United States

Environmental Protection

Agency

EPA 231-R-06-001 

January 2006 

www.epa.gov/smartgrowth

PROTECTING WATER RESOURCES

WITH HIGHER-DENSITY DEVELOPMENT

Office of Sustainable Communities

Smart Growth Program
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 
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

Assistant Administrator

Office of Water

Brian F. Mannix

Associate Administrator

Office of Policy, Economics, and 

Innovation
PROTECTING WATER RESOURCES

WITH HIGHER-DENSITY DEVELOPMENT
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
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
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

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 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 st

1997, when population in the contiguous United States

grew by about 15 percent, developed land increased by

25 million acres, or 34 percent. Most of this growth is taking place at the edge of developed areas, on greenfield

sites, which can include forestland, meadows, pasture,

and rangeland (USDA, 1997a). Indeed, in one analysis of

building permits in 22 metropolitan areas between 1989

and 1998, approximately 95 percent of building permits

ates (USDA, 1997b). From 1982 to

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

rated

 more

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

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

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

Which is a better strategy

to protect water quality:

low- or high-density

development?

Between 1954 and 1997,

urban land area almost

quadrupled, from 18.6 million acres to about 74 

million acres in the contiguous 48 states. 
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

3

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. 
Protecting Water Resources with Higher-Density Development

4

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
5

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

Wetlands, such as this one in Butte County, California, provide sedimentation and turbidity. Organic critical watershed services for the region. 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. 

A NRCS tesy of USD ouro cPhot
Protecting Water Resources with Higher-Density Development

6

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, accumuPhot

lated pollutants are quickly washed off

o cour

and rapidly delivered to aquatic sys- t

tems. Studies have demonstrated that

esy of USD

at 10 percent imperviousness,2 a

A NRCS

watershed is likely to become

impaired (Schueler, 1996; Caraco, 1998; Current construction practices generally disturb the entire

development site, as shown by this site in Des Moines, Iowa. 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.
7

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

provide “open space” for infiltration of water. However,

because of construction practices, the soil becomes

compacted by heavy equipment and filling of depressions (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.

Lawns still contribute to

runoff because they are

compacted and disturbed.
Protecting Water Resources with Higher-Density Development

8

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

supporting those sites, such as roads and parking lots.

Lower-density development can require substantially

higher amounts of this infrastructure per house and

per acre than denser developments. Recent research

has demonstrated that on sites with two homes per

acre, impervious surfaces attributed to streets, driveways, and parking lots can represent upwards of 75

percent of the total site imperviousness (Cappiella,

2001). That number decreases to 56 percent on sites

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

to the region still comes, regardless of density limits in

a particular place. Forecasting future population

growth is a standard task for metropolitan planning

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 

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, and extension of 

infrastructure.

Growth is still coming 

to a region, regardless 

of density limits in a 

particular place.

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>. 
9

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.
Protecting Water Resources with Higher-Density Development

10

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.
11

• 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

Scenario A:

One house per

acre

1 house per acre

8 houses built 

on 8 acres 

Scenario B:

Four houses

per acre

4 houses per acre

8 houses built 

on 2 acres

Scenario C:

Eight houses

per acre

8 houses per acre

8 houses built 

on 1 acre
Protecting Water Resources with Higher-Density Development

12

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.
13

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

One Acre

Scenario A

1 house 

Scenario B

4 houses 

Scenario C

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 

Scenario A Scenario B Scenario C

Impervious cover = 20%

Runoff/acre = 18,700 ft3

/yr

Runoff/unit = 18,700 ft3

/yr

Impervious cover = 38%

Runoff/acre = 24,800 ft3

/yr

Runoff/unit = 6,200 ft3

/yr

Impervious cover = 65%

Runoff/acre = 39,600 ft3

/yr

Runoff/unit = 4,950 ft3

/yr
Protecting Water Resources with Higher-Density Development

14

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 (ft3

/yr). Scenario C, with eight houses, produces 39,600 ft3/yr, and

Scenario B falls between Scenarios A and C at 24,800 ft3

/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 ft3

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

Scenario C, however, each house produces 4,950 ft3

/yr average runoff. The middle scenario,

Scenario B, produces considerably less runoff—6,200

ft3

/yr—per house than Scenario A, but more than

Scenario C. Each house in Scenario B produces approximately 67 percent less runoff than a house in Scenario A,

and each house in Scenario C produces 74 percent less

runoff than a house in Scenario A. This is because the

houses in Scenarios B and C create less impervious surface per house than the house in Scenario A. Therefore,

per house, each home in the higher-density communities

results in less stormwater runoff. 

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

Each house in Scenario B

produces approximately 

67 percent less runoff than 

a house in Scenario A, and

each house in Scenario C

produces 74 percent less

runoff than a house in

Scenario A.

Scale of Analysis Scenario A Scenario B Scenario C

Lot 8 houses built on

8 acres 

8 houses built on

2 acres

8 houses built on

1 acre
EXHIBIT 4: Each Scenario Accommodates Eight Houses

15

Scenario A

Impervious cover = 20% Total runoff (18,700 ft3

/yr x 

8 acres) = 149,600 ft3

/yr

Runoff/house =

18,700 ft3/yr

Scenario B

Impervious cover = 38% Total runoff (24,800 ft3

/yr x 

2 acres) = 49,600 ft3

/yr

Runoff/house = 

6,200 ft3/yr

Scenario C

Impervious cover = 65% Total runoff = 39,600 ft3/yr Runoff/house = 

4,950 ft3

/yr
Protecting Water Resources with Higher-Density Development

16

For each development to accommodate the same number of houses, the lower-density scenarios require more

land to accommodate the same number of houses that

Scenario C has accommodated on one acre. Specifically,

Scenario A must develop seven additional acres, or eight

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 ft3

/yr (18,700 ft3/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 ft3

/yr (24,800 ft3/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.

The increase in runoff 

for Scenario A is due to 

the additional land 

consumption.
17

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

Scenario A

10,000 houses built on

10,000 acres produce:

10,000 acres x 1 house

x 18,700 ft3/yr of 

runoff =

187 million ft3

/yr of

stormwater runoff

Site: 20% impervious

cover 

Watershed: 20%

impervious cover

Scenario B

10,000 houses built on

2,500 acres produce: 

2,500 acres x 4 houses

x 6,200 ft3/yr of 

runoff =

62 million ft3

/yr 

of stormwater runoff

Site: 38% impervious

cover 

Watershed: 9.5%

impervious cover

Scenario C

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 ft3

/yr of

stormwater runoff

Site: 65% impervious

cover 

Watershed: 8.1%

impervious cover

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 ft3/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

impervious cover for the watershed decreases as site density increases. Scenario C, which has a lot-level imperviousness of 65 percent, has a watershed-level imperviousness of only 8.1 percent, which is lower than the 10 

Overall impervious 

cover for the watershed decreases as site 

density increases.
Protecting Water Resources with Higher- Density Development

18

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

stormwater impacts because they consume less land than

their lower-density counterparts. For example, imagine if

Manhattan, which accommodates 1.54 million people on

14,720 acres (23 square miles) (Census, 2000), were developed not at its current density of 52 houses per acre, but

at one or four houses per acre. At one house per acre,

Manhattan would need approximately 750,000 more

acres, or an additional 1,170 square miles, to accommodate 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.

At one house per acre,

Manhattan would need

approximately 750,000

more acres, or an additional 1,170 square miles, 

to accommodate its current

population at two people

per household.

Scale of Analysis

Watershed—Each 10,000-

acre watershed is fully built

out at different densities

Scenario A

10,000 houses

built on 10,000

acres

Scenario B

40,000 houses

built on 10,000

acres

Scenario C

80,000 houses

built on 10,000

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

Scenario A

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

Scenario B

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

Scenario C

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

19
Protecting Water Resources with Higher-Density Development

20

Each watershed is fully built out, and the watershed

developed at the highest density (Scenario C) is generating approximately double the total stormwater runoff of

Scenario A. Scenario B is generating approximately onethird more runoff than Scenario A. Similar to the acrelevel and lot-level results, Scenario C has the highest

degree of impervious cover at 65 percent, while Scenario

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

1 house per

acre—80,000

houses consume 8 

watersheds

Scenario B

4 houses per

acre—80,000

houses consume 2 

watersheds

Scenario C

8 houses per

acre—80,000

houses consume 1 

watershed

Watershed—Each scenario

accommodates the same

number of houses

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.

Scenarios A and B accommodate only a small portion of the expected

growth. The rest will 

have to be built in 

other watersheds.
EXHIBIT 7: 80,000 Houses Accommodated 

Scenario A

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

Scenario B

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

Scenario C

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

22

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 accommodate the additional 70,000 homes already accommodated in Scenario C, it must develop another seven

watersheds. This generates 1.496 billion ft3

/yr of

stormwater runoff. Scenario C, with a development density of eight houses per acre, has still developed just one

watershed and is generating approximately 74 percent

less stormwater runoff than Scenario A—or 396 million

ft3

/yr. Scenario B, at four houses per acre, is generating

496 million ft3

/yr runoff, or two-thirds less runoff than

Scenario A, but 100 million ft3

/yr more than Scenario C.

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

Scale of Analysis Scenario A

10,000 houses

built on 10,000

acres

Scenario B

10,000 houses

built on 2,500

acres

Scenario C

10,000 houses

built on 1,250

acres

Hypothetical build-out in 

the year 2000

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.

Scenario A would need to 

develop an additional seven

watersheds and Scenario B

would need to develop one

additional watershed in

order to accommodate 

the same growth found 

in Scenario C.
23

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

Scenario A

10,000 houses on

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

consume 1 entire

watershed. 

Scenario B

10,000 houses on 

2,500 acres at a density

of 4 houses per acre

consume 1⁄4 of 1 

watershed. 

Scenario C

10,000 houses on

1,250 acres at a density

of 8 houses per acre

consume 1

/8 of 1

watershed. 

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

Hypothetical build-out in the

year 2020

Scenario A

20,000 houses

built on 20,000

acres, or 2

watersheds

Scenario B

20,000 houses

built on 5,000

acres, or 1⁄2 of 1

watershed

Scenario C

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. 
Protecting Water Resources with Higher-Density Development

24

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

Scenario A

20,000 houses accommodated on 20,000

acres at a density of 1

house per acre will consume 2 watersheds. 

Scenario B

20,000 houses accommodated on 5,000

acres at a density of 4

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

Scenario C

20,000 houses accommodated on 2,500

acres at a density of

eight houses per acre

will consume 1⁄4 of 1

watershed. 

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

Hypothetical build-out in 

the year 2040

Scenario A

40,000 houses

built on 40,000

acres, or 4

watersheds

Scenario B

40,000 houses

built on 10,000

acres, or 1

watershed

Scenario C

40,000 houses

built on 5,000

acres, or 1⁄2 of 1

watershed
25

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 

Scenario A

40,000 houses on

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

will consume 4 

watersheds. 

Scenario B

40,000 houses on

10,000 acres at a density of 4 houses per

acre will consume 1

watershed. 

Scenario C

40,000 houses on 

5,000 acres at a density

of 8 houses per acre

will consume 1⁄2 of 1

watershed. 

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. 

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. 

Lower-density development always requires

more land than higher

densities to accommodate

the same amount of

growth.
Protecting Water Resources with Higher-Density Development

26

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. 
EXHIBIT 11: Summary of Findings

27

Scenario Number of

Acres 

Developed

Impervious

Cover 

(%)

Total

Runoff

(ft3

/yr)

Runoff

Per Unit

(ft3

/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

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

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

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

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

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

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

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
Protecting Water Resources with Higher-Density Development

28

Specifically, this analysis demonstrates: 

• With more dense development (Scenario C), runoff

rates per house decrease by approximately 74 percent from the least dense scenario (Scenario A); 

• For the same amount of development, denser development produces less runoff and less impervious

cover than low-density development; and

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

EPA found that the higherdensity scenarios generate

less stormwater runoff per

house at all scales—one

acre, lot, watershed—and

time series build-out

examples.

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
29

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

include 1,200 homes (public housing, affordable and market rate rentals, and for-sale units), local retail, a farmers

market, a senior housing facility, a daycare center, a

health clinic, commercial office space, and an expanded

community center. Among the most important priorities

for the redevelopment is restoring the water quality of

Swan Creek, which forms the eastern edge of Salishan.

The creek is a spawning ground for indigenous salmon

populations that feed into the Puyallup River and Puget

Sound. The site plan seeks to restore 65 percent of the

land to forest and pervious landscape. In addition, the

streets will be narrowed to reduce impervious surfaces

and also make the neighborhood more inviting for walking. Some streets may be eliminated and replaced with

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. 

Salishan Housing District 

is replacing 855 public

housing units with 1,200

units. Numerous site-level

strategies, such as integrating uses, narrowing

the streets, installing rain

gardens, and daylighting a

stream, are used to restore

the water quality of Swan

Creek and revitalize an

existing neighborhood. 

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.
Protecting Water Resources with Higher-Density Development

30

EXHIBIT 12: Redevelopment of a Greyfield Property

Before Redevelopment After Redevelopment

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

Photos courtesy of Juan Ayala, Invisioneering, for the New Jersey Office of State Planning
31

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

is an opportunity to accommodate new growth without

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

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

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 

Redeveloping brownfield

and greyfield sites can

reduce regional land 

consumption.
Protecting Water Resources with Higher-Density Development

32

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
33

are good reasons to consider higher-density development

as a strategy that can better protect water resources than

lower-density development. However, any bias toward

either is inappropriate from a water perspective. A superior

approach to protect water resources locally is likely to be

some combination of development densities, based on

local factors, incorporating adequate open space, preserving critical ecological and buffer areas, and 

minimizing land disturbance. 

These conclusions have implications for how communities

can enjoy the benefits of growth and development while

also protecting their water quality. Additional relevant information can be found in other resources, such as Protecting

Water Resources with Smart Growth and Using Smart Growth

Techniques as Stormwater Best Management Practices.13 Both

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.

Additional relevant information can be found in

these resources: 

• Protecting Water Resources

with Smart Growth, available

at: www.epa.gov/smartgrowth/pdf/waterresources

_with_sg.pdf.

• Creating Great Neighborhoods: Density in Your

Community, available at:

www.epa.gov/smart

growth/pdf/density.pdf. 

13 Forthcoming EPA publication.
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