PROTECTING WATER RESOURCES
WITH HIGHER-DENSITY DEVELOPMENT
United States
Environmental Protection
Agency
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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 or .
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 cour-
tesy 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 for-
merly a large department store surrounded by surface parking. Photo courtesy of US EPA.
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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 informa-
tion necessary to address current environmental challenges. It is our belief that good envi-
ronmental 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 neighbor-
hoods, 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 pro-
fessionals, communities, local governments, and state and regional planners who are grap-
pling with protecting or enhancing their water resources while accommodating growing
populations. We hope that you find this report informative as your community strives to
enjoy the many benefits of growth and development and cleaner water.
For additional free copies, please send an e-mail to ncepimal@one.net or call (800) 490-9198
and request EPA publication 231-R-06-001. If you have any questions concerning this study,
please do not hesitate to contact Lynn Richards at (202) 566-2858.
Sincerely,
Ben Grumbles Brian F. Mannix
Assistant Administrator Associate Administrator
Office of Water Office of Policy, Economics, and
Innovation
Internet Address (URL) • http://www.epa.gov
Recycled/Recyclable .Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 50% Postconsumer content)
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PROTECTING WATER RESOURCES
WITH HIGHER-DENSITY DEVELOPMENT
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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
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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
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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-den-
sity development will best protect water resources. However, some water-quality experts
argue that this strategy can backfire and actually harm water resources. Higher-density devel-
opment, 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.
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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 im-
pacts of development can make it more difficult for communities to protect their natural
resources. Where and how communities accommodate growth has a profound impact on the
quality of their streams, rivers, lakes, and beaches. Development that uses land efficiently and
protects undisturbed natural lands allows a community to grow and still protect its
water resources.
The U.S. Census Bureau projects that the U.S. population Which is a better Strategy
will grow by 50 million people, or approximately 18 per- tQ protect water quality:
cent, between 2000 and 2020. Many communities are . •''
asking where and how they can accommodate this low~ or high-density
growth while maintaining and improving their water development?
resources. Some communities have interpreted 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.
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 mil-
lion acres to about 74 million acres in the contiguous 48 states (USDA, 1997b). From 1982 to
1997, when population in the contiguous United States
grew by about 15 percent, developed land increased by Between 1954 and 1997,
25 million acres, or 34 percent. Most of this growth is tak- urban land area almost
ing place at the edge of developed areas, on greenfield , , , c -nr .,
•4 u- u • \A t *i A A * quadrupled, from 18.6 mil-
sites, which can include forestland, meadows, pasture, ^ ^
and rangeland (USDA, 1997a). Indeed, in one analysis of lion acres to about 74
building permits in 22 metropolitan areas between 1989 million acres in the COn-
and 1998, approximately 95 percent of building permits tidUOUS 48 States
were on greenfield sites (Farris, 2001).
According to the American Housing Survey, 35 percent of new housing is built on lots
between two and five acres, and the median lot size is just under one-half acre (Census,
2001). Local zoning may encourage building on relatively large lots, in part because local
governments often believe that it helps protect their water quality. Indeed, research has
revealed that more impervious cover can degrade water quality. Studies have demonstrated
that at 10 percent imperviousness, a watershed is likely to become impaired and grows more
so as imperviousness increases (Arnold, 1996; Schueler, 1994).This research has prompted
many communities to adopt low-density zoning and site-level imperviousness limits, e.g.,
establishing a percentage of the site, such as 10 or 20 percent, that can be covered by
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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
25% shallow
infiltration
Natural Ground Cover
25% deep
infiltration
35% evapotranspiration
21% deep
infiltration
1O%-2O% Impervious Surface
3O% evapotranspiration
m , _,
||||
= 5 :•
2O% shallow
infiltration
35%-5O% Impervious Surface
deep
infiltration
1G% shallow
infiltration
75%-1OO% Impervious Surface
deep
infiltration
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-million-
square-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.
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Protecting Water Resources with Higher-Density Development
diverse aquatic communities and perform the vital ecological roles of processing the carbon,
sediments, and nutrients upon which downstream ecosystems depend. Healthy, functioning
watersheds naturally filter pollutants and moderate water quality by slowing surface runoff
and increasing the infiltration of water into soil. The result is less flooding and soil erosion,
cleaner water downstream, and greater ground water reserves.
Land development directly affects watershed functions. When development occurs in previ-
ously 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, develop-
ment 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 ade-
quate 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 dif-
ferent levels of ecological value. For example, a nutrient-rich floodplain has a higher ecologi-
cal 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, con-
tinuous 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 rain-
water. In most regions, this undeveloped land comprises large portions of a watershed, filtering
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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 opportuni-
ties, 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 multi-
faceted 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 ecologi-
cally important land, such as wetlands, buffer zones, riparian corridors, and floodplains, is crit-
ical 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 protec-
tion. For example, if soil conditions are
right, a 20- to 30-foot-wide strip of
woodland removes 90 percent of the
nitrates in stormwater runoff (Trust for
Public Land, 1997). These buffer zones
decrease the amount of pollution
entering the water system. Tree and
shrub roots hold the bank in place,
preventing erosion and its resulting
sedimentation and turbidity. Organic
matter and grasses slow the flow of
runoff, giving the sediment time to settle and water time to percolate, filter through the soil,
and recharge underlying ground water. Research has shown that wetlands and buffer zones,
by slowing and holding water, increase ground water recharge, which directly reduces the
potential for flooding (Schueler, 1994). By identifying and preserving these critical ecological
areas, communities are actively protecting and enhancing their water quality.
Wetlands, such as this one in Butte County, California, provide
critical watershed services for the region.
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Protecting Water Resources with Higher-Density Development
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 com-
pacted 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 com-
paction 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 rela-
tionship between impervious cover
and water quality (Arnold, 1996;
Schueler, 1994; EPA, 1997). Impervious
surfaces collect and accumulate pollu-
tants deposited from the atmosphere,
leaked from vehicles, or derived from
other sources. During storms, accumu-
lated pollutants are quickly washed off
and rapidly delivered to aquatic sys-
tems. Studies have demonstrated that
at 10 percent imperviousness,2a
watershed is likely to become
impaired (Schueler, 1996; Caraco, 1998;
Montgomery County, 2000), the
Current construction practices generally disturb the entire
development site, as shown by this site in Des Moines, Iowa.
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 water-
shed 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 den-
sities 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 meet-
ing its designated water quality standard. However, the term can also imply a decline or absence of biological integrity; for example, the water-
body 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.
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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 exam-
ines 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 low-
density 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 foot-
print 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 developmen t often acts like impervious surface.
In general, impervious surfaces, such as a structure's footprint, driveways, and roads, have
higher amounts of runoff and associated pollutants than pervious surfaces. However,
most lawns, though pervious, still contribute to runoff
because they are compacted. Lawns are thought to Lawns Still contribute to
provide "open space"for infiltration of water. However, runoff because they are
because of construction practices, the soil becomes
compacted by heavy equipment and filling of depres- compacted and disturbed.
sions (Schueler, 1995, 2000). The effects of this com-
paction 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 develop-
ment (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.
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Protecting Water Resources with Higher-Density Development
vary dramatically between housing types. For example, in some dispersed, low-density
communities, such as Fairfax County, Virginia, some homeowners are paving their front
lawns to create more parking for their cars (Rein, 2002).
3. Low-density developments often mean more off-site impervious infrastructure. Development
in the watershed is not simply the sum of the sites within it. Rather, total impervious area
in a watershed is the sum of site developments plus
the impervious surface associated with infrastructure Water quality Suffers not
supporting those sites, such as roads and parking lots, only from the increase in
Lower-density development can require substantially j vjous surf but a|SQ
higher amounts of this infrastructure per house and r
per acre than denser developments. Recent research from tne associated activi-
has demonstrated that on sites with two homes per ties: construction, increased
acre, impervious surfaces attributed to streets, drive- travel to and from the devel-
ways, and parking lots can represent upwards of 75 Q t and extensjon of
percent of the total site imperviousness (Cappiella,
2001). That number decreases to 56 percent on sites infrastructure.
with eight homes per acre.This research indicates
that low densities often require more off-site transportation-related impervious infra-
structure, 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 develop-
ment, extension of infrastructure, and chemical maintenance of the areas in and sur-
rounding the development. Oil and other waste products, such as heavy metals, from
motor vehicles, lawn fertilizers, and other common solvents, combined with the increased
flow of runoff, contribute substantially to water pollution. As imperviousness increases, so
do associated activities, thereby increasing the impact on water quality.
4. If growth is coming to the region, limiting density on a given site does not eliminate that
growth. Density limits constrain the amount of development on a site but have little
effect on the region's total growth (Pendall, 1999,
2000). The rest of the growth that was going to come Growth is Still coming
to the region still comes, regardless of density limits in to a region, regardless
a particular place. Forecasting future population of density limits in a
growth is a standard task for metropolitan planning .. • •
organizations as they plan where and how to accom- ^ ^
modate growth in their region. They project future
population growth based on standard regional population modeling practices, where
wage or amenity differentials, such as climate or culture (Mills, 1994)—and not zoning
practices such as density limits—account for most of a metropolitan area's population
gain or loss.5 While estimates of future growth within a particular time frame are rarely
precise, a region must use a fixed amount of growth to test the effects of adopting
5 The most widely-used such model—the REMI8 Policy Insight™ model—uses an amenity variable. However, even this is implemented as an
additional change in the wage rate. See Remi Model Structure. .The in-
house model used by the San Diego Association of Governments is an advanced example of the type used by councils of governments
around thecountry..
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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 sin-
gle-family, detached housing. EPA examined the stormwater impacts from each density sce-
nario 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 num-
ber 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 sim-
ple 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 ) to Cobb
County, Georgia, outside of Atlanta, which requires between one and two units per acre in its low-density residential districts (). 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 (), 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 fora 10-year period (1992-2001, inclusive) were used.
-------
Protecting Water Resources with Higher-Density Development
can be significant since the majority of storms are small storms. In addition, the curve num-
bers 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 develop-
ment 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 addi-
tional stormwater benefits, that may be realized by appropriately locating the develop-
ment 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.11 Whether
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 fre-
quent 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 hous-
ing 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-densi-
ty scenarios become relatively less dense while the higher-density scenarios become relatively more dense. In general, low-density residen-
tial development tends to be associated with low-density commercial development, characterized by large retail spaces, wide roads, large
parking lots, and minimal public transportation. Higher-density residential areas are more likely to have high-density commercial options,
with smaller retail spaces, mixed land uses, narrower streets, parking garages, on-street parking, and sometimes a well-developed public
transportation system, which can reduce parking needs.
10
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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 accom-
modates 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 build-
ing 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
Scenario A:
One house per
Scenario B:
Four houses
per acre
Scenario C:
Eight houses
per acre
1 house per acre 4 houses per acre 8 houses per acre
Example 2: Lot—Each deve-
lopment lot accommodates
the same number of houses
8 houses built
on 8 acres
8 houses built
on 2 acres
8 houses built
on 1 acre
11
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Protecting Water Resources with Higher-Density Development
Example 3: Watershed—
Each 10,000-acre water-
shed accommodates the
same number of houses
10,000 houses
built on 10,000
acres
10,000 houses
built on 2,500
acres or 14 of
the watershed
10,000 houses
built on 1,250
acres or V8 of
the watershed
Example 4: Watershed—
Each 10,000-acre water-
shed is fully built out at
different densities
Example 5: Watershed—
Each scenario accommo-
dates the same number
of houses
Example 6: Hypothetical
build-out in the year 2000
Example 7: Hypothetical
build-out in the year
2020
Example 8: Hypothetical
build-out in the year
2040
10,000 houses
built on 10,000
acres
80,000 houses
consume 8
watersheds
10,000 ho uses
built on 10,000
acres
20,000 houses
built on 20,000
acres, or 2 water-
sheds
40,000 houses
built on 40,000
acres, or 4 water-
sheds
40,000 houses
built on 10,000
acres
80,000 house
consume 2
watersheds
10,000 houses
built on 2,500
acres
20,000 houses
built on 5,000
acres, or 1/2 of 1
watershed
40,000 houses
built on 10,000
acres, or 1
watershed
80,000 houses
built on 10,000
acres
80,000 houses
consume 1
watershed
10,000 houses
built on 1,250
acres
20,000 houses
built on 2,500
acres, or 14 of 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 underly-
ing 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 popu-
lation increases in their regions.
2. Housing density affects the distribution of new growth within a given region, not the
amount of growth. Individual states and regions grow at different rates depending on
a variety of factors, including macroeconomic trends (e.g., the technology boom in the
1980s spurring development in the Silicon Valley region in California) and demographic
shifts. Distribution and density of new development do not significantly affect these factors.
12
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3. The model focuses on the comparative differences in stormwater runoff between scenar-
ios, not absolute values. As discussed, using the curve number and event mean concen-
tration 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 simplifica-
tions 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
Scenario B
Scenario C
1 house
4 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
Impervious cover = 20%
Runoff/acre = 18,700 ftVyr
Runoff/unit = 18,700 ftVyr
Scenario B
Impervious cover = 38%
Runoff/acre = 24,800 ftVyr
Runoff/unit = 6,200 ftVyr
Scenario C
Impervious cover = 65%
Runoff/acre = 39,600 ftVyr
Runoff/unit = 4,950 ftVyr
13
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Protecting Water Resources with Higher-Density Development
Examining the estimated average annual runoff at the acre level, as illustrated in Exhibit 4,
the low-density Scenario A, with just one house, produces an average runoff volume of
18,700 cubic feet per year (ftVyr). Scenario C, with eight houses, produces 39,600 ftVyr, and
Scenario B falls between Scenarios A and C at 24,800 ftVyr. In short, looking at the compara-
tive differences between scenarios, runoff roughly doubles as the number of houses increas-
es 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 ftVyr, the same as the per acre level. In the denser
Scenario C, however, each house produces 4,950 ftVyr average runoff.The middle scenario,
Scenario B, produces considerably less runoff—6,200
ftVyr—per house than Scenario A, but more than
Scenario C. Each house in Scenario B produces approxi-
mately 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 sur-
face per house than the house in Scenario A. Therefore,
per house, each home in the higher-density communities
results in less stormwater runoff.
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.
Modeling at the acre level demonstrates that, in this
example, when density is quadrupled (from one house
to four houses), stormwater runoff increases by one-
third 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 mini-
mize water quality impacts compared to the higher-density scenarios. However, when
measured by the house, higher densities produce less stormwater runoff.
EXAMPLE 2: LOT LEVEL
Scale of Analysis
Scenario A
Scenario B
Scenario C
8 houses built on
8 acres
8 houses built on
2 acres
8 houses built
1 acre
14
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EXHIBIT 4: Each Scenario Accommodates Eight Houses
Impervious cover = 20%
Total runoff (18,700 ftYyrx
8 acres) = 149,600 ftVyr
Runoff/house =
18,700 ftVyr
Scenario B
Impervious cover = 38%
Total runoff (24,800 ftVyr x
2 acres) = 49,600 ftVyr
Runoff/house
6,200 ftVyr
Scenario C
****
Impervious cover = 65% Total runoff = 39,600 ftVyr
Runoff/house
4,950 ftVyr
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Protecting Water Resources with Higher-Density Development
For each development to accommodate the same num-
ber 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.
The increase in runoff
for Scenario A is due to
the additional land
consumption.
With each scenario accommodating the same number of houses, this analysis shows that
total average runoff in Scenario A is 149,600 ftVyr (18,700 ftVyrx 8 acres), which is a 278 per-
cent increase from the 39,600 ft3/yr total runoff in Scenario C. Total average runoff from eight
houses in Scenario B is 49,600 ftVyr (24,800 ftVyr 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 con-
sumption 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
Watershed—Each 10,000-acre
watershed accommodates
the same number of houses
Scenario A
Scenario B
Scenario C
10,000 houses 10,000 houses 10,000 houses
built on 10,000 built on 2,500 built on 1,250
Taking the analysis to the watershed level, EPA examined the comparative watershed
stormwater runoff impacts from accommodating growth at different densities. The water-
shed used in this analysis is a hypothetical 10,000-acre watershed accommodating only
houses. As discussed, the modeling does not include retail, business centers, farms, or any
other land uses typically seen in communities, nor does it take into consideration where the
development occurs within the watershed. Research has shown that upper sub-watersheds,
which contain smaller streams, are generally more sensitive to development than lower
sub-watersheds (Center for Watershed Protection, 2001).
Accommodating 10,000 houses at one house per acre in the 10,000-acre watershed would
fully build out the watershed. At the higher density of four houses per acre, one-quarter of the
watershed would be developed, and at eight houses per acre, one-eighth of the watershed
would be developed. Exhibit 5 shows the runoff associated with each of these scenarios.
16
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EXHIBITS: 10,000-Acre Watershed Accommodating 10,000 Houses
Scenario A
Scenario B
Scenario C
10,000 houses built on
10,000 acres produce:
10,000 acres x 1 house
x18,700 ft3/yr of
runoff =
187 million ft3/yr of
stormwater runoff
Site: 20% impervious
cover
Watershed: 20%
impervious cover
10,000 houses built on
2,500 acres produce:
2,500 acres x 4 houses
x 6,200 ft3/yr of
runoff =
62 million ft3/yr
of stormwater runoff
Site: 38% impervious
cover
Watershed: 9.5%
impervious cover
10,000 houses built on
1,250 acres produce:
1,250 acres x 8 houses
x 4,950 ft3/yr of
runoff =
49.5 million 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 mil-
lion 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 den-
sity increases. Scenario C, which has a lot-level impervi-
ousness of 65 percent, has a watershed-level impervious-
ness of only 8.1 percent, which is lower than the 10
Overall impervious
cover for the water-
shed decreases as site
density increases.
17
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Protecting Water Resources with Higher- Density Development
percent threshold discussed earlier. Scenario B, with a density of four houses per acre, has a
site-level impervious cover of 38 percent, but a watershed imperviousness of 9.5 percent, which
is still lower than the 10 percent threshold. Finally, Scenario A, at a lot-level imperviousness of
20 percent, has the same overall imperviousness at the watershed level. Both of the higher-
density 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 devel-
oped 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 accommo-
At one house per acre,
Manhattan would need
approximately 750,000
more acres, or an addi-
tional 1,170 square miles,
to accommodate its current
population at two people
per household.
date its current population at two people per household.
That's approximately the size of Rhode Island. At four houses per acre, Manhattan would
need approximately 175,000 more acres, or an additional 273 square miles.
Reducing land consumption is crucial to preserving water quality because, as discussed pre-
viously, 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 headwa-
ters, wetlands, riparian corridors, and floodplains.
EXAMPLE 4: REMAINING LAND IN THE WATERSHED DEVELOPED
What happens if the remaining undeveloped parts of the watershed in Scenarios B and C are
developed? Exhibit 6 considers this situation.
Scale of Analysis
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
uuu
18
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EXHIBIT 6:10,000-Acre Watershed Accommodating Different Numbers of Houses
I
Scenario A
t
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
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Protecting Water Resources with Higher-Density Development
Each watershed is fully built out, and the watershed
developed at the highest density (Scenario C) is generat-
ing approximately double the total stormwater runoff of
Scenario A. Scenario B is generating approximately one-
third more runoff than Scenario A. Similar to the acre-
level 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.
Scenarios A and B accom-
modate only a small por-
tion of the expected
growth. The rest will
have to be built in
other watersheds.
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 commu-
nity 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 accommo-
dated 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 accom-
modate 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
Watershed—Each scenario
accommodates the same
number of houses
Scenario A
1 house per
acre—80,000
houses con-
sume 8
watersheds
Scenario B
4 houses per
acre—80,000
houses con-
sume 2
watersheds
Scenario C
8 houses per
acre—80,000
houses con-
sume 1
watershed
As discussed, the U.S. population will increase by an estimated 50 million people by 2020.
Different areas of the country will grow at different rates in the future. Whether a region
anticipates 1,000 or 80,000 new households to come to the region over the next 10 years,
comparisons between build-out scenarios must keep the number of homes consistent. In this
case, if Scenario C is developed so that its entire watershed is built out to 80,000 houses, then
for a fair comparison, Scenarios A and B must also include 80,000 houses. Exhibit 7 illustrates
this situation.
20
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EXHIBIT 7:80,000 Houses Accommodated
Scenario A
Scenario B
At 1 house per acre,
80,000 houses require
80,000 acres, or 8 wafer-
sheds, 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 C
At 4 houses per acre,
80,000 houses require
20,000 acres, or 2 wafer-
sheds, translating to:
20,000 acres x 4 houses x
6,200 ft3/yr of runoff =
496 million ft3/yr of
stormwater runoff
2 watersheds at 38%
impervious cover
At 8 houses per acre,
80,000 houses require
10,000 acres, or 7 wafer-
shed, 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
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Protecting Water Resources with Higher-Density Development
When the number of houses is kept consistent, Scenario A would need to develop an addi-
tional seven watersheds (assuming the same size watersheds) and Scenario B would need to
develop one additional watershed to accommodate the same growth found in Scenario C.
As Exhibit 7 demonstrates, for Scenario A to accommo-
date the additional 70,000 homes already accommodat-
ed in Scenario C, it must develop another seven
watersheds.This generates 1.496 billion ftVyr of
stormwater runoff. Scenario C, with a development den-
sity 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
ftVyr. Scenario B, at four houses per acre, is generating
496 million ftVyr runoff, or two-thirds less runoff than
Scenario A, but 100 million ftVyr more than Scenario C.
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.
EXAMPLE 6: TIME SERIES BUILD-OUT ANALYSIS: BUILD-OUT IN 2000
Scale of Analysis
Hypothetical build-out in
the year 2000
Scenario A
Scenario B
Scenario C
10,000 houses
built on 10,000
acres
10,000 houses
built on 2,500
acres
10,000 houses
built on 1,250
acres
Another way to examine this issue is to look at what happens to build-out of the three sce-
narios 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 dif-
ference between the watersheds is the densities at which the building occurs. In 2000, they
might look something like Exhibit 8.
22
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EXHIBIT 8: Time Series Build-out Analysis: Build-out in 2000
Scenario A
Scenario B
Scenario C
t
10,000 houses on
10,000 acres at a densi-
ty of 1 house per acre
consume 1 entire
watershed.
10,000 houses on
2,500 acres at a density
of 4 houses per acre
consume 14 of 1
watershed.
10,000 houses on
1,250 acres at a density
of 8 houses per acre
consume Vsof 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 V* of 1
watershed
Scenario C
20,000 houses
built on 2,500
acres, or 14 of 1
watershed
Fast-forwarding 20 years, the population in the hypothetical community has doubled from
10,000 houses to 20,000 houses. Each scenario must accommodate this additional growth at
different development densities. Exhibit 9 demonstrates how this development might look.
23
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Protecting Water Resources with Higher-Density Development
EXHIBIT 9: Time Series Build-out Analysis: Build-out in 2020
Scenario A
Scenario B
Scenario C
20,000 houses accom-
modated on 20,000
acres at a density of 7
house per acre will con-
sume 2 watersheds.
20,000 houses accom-
modated on 5,000
acres at a density of 4
houses per acre will con-
sume !/2 of 1 watershed.
20,000 houses accom-
modated on 2,500
acres at a density of
eight houses per acre
will consume 14 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
Scenario C
40,000 houses
built on 10,000
acres, or 1
watershed
40,000 houses
built on 5,000
acres, or V-i of 1
watershed
24
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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
Scenario B
Scenario C
40,000 houses on
40,000 acres at a den-
sity of 1 house per acre
will consume 4
watersheds.
40,000 houses on
10,000 acres at a den-
sity of 4 houses per
acre will consume 1
watershed.
40,000 houses on
5,000 acres at a density
of 8 houses per acre
will consume V* of 1
watershed.
As Exhibit 10 demonstrates, Scenario A, developing at
one house per acre, must develop land in four water-
sheds, 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 pro-
vide watershed services. Scenario A and B do not. Any
land for watershed services would need to come from
additional watersheds.
Lower-density develop-
ment always requires
more land than higher
densities to accommodate
the same amount of
growth.
This build-out analysis can continue indefinitely with the same result: lower-density
development always requires more land than higher densities to accommodate the same
amount of growth. Because more land is required, more undeveloped land is converted.
25
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Protecting Water Resources with Higher-Density Development
Findings/Discussion
The results indicate when runoff is measured by the acre, limiting density does produce less
stormwater runoff when compared to the higher-density scenarios. However, when meas-
ured 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 water-
shed rather than from particular sites—which is more consistent with the science indicat-
ing that imperviousness within the watershed is critical.
To more fully explore this dynamic, EPA modeled scenarios at three scales—one acre, lot, and
watershed—and at three different time series build-out examples to examine the premise
that lower-density development better protects water quality. EPA examined stormwater
runoff from different development densities to determine the comparative difference
between scenarios. The higher-density scenarios generated less stormwater runoff per house
at all scales and time series build-out examples. Exhibit 11 summarizes these findings.
26
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EXHIBIT 11: Summary of Findings
Scenario
Number of Impervious Total Runoff
Acres Cover Runoff Per Unit
Developed (%) (ftVyr) (ftVyr)
Savings
Over
Scenario A:
runoff per
unit (%)
One-Acre Level: Different densities developed on one acre
20.0 18,700
A: One house/acre
B: Four houses/acre
C: Eight houses/acre
18,700
38.0
65.0
24,800 6,200
39,600 4,950
Lot Level: Eight houses accommodated at different density levels
Scenario A
Scenario B
Scenario C
20.0
38.0
65.0
149,600
49,600
9,600
Watershed Level: 10,000 houses accommodated In one 10,000-acre watershed
10,000 20.0 187M 18,700
2,500 9.5 62 M 6,200
1,2f
Scenario A
Scenario B
Scenario C
Scenario
49.5 M
4,950
Summary of Build-out Examples
Scenario A
Watershed Level: Time Series Build-out Analysis: Build-out in 2000
10,000 houses built on 10,000 acres: 1 watershed is consumed
10,000 houses built on 2,500 acres: 14 of 1 watershed is consumed
10,000 houses built on 1,250 acres: Vs of 1 watershed is consumed
Scenario C
Watershed Level: Time Series Build-out Analysis: Build-out in 2020
Scenario A
Scenario B
Scenario C
20,000 houses built on 20,000 acres: 2 watersheds are consumed
20,000 houses built on 5,000 acres: 1/2 of 1 watershed is consumed
20,000 houses built on 2,500 acres: 14 of 1 watershed is consumed
Scenario A
Watershed Level: Time Series Build-out Analysis: Build-out in 2040
40,000 houses built on 40,000 acres: 4 watersheds are consumed
40,000 houses built on 10,000 acres: 1 watershed is consumed
40,000 houses built on 5,000 acres: 1/2 of 1 watershed is consumed
Scenario C
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Protecting Water Resources with Higher-Density Development
Specifically, this analysis demonstrates:
• With more dense development (Scenario C), runoff
rates per house decrease by approximately 74 per-
cent from the least dense scenario (Scenario A);
• For the same amount of development, denser devel-
opment produces less runoff and less impervious
cover than low-density development; and
• For a given amount of growth, lower-density devel-
opment uses more of the watershed.
EPA found that the higher-
density scenarios generate
less stormwater runoff per
house at all scales—one
acre, lot, watershed—and
time series build-out
examples.
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, con-
tinuous 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 water-
bodies. To address this increased impervious cover, numerous site-level techniques are avail-
able 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 interest-
ing 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 stormwa-
ter management strategies in three
common neighborhood development Thecityof Portland, Oregon, is developing urban stormwater
patterns. For example, best manage- strategies, such as these curb extensions that can absorb the
ment practices, such as disconnecting street's runoff from large storm events.
28
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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, consid-
erably 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 address-
ing stormwater at the site level can work effectively. The Salishan Housing District was built
on Tacoma's eastern edge in the 1940s as temporary housing for ship workers. It is currently a
public housing community with 855 units.
Redevelopment of Salishan will increase densities to Salishan Housinq District
include 1,200 homes (public housing, affordable and mar- . rpniarjnn occ ni|hlir
ket rate rentals, and for-sale units), local retail, a farmers . . .
market, a senior housing facility, a daycare center, a housing units with 1,200
health clinic, commercial office space, and an expanded units. Numerous site-level
community center. Among the most important priorities strategies Such as inte-
for the redevelopment is restoring the water quality of . ^ narrowj
Swan Creek, which forms the eastern edge of Salishan. ^ ^ . .
The creek is a spawning ground for indigenous salmon *ne Streets, installing rain
populations that feed into the Puyallup River and Puget gardens, and daylighting a
Sound. The site plan seeks to restore 65 percent of the Stream, are used to restore
land to forest and pervious landscape. In addition, the thfi ^ef ^ Qf $wan
streets will be narrowed to reduce impervious surfaces . '
and also make the neighborhood more inviting for walk- Creek and revitalize an
ing. Some streets may be eliminated and replaced with existing neighborhood.
pedestrian paths. The remaining streets will be bordered
by rain gardens that would accept, filter, and evapotranspire runoff. Most existing street sur-
faces 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 shop-
ping 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 impervi-
ous cover. In many cases, redevelopment of these properties breaks up or removes some
portion of the impervious cover, converting it to pervious cover and allowing for some
stormwater infiltration. In this case, redevelopment of these properties can produce a
net improvement in regional water quality by decreasing total runoff. Exhibit 12
illustrates this opportunity.
12 Greyfield sites generally refer to abandoned or underutilized shopping malls, strip malls, or other areas that have significant paved sur-
face and little or no contamination.
29
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Protecting Water Resources with Higher-Density Development
EXHIBIT 12: Redevelopment of a Greyfield Property
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 con-
sumption. 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 redevel-
oping brownfield sites, regions can
identify underused properties or land,
such as infill or greyfield sites, and tar-
get 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 shop-
ping mall, decreased impervious cover by 15 per-
cent through the addition of this central plaza.
30
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500,000 people (Pryne, 2002). Redeveloping this property Redeveloping brownfield
is an opportunity to accommodate new growth without anc| Qrevfield sites can
expanding into other watersheds. As Kurt Zwikl, execu- . . .
tive director of the Pottstown, Pennsylvania-based reduce regional land
Schuylkill River Greenway Association, said, "Certainly, if we consumption.
can get redevelopment going in brownfields and old indus-
trial 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 differ-
ent 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 neighbor-
hood 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 densi-
ties in the state by directing development to existing communities and existing infrastruc-
ture. Researchers at Rutgers University analyzed the water quality impacts from current
development trends and compared them to water quality impacts from the proposed com-
pact development. The study found that compact development would generate significantly
less water pollution than current development patterns, which are mostly characterized by
low-density development, for all categories of pollutants (Rutgers University, 2000). The
reductions ranged from over 40 percent for phosphorus and nitrogen to 30 percent for
runoff. These conclusions supported a similar statewide study completed in 1992 that
31
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Protecting Water Resources with Higher-Density Development
concluded that compact development would result in 30 percent less runoff and 40 percent
less water pollution than would a lower-density scenario (Burchell, 1995).
Researchers at Purdue University examined two possible project sites in the Chicago area
(Harbor, 2000). The first site was in the city; the second was on the urban fringe. The study
found that placing a hypothetical low-density development on the urban fringe would pro-
duce 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 conven-
tional 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 neighbor-
hoods, 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 develop-
ment'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 automati-
cally 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 pre-
ferred 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 neces-
sarily more protective. This study has not considered all factors, such as location of development
within the watershed, varying soil types, slope, advanced post-construction controls (and their
performance over time), and many other factors. In that sense, this study concludes that there
32
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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, preserv-
ing critical ecological and buffer areas, and
minimizing land disturbance.
Additional relevant infor-
mation can be found in
these resources:
• Protecting Water Resources
with Smart Growth, available
at: www.epa.gov/smart-
g rowth/pdf/waterresou rces
_with_sg.pdf.
• Creating Great Neighbor-
hoods: Density in Your
Community, available at:
www.epa.gov/smart
growth/pdf/density.pdf.
These conclusions have implications for how communities
can enjoy the benefits of growth and development while
also protecting their water quality. Additional relevant infor-
mation can be found in other resources, such as Protecting
Water Resources with Smart Growth and Using Smart Growth
Techniques as Stormwater Best Management Practices.^3 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 water-
sheds. EPA believes that increasing development densities is one strategy communities can use
to minimize regional water quality impacts.
5 Forthcoming EPA publication.
33
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j Recycled/Recyclable-Printed with vegetable oil based inks on 100% (minimum 50% postconsumer) recycled paper.
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•* -
United States
Environmental Protection Agency
(1807-T)
Washington, DC 20460
Official Business
Penalty for Private Use $300
EPA231-R-06-001
January 2006
www.epa.gov/smartgrowth
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