY OF FINDINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

EXAMPLES 

1: ONE-ACRE LEVEL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

2: LOT LEVEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

3: WATERSHED LEVEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

4: REMAINING LAND IN THE WATERSHED DEVELOPED . . . . . . . . . . . . . . . . . . . . . . . . . 18 

5: ACCOMMODATING THE SAME NUMBER OF HOUSES . . . . . . . . . . . . . . . . . . . . . . . . . 20 

6: TIME SERIES BUILD-OUT ANALYSIS: 

BUILD-OUT IN 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

7: TIME SERIES BUILD-OUT ANALYSIS: 

BUILD-OUT IN 2020 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

8: TIME SERIES BUILD-OUT ANALYSIS: 

BUILD-OUT IN 2040 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

v 
Executive Summary 

Growth and development expand communities’ opportunities by bringing in new residents, 

businesses, and investments. Growth can give a community the resources to revitalize a 

downtown, refurbish a main street, build new schools, and develop vibrant places to live, 

work, shop, and play. However, with the benefits come challenges. The environmental 

impacts of development can make it more difficult for communities to protect their natural 

resources. Where and how communities accommodate growth has a profound impact on the 

quality of their streams, rivers, lakes, and beaches. Development that uses land efficiently and 

protects undisturbed natural lands allows a community to grow and still protect its 

water resources. 

The U.S. Census Bureau projects that the U.S. population will grow by 50 million people, or 

approximately 18 percent, between 2000 and 2020. Many communities are asking where and 

how they can accommodate this growth while maintaining and improving their water 

resources. Some communities have interpreted water-quality research to mean that low-density development will best protect water resources. However, some water-quality experts 

argue that this strategy can backfire and actually harm water resources. Higher-density development, they believe, may be a better way to protect water resources. This study intends to 

help guide communities through this debate to better understand the impacts of high- and 

low-density development on water resources. 

To more fully explore this issue, EPA modeled three scenarios of different densities at three 

scales—one-acre level, lot level, and watershed level—and at three different time series 

build-out examples to examine the premise that lower-density development is always better 

for water quality. EPA examined stormwater runoff from different development densities to 

determine the comparative difference between scenarios. This analysis demonstrated: 

• The higher-density scenarios generate less stormwater runoff per house at all scales—


one acre, lot, and watershed—and time series build-out examples; 


• For the same amount of development, higher-density development produces less


runoff and less impervious cover than low-density development; and


• For a given amount of growth, lower-density development impacts more of the


watershed. 


Taken together, these findings indicate that low-density development may not always be the 

preferred strategy for protecting water resources. Higher densities may better protect water 

quality—especially at the lot and watershed levels. To accommodate the same number of 

houses, denser developments consume less land than lower density developments. 

Consuming less land means creating less impervious cover in the watershed. EPA believes 

that increasing development densities is one strategy communities can use to minimize 

regional water quality impacts. To fully protect water resources, communities need to employ 

a wide range of land use strategies, based on local factors, including building a range of 

development densities, incorporating adequate open space, preserving critical ecological 

and buffer areas, and minimizing land disturbance. 

1 
Protecting Water Resources with Higher-Density Development


Introduction 

Growth and development expand communities’ opportunities by bringing in new residents, 

businesses, and investments. Growth can give a community the resources to revitalize a 

downtown, refurbish a main street, build new schools, and develop vibrant places to live, 

work, shop, and play. However, with the benefits come challenges. The environmental impacts of development can make it more difficult for communities to protect their natural 

resources. Where and how communities accommodate growth has a profound impact on the 

quality of their streams, rivers, lakes, and beaches. Development that uses land efficiently and 

protects undisturbed natural lands allows a community to grow and still protect its 

water resources. 

The U.S. Census Bureau projects that the U.S. population Which is a better strategy will grow by 50 million people, or approximately 18 per- to protect water quality: 

cent, between 2000 and 2020. Many communities are 

asking where and how they can accommodate this low- or high-density 

growth while maintaining and improving their water development? 

resources. Some communities have interpreted waterquality research to mean that low-density development will best protect water resources. 

However, some water-quality experts argue that this strategy can backfire and actually harm 

water resources. Higher-density development, they believe, may be a better way to protect 

water resources. This study intends to help guide communities through this debate to better 

understand the impacts of high- and low-density development on water resources. 

Virtually every metropolitan area in the United States has expanded substantially in land area 

in recent decades. According to the U.S. Department of Agriculture’s National Resources 

Inventory (NRI), between 1954 and 1997, urban land area almost quadrupled, from 18.6 million acres to about 74 million acres in the contiguous 48 states (USDA, 1997b). From 1982 to 

1997, when population in the contiguous United States 

grew by about 15 percent, developed land increased by Between 1954 and 1997, 

25 million acres, or 34 percent. Most of this growth is tak- urban land area almost 

ing place at the edge of developed areas, on greenfield quadrupled, from 18.6 mil- sites, which can include forestland, meadows, pasture, 

and rangeland (USDA, 1997a). Indeed, in one analysis of lion acres to about 74 

building permits in 22 metropolitan areas between 1989 million acres in the conand 1998, approximately 95 percent of building permits tiguous 48 states. were on greenfield sites (Farris, 2001). 

According to the American Housing Survey, 35 percent of new housing is built on lots 

between two and five acres, and the median lot size is just under one-half acre (Census, 

2001). Local zoning may encourage building on relatively large lots, in part because local 

governments often believe that it helps protect their water quality. Indeed, research has 

revealed that more impervious cover can degrade water quality. Studies have demonstrated 

that at 10 percent imperviousness, a watershed is likely to become impaired and grows more 

so as imperviousness increases (Arnold, 1996; Schueler, 1994). This research has prompted 

many communities to adopt low-density zoning and site-level imperviousness limits, e.g., 

establishing a percentage of the site, such as 10 or 20 percent, that can be covered by 

2 
impervious surfaces such as houses, garages, and driveways. These types of zoning and 

development ordinances are biased against higher-density development because it has 

more impervious cover. But do low-density approaches protect our water resources? 

This study examines the assumption that low-density development is always better for water 

quality.

1 EPA modeled stormwater runoff from different development densities at the site 

level and then extrapolated and analyzed these findings at the watershed level. Modeling 

results were used to compare stormwater runoff associated with several variations of 

residential density. 

Impacts from Development on Watershed Functions 

A watershed is a land area that drains to a given body of water. Precipitation that falls in the 

watershed will either infiltrate into the ground, evapotranspirate back into the air, or run off 

into streams, lakes, or coastal waters. This dynamic is described in Exhibit 1. 

EXHIBIT 1: Watershed Services 

Graphic courtesy of U.S. EPA 

As land cover changes, so does the amount of precipitation that absorbs into the 

ground, evaporates into the air, or runs off. 

A watershed may be large or small. The Mississippi River, for example, drains a one-millionsquare-mile watershed made up of thousands of smaller watersheds, such as the drainage 

basins of the creeks that flow into tributaries of the Mississippi. In smaller watersheds, a few 

acres of land may drain into small streams, which flow into larger streams or rivers; the lands 

drained by these streams or rivers make up a larger watershed. These streams support 

1 Stormwater runoff was used as a proxy for overall water quality. In general, the more stormwater runoff a region experiences, the more 

associated pollutants, such as total nitrogen, phosphorus, and suspended solids, will enter receiving waterbodies. 

3 
Protecting Water Resources with Higher-Density Development


diverse aquatic communities and perform the vital ecological roles of processing the carbon, 

sediments, and nutrients upon which downstream ecosystems depend. Healthy, functioning 

watersheds naturally filter pollutants and moderate water quality by slowing surface runoff 

and increasing the infiltration of water into soil. The result is less flooding and soil erosion, 

cleaner water downstream, and greater ground water reserves. 

Land development directly affects watershed functions. When development occurs in previously undeveloped areas, the resulting alterations to the land can dramatically change how 

water is transported and stored. Residential and commercial development create impervious 

surfaces and compacted soils that filter less water, which increases surface runoff and 

decreases ground water infiltration. These changes can increase the volume and velocity of 

runoff, the frequency and severity of flooding, and peak storm flows. 

Moreover, during construction, exposed sediments and construction materials can be 

washed into storm drains or directly into nearby bodies of water. After construction, development usually replaces native meadows, forested areas, and other natural landscape features 

with compacted lawns, pavement, and rooftops. These largely impervious surfaces generate 

substantial runoff. For these reasons, limiting or minimizing the amount of land disturbed 

and impervious cover created during development can help protect water quality. 

Critical Land Use Components for Protecting Water 

Quality for Both Low- and High-Density Development 

What strategies can communities use to continue to grow while protecting their water quality? 

Watershed hydrology suggests that three primary land use strategies can help to ensure adequate water resource protection: 

•	 Preserve large, continuous areas of absorbent open space; 

•	 Preserve critical ecological areas, such as wetlands, floodplains, 


and riparian corridors; and


•	 Minimize overall land disturbance and impervious surface associated 

with development. 

These approaches work because, from a watershed perspective, different land areas have different levels of ecological value. For example, a nutrient-rich floodplain has a higher ecological value than a grass meadow. Communities should view these strategies as basic steps to 

preserve watershed function and as the framework within which all development occurs. 

PRESERVING OPEN SPACE 

Preserving open space is critical to maintaining water quality at the regional level. Large, continuous areas of open space reduce and slow runoff, absorb sediments, serve as flood control, 

and help maintain aquatic communities. To ensure well-functioning watersheds, regions 

should set aside sufficient amounts of undisturbed, open space to absorb, filter, and store rainwater. In most regions, this undeveloped land comprises large portions of a watershed, filtering 

4 
Photo courtesy of USDA NRCS 

out trash, debris, and chemical pollutants before they enter a community’s water system. Open 

space provides other benefits, including habitat for plants and animals, recreational opportunities, forest and ranch land, places of natural beauty, and community recreation areas. 

To protect these benefits, some communities are preserving undeveloped parcels or regional 

swaths of open space. One of the most dramatic examples is the New York City Watershed 

Agreement. New York City, New York State, over 70 towns, eight counties, and EPA signed the 

agreement to support an enhanced watershed protection program for the New York City 

drinking water supply. The city-funded, multi-year, $1.4-billion agreement developed a multifaceted land conservation approach, which includes the purchase of 80,000 acres within the 

watershed as a buffer around the city’s drinking water supply. This plan allows the city to 

avoid the construction of filtration facilities estimated to cost six to eight billion dollars (New 

York City, 2002). 

PRESERVING ECOLOGICALLY SENSITIVE AREAS 

Some types of land perform watershed functions better than others do. Preserving ecologically important land, such as wetlands, buffer zones, riparian corridors, and floodplains, is critical for regional water quality. Wetlands are natural filtration plants, slowing water flow and 

allowing sediments to settle and the water to clarify. Trace metals bound to clay carried in 

runoff also drop out and become sequestered in the soils and peat at the bed of the marsh 

instead of entering waterbodies, such as streams, lakes, or rivers. Preserving and maintaining 

wetlands are critical to maintain water quality. 

In addition, strips of vegetation along 

streams and around reservoirs are 

important buffers, with wooded 

buffers offering the greatest protection. For example, if soil conditions are 

right, a 20- to 30-foot-wide strip of 

woodland removes 90 percent of the 

nitrates in stormwater runoff (Trust for 

Public Land, 1997). These buffer zones 

decrease the amount of pollution 

entering the water system. Tree and 

shrub roots hold the bank in place, 

preventing erosion and its resulting 

sedimentation and turbidity. Organic 

matter and grasses slow the flow of 

runoff, giving the sediment time to settle and water time to percolate, filter through the soil, 

and recharge underlying ground water. Research has shown that wetlands and buffer zones, 

by slowing and holding water, increase ground water recharge, which directly reduces the 

potential for flooding (Schueler, 1994). By identifying and preserving these critical ecological 

areas, communities are actively protecting and enhancing their water quality. 

Wetlands, such as this one in Butte County, California, provide 

critical watershed services for the region. 

5 
Protecting Water Resources with Higher-Density Development


Current construction practices generally disturb the entire 

development site, as shown by this site in Des Moines, Iowa. 

MINIMIZING LAND DISTURBANCE AND IMPERVIOUS COVER 

Minimizing land disturbance and impervious cover is critical to maintaining watershed 

health. The amount of land that is converted, or “disturbed,” from undeveloped uses, such as 

forests and meadows, to developed uses, such as lawns and playing fields, significantly 

affects watershed health. Research now shows that the volume of runoff from highly compacted lawns is almost as high as from paved surfaces (Schueler, 1995, 2000; USDA, 2001). 

This research indicates that lawns and other residential landscape features do not function, 

with regard to water, in the same way as nondegraded natural areas. In part, the difference 

arises because developing land in greenfield areas involves wholesale grading of the site and 

removal of topsoil, which can lead to severe erosion during construction, and soil compaction by heavy equipment. However, most communities focus not on total land disturbed, 

but on the amount of impervious cover created. 

Research has revealed a strong relationship between impervious cover 

and water quality (Arnold, 1996; 

Schueler, 1994; EPA, 1997). Impervious 

surfaces collect and accumulate pollutants deposited from the atmosphere, 

leaked from vehicles, or derived from 

other sources. During storms, accumulated pollutants are quickly washed off 

and rapidly delivered to aquatic systems. Studies have demonstrated that 

at 10 percent imperviousness,

2 a 

watershed is likely to become 

impaired (Schueler, 1996; Caraco, 1998; 

Montgomery County, 2000), the 

stream channel becomes unstable due to increased water volumes and stream bank erosion, 

and water quality and stream biodiversity decrease. At 25 percent imperviousness, a watershed becomes severely impaired, the stream channel can become highly unstable, and water 

quality and stream biodiversity are poor3 (Schueler, 2000). The amount of impervious cover is 

an important indicator of watershed health, and managing the degree to which a watershed is 

developed is critical to maintaining watershed function. 

Although the 10 percent threshold refers to overall imperviousness within the watershed, 

municipalities have applied it to individual sites within the watershed, believing that lower densities better protect watershed functions. Indeed, as mentioned earlier, some localities have 

gone so far as to create strong incentives for, or even require, low densities—with water 

resource protection as an explicit goal. These communities are attempting to minimize hard 

2 The 10 percent figure is not an absolute threshold. Recent studies have indicated that in some watersheds, serious degradation may begin 

well below 10 percent. However, the level at which watershed degradation begins is not the focus of this study. For purposes of our analysis, 

EPA uses the 10 percent threshold as an indicator that water resources might be impacted. 3 There are different levels of impairment. In general, when the term is used in EPA publications, it usually means that a waterbody is not meeting its designated water quality standard. However, the term can also imply a decline or absence of biological integrity; for example, the waterbody can no longer sustain critical indicator species, such as trout or salmon. Further, there is a wide breadth of levels of impairment, from 

waterbodies that are unable to support endangered species to waterbodies that cannot support any of the beneficial-use designations. 

Photo courtesy of USDA NRCS 

6 
surfaces at the site level. They believe that limiting densities within particular development sites 

limits regional imperviousness and thus protects regional water quality. The next section examines this proposition and finds that low-density development can, in fact, harm water quality. 

Low-Density Development—Critiquing 

Conventional Wisdom 

As discussed, studies have demonstrated that watersheds can suffer impairment at 10 percent 

impervious cover and that at 25 percent imperviousness, the watershed is typically considered 

severely impaired. Communities have often translated these findings into the notion that lowdensity development at the site level results in better water quality. Such conclusions often 

come from analysis such as: a one-acre site has one or two homes with a driveway and a road 

passing by the property. The remainder of the site is lawn. Assuming an average housing footprint of 2,265 square feet4 (National Association of Home Builders, 2001), the impervious 

cover for this one-acre site is approximately 35 percent (Soil Conservation Service, 1986). By 

contrast, a higher-density scenario might have eight to 10 homes per acre and upwards of 85 

percent impervious cover (Soil Conservation Service, 1986). The houses’ footprints account 

for most of the impervious cover. Thus, low-density zoning appears to create less impervious 

cover, which ought to protect water quality at the site and regional levels. However, this logic 

overlooks several key caveats. 

1.	 The “pervious” surface left in low-density development often acts like impervious surface. 

In general, impervious surfaces, such as a structure’s footprint, driveways, and roads, have 

higher amounts of runoff and associated pollutants than pervious surfaces. However, 

most lawns, though pervious, still contribute to runoff 

because they are compacted. Lawns are thought to Lawns still contribute to 

provide “open space” for infiltration of water. However, runoff because they are because of construction practices, the soil becomes 

compacted by heavy equipment and filling of depres- compacted and disturbed. 

sions (Schueler, 1995, 2000). The effects of this compaction can remain for years and even increase due to mowing and the presence of a 

dense mat of roots. Therefore, a one- or two-acre lawn does not offer the same infiltration 

or other water quality functions as a one- or two-acre undisturbed forest. Minimizing 

impervious surfaces by limiting the number of houses but allowing larger lawns does not 

compensate for the loss of watershed services that the area provided before development (USDA, 2001). 

2.	 Density and imperviousness are not equivalent. Depending on the design, two houses may 

actually create as much imperviousness as four houses. The impervious area per home 

can vary widely due to road infrastructure, housing design (single story or multistory), or 

length and width of driveways. To illustrate, a three-story condominium building of 10 

units on one acre can have less impervious surface than four single-family homes on the 

same acre. Furthermore, treatment of the remaining undeveloped land on that acre can 

4 The average house built in 2001 included three or more bedrooms, two and a half baths, and a two-car garage. 

7 
Protecting Water Resources with Higher-Density Development


vary dramatically between housing types. For example, in some dispersed, low-density 

communities, such as Fairfax County, Virginia, some homeowners are paving their front 

lawns to create more parking for their cars (Rein, 2002). 

3.	 Low-density developments often mean more off-site impervious infrastructure. Development 

in the watershed is not simply the sum of the sites within it. Rather, total impervious area 

in a watershed is the sum of site developments plus 

the impervious surface associated with infrastructure Water quality suffers not 

supporting those sites, such as roads and parking lots. only from the increase in 

Lower-density development can require substantially impervious surface, but also higher amounts of this infrastructure per house and 

per acre than denser developments. Recent research from the associated activihas demonstrated that on sites with two homes per ties: construction, increased 

acre, impervious surfaces attributed to streets, drive- travel to and from the develways, and parking lots can represent upwards of 75 opment, and extension of 

percent of the total site imperviousness (Cappiella, 

2001). That number decreases to 56 percent on sites infrastructure. 

with eight homes per acre. This research indicates 

that low densities often require more off-site transportation-related impervious infrastructure, which is generally not included when calculating impervious cover. 

Furthermore, water quality suffers not only from the increase in impervious surface, but 

also from the associated activities: construction, increased travel to and from the development, extension of infrastructure, and chemical maintenance of the areas in and surrounding the development. Oil and other waste products, such as heavy metals, from 

motor vehicles, lawn fertilizers, and other common solvents, combined with the increased 

flow of runoff, contribute substantially to water pollution. As imperviousness increases, so 

do associated activities, thereby increasing the impact on water quality. 

4.	 If growth is coming to the region, limiting density on a given site does not eliminate that 

growth. Density limits constrain the amount of development on a site but have little 

effect on the region’s total growth (Pendall, 1999, 

2000). The rest of the growth that was going to come Growth is still coming 

to the region still comes, regardless of density limits in to a region, regardless 

a particular place. Forecasting future population of density limits in a 

growth is a standard task for metropolitan planning particular place. organizations as they plan where and how to accommodate growth in their region. They project future 

population growth based on standard regional population modeling practices, where 

wage or amenity differentials, such as climate or culture (Mills, 1994)—and not zoning 

practices such as density limits—account for most of a metropolitan area’s population 

gain or loss.

5 While estimates of future growth within a particular time frame are rarely 

precise, a region must use a fixed amount of growth to test the effects of adopting 

5 The most widely-used such model—the REMI® Policy InsightTM model—uses an amenity variable. However, even this is implemented as an 

additional change in the wage rate. See Remi Model Structure. <www.remi.com/Overview/Evaluation/Structure/structure.html>. The inhouse model used by the San Diego Association of Governments is an advanced example of the type used by councils of governments 

around the country.<www.sandag.cog.ca.us/resources/demographics_and_other_data/demographics/forecasts/index.asp>. 

8 
different growth planning strategies because it still must understand the economic, 

social, and environmental impacts of accommodating a growing population. Absent 

regional coordination and planning, covering a large part of a region with density limits 

will likely drive growth to other parts of the region. Depending on local conditions, water 

quality may be more severely impaired than if the growth had been accommodated at 

higher densities on fewer sites. 

Testing the Alternative: Can Compact Development 

Minimize Regional Water Quality Impacts? 

To more fully understand the potential water quality impacts of different density levels, this 

section compares three hypothetical communities, each accommodating development at 

different densities—one house per acre, four houses per acre, and eight houses per acre.

6 

To assess regional water quality impacts, EPA modeled the stormwater impacts from different 

development densities. In general, the more stormwater runoff generated within a region, 

the more associated pollutants, such as total nitrogen, phosphorus, and suspended solids, 

will enter receiving waterbodies. The three density levels capture some of the wide range of 

zoning practices in use throughout the country. All of these densities are consistent with single-family, detached housing. EPA examined the stormwater impacts from each density scenario at various scales of residential development7—one-acre, lot, and watershed 

levels—and through a 40-year time series build-out analysis. 

The Model and Data Inputs 

The model used to compare the stormwater impact from the scenarios is the Smart Growth 

Water Assessment Tool for Estimating Runoff (SG WATER), which is a peer-reviewed sketch 

model that was developed specifically to compare water quantity and quality differences 

among different development patterns (EPA, 2002). SG WATER’s methodology is based on the 

Natural Resources Conservation Service (NRCS) curve numbers (Soil Conservation Service, 

1986), event mean concentrations, and daily rainfall data.8 The model requires the total number of acres developed at a certain development density. If density is unknown, total percent 

imperviousness can be used. The model was run using overall percent imperviousness. 

EPA believes that the results presented here are conservative. SG WATER uses a general and simple methodology based on curve numbers. One limitation of curve numbers is that they tend 

to underestimate stormwater runoff for smaller storms (less than one inch). This underestimate 

6 Densities at one, four, and eight residential units per acre are used here for illustrative purposes only. Many communities now are zoning 

for one unit per two acres at the low-density end of the spectrum. Low-density residential zoning exists in places as diverse as Franklin 

County, Ohio, which requires no less than two acres per unit <www.co.franklin.oh.us/development/franklin_co/LDR.html#304.041>) to Cobb 

County, Georgia, outside of Atlanta, which requires between one and two units per acre in its low-density residential districts (<www.cobbcounty.org/community/plan_bza_commission.htm>). By comparison, some communities are beginning to allow higher densities, upwards 

of 20 units per acre. For example, the high-density residential district in Sonoma County, California permits between 12 and 20 units per 

acre (<www.sonoma-county.org/prmd/Zoning/article_24.htm>), and the city of Raleigh, North Carolina, allows up to 40 units per acre in 

planned development districts. 7 This example and others throughout this study compare residential units, but a similar comparison including commercial development could also 

be done . 

8 Daily time-step rainfall data for a 10-year period (1992-2001, inclusive) were used. 

9 
Protecting Water Resources with Higher-Density Development


can be significant since the majority of storms are small storms. In addition, the curve numbers tend to overestimate runoff for large storms. However, curve numbers more accurately 

predict runoff in areas with more impervious cover.

9 For the analysis here, the runoff from the 

low-density site is underestimated to a larger degree than the runoff from the higher-density 

site because the higher-density site has more impervious cover. Simply put, because of 

methodology, the difference in the numbers presented here is conservative—it is likely that 

the comparative difference in runoff between the sites would be greater if more extensive 

modeling were used. 

To isolate the impacts that developing at different densities makes on stormwater runoff, EPA 

made several simplifying assumptions in the modeling: 

•	 EPA modeled only residential growth and not any of the corresponding commercial, 

retail, or industrial growth that would occur in addition to home building. Moreover, EPA 

assumed that all the new growth would occur in greenfields (previously undeveloped 

land). Infill development, brownfield redevelopment, and other types of urban development were not taken into consideration, nor were multifamily housing, apartments, or 

accessory dwelling units.

10 

•	 The modeling did not take into account any secondary or tertiary impacts, such as additional stormwater benefits, that may be realized by appropriately locating the development within the watershed. For example, siting development away from headwaters, 

recharge areas, or riparian corridors could better protect these sensitive areas. Denser 

development makes this type of protective siting easier since less land is developed. 

However, these impacts are not captured or calculated within the modeling. 

•	 Whether developed at one, four, or eight houses per acre, when one acre is developed, 

EPA assumed the entire acre is disturbed land (e.g., no forest or meadow cover would be 

preserved), which is consistent with current construction practices. 

•	 All the new growth is assumed to be single-family, detached houses.

11Whether 

developed at one, four, or eight houses per acre, each home has a footprint of 2,265 

square feet, roughly the current average size for new houses (National Association of 

Home Builders, 2001). 

9 Most existing stormwater models incorrectly predict flows associated with small rains in urban areas. Most existing urban runoff models 

originated from drainage and flooding evaluation procedures that emphasized very large rains (several inches in depth). These large storms 

contribute only very small portions of the annual average discharges. Moderate storms, occurring several times a year, are responsible for 

the majority of the pollutant discharges. These frequent discharges cause mostly chronic effects, such as contaminated sediment and frequent high flow rates, and the inter-event periods are not long enough to allow the receiving water conditions to recover. 10 Single-family, detached housing dominates many low-density residential developments. However, higher-density developments support 

a range of housing types, including townhouses, apartments, and other forms of multifamily housing. These housing types generally have a 

smaller footprint per house than 2,265 square feet. Therefore, a more realistic situation for the higher-density scenarios would either be a 

smaller housing footprint or an increase in the number of homes accommodated on one acre. In either case, including these different housing types in the analysis would produce less overall stormwater runoff and less per house runoff for the higher-density scenarios. 11 It is possible that when additional land uses, such as commercial, transportation, or recreation, are included in the analysis, the low-density scenarios become relatively less dense while the higher-density scenarios become relatively more dense. In general, low-density residential development tends to be associated with low-density commercial development, characterized by large retail spaces, wide roads, large 

parking lots, and minimal public transportation. Higher-density residential areas are more likely to have high-density commercial options, 

with smaller retail spaces, mixed land uses, narrower streets, parking garages, on-street parking, and sometimes a well-developed public 

transportation system, which can reduce parking needs. 

10 
•	 The same percentage of transportation-associated infrastructure, such as roads, parking 

lots, driveways, and sidewalks, is allocated to each community acre, based on the curve 

number methodology from the NRCS. For example, each scenario has the same width of 

road, but because the higher-density scenario is more compact, it requires fewer miles of 

roads than the lower-density scenarios. So while the same percentage is applied, the 

amounts differ by scenario. Collector roads or arterials that serve the development are 

not included. 

•	 The modeled stormwater runoff quantity for each scenario is assumed to come from one 

hypothetical outfall. 

•	 The model does not take into account wastewater or drinking water infrastructure, slope, 

or other hydrological interactions that the more complex water modeling tools use. 

Summary of Scenarios 

Example 1 examines the stormwater runoff impacts on a one-acre lot that accommodates one 

house (Scenario A), four houses (Scenario B), or eight houses (Scenario C). Example 2 expands 

the analysis to examine stormwater runoff impacts within a lot-level development that accommodates the same number of houses. Because of different development densities, this growth 

requires different amounts of land. Scenario A requires eight acres for eight houses, Scenario B 

requires two acres for eight houses, and Scenario C requires one acre for eight houses. 

Examples 3, 4, and 5 explore the relationship between density and land consumption by building in a watershed at different densities. Again, different amounts of land are required 

to support the same amount of housing. Examples 6, 7, and 8 examine how the hypothetical 

community grows over a 40-year timeframe with different development densities. 

The scenarios and scales of development are summarized in Exhibit 2. EPA expects to capture 

the differences in stormwater runoff associated with different development densities by using 

these three scenarios (Scenarios A, B, and C) at four different scales (one acre, lot, watershed, 

and build-out). 

EXHIBIT 2: Summary of Scenarios 

Scale of Analysis 

Example 1: One acre 

Example 2: Lot—Each development lot accommodates 

the same number of houses 

Sc enario A: Scenario B: Scenario C: 

One house per Four houses Eight houses 

acre per acre per acre 

1 house per acre 4 houses per acre 8 houses per acre 

8 houses built 8 houses built 8 houses built 

on 8 acres on 2 acres on 1 acre 

11 
Protecting Water Resources with Higher-Density Development


Example 3: Watershed— 

Each 10,000-acre watershed accommodates the 

same number of houses 

10,000 houses 

built on 10,000 

acres 

10,000 houses 

built on 2,500 

acres or 1⁄4 of 

the watershed 

10,000 houses 

built on 1,250 

acres or 1

/

8 of 

the watershed 

Example 4: Watershed— 

Each 10,000-acre watershed is fully built out at 

different densities 

10,000 houses 

built on 10,000 

acres 

40,000 houses 

built on 10,000 

acres 

80,000 houses 

built on 10,000 

acres 

Example 5: Watershed— 

Each scenario accommodates the same number 

of houses 

80,000 houses 

consume 8 

watersheds 

80,000 houses 

consume 2 

watersheds 

80,000 houses 

consume 1 

watershed 

Example 6: Hypothetical 

build-out in the year 2000 

10,000 houses 

built on 10,000 

acres 

10,000 houses 

built on 2,500 

acres 

10,000 houses 

built on 1,250 

acres 

Example 7: Hypothetical 

build-out in the year 

2020 

20,000 houses 

built on 20,000 

acres, or 2 watersheds 

20,000 houses 

built on 5,000 

acres, or 1⁄2 of 1 

watershed 

20,000 houses 

built on 2,500 

acres, or 1⁄4 of 1 

watershed 

Example 8: Hypothetical 

build-out in the year 

2040 

40,000 houses 

built on 40,000 

acres, or 4 watersheds 

40,000 houses 

built on 10,000 

acres, or 1 

watershed 

40,000 houses 

built on 5,000 

acres, or 1⁄2 of 1 

watershed 

Before analyzing the impacts of these different scenarios, it is useful to clarify some underlying premises. This analysis assumes that: 

1.	 Metropolitan regions will continue to grow. This assumption is consistent with U.S. Census 

Bureau projections that the U.S. population will grow by roughly 50 million people by 

2020 (Census, 2000). Given this projected population growth, most communities across 

the country are or will be determining where and how to accommodate expected population increases in their regions. 

2.	 Housing density affects the distribution of new growth within a given region, not the 

amount of growth. Individual states and regions grow at different rates depending on 

a variety of factors, including macroeconomic trends (e.g., the technology boom in the 

1980s spurring development in the Silicon Valley region in California) and demographic 

shifts. Distribution and density of new development do not significantly affect these factors. 

12 
3.	 The model focuses on the comparative differences in stormwater runoff between scenarios, not absolute values. As discussed, using the curve number and event mean concentration approach can underestimate the total quantity of stormwater runoff for smaller 

storm events and in areas of lower densities. Because of this and other model simplifications discussed above, the analysis does not focus on the absolute value of stormwater 

runoff generated for each scenario but instead focuses on the comparative difference, or 

the delta, in runoff between scenarios. 

Results 

The results from the eight examples for all three scenarios are presented below. 

EXAMPLE 1: ONE-ACRE LEVEL 

Scale of Analysis Scenario A Scenario B Scenario C 

One Acre 1 house 4 houses 8 houses 

EPA examined one acre developed at three different densities: one house, four houses, and 

eight houses. The results are presented in Exhibit 3. As Exhibit 3 demonstrates, the overall 

percent imperviousness for Scenario A is approximately 20 percent with one house per acre, 

38 percent for Scenario B with four houses per acre, and 65 percent for Scenario C with eight 

houses per acre (Soil Conservation Service, 1986). 

EXHIBIT 3: Total Average Annual Stormwater Runoff for All Scenarios 

Impervious cover = 20% 

Runoff/acre = 18,700 ft

3

/yr 

Runoff/unit = 18,700 ft

3

/yr 

Impervious cover = 38% 

Runoff/acre = 24,800 ft

3

/yr 

Runoff/unit = 6,200 ft

3

/yr 

Impervious cover = 65% 

Runoff/acre = 39,600 ft

3

/yr 

Runoff/unit = 4,950 ft

3

/yr 

Scenario A Scenario B Scenario C 

13 
Protecting Water Resources with Higher-Density Development


Examining the estimated average annual runoff at the acre level, as illustrated in Exhibit 4, 

the low-density Scenario A, with just one house, produces an average runoff volume of 

18,700 cubic feet per year (ft

3

/yr). Scenario C, with eight houses, produces 39,600 ft

3

/yr, and 

Scenario B falls between Scenarios A and C at 24,800 ft

3

/yr. In short, looking at the comparative differences between scenarios, runoff roughly doubles as the number of houses increases from one house per acre to eight houses per acre. Scenario C, with more houses on the 

acre, has the greatest amount of impervious surface cover and thus generates the most 

runoff at the acre level. 

Looking at the comparative difference of how much runoff each individual house produces, 

in Scenario A, one house yields 18,700 ft

3

/yr, the same as the per acre level. In the denser 

Scenario C, however, each house produces 4,950 ft

3

/yr average runoff. The middle scenario, 

Scenario B, produces considerably less runoff—6,200 

ft3

/yr—per house than Scenario A, but more than Each house in Scenario B 

Scenario C. Each house in Scenario B produces approxi- produces approximately 

mately 67 percent less runoff than a house in Scenario A, 67 percent less runoff than 

and each house in Scenario C produces 74 percent less a house in Scenario A, and runoff than a house in Scenario A. This is because the 

houses in Scenarios B and C create less impervious sur- each house in Scenario C 

face per house than the house in Scenario A. Therefore, produces 74 percent less 

per house, each home in the higher-density communities runoff than a house in 

results in less stormwater runoff. Scenario A. 

Modeling at the acre level demonstrates that, in this 

example, when density is quadrupled (from one house 

to four houses), stormwater runoff increases by onethird per acre, but decreases by two-thirds per house. Moreover, when density increases by a 

factor of eight—from one house to eight houses—stormwater runoff doubles per acre, but 

decreases by almost three-quarters per house. 

These results indicate when runoff is measured by the acre, limiting density does minimize water quality impacts compared to the higher-density scenarios. However, when 

measured by the house, higher densities produce less stormwater runoff. 

EXAMPLE 2: LOT LEVEL 

Scale of Analysis Scenario A Scenario B Scenario C 

Lot 8 houses built on 8 houses built on 8 houses built on 

8 acres 2 acres 1 acre 

14 
EXHIBIT 4: Each Scenario Accommodates Eight Houses


Impervious cover = 20% Total runoff (18,700 ft

3

/yr x 

8 acres) = 149,600 ft

3

/yr 

Runoff/house = 

18,700 ft

3

/yr 

Scenario A 

Impervious cover = 38% Total runoff (24,800 ft

3

/yr x 

2 acres) = 49,600 ft

3

/yr 

Runoff/house = 

6,200 ft

3

/yr 

Scenario B 

Impervious cover = 65% Total runoff = 39,600 ft

3

/yr Runoff/house = 

4,950 ft

3

/yr 

Scenario C 

15 
Protecting Water Resources with Higher-Density Development


For each development to accommodate the same num- The increase in runoff 

ber of houses, the lower-density scenarios require more for Scenario A is due to land to accommodate the same number of houses that 

Scenario C has accommodated on one acre. Specifically, the additional land 

Scenario A must develop seven additional acres, or eight consumption. 

acres total, to accommodate the same number of houses 

as Scenario C. Scenario B must develop two acres to accommodate the same number of 

houses. Exhibit 4 illustrates. 

With each scenario accommodating the same number of houses, this analysis shows that 

total average runoff in Scenario A is 149,600 ft

3

/yr (18,700 ft

3

/yr x 8 acres), which is a 278 percent increase from the 39,600 ft3/yr total runoff in Scenario C. Total average runoff from eight 

houses in Scenario B is 49,600 ft

3

/yr (24,800 ft

3

/yr x 2 acres), which is a 25 percent increase in 

runoff from Scenario C. The increase in runoff for Scenario A is due to the additional land consumption and associated runoff. The impervious cover for Scenario A remains the same at 20 

percent, but now, seven additional acres have 20 percent impervious cover. 

Examining the comparative difference in runoff between scenarios shows that lower 

densities can create less total impervious cover, but produce more runoff when the 

number of houses is kept consistent between scenarios. Furthermore, the higher-density 

scenario produces less runoff per house and per lot. 

EXAMPLE 3: WATERSHED LEVEL 

Scale of Analysis Scenario A Scenario B Scenario C 

Watershed—Each 10,000-acre 

watershed accommodates 

the same number of houses 

10,000 houses 

built on 10,000 

acres 

10,000 houses 

built on 2,500 

acres 

10,000 houses 

built on 1,250 

acres 

Taking the analysis to the watershed level, EPA examined the comparative watershed 

stormwater runoff impacts from accommodating growth at different densities. The watershed used in this analysis is a hypothetical 10,000-acre watershed accommodating only 

houses. As discussed, the modeling does not include retail, business centers, farms, or any 

other land uses typically seen in communities, nor does it take into consideration where the 

development occurs within the watershed. Research has shown that upper sub-watersheds, 

which contain smaller streams, are generally more sensitive to development than lower 

sub-watersheds (Center for Watershed Protection, 2001). 

Accommodating 10,000 houses at one house per acre in the 10,000-acre watershed would 

fully build out the watershed. At the higher density of four houses per acre, one-quarter of the 

watershed would be developed, and at eight houses per acre, one-eighth of the watershed 

would be developed. Exhibit 5 shows the runoff associated with each of these scenarios. 

16 
EXHIBIT 5: 10,000-Acre Watershed Accommodating 10,000 Houses


10,000 houses built on 

10,000 acres produce: 

10,000 acres x 1 house 

x 18,700 ft3/yr of 

runoff = 

187 million ft

3

/yr of 

stormwater runoff 

Site: 20% impervious 

cover 

Watershed: 20% 

impervious cover 

10,000 houses built on 

2,500 acres produce: 

2,500 acres x 4 houses 

x 6,200 ft3/yr of 

runoff = 

62 million ft

3

/yr 

of stormwater runoff 

Site: 38% impervious 

cover 

Watershed: 9.5% 

impervious cover 

10,000 houses built on 

1,250 acres produce: 

1,250 acres x 8 houses 

x 4,950 ft3/yr of 

runoff = 

49.5 million ft

3

/yr of 

stormwater runoff 

Site: 65% impervious 

cover 

Watershed: 8.1% 

impervious cover 

Scenario A Scenario B Scenario C 

As Exhibit 5 illustrates, if development occurs at a lower density, e.g., one house per acre, 

the entire watershed will be built out, generating 187 million ft

3/yr of stormwater runoff. 

Scenario B, at four houses per acre, consumes less land and produces approximately 62 million ft3/yr of stormwater runoff, while Scenario C, at the highest density, consumes the least 

amount of land and produces just 49.5 million ft3/yr of stormwater runoff. Looking at the 

comparative differences, Scenario A generates approximately three times as much runoff 

from development as Scenario B, and approximately four times as much stormwater 

runoff as Scenario C. 

Exhibit 5 also illustrates that, in this example, overall Overall impervious 

impervious cover for the watershed decreases as site den- cover for the watersity increases. Scenario C, which has a lot-level impervi­ shed decreases as site ousness of 65 percent, has a watershed-level impervious­


ness of only 8.1 percent, which is lower than the 10 density increases.


17 
Protecting Water Resources with Higher- Density Development 

percent threshold discussed earlier. Scenario B, with a density of four houses per acre, has a 

site-level impervious cover of 38 percent, but a watershed imperviousness of 9.5 percent, which 

is still lower than the 10 percent threshold. Finally, Scenario A, at a lot-level imperviousness of 

20 percent, has the same overall imperviousness at the watershed level. Both of the higherdensity scenarios consume less land and maintain below-the-threshold imperviousness. 

This simplistic illustration demonstrates a basic point of 

this analysis—higher-density developments can minimize At one house per acre, 

stormwater impacts because they consume less land than Manhattan would need 

their lower-density counterparts. For example, imagine if approximately 750,000 

Manhattan, which accommodates 1.54 million people on more acres, or an addi­ 14,720 acres (23 square miles) (Census, 2000), were developed not at its current density of 52 houses per acre, but tional 1,170 square miles, 

at one or four houses per acre. At one house per acre, to accommodate its current 

Manhattan would need approximately 750,000 more population at two people 

acres, or an additional 1,170 square miles, to accommo- per household. date its current population at two people per household. 

That’s approximately the size of Rhode Island. At four houses per acre, Manhattan would 

need approximately 175,000 more acres, or an additional 273 square miles. 

Reducing land consumption is crucial to preserving water quality because, as discussed previously, preserving large, continuous areas of open space and sensitive ecological areas is 

critical for maintaining watershed services. In addition, because of their dense development 

pattern, Scenarios B and C may realize additional stormwater benefits if the developed land is 

appropriately sited in the watershed to protect sensitive ecological areas, such as headwaters, wetlands, riparian corridors, and floodplains. 

EXAMPLE 4: REMAINING LAND IN THE WATERSHED DEVELOPED 

What happens if the remaining undeveloped parts of the watershed in Scenarios B and C are 

developed? Exhibit 6 considers this situation. 

Scale of Analysis Scenario A Scenario B Scenario C 

Watershed—Each 10,000­

acre watershed is fully built 

out at different densities 

10,000 houses 

built on 10,000 

acres 

40,000 houses 

built on 10,000 

acres 

80,000 houses 

built on 10,000 

acres 

18 
EXHIBIT 6: 10,000-Acre Watershed Accommodating Different Numbers of Houses


The watershed is fully 

built out at 1 house per 

acre. 10,000 acres 

accommodates 10,000 

houses, translating to: 

10,000 acres x 1 house x 

18,700 ft3/yr of runoff = 

187 million ft3/yr 

stormwater runoff 

Site: 20% impervious 

cover 

Watershed: 20% 

impervious cover 

The watershed is fully 

built out at 4 houses per 

acre. 10,000 acres 

accommodates 40,000 

houses, translating to: 

10,000 acres x 4 houses 

x 6,200 ft3/yr of runoff = 

248 million ft3/yr 

stormwater runoff 

Site: 38% impervious 

cover 

Watershed: 38% 

impervious cover 

The watershed is fully 

built out at 8 houses per 

acre. 10,000 acres 

accommodates 80,000 

houses, translating to: 

10,000 acres x 8 houses x 

4,950 ft3/yr of runoff = 

396 million ft3/yr 

stormwater runoff 

Site: 65% impervious 

cover 

Watershed: 65% 

impervious cover 

Scenario A Scenario B Scenario C 

19 
Protecting Water Resources with Higher-Density Development 

Each watershed is fully built out, and the watershed Scenarios A and B accomdeveloped at the highest density (Scenario C) is generat- modate only a small por­ ing approximately double the total stormwater runoff of 

Scenario A. Scenario B is generating approximately one- tion of the expected 

third more runoff than Scenario A. Similar to the acre- growth. The rest will 

level and lot-level results, Scenario C has the highest have to be built in 

degree of impervious cover at 65 percent, while Scenario other watersheds. A maintains the lowest level at 20 percent. 

The higher densities found in Scenario B and C are degrading their watershed services to a 

greater extent than Scenario A. However, the number of houses accommodated in each community is not the same. Scenario B is accommodating 30,000 more houses (four times the number 

of Scenario A), and Scenario C is accommodating 70,000 more houses (eight times the number 

of Scenario A). Recall that density limits shift growth and do not generally affect the total 

amount of growth in a given time period. Therefore, this is not a fair comparison. Scenarios A 

and B accommodate only one-eighth and one-half, respectively, of the 80,000 houses accommodated in Scenario C. Where do the other houses, households, and families go? To get a true 

appreciation for the effects of density, Scenarios A and B must also show where those homes 

will be accommodated. It is likely that they would be built in nearby or adjacent watersheds. 

Our hypothetical community that develops at one house per acre (Scenario A) is able to accommodate only 10,000 houses. For the community that develops at that density to accommodate 

the same number of houses that Scenario C contains, it must disturb and develop land from 

nearby or adjacent watersheds. 

EXAMPLE 5: ACCOMMODATING THE SAME NUMBER OF HOUSES 

Scale of Analysis Scenario A Scenario B Scenario C 

Watershed—Each scenario 

accommodates the same 

number of houses 

1 house per 

acre—80,000 

houses consume 8 

watersheds 

4 houses per 

acre—80,000 

houses consume 2 

watersheds 

8 houses per 

acre—80,000 

houses consume 1 

watershed 

As discussed, the U.S. population will increase by an estimated 50 million people by 2020. 

Different areas of the country will grow at different rates in the future. Whether a region 

anticipates 1,000 or 80,000 new households to come to the region over the next 10 years, 

comparisons between build-out scenarios must keep the number of homes consistent. In this 

case, if Scenario C is developed so that its entire watershed is built out to 80,000 houses, then 

for a fair comparison, Scenarios A and B must also include 80,000 houses. Exhibit 7 illustrates 

this situation. 

20 
EXHIBIT 7: 80,000 Houses Accommodated 


Scenario A Scenario B Scenario C


At 1 house per acre, 

80,000 houses require 

80,000 acres, or 8 watersheds, translating to: 

80,000 acres x 1 house x 

18,700 ft3/yr of runoff = 

1.496 billion ft3/yr of 

stormwater runoff 

8 watersheds at 20% 

impervious cover 

At 4 houses per acre, 

80,000 houses require 

20,000 acres, or 2 watersheds, translating to: 

20,000 acres x 4 houses x 

6,200 ft3/yr of runoff = 

496 million ft3/yr of 

stormwater runoff 

2 watersheds at 38% 

impervious cover 

At 8 houses per acre, 

80,000 houses require 

10,000 acres, or 1 watershed, translating to: 

10,000 acres x 8 houses x 

4,950 ft3/yr of runoff = 

396 million ft3/yr of 

stormwater runoff 

1 watershed at 65% 

impervious cover 

21 
Protecting Water Resources with Higher-Density Development 

When the number of houses is kept consistent, Scenario A would need to develop an additional seven watersheds (assuming the same size watersheds) and Scenario B would need to 

develop one additional watershed to accommodate the same growth found in Scenario C. 

As Exhibit 7 demonstrates, for Scenario A to accommo- Scenario A would need to date the additional 70,000 homes already accommodated in Scenario C, it must develop another seven develop an additional seven 

watersheds. This generates 1.496 billion ft

3

/yr of watersheds and Scenario B 

stormwater runoff. Scenario C, with a development den- would need to develop one 

sity of eight houses per acre, has still developed just one additional watershed in watershed and is generating approximately 74 percent 

less stormwater runoff than Scenario A—or 396 million order to accommodate 

ft3

/yr. Scenario B, at four houses per acre, is generating the same growth found 

496 million ft

3

/yr runoff, or two-thirds less runoff than in Scenario C. 

Scenario A, but 100 million ft

3

/yr more than Scenario C. 

EXAMPLE 6: TIME SERIES BUILD-OUT ANALYSIS: BUILD-OUT IN 2000 

Scale of Analysis Scenario A Scenario B Scenario C 

Hypothetical build-out in 

the year 2000 

10,000 houses 

built on 10,000 

acres 

10,000 houses 

built on 2,500 

acres 

10,000 houses 

built on 1,250 

acres 

Another way to examine this issue is to look at what happens to build-out of the three scenarios over time. A basic assumption for EPA’s modeling is that growth is coming to the 

hypothetical community, and that growth will be accommodated within a fixed time 

horizon. But what happens to growth in the hypothetical community over several, 

sequential time horizons? 

Given the dynamic nature of population growth, what will build-out look like in the 

hypothetical community in 2000, 2020, and 2040 at different development densities? The 

next several examples examine the amount of land required to accommodate increasing 

populations within a watershed that develops at different densities. The purpose of this 

time series build-out is to examine how much land is consumed as the population grows 

in 20-year increments. 

Starting in the year 2000, the three watersheds each begin with 10,000 homes. The only difference between the watersheds is the densities at which the building occurs. In 2000, they 

might look something like Exhibit 8. 

22 
EXHIBIT 8: Time Series Build-out Analysis: Build-out in 2000 


10,000 houses on 

10,000 acres at a density of 1 house per acre 

consume 1 entire 

watershed. 

10,000 houses on 

2,500 acres at a density 

of 4 houses per acre 

consume 1⁄4 of 1 

watershed. 

10,000 houses on 

1,250 acres at a density 

of 8 houses per acre 

consume 1

/8 of 1 

watershed. 

Scenario A Scenario B Scenario C 

As previously demonstrated in Example 3, building at higher densities consumes, or converts, 

less land within the watershed. Scenario A, developing at one unit per acre, requires the 

entire 10,000-acre watershed to accommodate 10,000 houses. Scenario C, on the other hand, 

developing at eight units an acre, requires significantly less land to accommodate the same 

amount of development. 

EXAMPLE 7: TIME SERIES BUILD-OUT ANALYSIS: BUILD-OUT IN 2020 

Scale of Analysis Scenario A Scenario B Scenario C 

Hypothetical build-out in the 

year 2020 

20,000 houses 

built on 20,000 

acres, or 2 

watersheds 

20,000 houses 

built on 5,000 

acres, or 1⁄2 of 1 

watershed 

20,000 houses 

built on 2,500 

acres, or 1⁄4 of 1 

watershed 

Fast-forwarding 20 years, the population in the hypothetical community has doubled from 

10,000 houses to 20,000 houses. Each scenario must accommodate this additional growth at 

different development densities. Exhibit 9 demonstrates how this development might look. 

23 
Protecting Water Resources with Higher-Density Development


EXHIBIT 9: Time Series Build-out Analysis: Build-out in 2020


20,000 houses accommodated on 20,000 

acres at a density of 1 

house per acre will consume 2 watersheds. 

20,000 houses accommodated on 5,000 

acres at a density of 4 

houses per acre will consume 1⁄2 of 1 watershed. 

20,000 houses accommodated on 2,500 

acres at a density of 

eight houses per acre 

will consume 1⁄4 of 1 

watershed. 

Scenario A Scenario B Scenario C 

As Exhibit 9 demonstrates, Scenario A, developing at one house per acre, requires another 

whole watershed to accommodate the additional growth. Scenarios B and C, developing 

at higher densities, can accommodate the additional growth within the same watershed. 

Moreover, by developing at higher densities within the watershed, ample open space or 

otherwise undeveloped land remains to perform critical watershed functions. No such land 

exists in Scenario A, and, as previously discussed, lawns typically associated with one house 

per acre are not able to provide the same type of watershed services as forests, meadows, 

or other types of unconverted land. 

EXAMPLE 8: TIME SERIES BUILD-OUT ANALYSIS: BUILD-OUT IN 2040 

Scale of Analysis Scenario A Scenario B Scenario C 

Hypothetical build-out in 

the year 2040 

40,000 houses 

built on 40,000 

acres, or 4 

watersheds 

40,000 houses 

built on 10,000 

acres, or 1 

watershed 

40,000 houses 

built on 5,000 

acres, or 1⁄2 of 1 

watershed 

24 
The hypothetical community continues to grow and, in another 20 years, population has 

doubled again, requiring each scenario to accommodate 20,000 more homes at different 

development densities. Exhibit 10 demonstrates how this development might look. 

EXHIBIT 10: Time Series Build-out Analysis: Build-out in 2040 

40,000 houses on 

40,000 acres at a density of 1 house per acre 

will consume 4 

watersheds. 

40,000 houses on 

10,000 acres at a density of 4 houses per 

acre will consume 1 

watershed. 

40,000 houses on 

5,000 acres at a density 

of 8 houses per acre 

will consume 1⁄2 of 1 

watershed. 

Scenario A Scenario B Scenario C 

As Exhibit 10 demonstrates, Scenario A, developing at 

one house per acre, must develop land in four watersheds, or 40,000 acres, to accommodate all its houses. 

Scenario B, developing at a slightly higher density, uses 

its remaining land to accommodate the additional 

growth. Scenario C is still developing within the same 

watershed and still has additional land available to provide watershed services. Scenario A and B do not. Any 

land for watershed services would need to come from 

additional watersheds. 

Lower-density development always requires 

more land than higher 

densities to accommodate 

the same amount of 

growth. 

This build-out analysis can continue indefinitely with the same result: lower-density 

development always requires more land than higher densities to accommodate the same 

amount of growth. Because more land is required, more undeveloped land is converted. 

25 
Protecting Water Resources with Higher-Density Development


Findings/Discussion 

The results indicate when runoff is measured by the acre, limiting density does produce less 

stormwater runoff when compared to the higher-density scenarios. However, when measured by the house, higher densities produce less stormwater runoff. So, which is the 

appropriate measure? 

Typically, a planning department analyzes the projected stormwater runoff impacts of a 

developer’s proposal based on the acreage, not the number of houses being built. Based on 

the results from the one-acre level example, communities might conclude that lower-density 

development would minimize runoff. Runoff from one house on one acre is roughly half the 

runoff from eight houses. However, where did the other houses, and the people who live in 

those houses, go? The answer is almost always that they went somewhere else in that 

region—very often somewhere within the same watershed. Thus, those households still have 

a stormwater impact. To better understand the stormwater runoff impacts from developing 

at low densities, the impacts associated with those houses locating elsewhere need to be 

taken into account. This approach has two advantages: 

•	 It acknowledges that the choice is not whether to grow by one house or eight but is


instead where and how to accommodate the eight houses (or whatever number by


which the region is expected to grow).


•	 It emphasizes minimization of total imperviousness and runoff within a region or watershed rather than from particular sites—which is more consistent with the science indicating that imperviousness within the watershed is critical. 

To more fully explore this dynamic, EPA modeled scenarios at three scales—one acre, lot, and 

watershed—and at three different time series build-out examples to examine the premise 

that lower-density development better protects water quality. EPA examined stormwater 

runoff from different development densities to determine the comparative difference 

between scenarios. The higher-density scenarios generated less stormwater runoff per house 

at all scales and time series build-out examples. Exhibit 11 summarizes these findings. 

26 
EXHIBIT 11: Summary of Findings


Scenario Number of 

Acres 

Developed 

Impervious 

Cover 

(%) 

Total 

Runoff 

(ft

3

/yr) 

Runoff 

Per Unit 

(ft

3

/yr) 

Savings 

Over 

Scenario A: 

runoff per 

unit (%) 

One-Acre Level: Different densities developed on one acre 

A: One house/acre 1 20.0 18,700 18,700 0 

B: Four houses/acre 1 38.0 24,800 6,200 67 

C: Eight houses/acre 1 65.0 39,600 4,950 74 

Lot Level: Eight houses accommodated at different density levels 

Scenario A 8 20.0 149,600 18,700 0 

Scenario B 2 38.0 49,600 6,200 67 

Scenario C 1 65.0 39,600 4,950 74 

Watershed Level: 10,000 houses accommodated in one 10,000-acre watershed 

Watershed Level: Time Series Build-out Analysis: Build-out in 2000 

Watershed Level: Time Series Build-out Analysis: Build-out in 2020 

Watershed Level: Time Series Build-out Analysis: Build-out in 2040 

Scenario A 10,000 20.0 187 M 18,700 0 

Scenario B 2,500 9.5 62 M 6,200 67 

Scenario C 1,250 8.1 49.5 M 4,950 74 

Scenario Summary of Build-out Examples 

Scenario A 10,000 houses built on 10,000 acres: 1 watershed is consumed 

Scenario B 10,000 houses built on 2,500 acres: 1⁄4 of 1 watershed is consumed 

Scenario C 10,000 houses built on 1,250 acres: 1/8 of 1 watershed is consumed 

Scenario A 20,000 houses built on 20,000 acres: 2 watersheds are consumed 

Scenario B 20,000 houses built on 5,000 acres: 1⁄2 of 1 watershed is consumed 

Scenario C 20,000 houses built on 2,500 acres: 1⁄4 of 1 watershed is consumed 

Scenario A 40,000 houses built on 40,000 acres: 4 watersheds are consumed 

Scenario B 40,000 houses built on 10,000 acres: 1 watershed is consumed 

Scenario C 40,000 houses built on 5,000 acres: 1⁄2 of 1 watershed is consumed 

27 
Protecting Water Resources with Higher-Density Development


Specifically, this analysis demonstrates: 	 EPA found that the higher­

•	 With more dense development (Scenario C), runoff density scenarios generate 

rates per house decrease by approximately 74 per- less stormwater runoff per 

cent from the least dense scenario (Scenario A); house at all scales—one 

•	 For the same amount of development, denser devel- acre, lot, watershed—and 

opment produces less runoff and less impervious time series build-out 

cover than low-density development; and examples. •	 For a given amount of growth, lower-density devel­


opment uses more of the watershed. 


Taken together, these findings indicate that low-density development may not always be 

the preferred strategy for reducing stormwater runoff. In addition, the findings indicate that 

higher densities may better protect water quality—especially at the lot and watershed levels. 

Higher-density developments consume less land to accommodate the same number of 

houses as lower density. Consuming less land means less impervious cover is created within 

the watershed. To better protect watershed function, communities must preserve large, continuous areas of open space and protect sensitive ecological areas, regardless of how densely 

they develop. 

However, while increasing densities on a regional scale can, on the whole, better protect 

water resources at a regional level, higher-density development can have more site-level 

impervious cover, which can exacerbate water quality problems in nearby or adjacent waterbodies. To address this increased impervious cover, numerous site-level techniques are available to mitigate development impacts. When used in combination with regional techniques, 

these site-level techniques can prevent, treat, and store runoff and associated pollutants. 

Many of these practices incorporate some elements of low-impact development techniques 

(e.g., rain gardens, bioretention areas, and grass swales), although others go further to 

include changing site-design practices, such as reducing parking spaces, narrowing streets, 

and eliminating cul-de-sacs. 

Incorporating these techniques can 

help communities meet their water 

quality goals and create more interesting and enjoyable neighborhoods. 

A University of Oregon study, 

Measuring Stormwater Impacts of 

Different Neighborhood Development 

Patterns (University of Oregon, 2001), 

supports this conclusion. The study, 

which included a study site near 

Corvallis, Oregon, compared stormwater management strategies in three 

common neighborhood development 

patterns. For example, best management practices, such as disconnecting 

The city of Portland, Oregon, is developing urban stormwater 

strategies, such as these curb extensions that can absorb the 

street’s runoff from large storm events. 

Photo courtesy of the City of Portland, Oregon 

28 
residential roofs and paved areas from the stormwater system, introducing swales and water 

detention ponds into the storm sewer system, and strategically locating open space, considerably reduced peak water runoff and improved infiltration. The study concluded that “some 

of the most effective opportunities for reducing stormwater runoff and decreasing peak flow 

are at the site scale and depend on strategic integration with other site planning and design 

decisions.” The study also found that planting strips and narrower streets significantly 

reduced the amount of pavement and, as a result, runoff in developed areas. 

A development in Tacoma, Washington, demonstrates that increasing densities and addressing stormwater at the site level can work effectively. The Salishan Housing District was built 

on Tacoma’s eastern edge in the 1940s as temporary housing for ship workers. It is currently a 

public housing community with 855 units. 

Redevelopment of Salishan will increase densities to Salishan Housing District 

include 1,200 homes (public housing, affordable and mar- is replacing 855 public ket rate rentals, and for-sale units), local retail, a farmers 

market, a senior housing facility, a daycare center, a housing units with 1,200 

health clinic, commercial office space, and an expanded units. Numerous site-level 

community center. Among the most important priorities strategies, such as inte­ for the redevelopment is restoring the water quality of grating uses, narrowing Swan Creek, which forms the eastern edge of Salishan. 

The creek is a spawning ground for indigenous salmon the streets, installing rain 

populations that feed into the Puyallup River and Puget gardens, and daylighting a 

Sound. The site plan seeks to restore 65 percent of the stream, are used to restore 

land to forest and pervious landscape. In addition, the the water quality of Swan 

streets will be narrowed to reduce impervious surfaces 

and also make the neighborhood more inviting for walk- Creek and revitalize an 

ing. Some streets may be eliminated and replaced with existing neighborhood. 

pedestrian paths. The remaining streets will be bordered 

by rain gardens that would accept, filter, and evapotranspire runoff. Most existing street surfaces would be reused, although some may be replaced with pervious pavers. 

Communities can enjoy a further reduction in runoff if they take advantage of underused 

properties, such as infill, brownfield, or greyfield12 sites. For example, an abandoned shopping center (a greyfield property) is often almost completely impervious cover and is already 

producing high volumes of runoff (Sobel, 2002). If this property were redeveloped, the net 

runoff increase would likely be zero since the property was already predominately impervious cover. In many cases, redevelopment of these properties breaks up or removes some 

portion of the impervious cover, converting it to pervious cover and allowing for some 

stormwater infiltration. In this case, redevelopment of these properties can produce a 

net improvement in regional water quality by decreasing total runoff. Exhibit 12 

illustrates this opportunity. 

12 Greyfield sites generally refer to abandoned or underutilized shopping malls, strip malls, or other areas that have significant paved surface and little or no contamination. 

29 
Protecting Water Resources with Higher-Density Development 

EXHIBIT 12: Redevelopment of a Greyfield Property 

Before Redevelopment After Redevelopment 

Photos courtesy of Juan Ayala, Invisioneering, for the New Jersey Office of State Planning 

Redevelopment of a former shopping mall in Boca Raton, Florida, provides an example of this 

type of opportunity. The Mizner Park shopping mall was redesigned from its original pattern 

of a large retail structure surrounded by surface parking lots; the 29-acre site now includes 

272 apartments and townhouses, 103,000 square feet of office space, and 156,000 square feet 

of retail space. Most parking is accommodated in four multistory parking garages. Designed 

as a village within a city, the project has a density five times higher than the rest of the city 

and a mix of large and small retailers, restaurants, and entertainment venues (Cooper, 2003). 

Most significantly, the final build-out of Mizner Park decreased overall impervious surface on 

the site by 15 percent through the addition of a central park plaza, flower and tree planters, 

and a large public amphitheater. 

Redeveloping brownfield and greyfield 

sites can reduce regional land consumption. A recent George Washington 

University study found that for every 

brownfield acre that is redeveloped, 4.5 

acres of open space are preserved 

(Deason, 2001). In addition to redeveloping brownfield sites, regions can 

identify underused properties or land, 

such as infill or greyfield sites, and target those areas for redevelopment. For 

example, a recent analysis by King 

County, Washington, demonstrated 

that property that is vacant and eligible 

for redevelopment in the county’s 

growth areas can accommodate 

263,000 new houses—enough for 

The redevelopment of Mizner Park, a former shopping mall, decreased impervious cover by 15 percent through the addition of this central plaza. 

Photo courtesy of U.S. EPA 

30 
500,000 people (Pryne, 2002). Redeveloping this property Redeveloping brownfield 

is an opportunity to accommodate new growth without and greyfield sites can 

expanding into other watersheds. As Kurt Zwikl, executive director of the Pottstown, Pennsylvania-based reduce regional land 

Schuylkill River Greenway Association, said, “Certainly, if we consumption. 

can get redevelopment going in brownfields and old industrial sites in older riverfront boroughs like Pottstown and Norristown, that’s a greenfield further 

out in the watershed that has been preserved to absorb more stormwater” (Brandt, 2004). 

Other Research 

Current research supports the findings of this study. Several site-specific studies have been 

conducted across the United States and in Australia that examine stormwater runoff and 

associated pollutants in relation to different development patterns and densities. Several 

case studies approach the research question with varying levels of complexity. Studies of 

Highland Park, Australia; Belle Hall, South Carolina; New Jersey; Chicago, Illinois; and the 

Chesapeake Bay each analyze the differences in runoff and associated water pollution from 

different types of development patterns. 

Queensland University of Technology, Gold Coast City Council, and the Department of Public 

Works in Brisbane, Australia, examined the relationship between water quality and six different land uses to offer practical guidance in planning future developments. When comparing 

monitored runoff and associated pollutants from six areas, they found the most protective 

strategy for water quality was high-density residential development (Goonetilleke, 2005). 

The Belle Hall study, by the South Carolina Coastal Conservation League, examined the water 

quality impacts of two development alternatives for a 583-acre site in Mount Pleasant, South 

Carolina. The town planners used modeling to examine the potential water quality impacts of 

each site design. In the “Sprawl Scenario,” the property was analyzed as if it developed along 

a conventional suburban pattern. The “Town Scenario” incorporated traditional neighborhood patterns. In each scenario, the overall density and intensity (the number of homes and 

the square feet of commercial and retail space) were held constant. The results found that the 

“Sprawl Scenario” consumed eight times more open space and generated 43 percent more 

runoff, four times more sediment, almost four times more nitrogen, and three times more 

phosphorous than the “Town Scenario” development (South Carolina Coastal Conservation 

League, 1995). 

These findings hold at a larger, state scale. New Jersey’s State Plan calls for increasing densities in the state by directing development to existing communities and existing infrastructure. Researchers at Rutgers University analyzed the water quality impacts from current 

development trends and compared them to water quality impacts from the proposed compact development. The study found that compact development would generate significantly 

less water pollution than current development patterns, which are mostly characterized by 

low-density development, for all categories of pollutants (Rutgers University, 2000). The 

reductions ranged from over 40 percent for phosphorus and nitrogen to 30 percent for 

runoff. These conclusions supported a similar statewide study completed in 1992 that 

31 
Protecting Water Resources with Higher-Density Development


concluded that compact development would result in 30 percent less runoff and 40 percent 

less water pollution than would a lower-density scenario (Burchell, 1995). 

Researchers at Purdue University examined two possible project sites in the Chicago area 

(Harbor, 2000). The first site was in the city; the second was on the urban fringe. The study 

found that placing a hypothetical low-density development on the urban fringe would produce 10 times more runoff than a higher-density development in the urban core. 

Finally, a study published by the Chesapeake Bay Foundation in 1996 comparing conventional and clustered suburban development on a rural Virginia tract found that clustering 

would convert 75 percent less land, create 42 percent less impervious surface, and produce 

41 percent less stormwater runoff (Pollard, 2001). These studies suggest that a low-density 

approach to development is not always the preferred strategy for protecting water resources. 

Conclusions 

Our regions, cities, towns, and neighborhoods are growing. Every day, new buildings or 

houses are proposed, planned, and built. Local governments, working with planners, citizen 

groups, and developers, are thinking about where and how this new development can 

enhance existing neighborhoods and also protect the community’s natural environment. 

They are identifying the characteristics of development that can build vibrant neighborhoods, rich in natural and historic assets, with jobs, housing, and amenities for all types of 

people. They are directing growth to areas that will maintain and improve the buildings and 

infrastructure in which they have already invested. In addition to enjoying the many benefits 

of growth, communities are also grappling with growth’s challenges, including development’s impact on water resources. 

Many communities assume that low-density development automatically protects water 

resources. This study has shown that this assumption is flawed and that pursuit of low-density 

development can in fact be counterproductive, contributing to high rates of land conversion 

and stormwater runoff and missing opportunities to preserve valuable land within watersheds. 

The purpose of this study is to explore the effects of development density on stormwater runoff 

and to illustrate the problems with the assumption that low-density development is automatically a better strategy to protect water quality. To that end, three different development densities 

were modeled at the one-acre, lot, and watershed levels, as well as in the time series build-out 

examples. The modeling results suggest that low-density development is not always the preferred strategy for protecting water resources. Furthermore, the results seem to suggest that 

higher-density development could better protect regional water quality because it consumes 

less land to accommodate the same number of homes. 

However, while this study shows that low-density development does not automatically better 

protect water resources, it does not conclude that high-density development is therefore necessarily more protective. This study has not considered all factors, such as location of development 

within the watershed, varying soil types, slope, advanced post-construction controls (and their 

performance over time), and many other factors. In that sense, this study concludes that there 

32 
are good reasons to consider higher-density development Additional relevant inforas a strategy that can better protect water resources than mation can be found in lower-density development. However, any bias toward these resources: either is inappropriate from a water perspective. A superior 

approach to protect water resources locally is likely to be • Protecting Water Resources 

some combination of development densities, based on with Smart Growth, available 

local factors, incorporating adequate open space, preserv- at: www.epa.gov/smarting critical ecological and buffer areas, and growth/pdf/waterresources minimizing land disturbance. _with_sg.pdf. 

These conclusions have implications for how communities 

• Creating Great Neighbor- can enjoy the benefits of growth and development while 

also protecting their water quality. Additional relevant inforhoods: Density in Your 

mation can be found in other resources, such as Protecting Community, available at: 

Water Resources with Smart Growth and Using Smart Growth www.epa.gov/smart 

Techniques as Stormwater Best Management Practices.

13 Both growth/pdf/density.pdf. 

publications draw on the experience of local governments, 

which has shown that regional and site-specific strategies are most effective when implemented 

together. In addition, Creating Great Neighborhoods: Density in Your Community, by the Local 

Government Commission and the National Association of Realtors, can provide 

information on some of the other benefits from density that communities can enjoy. 

Nationwide, state and local governments are considering the environmental implications of 

development patterns. As low-density development and its attendant infrastructure consume 

previously undeveloped land and create stretches of impervious cover throughout a region, the 

environment is increasingly affected. In turn, these land alterations are not only likely to degrade 

the quality of the individual watershed, but are also likely to degrade a larger number of watersheds. EPA believes that increasing development densities is one strategy communities can use 

to minimize regional water quality impacts. 

13 Forthcoming EPA publication. 

33 
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