EPA 841 B 13 001
Engineering Urban Forests for
Stormwater Management
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Stormwater
to
Street Trees
Engineering Urban Forests for
Stormwater Management
Mention of trade names, products, or services does not convey official EPA approval,
endorsement, or recommendation.
EPA 841 B 13 001
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Table of Contents
Introduction 1
Using This Guide 2
Section 1. Urban Stormwater Runoff 3
Why Is Urban Stormwater Runoff A Problem? 3
Nonpoint Source Pollution 3
Section 2. The Role Of Trees In Stormwater Management 4
Trees Can Be More Than Just Landscaping 5
Design Sites For Success 6
Grow Bigger Trees to Reduce More Runoff! 6
Street Tree Design Failures 8
In A Well-designed System, Your Street Trees Can 9
Section 3. Stormwater Management Systems with Trees 12
SUSPENDED PAVEMENT AND STRUCTURAL CELLS 12
How Suspended Pavement Benefits Stormwater Management and Trees 12
Design Considerations 14
STRUCTURAL SOIL 15
How Structural Soil Benefits Stormwater Management and Trees 15
pH and Stone Type 16
Design Considerations 16
STORMWATER TREE PITS 18
How Stormwater Tree Pits Benefit Stormwater Management and Trees 18
Design Considerations 18
PERMEABLE PAVEMENTS 20
How Permeable Pavement Benefits Stormwater Management and Trees 20
Design Considerations 20
Other Vegetated Systems Designed to Mimic Nature 23
FORESTED BIOSWALES 23
GREEN ROOFS 24
GREEN STREETS 24
Know the Rules 25
References 25
Section 4. Case Studies 26
Minneapolis, Minnesota: Structural Cells 27
Charlotte, North Carolina: Suspended Pavement 28
Ithaca, New York: Structural Soil 29
Olympia, Washington: Structural Soil 29
Chattanooga, Tennessee: Permeable Pavement 29
Additional Resources and Information 31
Green Building 31
Green Infrastructure 31
Green Streets 31
Greening 31
Low-Impact Development (LID) 31
Technical Guide Assistance 31
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Introduction
The presence of trees in a streetscape,
neighborhood, and community can decrease
the amount of stormwater runoff and pollutants
that reach local waters.
*> Trees reduce stormwater runoff by
capturing and storing rainfall in their
canopy and releasing water into the
atmosphere.
*> Tree roots and leaf litter create soil
conditions that promote the infiltration of
rainwater into the soil.
«J« Trees help slow down and temporarily
store runoff and reduce pollutants by
taking up nutrients and other pollutants
from soils and water through their roots.
*> Trees transform pollutants into less harmful
substances.
Cities employ a variety of measures to manage
stormwater runoff. However, most do not take
advantage of the stormwater utility benefits
trees provide. Grey stormwater systems use
curbs, gutters, drains, pipes, ponds, vaults, and
outfalls to move water quickly to containment
and/or treatment areas or to receiving waters.
Alternatively, green stormwater systems
manage stormwater on site with overflow
ability, creating areas that mimic nature.
Vegetation, swales, wetlands, buffer zones, and
pervious surfaces capture, filter, and slow
stormwater runoff. Volume is managed through
evapotranspiration, infiltration, and soil
moisture recharge.
Trees are natural managers of stormwater.
When included as part of a system engineered
to manage stormwater, they can improve
infiltration and capacity, reducing the overall
amount of runoff. Photo courtesy of Davey
Resource Group.
Stormwater is a problem for cities across the
country. Existing grey and green stormwater
management systems are often not enough to
accommodate runoff. Adding trees to those
systems is a cost-effective way to improve
their function and reduce runoff. Photo
courtesy of Davey Resource Group.
Trees are typically not considered part of either grey or green stormwater management
systems; they are generally, and falsely, considered of landscaping value. Planting a tree as just
landscaping is not taking advantage of the stormwater utility benefits and other environmental
services it provides.
In urban areas, trees are part of the managed municipal infrastructure. A street tree,
which is generally a publicly managed tree found growing within the right-of-way, offers unique
opportunities to increase the effectiveness of grey and green stormwater systems.
With urbanization on the rise and impervious surfaces dominating our urban cores, existing
stormwater and sewer systems are often inadequate to handle peak flows. When a system is
overtaxed, peak flows can blow manhole covers from the ground and back up stormwater and,
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in some cases, even sewage into the streets. To reduce pressure on existing systems and
increase capacity, cities must consider every available option, especially using trees to help
manage stormwater.
Installing trees in locations that are engineered to retain stormwater is a great way to augment
existing stormwater management systems, increasing their capacity and improving water
quality while greatly improving the urban forest canopy. This guide is an introduction to those
engineered systems available, and in use today, that utilize trees to manage a volume of
stormwater. These systems, in addition to providing a solution for managing runoff, also grow
big trees.
USING THIS GUIDE
This guide is divided into the following four sections:
Section 1. Urban Stormwater Runoff defines urban stormwater runoff and explains why it is a
problem.
Section 2. The Role of Trees in Stormwater Management discusses what trees need to grow in
urban environments and how they help manage stormwater.
Section 3. Stormwater Management Systems with Trees provides an introduction to
engineered systems available that utilize trees to manage a volume of stormwater.
Section 4. Case Studies presents projects from throughout the country that have successfully
used trees in the engineered systems discussed.
This guide is intended to help engineers, planners, developers, architects, arborists, and public
officials understand how trees perform and interact in a stormwater management system, and
the new technologies that are being used to increase the stormwater utility function of the
urban forest, even in the densest urban environments.
The illustrations in this guide are not intended to serve as construction drawings. They should
be used to communicate concepts about how properly designed urban tree systems help
reduce stormwater runoff, while concurrently improving tree health.
Although each system is presented independently, a combination of systems uniquely
designed for a specific site will provide the greatest benefits.
Because every project and installation is different, appropriate consideration of the ecoregion,
site, and project goals are a must for a successful outcome.
Always consult with regulatory bodies, engineers, arborists, planners, landscape architects, and
other stakeholders to ensure development plans and project implementation meet site needs
as well as all local, state, and federal regulations and requirements regarding the capture,
detainment, storage, and/or manipulation of stormwater runoff.
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Section 1. Urban Stormwater Runoff
By design and function, urban areas are covered with impervious surfaces such as roofs,
streets, sidewalks, and parking lots. All of those surfaces contribute to urban stormwater
runoff. Urban stormwater runoff is caused when precipitation from rain and snowmelt flows
over land and impervious surfaces without infiltrating the ground. Stormwater runoff is a
problem for everyone.
WHY IS URBAN STORMWATER RUNOFF A PROBLEM?
As stormwater flows over city streets and sidewalks and through parking lots, it collects debris,
chemicals, sediment, and other pollutants that can seriously impair water quality. On the
ground, rainwater mixes with these pollutants to create natural and human-made pollutants,
which can include contaminants like:
Oil, grease, metals, and coolants from vehicles
Fertilizers, pesticides, and other chemicals from farms, gardens, and homes
Bacteria from pet wastes and failing septic systems
Soil from construction sites and other bare ground
Detergents from car and equipment washing
Accidental spills, leaky storage containers, and whatever else ends up on the ground
The polluted runoff then rushes into nearby gutters and storm drains, where it is commonly
conveyed through Municipal Separate Storm Sewer Systems (MS4s) and eventually discharged
into streams, lakes, rivers, bays, and oceans—the same bodies of water we use for swimming,
fishing, and drinking water. In many areas, stormwater runoff enters vital surface waters
without treatment, conveying contaminants that were collected along the way. Stormwater is
a major contributor to urban nonpoint source pollution.
NONPOINT SOURCE POLLUTION
Nonpoint source pollution results from stormwater
and snowmelt carrying and depositing
contaminants into surface and ground waters.
Nonpoint source pollution is detrimental to fresh
water supplies, often contaminating drinking water
sources and adversely affecting the health of plants,
fish, animals, and people. Excess volumes of runoff
from impervious surfaces also cause stream
scouring, causing significant property damage as
well as loss of aquatic habitat and floodplain Stormwater runoff is a cause of nonpoint
connectivity S°"rce P°llution' /f Contaminates the
" ' ""•'• waterways that we use for swimming,
fishing, and even drinking water supplies.
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Section 2. The Role Of Trees In Stormwater
Management
In nature, trees play critical roles in controlling stormwater runoff and protecting surface
waters from sediment and nutrient loading. In cities, trees can play an important role in
stormwater management by reducing the amount of runoff that enters stormwater and
combined sewer systems. Trees, acting as mini-reservoirs, control stormwater at the source.
A healthy urban forest can reduce runoff
in the following ways:
«> Transpiration—Trees draw large
quantities of water from the soil for
use in photosynthesis. The water is
eventually released into the
atmosphere as vapor from the
canopy, a process termed
transpiration.
«J« Interception—Leaves, branches, and
trunk surfaces intercept and absorb
rainfall, reducing the amount of water
that reaches the ground, delaying the
onset and reducing the volume of
peak flows.
«> Reduced Throughfall—Tree canopies
reduce soil erosion by diminishing the
volume and velocity of rainfall as it
falls through the canopy, lessening
the impact of raindrops on barren
surfaces.
Precipitation
Transpiration
Canopy interception
& evaporation
Infiltration
Throughfall
Evapotranspiration
Runoff
Roots take up soil
moisture, increasing
runoff storage potential
Increased Infiltration—Root growth
and decomposition increase soil infiltration capacity and rate.
Phytoremediation—Along with water, trees take up trace amounts of harmful chemicals,
including metals, organic compounds, fuels, and solvents from the soil. Inside the tree,
these chemicals may be transformed into less harmful substances, used as nutrients
and/or stored in roots, stems, and leaves.
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TREES CAN BE MORE THAN JUST LANDSCAPING
While trees have long been recognized for their ability to help clean the air, reduce energy
needs, raise property values, and mitigate heat island effects, their innate ability to absorb and
divert rainfall has been underutilized. Trees have proven value in reducing runoff and
mitigating the costs of stormwater management. In fact, research by the United States
Department of Agriculture (USDA) Forest Service has shown the environmental and
economical values trees contribute to the community.
The USDA Forest Service software suite, i-Tree, provides urban forestry analyses and benefit
assessment tools. Specific to stormwater management are the i-Tree applications Streets and
Hydro. i-Tree Streets was developed to estimate the environmental and economical impacts
street trees have on a community. i-Tree Hydro was designed to simulate the effects of tree
and impervious cover changes on stream flow and water quality within a defined watershed.
In 2010, the State of Indiana Department of Natural Resources conducted a statewide street
tree benefit study using i-Tree Streets. The study showed that Indiana's street trees returned a
multitude of environmental services and economic benefits annually to the community,
including services that conserved energy ($9.7 million), managed stormwater ($24.1 million),
improved air quality ($2.8 million), and sequestered carbon dioxide ($1.1 million). Less tangible
but equally significant, the aesthetic and social benefits and increased property values gained
because of the presence of street trees were estimated at $41 million dollars per year to
Indiana communities.
$24,114,485
31%
$9,703,847
$41,009,739 ^ ^ ,"^^^P 12%
52%
•^^kk^ ^ r
$2,820,721
4%
u $1,158,214
1%
• Aesthetic/Other a Stormwater • Energy • Air Quality ->CO2
Figure 1. Environmental and economic benefits extrapolated for 567 Indiana
communities using i-Tree Streets. htto://www. itreetools. ora/
resources/reoorts/lndiana Statewide Street Tree Ana lysis, odf viewed 11 May,
2011.
For the 23 communities involved in this statewide project, street trees provided
approximately $30 million of functional benefits each year. Applied to all 567 Indiana
communities, the annual benefits afforded by street trees were nearly $79 million
(Figure 1). Reductions in stormwater management costs accounted for 64% of the
environmental services (stormwater, energy, air quality, and CO2) provided by street
trees.
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i-Tree Streets studies performed in communities of all sizes in every ecoregion show a
similar saving trend in stormwater management costs because of the presence of street
trees. Using trees to help manage stormwater, rather than as just landscaping,
significantly reduces stormwater management costs, as well as provides other valuable
environmental services such as improvements in air quality and reductions in carbon
dioxide.
DESIGN SITES FOR SUCCESS
To effectively use trees to manage stormwater runoff, the site must be designed
properly. Site design is critical to the success of any project, even when the project
seems as simple as planting a tree. Urban trees require space, proper soil, drainage, and
irrigation. Soil properties and soil volume are keys to growing trees in urban landscapes
and using them successfully as a means to managing runoff.
A soil's porosity (amount of available pore space), permeability (how interconnected pore
spaces are), and infiltration rate (how quickly the water moves through the soil) are critical to
the success of a street tree and its ability to absorb stormwater. These soil properties affect the
amount of air, moisture, and nutrients that are available in the root zone and how much runoff
is absorbed into the ground instead of flowing over the ground.
Impervious surfaces and compacted soils in urban areas create challenges for both stormwater
managers and urban foresters by preventing the infiltration of runoff into the ground. One way
to address these problems, providing a solution for both, is to design tree planting areas to
increase infiltration and limit compaction, and engineer them to receive and process street and
rooftop runoff.
Designing the tree planting to accommodate the largest size tree possible will increase its
stormwater utility function. Big trees with their large, dense canopies manage the most
stormwater, and should be considered where the location is appropriate
GROW BIGGER TREES TO
REDUCE MORE RUNOFF!
Engineering a tree planting area which
enables trees to grow to their full size,
and where space allows to grow big trees,
takes planning. Big trees require large
volumes of soil and aboveground and
belowground space to grow. Much
research has been done to determine the
relationship between soil volume and
mature tree size. And although no
universal standard for soil volume
requirements for expected mature tree
exists in arboriculture, it is generally
accepted that a large-sized tree (16
inches diameter at breast height) needs
at least 1,000 cubic feet of uncompacted
soil (Figure 2).
Mature Tree
Size(DBH[in])
n -
/
X
.
'
X
/
/
/
x
0 200 400 600 800 1000 1200 1400 1600
Soil Volume Required (Cu Ft)
figure 2. James Urban (1992) synthesized data from
Bassuk and Lindsey (1991) and others to determine a
relationship between soil volume requirements and
mature tree size. The larger the tree, the more soil
volume it needs.
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A tree's ability to establish, grow to its full potential, and remain healthy is largely
dependent upon soil volume. If too little soil is available, the tree will not reach full stature,
regardless of what species of tree is planted. Trees without adequate soil volume tend to
be short-lived and don't function as useful components of a city's infrastructure. Poorly
designed sites—those lacking adequate soil and space—generally require continual, costly
plant healh care and often continual replantment of trees. Designing a site for success—
providing both soil and space—will grow the biggest tree the site can accommodate and,
thus, divert and absorb the most stormwater (Figure 3).
Figure 3. Tree growth is limited by soil volume. To grow big trees, large amounts of uncompacted soil are
needed. For a mature tree with a canopy spread of approximately 30 feet, 1,000 cubic feet of soil is
needed. Illustration from Casey Trees, 2008.
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STREET TREE DESIGN FAILURES
Streetscape designs and even individual tree planting spaces often fail to address the needs of
trees. Common design failures include compacted soil, improper (too small) tree pit size, a lack
of soil for root growth, and impervious surfaces directly above the tree. Compaction—under
the entire extent of the tree canopy—destroys soil porosity and permeability, limiting water
infiltration and tree root growth. A tree pit that is too small and lacks the needed soil volume is
inadequate for growing trees to their full potential, or even sustaining them for more than a
decade. Trees placed in an area that lack space and soil and regular access to rainfall or
stormwater are destined for failure. Impervious surfaces cause stormwater to run off the site,
preventing infiltration and forcing tree roots to grow towards the surface in search of air and
moisture, oftern resulting in the common problem of sidewalk upheaval as roots grow upward
to reach all important oxygen and water.
Tree in decline as a
result of poor root
health and water deficit
Stormwater pools,
then runs off site
carrying sediments
and pollutants
Roots lift and
crack pavement in
search of moisture
and air
Impervious
pavement
prevents
( infiltration
Traditional sidewalk construction
over compacted sub-base
Soil under sidewalk contains little
pore space for either stormwater
storage or healthy root growth
Eventually, and because of poor root health due to site design failures, a tree shows signs of
decline and may present an increased risk of limb or tree failure and liability. Often these
declining trees are removed because the tree was a problem; however, in reality, it was the site
design that was the problem, not the tree. The cost of removing and replacing this tree—a
significant investment—is a waste of a community's money and time and it could have been
prevented with proper design.
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IN A WELL-DESIGNED SYSTEM, YOUR STREET TREES CAN...
With proper design, street trees can grow to their full size and live for many decades,
enhancing streetscapes and providing stormwater utility services to the community.
Well-designed street tree systems are being achieved in even the most challenging urban
environments. Creative designs are engineered to provide both space for trees to grow and for
stormwater to be managed. Pavements can be supported by pillars, piles, and structural cells,
allowing for large volumes of uncompacted soil below ground. Structural soils are engineered
to be compactable enough to support vehicle traffic, yet gravel with a soil media adhered to
the stone provides porosity and enough soil for root growth. Surface treatments that are
permeable and inlets provided to increase infiltration into the soil profile can enhance tree
survival and manage runoff. Storage areas can be created and fed by downspouts and curb
inlets. Overflow pipes direct excess flows from large storms to high-flow management systems.
Runoff flows to
underground storage
system & tree roots
c
Struct ural/Engineered
soil under pavement
H2O capacity ~25%
Suspended pavement over
uncompacted soil
H2O capacity ~2O%
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When tree planting areas are designed and engineered with healthy trees as a goal, not an
afterthought, trees grow to their maximum size, extending dense canopies and providing the
greatest stormwater utility benefits to our cities, as well as other environmental and
economical benefits including:
«J« Improving Air Quality. Trees absorb
gaseous pollutants including ozone
and nitrogen dioxide; intercept
particulate matter such as dust,
smoke, and pollen; and increase
oxygen levels.
«> Saving Energy. Shade from tree
canopies reduces heat island effects;
transpiration cools the air by adding
moisture.
«> Increasing Property Values. Trees
provide beauty, privacy, and a sense
of place.
* Reducing Carbon Dioxide (CO2).
Trees reduce CO2 directly by
sequestration and indirectly by
lowering the demand for energy.
«> Providing Socioeconomic Benefits.
Trees can reduce crime (Kuo and
Urban trees can grow big and provide more
than beauty and shade to a city. Engineered
tree planting areas that provide the soil
volume needed for tree growth and integrate
with stormwater infrastructure for increased
runoff management are being used in cities
throughout the country to control stormwater
and reduce runoff. Photo courtesy of Davey
Resource Group.
Sullivan, 2001), speed up recovery
time (Ulrich, 1984 and 1986), and improve perceptions of business districts (Wolf,
2000).
*> Protecting Water Quality. Trees protect water quality by filtering and reducing
stormwater runoff.
In cities around the world and in every ecoregion in the United States, street trees are
providing measurable environmental benefits. Their innate ability to capture stormwater
reduces the load on existing stormwater management systems, which in turn reduces
treatment costs as well as the need for additional facilities. The following table, using data
from the USDA Forest Service i-Tree Streets Reference Cities, illustrates just how much rainfall
trees intercept and the savings to cities in stormwater management costs because of them.
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Year i-Tree
Completed Reference City
2006 Albuquerque, N.M.
2005 Berkeley, Calif.
2004 Bismarck, N.D.
2007 Boise, Idaho
2005 Boulder, Colo.
2006 Charleston, S.C.
2005 Charlotte, N.C.
2004 Cheyenne, Wyo.
2003 Fort Collins, Colo.
2005 Glendale, Ariz.
2007 Honolulu, Hawaii
2008 Indianapolis, Ind.
2005 Minneapolis, Minn.
2007 New York City, N.Y.
2009 Orlando, Fla.
2003 San Francisco, Calif.
2001 Santa Monica, Calif.
Number of
Trees
Studied
Annual
Stormwater
Benefits (dollars)
Rainfall
Intercepted
Annually by
Trees
(million gallons)
4,586
36,485
17,821
23,262
25,281
15,244
85,146
17,010
31,000
21,480
235,800
117,525
198,633
592,130
68,211
2,625
29,229
$55,833
$215,645
$496,227
$96,238
$357,255
$171,406
$2,077,393
$55,301
$403,597
$18,198
$350,104
$1,977,467
$9,071,809
$35,628,220
$539,151
$466,554
$110,784
11.1
53.9
7.1
19.2
44.9
28.3
209.5
5.7
37.4
1.0
35.0
318.9
334.8
890.6
283.7
99.2
3.2
Street trees in cities throughout America make a difference in the amount of runoff entering
combined sewer systems and Stormwater drains. The mere presence of street trees reduces runoff
by millions of gallons and saves cities tens of thousands to millions of dollars annually in
Stormwater management facility costs. Data from the United States Forest Service i-Tree Streets
Reference Cities Guides are available at: http://www.fs.fed.us/psw/programs/uesd/
uep/tree_ guides, php.
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Section 3. Stormwater Management Systems with
Trees
SUSPENDED PAVEMENT AND STRUCTURAL CELLS
In a suspended pavement or structural cell
system, pavement or the intended ground
surface is supported by a network of pillars,
piles, or structural cells. The suspension
system supports the weight and forces of the
pavement above and allows the soil below to
remain uncompacted, accommodating tree
roots and filtering and managing stormwater
runoff. Suspended pavement can
accommodate large volumes of soil needed
for big tree growth.
Depending on engineering and design,
suspended pavement and structural cells can
support varying surface loads, including
vehicular.
Practices: new construction/ redevelopment/
retrofit.
Applications: streetscapes, green streets,
plazas, parking areas, green roofs, and tree
pits and lawns.
Suspended pavement/structural cell system
(black pillars above) supports pavement, creating
large subsurface areas of uncompacted soil for
root growth, bioremediation, and storage of
stormwater.
How Suspended Pavement Benefits Stormwater Management and Trees
Soil Volume. Large volumes of soil are needed for tree growth—this system does that well.
Load Bearing. Can be engineered to meet loading standards, including some vehicles.
Bioremediation. Soil, roots, and soil biota filter stormwater, removing trace amounts of
harmful chemicals including metals, organic compounds, fuels, and solvents.
Helps Trees Grow. Field studies found that trees in uncompacted, suspended pavement
systems had better color, more root growth, and grew faster and larger than most other
treatments, including structural soil (Smiley et al., 2006).
Transpiration. Stormwater is dissipated by trees through transpiration. Since this system
actually gets stormwater to the tree roots, more transpiration can occur.
Tree Preservation. Existing trees can be preserved by suspending pavement over lateral roots.
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Downspout:
transfers roof
ran off bo
underground
storage system
Runoff flows bo
underground
storage system
and tree roots
einforced concrete
sidewalk
Undisturbed soil
sub-base
Uncompacbed soil supports
a healthy root system and
holds more sbormwaber
Excess water overflows
into sbormwaber
distribution pipe
Concrete pillars
or prefabricated
modules set on
undisturbed
sub-base provide
support ot sidewalk
over uncompacted
soil layer
World Trade Center Memorial Plaza
Designed by Michael Arad and Peter Walker and Partners, a California-based
landscape architecture company, the World Trade Center Memorial Plaza was
designed to be one of the most sustainable green plazas ever built. The Memorial
will feature approximately 400 swamp white oak and sweet gum trees planted
using a suspended pavement system. The memorial is designed to collect the
stormwater that falls into tanks below the plaza surface. The stormwater storage
potential will exceed the irrigation needs of the plaza so daily and monthly
irrigation requirements for the trees will be met by the harvested stormwater.
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Design Considerations
Engineered designs should include drainage and inlet and outlet locations along with
elevations.
Overflow outlets are needed to prevent flooding.
Since suspended pavement sits on top of a sub-base, underdrains may be beneficial if the
underlying surface is impervious and tends to pond water. Ponding water may suffocate trees.
Grasses, permeable pavers or pavements, and other surface treatments are appropriate for
use with the system.
Underground utilities can be placed around and even through suspended pavement systems.
However, all underground utilities should be protected from water and root penetration.
For system repair or utility access requiring excavation of suspended sections, to date,
backfilling with soil, structural soil, or other aggregate has been done where the suspended
pavement removed cannot be replaced.
Build the system large enough to grow many trees and manage a desired amount of runoff or
storm event. A few structural cells or strips of suspended pavement may be good enough for
growing a tree, but to manage stormwater runoff, contiguous areas of suspended pavement
interconnected with other green and grey infrastructure is needed.
Consult engineers and landscape architects for design and arborists for tree specifications.
Structural cell installation in an urban
streetscape; frames and decks pictured
(below). Completed installation showing trees
growing in the structural cell system (right).
Photos courtesy of Deep Root Partners, L.P.
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STRUCTURAL SOIL
Structural soil refers to a group of soil-on-gravel
mixes that are designed to support tree growth and
serve as a sub-base for pavements. Structural soils
are highly porous, engineered aggregate mixes
designed to be used under asphalt and concrete
pavements as the load-bearing and leveling layer. In
addition to providing a compactable base for
pavements, structural soil provides a soil component
(i.e., engineered dirt) to the aggregate mix that
facilitates root growth—common road bases do not
have this tree-friendly component.
Structural soils are typically composed of 70% to 80%
angular, gravel and 20% to 30% clay loam soil and a
small amount of hydrogel
Structural soil (grey stones above) is
compactable to some roadway base
standards yet provides pore space and
soil for root growth and the storage of
stormwater.
Conceptual diagram of structural
soil (Bassuk, 2007)
3%) to prevent
separation
during mixing.
Structural soils
have 20% to
25% void space which supports root growth and
accommodates stormwater runoff.
Practices: new construction/redevelopment/retrofit.
Applications: streets, streetscapes, green streets, plazas,
parking areas, green roofs, and tree planting areas, and
tree pits and lawns.
How Structural Soil Benefits Stormwater Management and Trees
Load Bearing. Structural soil can be compacted to meet load-bearing requirements, and can
even support some roadways, while preserving porosity and permeability.
Reduces Runoff—Manages Stormwater. The aggregate mix has void space that
accommodates runoff.
Helps Trees Grow. Structural soils provide pore space for tree roots and a clay loam or climate-
specific soil component that supports tree growth. The high volume of rock may preclude a
large tree from growing to full size; however, continued fertilization of the tree will enhance
the aggregate mix with nutrients and promote growth.
Stormwater Storage. A reservoir can be created underneath pavements to store runoff and
desired storm events, shaving peak flows and reducing overall volume of runoff.
Infiltration. Structural soils have high porosity that allows tree roots to penetrate it freely, and
stormwater to infiltrate it rapidly.
Easy Tree Planting. Trees are planted as they would be in normal soil. A 24-inch to 36-inch
reservoir depth is generally considered optimal for tree growth, storing 6.25 to 9.36 inches of
rain, respectively (Bassuk, 2007).
Multi-tasking. When planting tree grow spaces, structural soil can rise to surface grade, acting
as a groundcover, maximizing opportunities for infiltration, aeration, and transpiration.
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pH and Stone Type
Trees are sensitive to pH (acidity or alkalinity). pH can significantly affect the life and health of a
tree and its ability to absorb nutrients. When using structural soil, the pH of the soil and water
will be influenced by the type of stone used in the mix, whether limestone, granite, lava rock,
or other stone. In systems that incorporate concrete products, the pH will continue to rise over
time as concrete deteriorates. Always plant tree species that are compatible with the growing
environments and structural soil's pH.
In some cases, the addition of chemicals may be necessary to help offset pH conditions. These
chemicals should be selected so as to not damage the concrete or other materials. Designs
should consider this aspect of long-term maintenance and try to minimize these effects.
Design Considerations
Because structural soils are only 20%
to 30% soil, large volumes may be
needed to provide sufficient
resources for trees (Loh et al., 2003).
A proper gravel gradation is critical
for road base applications. Testing by
pavement and geotechnical
engineers is necessary and ensures
soundness.
The sub-grade may need to be
compacted and be impermeable to
meet the installation's overall
requirements for traffic loading, etc.
In this case, a sub-drain system
between the structural soil and
compacted sub-grade is necessary to
prevent standing water that could
suffocate the tree roots.
When structural soil is being used as a
reservoir for stormwater, the sub-soil
may become saturated at times,
resulting in lower soil strength.
Consult a geotechnical engineer to
determine if a separation geotextile is
necessary.
Lateral flow through structural soil is
extremely rapid. If the sub-base is
permeable, or has some permeable areas, throughflow is likely to be fast. If surrounding areas
are impermeable, ponding is possible. Provide overflow and underdrain outlets as needed.
Suppliers of structural soil should ensure that the mix used has the correct soil-to-gravel ratio,
stone composition, and size and shape for the site and ecoregion.
Typical structural soil streetscape installation (above).
Trees growing in structural soil in high-use area (below).
Photos courtesy of Dr. Nina Bassuk, Cornell University.
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For roads and other surfaces intended to support vehicular use, measure the bearing capacity
of the structural soil used to ensure it meets regional Department of Transportation standards.
Build the system large enough to grow many trees and manage a desired amount of runoff or
storm event. A few areas or strips of structural soil may be good enough for growing a tree, but
to manage stormwater runoff, contiguous areas of structural soil interconnected with other
green and grey infrastructure is needed.
Consult engineers and landscape architects for design and arborists for tree specifications.
Downspout transfers
runoff from roof to
underground storage
system
Runoff flows to
underground storage
system and tree
roots
Excess water overflows
into stormwater
distribution pipe
Structural soil provides support for pavement
and sidewalk while preserving pore space for
healthy tree roots
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STORMWATER TREE PITS
Street trees provide natural stormwater
management. When tree pits provide enough
uncompacted soil volume to grow large-sized
trees, they become an integral part of stormwater
management. Trees act as mini-reservoirs
absorbing, diverting, and purifying rainfall on the
spot. While tree pits can be individual, connecting
multiple tree pits by soil paths or drains can
increase soil volume for both trees and
stormwater management opportunities.
Stormwater tree pits are similar to traditional
street tree pits in that they are modified to have
increased growing space, be interconnected, and
receive and treat stormwater runoff. Stormwater
benefits increase with the number of stormwater
tree pits installed and connected.
Practices: new construction/redevelopment/ Stormwater tree pits are designed to
retrofit.
Applications: tree lawns, medians, plazas,
streetscapes, parking areas, green roofs, and green
streets.
increase infiltration through inlets and
pervious surfaces. Trees transpire water,
reducing the amount of water entering
constructed runoff management systems.
How Stormwater Tree Pits Benefit Stormwater Management and Trees
Reduces Runoff—Manages Stormwater. The connection between tree pits and the
integration of other grey and green stormwater management systems reduces runoff and
increases the amount of stormwater managed.
Helps Trees Grow. Stormwater tree pits have additional soil volume and grow space, regular
irrigation, and improved drainage. They provide an improved growing environment for trees.
Bioremediation. Soil, roots, and soil biota filter stormwater, removing trace amounts of
harmful chemicals including metals, organic compounds, fuels, and solvents.
Design Considerations
Stormwater tree pits are constructed similar to traditional street tree pits, but are engineered
to accept and treat runoff. A continuous soil trench, drains, or other grey or green
infrastructure should connect individual tree pits, maximizing capacity.
Stormwater tree pits are useful in streetscape retrofits when existing soils are very compacted
or poor and underground space is limited.
Can be installed in conjunction with repair of underground utilities or streetscape retrofits.
Tree species selection is critical for stormwater tree pits. Plant trees that are adapted to soil
and site conditions. Arborists should be consulted for tree specifications.
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Directing runoff into grow spaces and tree pits with grading, inlets, and pervious surfaces can
maximize infiltration and reduce stormwater runoff.
Connect enough stormwater tree pits to manage a desired amount of runoff or storm event. A
few stormwater tree pits may be good enough for growing trees along a street, but to manage
stormwater runoff, contiguous strips of stormwater tree pits interconnected with other green
and grey infrastructure are needed.
Consult engineers and landscape architects for design. As with other designs, overflows and
ponding require management.
Stormwater tree pits are
connected by a continuous
underground trench.
Excess runoff flows into
combined sewer system
Pipes drain excess
runoff from green
roofs into tree pits
Connected tree pits
increase soil volume for
tree root expansion and
stormwater detention
Green roofs reduce impervious surface
and mitigate heat island effects
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PERMEABLE PAVEMENTS
Permeable pavement refers to a wide variety of
surfaces, including concretes, asphalts, and various
types of grid and paver systems, that allow for
rapid infiltration of water. Permeable pavement
has a network of voids or spaces that allow water
to pass through. Installations typically include a
belowground, load-bearing stone reservoir that
can store runoff until it percolates and interflows
through the subsurface.
When combined with other engineered systems
that promote tree growth, such as structural soil,
suspended pavement, and stormwater tree pits,
the volume of runoff infiltrating into the system
can be increased significantly and tree growth
maximized.
Practices:
retrofit.
new construction/redevelopment/
Permeable pavements (surface shown
above) increase infiltration, allowing more
runoff to be absorbed and available for use
by the tree.
Applications: curbs, cutouts, sidewalks, plazas and parking areas, and low-traffic areas.
How Permeable Pavement Benefits Stormwater Management and Trees
Infiltration. Reduces impervious surface. Rainfall enters the ground directly, almost where it
falls. May reduce or eliminate the requirement for land for stormwater ponds.
Reduces Runoff—Manages Stormwater. Permeable surface reduces runoff.
Filtration. Some permeable pavements eventually build up a film of biomass that naturally
reduces trace amounts of hydrocarbons, nitrogen, and other biodegradable pollutants.
Recycled. Many pavers and pavements are manufactured using recycled materials.
Helps Trees Grow. Increases the amount of oxygenated water directly entering the root zone,
improving tree health.
Tree Preservation. Can be used around trees where a load-carrying surface is required.
Reduces Puddling. Because water freely drains through the pavement, water is less likely to
accumulate on the surface of the pavement.
Design Considerations
Proper design, construction, and maintenance is required to reduce clogging and failure.
Do not seal or repave with non-permeable materials, as these clog the surface and prevent the
pavement from allowing water to infiltrate.
Vacuuming of debris is recommended to ensure void spaces do not clog. Sweeping can push
debris into the void spaces.
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Permeable pavement allows stormwater
to infiltrate surface, recharging sub-soil
and irrigating trees
Aggregate layer provides temporary
storage while stormwater
infiltrates sub-soil
Do not cover the surface with toxic materials as they will pollute the underlying soils and water.
Sand is not recommended as joint filler for pervious, interlocking concrete paver (PICP)
systems. Sand is a growing medium that will support mold, moss, and other vegetation, which
can render the surface impervious.
Avoid installing in areas where activities generate sediment or contaminated runoff. Areas
where sand is applied should not be considered for permeable pavement installations.
A common failure of permeable pavement is sediment accumulation during construction.
Ensure that the surrounding construction area is completely stabilized before installing
permeable pavements.
Proper jointing for contraction and expansion is required.
Snow plows must avoid surface contact.
Consult engineers and landscape architects for appropriate design and arborists for tree
specifications.
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Permeable pavements increase infiltration, helping trees receive oxygenated water and reduce
stormwater runoff. When used in conjunction with other engineered systems designed to grow
big trees and manage stormwater, permeable pavements can boost the infiltration rate and
amount of runoff entering the system. Photo courtesy ofDavey Resource Group.
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Other Vegetated Systems Designed to Mimic
Nature
Stormwater management systems designed to mimic natural areas can be integrated into
community, street, building, and even site developments to reduce the damaging effects of
urbanization on rivers and streams and relieve pressures on combined sewer and stormwater
systems. Bioswales, green streets, and green roofs are three such designed systems that
incorporate a variety of green and grey infrastructure components to increase on-site
infiltration and filtering of stormwater by natural processes. These created naturalistic systems
disconnect flow from storm sewers and force runoff to areas such as landscaped planters,
swales, and rain gardens. Vegetation, soils, and biota naturally filter stormwater while entry
into grey infrastructure is delayed or even prevented.
These systems, even though they mimic nature, are usually complexly designed and can
incorporate the engineered systems discussed in this guide.
FORESTED BIOSWALES
A bioswale is a graded depression designed to detain stormwater and promote infiltration.
Stormwater is filtered by trees, vegetation, and soil biota.
To function properly, a bioswale should be constructed with a mix of soil, engineered or native,
vegetation, and drainage. If the bioswale is surrounded by impervious surfaces, curbs, or
barriers, it should be positioned to direct runoff into storage areas before slowly releasing it
into storm drains. The swale takes advantage of a natural slope and reduces runoff speed. The
vegetation in a swale reduces the "gullywasher" effect by absorbing some of the water as it
moves downward. Check dams can be added along the length of the swale to slow runoff even
more.
6raded depression
detains stormwater
Sand ana qravel mix
stores and filters
stormwater
Increased soil volume and vegetation, including trees, maximizes potential
for absorption, bioremediation, and phytoremediation
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GREEN ROOFS
Green roofs reduce the amount of impervious
surface and increase transpiration opportunities in
even the densest urban cores. Photo courtesy of
Davey Resource Group.
GREEN STREETS
A green street is designed to integrate a
natural system of stormwater management
within a public right-of-way. Green streets are
generally planned to be visible components of
a system of "green infrastructure" and are
incorporated into the aesthetics of the
community. Green streets make use of
bioretention or bioswales and make the best
use of the street tree canopy for stormwater
interception as well as temperature
mitigation and air quality improvement.
Green streets designs will vary from
community to community or even street to
street, but they all have the same goal, to
reduce the amount of stormwater that
directly enters into streams and rivers. The
design and construction of a green street
should be one component of a larger
watershed approach to improving regional
water quality.
Green roofs can be effectively used to
reduce stormwater runoff from
commercial, industrial, and residential
buildings. In contrast to traditional asphalt
or metal roofing, green roofs absorb,
store, and later evapotranspire initial
precipitation. Overflow is directed into
stormwater and combined sewer systems.
A green roof manages stormwater on site
through retention in the media. It reduces
peak flow discharge to a stormwater
sewer system. Its design requires a careful
mix of impervious base materials to
prevent leakage, soils, plants, irrigation,
and drainage systems, making it and the
building below structurally sound and safe.
Green streets are an innovative way to manage
stormwater on site and combat urban heat
island effects. Photo courtesy of Davey Resource
Group.
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Know the Rules
The engineered systems that use trees presented here are not applicable everywhere.
When high concentrations of contaminants and/or pollutants are present in
stormwater, infiltration may not be appropriate due to the risk of groundwater
contamination, and the use of engineered systems may be regulated. Sites with very
rocky soils, high bedrock, water tables less than four feet from the surface, limited
drainage, and extreme slopes may not be suitable for increased infiltration rates
common with the engineered systems presented. Sites with Karst geology run the risk of
contaminating the groundwater. Before beginning a project, check and comply with
local, state, and federal rules, regulations, codes, and other restrictions or mandates
regarding the capture, manipulation, detention, and storage of stormwater.
References
Bassuk, Nina, Jason Grabosky, Ted Haffner, and Peter Trowbridge. 2007. Using
Porous Asphalt and CU-Structural Soil. Cornell University, Ithaca, New York.
Casey Trees. 2008. Tree Space Design Growing the Tree Out of the Box.
http://www.caseytrees.org/planning/design-resources/for-designers/tree-
space/documents/TreeSpaceDesignReport.pdf. Viewed 14 July, 2010.
Indiana Department of Natural Resources (IDNR). http://www.itreetools.org/
resources/reports/lndiana_Statewide_Street_Tree_ Analysis.pdf. Viewed
28 March, 2011.
Kuo, F., and W. Sullivan. 2001. Environment and Crime in the Inner City: Does
Vegetation Reduce Crime? Environment and Behavior 33(3): 343-367.
Lindsey, Patricia and Nina Bassuk. 1991. Specifying soil volumes to meet the water
needs of mature trees in containers. Journal of Arboriculture 17:141-149.
Loh, Felix, C.W., Jason C. Grabosky, and Nina L Bassuk. 2003. Growth response of
Ficus benjamina to limited soil volume and soil dilution in a skeletal soil
container study. Urban Forestry & Urban Greening 2: 53-62.
Smiley, Thomas E., Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. 2006.
Comparison of Structural and Noncompacted Soils for Trees Surrounded by
Pavement. Arboriculture & Urban Forestry 32(4): 164-169.
Ulrich, R. 1986. Human Responses to Vegetation and Landscapes. Landscape and
Urban Planning 13:29-44.
Ulrich, R. 1984. View through Window May Influence Recovery from Surgery.
Science 224(4647): 420-421.
Urban, James R. 1992. Bringing order to the technical dysfunction within the urban
forest. Journal of Arboriculture 18(2):85-90.
USGS. http://ga.water.usgs.gov/edu/watercycleevapotranspiration.html. Viewed 28
March, 2011.
Wolf, K. 2000. "Community Image - Roadside Settings and Public Perceptions."
University of Washington College of Forest Resources, Factsheet #32.
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Section 4. Case Studies
Minneapolis, Minnesota: Structural Cells
Charlotte, North Carolina: Suspended Pavement
Ithaca, New York: Structural Soil
Olympia, Washington: Structural Soil
Chattanooga, Tennessee: Permeable Pavement
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MINNEAPOLIS, MINNESOTA: STRUCTURAL CELLS
Year: 2010
Project Area: Marquette Avenue and 2
(MARQ2).
nd
Avenue
Goals: Reshape transportation corridor and address
capacity problem with urban stormwater runoff.
Modeled a 10% reduction in peak flows (peak storm
event) to City's stormwater system.
This downtown street project included installation of
structural cells or tree cells to create conditions that
promoted healthy mature trees and improved
stormwater management in the core of the downtown
district.
The project installed 173 trees along the new bus
corridor using a modular system of structural cells that
supported the sidewalk. The system created a void
space that held 10 cubic feet of soil per unit (10,800
units were installed), allowing for existing or future
utility pipes, protecting tree roots from compaction
and providing room for stormwater.
The system can temporarily hold large volumes of
stormwater that will either be used by the trees
(evapotranspiration) or will soak into the ground
(infiltration).
Photos courtesy of Deep Root
Partners, L.P.
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CHARLOTTE, NORTH CAROLINA: SUSPENDED PAVEMENT
Photo courtesy of Don McSween, City of Charlotte, North Carolina
Year: 1985
Project Area: Ten blocks of Tryon Street and two blocks of Trade Street; two of the
major downtown thoroughfares.
Goals: Major renovation of downtown thoroughfares. City wanted large stately trees in
its downtown area.
A custom, suspended pavement system using precast concrete pavement supported by
earthen trench sidewalks was designed to promote tree growth in downtown Charlotte.
This represents perhaps the lowest cost and simplest approach that may apply in
construction where trench integrity can bear the load. The entire system was topped by
nonpermeable pavers. The design included approximately 1,000 cubic feet of good
usable soil per tree; 170 willow oaks (Quercus phellos) trees were planted.
In 2009, the willow oaks planted had an average diameter at breast height of 16 inches
and an average height of 44 feet. In addition to growing big trees, the system modeled a
10% reduction in peak flows (peak storm event) to the City's stormwater system.
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ITHACA, NEW YORK: STRUCTURAL SOIL
Year: 2005
Project Area: Parking lot, Ithaca,
New York.
Goals: Improve tree growth,
reduce runoff, and improve water
quality in parking lots.
To show the benefits of the use of
structural soil to grow trees,
reduce runoff, and improve water
quality in parking lots, a parking
lot in Ithaca, NY was retrofit with
structural soil and porous asphalt.
Tree pits 3 feet X 18 feet were cut
into porous asphalt and filled with
structural soil. Bare-root Accolade
elms were planted in the tree pits.
Photo courtesy of Dr. Nina Bassuk, Cornell University
OLYMPIA, WASHINGTON: STRUCTURAL SOIL
Year: 2001
Project Area: Downtown block,
State Avenue.
Goals: Provide soil volume to grow
trees in downtown areas and
prevent sidewalk damage.
One hundred linear feet of
sidewalk and existing soil to a
depth of 36 inches were removed.
Structural soil, trees in cut outs,
and new sidewalks were installed.
Photo courtesy ofDavey Resource Group
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CHATTANOOGA, TENNESSEE: PERMEABLE PAVEMENT
Year: 1996
Photo courtesy of Gene Hyde, City of Chattanooga, Tennessee
Project Area: Finley Stadium Parking Lot.
Goals: Control stormwater runoff and irrigate parking lot trees.
The former brownfield site was retrofit with sections of permeable concrete. An existing
basement was re-purposed as a cistern to manage stormwater and grow big trees.
Approximately 40,000 square feet of permeable concrete was used. The pervious
concrete accounts for approximately Vs of the parking lot. The entire system has
perimeter drains for overflow. Runoff is intended to be harvested and stored in an
underground cistern, which was essentially the waterproofed basement of a demolished
building.
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Additional Resources and Information
GREEN BUILDING
www.epa.gov/greenbuilding/
GREEN INFRASTRUCTURE
www.epa.gov/greeninfrastructure/
GREEN STREETS
www.epa.gov/owow_keep/podcasts/greenstreetsusa.html
GREENING
WWW.EPA.GOV/OAINTRNT/LOW-IMPACT DEVELOPMENT (LID)
www.epa.gov/nps/lid
TECHNICAL GUIDE ASSISTANCE
For photograph, chart, or figure assistance, call 800-828-8312. Reference page
number, title, and Stormwater to Street Trees guide.
EPA 841 B 13 001
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