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Managing Stormwater for Urban Sustainability using Trees and Structural Soils

Authors:
Managing Stormwater
for Urban Sustainability
Using Trees and
Structural Soils


Susan Downing Day and
Sarah B. Dickinson, Editors
This manual was made possible in part by a grant from the United States
Department of Agriculture Forest Service Urban & Community Forestry
Program on the recommendation of the National Urban & Community
Forestry Advisory Council (NUCFAC).
Project title: “Development of a Green Infrastructure Technology that Links
Trees and Engineered Soil to Minimize Runoff from Pavement”.
Editors: Susan Downing Day, and Sarah Beth Dickinson
Contributing Authors: Nina Bassuk, Julia Bartens, Laurence Costello, Joseph
E. Dove, Jason Grabosky, Ted Haffner, J. Roger Harris, E. Gregory McPherson,
Peter Trowbridge, Theresa Wynn, and Qingfu Xiao
Design & Production: Sarah Beth Dickinson
How to cite this manual:
Day, S.D, and S.B. Dickinson (Eds.) 2008. Managing Stormwater for Urban
Sustainability using Trees and Structural Soils. Virginia Polytechnic Institute
and State University, Blacksburg, VA.
Copyright © 2008, Susan Downing Day and Sarah Beth Dickinson
Acknowledgements
This manual is the culmination of a four-year project that has relied on the hard
work and insight of many people. We appreciate the work of John O. James, Stephanie
Worthington, Mona Dollins, Liz Crawley, Velva Groover, Félix Rubén Arguedas, Andy
Hillman, and many others in bringing this project to completion.
Contributing Authors
Nina Bassuk, Ph.D., Professor and Program Leader of the Urban Horticulture Institute,
Cornell University
Julia Bartens, Graduate Research Assistant, Department of Horticulture, Virginia Tech
(current position: Ph.D. student, Department of Forestry, Virginia Tech)
Laurence Costello, Ph.D., Extension Specialist, University of California at Davis
Susan Downing Day, Ph.D., Assistant Professor, Departments of Forestry and
Horticulture, Virginia Tech
Sarah B. Dickinson, Research Associate, Department of Horticulture, Virginia Tech
Joseph E. Dove, Ph.D., P.E., Research Assistant Professor, Department of Civil and
Environmental Engineering, Virginia Tech
Jason Grabosky, Ph.D., Associate Professor, Department of Ecology, Evolution and
Natural Resources, Rutgers University
Ted Haffner, Graduate Research Assistant, Department of Horticulture, Cornell
University (current position: Associate Landscape Architect, Terry Guen Design
Associates, Chicago, IL)
J. Roger Harris, Ph.D., Professor and Head, Department of Horticulture, Virginia Tech
E. Gregory McPherson, Ph.D., Director, Center for Urban Forestry Research PSW, USDA
Forest Service
Peter Trowbridge, MLA, Professor and Chair, Landscape Architecture, Cornell
University
Theresa Wynn, Ph.D., Assistant Professor, Biological Systems Engineering, Virginia
Tech
Qingfu Xiao, Ph.D., Research Water Scientist, Department of Land, Air, and Water
Resources, University of California at Davis
Contents
Introduction 1
Chapter 1— Trees and Structural Soils- A System Overview 5
Trees— Mimicking the Hydrologic Benefits of a Forest in
the City 6
Structural Soils— Supporting Tree Growth and Pavement 7
Subsoils 10
Limitations concerning subsoil infiltration 11
Chapter 2— System Design to Meet Site Requirements 13
Specifications 13
Surface Treatments 13
Reservoir Sizing and Overflow Pipe Design 14
Geotextiles 18
By Joseph E. Dove
Trees and Other Plants 20
Chapter 3— Surface Treatments 25
Structural Soils and Turf 25
By Nina Bassuk, Ted Haffner, Jason Grabosky, and Peter
Trow br id ge
Using Porous Pavement on Structural Soils 30
By Ted Haffner, Nina Bassuk, Jason Grabosky, and Peter
Trow br id ge
Chapter 4— Research and Recommendations 33
Tree Root Penetration into Compacted Soils Increases
Infiltration 33
Based on Research by Julia Bartens, Susan Day, Joseph E.
Dove, J. Roger Harris, and Theresa Wynn, Virginia Tech
Tree Development in Structural Soils at Different Drainage
Rates 34
Based on Research by Julia Bartens, Susan Day, J. Roger
Harris, Joseph E. Dove, and Theresa Wynn, Virginia Tech
Drainage Rate at the Mini Parking Lot Demonstration Site
in Blacksburg, VA 35
Based on Research by Mona Dollins, Virginia Tech
System Effects on Water Quality 36
Based on Research by Qingfu Xiao, University of California
at Davis
Helpful Resources 39
Appendices 43
CU-Soil Specification and Mixing Procedure 44
Carolina Stalite Structural Soil Specification 51
Carolina Stalite Mixing Specification 54
Figures
Figure 1. Typical runoff from a parking lot going into a
storm sewer. 1
Figure 2. This system both serves as a parking lot and as a
stormwater management facility. 2
Figure 3. An example of a retention/detention pond
adjacent to a conference center on the Virginia Tech
campus in Blacksburg, Virginia. 5
Figure 4. This photograph shows the effect of soil volume
on tree growth. 7
Figure 5. Compacted soil from a typical construction site.
Lack of structure prohibits root penetration and growth. 8
Figure 6. CU-Soil, the structural soil developed at Cornell
University in the 1990s. 8
Figure 7. Conceptual diagram of structural soil including
stone-on-stone compaction and soil in interstitial spaces. 10
Figure 8. The top illustration shows a diversion mound
system as used on a roadway. The photo to the left shows
the installation of diversion mounds and the right photo
is the same divestion mounds with structural soil being
installed. 16
Figure 9. Enlarged view of woven and nonwoven
geotextiles. 19
Figure 10. Visual comparison of a healthy pin oak leaf
(left) and a chlorotic leaf (right). 20
Figure 11. Davis Soil, a non-loadbearing soil (i.e. not
a structural soil) with high infiltration rate and high
potential for water storage. 21
Figure 12. Area of park used for a weekly farmers market
in Chicago. 25
Figure 13. Photo simulation of turf-covered perimeter
parking at a big box lot in Ithaca, NY. 25
Figure 14. Aerial view of structural soil and turf
experimental plots at Cornell University in Ithaca, NY. 26
Figure 15. Construction detail for turfgrass and structural
soil profile. 27
Figure 16. In winter when the sod is dormant, the median
serves as additional storage and display space for the
dealership inventory. 29
Figure 17. The left figure shows rain on a traditional
asphalt parking lot. The right figure shows rain on a
porous asphalt parking lot. 30
Figure 18. A comparison of traditional asphalt (left) and
porous asphalt (right) when wet. 31
Figure 19. Ash roots penetrating geotextile after
compacted subsoil has been washed away. 33
Tables
Table 1 . Comparison of physical properties of CU-Soil,
Carolina Stalite and a silt-loam soil. 9
Table 2. Reservoir depths and the corresponding levels of
mitigated rain events based on the 30% void space within
the structural soil mix (assuming an empty reservoir). 14
Table 3. Pollutant removal of single storm event. 37
Table 4. Pollutant removal of multiple storm events. 37
Introduction 1
Introduction
U
and decreases vegetative cover. These disruptions increase stormwater
runoff at the expense of groundwater recharge, degrading water quality
and impairing aquatic habitats. The repercussions of this non point source
pollution are being felt worldwide. Creative Best Management Practices
(BMPs) that harness the ability of vegetation and soils to mitigate urban runoff
are needed. This material is a culmination of four years of research at Virginia
Tech, Cornell University and the University of California at Davis investigating
how a novel stormwater BMP that relies on shade trees and structural soils
can be designed and how it will function. We do not have the answer to every
question but the approach presented here works and is in place now at our
demonstration sites around the country. We developed this guide to assist
others in implementing this BMP. We hope it will expand your toolbox and
create new approaches for harnessing the power of trees in urban settings.
Challenges for Stormwater Management in Urban Areas
Urban areas are challenged by extensive impervious surfaces, damaged soils,
and little room for greenspace or for stormwater management facilities. The

remove pollutants. The system described in this manual addresses all three of
these goals by utilizing trees and structural soils to aid in water interception,

Figure 1. Typical runo from a parking lot going into a storm sewer. Noce
that traces of oil are visible to the naked eye. There are many other
pollutants in parking lot runo such as various metals, sediment, salts, and
lier.
Photo by Susan Day.
2Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Figure 2. This system both serves as a parking lot and as a stormwater management
facility. In addion to this double use of space, the structural soils also provide vastly
greater soil volumes for tree root growth than tradional parking lot construcon.
Note: Gravel base course is oponal, since the structural soil is designed to be as strong
as a base.
Figure by Sarah Dickinson.
Introduction 3
How Does This System Work?
The system guides water to a structural soil retention area beneath the
pavement where it is then temporarily stored. Water leaves the reservoir via

creates a large rooting volume, trees have the potential to develop full
canopies, allowing increased interception of precipitation. Tree roots take up

into the subsoil. Together, trees and structural soils can create a zero runoff



installations of this system. This is attributed to the distributed nature of the
system: because the reservoir is beneath the pavement, there is a one-to-one
ratio of land area receiving rainfall and land area treating stormwater.
Before deciding on any BMP, site constraints should be evaluated. This system is
designed to be installed beneath pavement and therefore stormwater management
is distributed throughout the site and not conned to unpaved porons of the site.
The system has not been evaluated for treang large amounts of collected runo from
adjacent areas. Inltraon BMP’s are not appropriate for sites that need to handle
highly polluted or contaminated water due to risk of groundwater contaminaon.
There are also some topographical and geological features that could limit the use of an
inltraon BMP (see the limitaons secon in Chapter 2).
Distributed Stormwater Management in Urban Settings
Distributed stormwater management techniques, such as bioswales, are
used to retain stormwater at many sites throughout the urban landscape
as opposed to collecting runoff at a more centralized facility, such as a
detention pond, or relying on a storm sewer system. But some sites do not

impervious surfaces in a dispersed fashion. In addition, sites that are largely


The system described in this manual can make it possible to use distributed
stormwater management that takes advantage of the stormwater mitigation

for stormwater management and vegetation are very limited. This system may

system also provides an alternative to detention ponds where lack of space is
not yet the primary concern.
4Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Project Background and Resources
This manual is the result of a series of research studies carried out at Virginia
Tech, Cornell University, and the University of California at Davis. This
research evaluated multiple aspects of the novel stormwater BMP described
here. Work at Virginia Tech focused on tree health and root development
in the system, as well as the ability of tree roots to enhance subsurface

physical characteristics of the structural soil mixes as they pertain to storing
stormwater, and the feasibility of a wide variety of surface treatments—
everything from porous asphalt to turf. Research at Davis in the Department
of Land and Water Resources produced baseline evaluations of the ability of
several structural soil mixes to remove typical urban runoff contaminants.
Each university partnered with private groups or municipalities and installed
one or more demonstration sites to evaluate the system as a whole. Overall,
the system presented here has been successful. We have prepared this manual
to help stormwater engineers, public works departments, and others to put
this new approach—or elements of it—into practice.
How this Manual is Organized
The manual is designed to guide you through the features of the system,
including its limitations, and how to design a system to suit the site’s needs.
Original research papers are referenced and are available from university
libraries or by contacting the authors. Brief summaries of this research appear
in the manual.
Chapter 1 introduces the stormwater management system, its attributes and
limitations.
Chapter 2 provides information on designing a system with structural soils
and trees based on the needs of individual sites.
Chapter 3 describes surface treatments that can be used in conjunction with
this stormwater management BMP, namely turf and porous pavement. All the
information in this section is based on a series of publications from Cornell
University’s Urban Horticulture Institute.
Chapter 4 summarizes several original research projects related to the
development and evaluation of this system which were conducted by the
contributors of this manual. The research in this section was made possible in
part through a grant from the United States Department of Agriculture Forest
Service Urban & Community Forestry Grants Program on the recommendation
of the National Urban & Community Forestry Advisory Council (NUCFAC).
Chapter 1— Trees and Structural Soils- A System Overview 5
Chapter 1— Trees and Structural
Soils- A System Overview
Stormwater management in urbanized settings faces special challenges:
paved surfaces and buildings generate high amounts of runoff while at
the same time leaving little space for constructed stormwater management
facilities or for the soil and vegetation combination that could reduce the need
for these facilities.
The system described in this manual seeks to address these limitations by
using structural soils to simultaneously allow healthy tree growth, water

the structural soil that supports them combine to form a shallow but extensive
reservoir for capturing and storing stormwater. Structural soils are engineered
soil mixes with a high porosity that allow tree roots to penetrate freely,

into the soil beneath. Tree canopies effectively intercept rainfall, reducing
throughfall to the ground and lengthening the time of runoff concentration
into stormwater systems. Trees also actively transpire, taking up water and

pollutants and contaminants can be removed from the stormwater via

This double use of land surface area (e.g. parking lot and stormwater

a large area, which more closely mimics natural hydrology than stormwater
Figure 3. An example of a retenon/detenon pond
adjacent to a conference center on the Virginia Tech
campus in Blacksburg, Virginia. This treatment uses
space that could be otherwise directed towards other
uses.
Photo by Susan Day.
6Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.

canopy increases opportunities for returning rainfall to the atmosphere
via evapotranspiration and through canopy interception and storage of

components of this system, namely trees and structural soils.
Addional benets of trees
• Shading, reducing
ambient temperature
• Removing pollutants
from the air
• Improve aesthecs
 Seehp://www.fs.fed.
us/psw/programs/cufr/
formoreinformaon







Trees— Mimicking the Hydrologic
Benets of a Forest in the City
Natural forests with their complete canopy
cover, large leaf areas, and permeable
soils handle rainwater effectively through

water to groundwater and the atmosphere
and protecting water quality in surface
waterways. Replicating elements of this
hydrologic cycle in urban settings, however, is

people, and other urban denizens compete
for land and soil resources.
Urban forests are also widely recognized as
an effective means of handling stormwater.
Like their forestland counterparts, urban
trees intercept rainfall, direct precipitation

take up stormwater through their roots.
In addition, urban tree roots penetrating
through typically impermeable urban soil
layers into more permeable zones have the

rates. However, urban canopy cover (and thus
rain interception) is greatly limited by urban
soil conditions such as compaction, reduced
rooting volume, and elevated pH. Even open
ground in urbanized areas is commonly
disturbed or compacted, limiting normal soil
hydrologic functions. This system directly
addresses the limitations of urban soils to
support vegetation and handle water. The
system provides a highly permeable rooting
environment that can support large trees,

the city.








Chapter 1— Trees and Structural Soils- A System Overview 7
Structural Soils— Supporting Tree Growth and Pavement
Why were structural soils
designed?
Typically, soils beneath
pavement are compacted to
meet engineering requirements
to support the loads from
vehicles, pavement and
structures. Unfortunately,
most plant life cannot survive
in soils compacted for these
purposes. Roots cannot
penetrate extremely strong
soils. In addition, compacting
soil destroys soil structure,
collapsing the large pore spaces
needed to provide the balance of
air and water that roots require.
The result is soil that can support
pavement but cannot support
trees. Structural soils were
designed to meet requirements
for pavement support while
still allowing adequate pore
space to support tree roots.
Structural soils must be carefully
constructed and tested according

to meet these requirements.
Figure 4. This photograph shows
the eect of soil volume on tree
growth. Both rows of willow oaks
were planted at the same me on
Pennsylvania Avenue, Washington,
D.C. The trees on the le are in tree
pits, and those on the right are in an
open grassed area.
Photo by Nina Bassuk.
A good structural soil will have known
water-holding, drainage, structural
and load-bearing characteriscs. It
should be able to be compacted to
95% of standard Proctor density and
sll support plant growth. It will also
have a research-based track record of
success and body of best pracces.
Just any mix of a stone and soil is
not a structural soil. Some so-called
structural soils have failed miserably
when praconers thought they were
purchasing a good soil but were just
purchasing an untested mix with no
research vericaon. The two discussed
here have been thoroughly tested yet
each product should sll be required
to undergo tesng aer installaon to
ensure that the nal product meets
the standards of the specicaon. In
the case of CU-Structural Soil it must
be purchased from licensed producers
who are required to test their materials
to adhere to a research-based
specicaon.
8Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
How do structural soils work ?
Structural soils are engineered to meet
compaction requirements for parking
lots, roads and other paved surfaces
and, at the same time, allow tree root
penetration under the pavement. Excavated
root systems from structural soils have
illustrated that deep rooting of trees in
these soils appears to prevent heaving of
sidewalks, curbs and gutters by tree roots.
Structural soil can therefore expand the
soil volume available for the roots of trees
in plazas and parking lots and other paved
areas.
There are many types of
structural soils, but they are
based on the same principal:
large “structural” particles,
typically an angular stone,
form a matrix that distributes
the load from pavement and
structures through stone-
to-stone contact ultimately
spreading the load across
the supporting subsoil. The
gaps between the structural

high quality mineral soil with
good water-holding capacity
and tilth. Hydrogel is often
used in addition to the mineral

segregation of the soil during
mixing and installation. When
structural soils are compacted, they form a rigid matrix while suspending
soil as a rooting medium within the interconnected voids of the stone matrix.
Roots are able to easily penetrate this uncompacted mineral soil within the
compacted stone matrix. As roots expand in the structural soil, they appear
to encapsulate, rather than displace the stone matrix or deform temporarily
to move between the smallest pores. Because stone is the load-bearing
component of the structural soil, the aggregates used should meet regional or
state department of transportation standards for pavement base courses.
-Adapted from Bassuk, et al. 2005
Figure 5. Compacted soil from
a typical construcon site.
Lack of structure prohibits root
penetraon and growth.
PhotobyJohnW.Layman.
Figure 6. CU-Soil, the structural soil developed
at Cornell University in the 1990s. Soil parcles
within the media are clearly visible and allow soil
nutrients and water holding capacity for healthy
root growth.
PhotobyTedHaner.
Chapter 1— Trees and Structural Soils- A System Overview 9
Table 1 . Comparison of physical properes of CU-Soil, Carolina Stalite and a silt-loam
soil. Note: The Stalite specicaons usually call for sandy loam but plant available
moisture with Stalite was tested using the same intersal silty clay loam as was used
with the CU-Soil.
TablebasedoninformaonfromHaner,E.C.2008.
The history of structural soil
This manual examines stormwater management techniques that detain

soils, CU-Soil (Amereq Inc., New York, NY) was developed at Cornell University

for tree root development. This new type of soil mix resulted from research
exploring a means to create a substrate that would both allow adequate tree
root growth and support pavement for sidewalks, streets, and parking lots. It

from other types of tree soils. Since then, other structural soils have been
developed that use other components (e.g. Carolina Stalite, a heat expanded
shale (Carolina Stalite Company, Salisbury, NC). The structural component of
Carolina Stalite is porous and lightweight in comparison to the gravel used

required to prevent segregation during mixing.
10 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Figure 7. Conceptual diagram of structural soil including stone-on-stone compacon
and soil in intersal spaces.
FigurebySarahDickinson,adaptedfromNinaBassuk.
Contact points where
load is transferred
Soil aggregate
Stone particle
Air or water pore spaces
around the soil aggregates
Compactive force
Subsoils

structural soil reservoir will be constructed. For optimum functioning of the
system, including healthy root development, the stormwater reservoir should
drain within two days. If the subsoil is permeable, or has some permeable

through structural soils is extremely rapid. If soils are impermeable but have
permeable layers beneath them, root penetration into the subsoil base may



page 15). Although a separation geotextile is not normally required below
structural soil sections, when the structural soil is being used as a reservoir
for stormwater, subsoil may be saturated at times, resulting in lower soil
strength. Therefore, a geotechnical engineer should always be consulted to
determine if a separation geotextile is advisable between the subsoil and
structural soil components (see Geotextiles section).
Chapter 1— Trees and Structural Soils- A System Overview 11
Limitations concerning subsoil infiltration


Some conditions only require minor adjustments to the design to use

Some of these limitations include:
 

to the risk of groundwater contamination. Always refer to local
regulations.
 
feet from the surface, limited drainage, and extreme slopes are not

 

the ground water without any contaminants or pollutants being

 
regions of the country. Carolina Stalite is produced in the eastern
United States and high transportation costs make its use in western
states impractical.
Citations
Bassuk, N.L., J. Grabosky, and P. Trowbridge. 2005. Using CU-Structural Soil
in the Urban Environment. Urban Horticulture Institute, Cornell University,
Ithaca, NY.
Haffner, E.C. 2008. Porous asphalt and turf: exploring new applications
through hydrological characterization of CU Structural Soil® and Carolina
Stalite Structural Soil. Master’s Thesis. Department of Horticulture, Cornell
University.
12 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 2— System Design to Meet Site Requirements 13
Chapter 2— System Design to
Meet Site Requirements
Specications
Surface Treatments
The intent of this BMP is to manage stormwater from the immediate
vicinity— it is not meant to handle large amounts of stormwater concentrated
from surrounding land areas. Regardless, the system requires that water be
directed into a structural soil reservoir beneath the soil surface. There are two
options for this that can be used alone or in combination:
• Local rainfall data and runo calculaons will determine the minimum depth
for the structural soil reservoir. The reservoir can be designed to store the
desired rain event (e.g. a 25-year storm).
• For opmal growth of trees, designs must provide adequate depth and
extent of structural soil (see Reservoir Sizing).
• Determine the type of soil and the seasonal water table levels underneath
the reservoir. Clay soils will drain much more slowly than sandy soils and will
inuence how much water the reservoir can take and will also determine
inltraon and groundwater recharge rates from the reservoir into the
subsoil below the reservoir.
• Inltrometer measurements may not accurately reect drainage rates of the
reservoir as a whole. This is because water moves laterally very quickly in
structural soils and zones of rapid inltraon can have a disproporonately
large eect.
Sustainable site design requires coordination and consultation with diverse
professions. For instance, a geotechnical engineer can determine if this

topography.
A stormwater engineer may determine the quantity of water that the system
will need to be able to handle. In addition to water quantity, they should be
familiar with the contaminants and pollutants that will be present in the
stormwater and local regulation and permit requirements.

consulted during the design process for choosing tree species and other
plantings that will perform well for a given system design and climate.
14 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Option 1: Pervious Pavement

through the wearing surface and into the structural soil reservoir below.

rates. There are many types of pervious pavement and the choices continue
to expand. For more information on alternatives to traditional impervious
pavement, see Chapter 3.
Option 2: Traditional, Impervious Pavement
Water can easily be directed beneath traditional pavement as well. Structural
soils allow rapid lateral water movement, so water entering at one point in a
structural soil system will seek its own level, spreading out in the reservoir
in accordance with the subsoil topography. Gravel swales on the edges of
impervious areas allow water to enter the system. This design also can be
used as a “backup” system for pervious pavement if there are concerns of
clogging.
Reservoir Sizing and
Overflow Pipe Design
In order to properly
mitigate any storm,
exact rainfall data must
be obtained from local
meteorological stations.
To help design the proper
reservoir depth to
accommodate any rain
event, the adjacent table
(Table 2) can be used
as a general aid. This
information is based on a
conservative estimation
of the total porosity
of any structural soil
of 30%. If actual total
porosity is calculated
for your particular structural soil mix, the chart can be adjusted accordingly.
It is important to note that while depths less than 24” will both support
and mitigate a storm event up to 5.4” in 24 hours, for larger tree species, a
reservoir depth of 24” to 36” is optimum.
Table 2. Reservoir depths and the corresponding levels
of migated rain events based on the 30% void space
within the structural soil mix (assuming an empty
reservoir). Numbers in the gray box illustrates the
depths necessary to accommodate opmum healthy
tree root development.
TablebyTedHaner.
Chapter 2— System Design to Meet Site Requirements 15
Although a structural soil
reservoir is a great way
to collect rainwater and
runoff as regulated by the
National Pollution Discharge
Elimination System (NPDES)
guidelines and decrease
demands on existing
municipal storm water
systems, there may be rain
events that generate more
runoff than the reservoir
below can handle. Installing

design stormwater retention
level of the reservoir can
prevent system failure during
extreme weather events.
Placement of the overflow pipe should be determined based on the




to remove water from the rooting zone (the top 18 to 24 inches of structural
soil) within 48 hours, the depth of the structural soil reservoir should be

level of the rooting zone it will be removed by the pipe.
Two systems combined insure against
system failure.
1. The structural soil reservoirs at a
predetermined depth allow water storage
and inltraon to recharge groundwater, if
soil condions below the reservoir permit.
2. Tradional piping infrastructure located
at a level high enough that water will not
backup under the pavement if the reservoir
is overlled by mulple storm events. The
combinaon of the two ensures the system
will work during storm events that are larger
than the design capacity of the system.
Helpful Hints
• Design to capture all the runo from the desired storm event. The system
can easily be designed to capture all of the runo from a 100— year storm in
most cases. At a minimum, design the reservoir to handle the “water quality
storm” for your region. This is the threshold which encompasses 90% of the
yearly runo producon.
• Inltraon expectaons: water should not stay in the upper 18 to 24 inches
of the reservoir for more than 48 hours. Longer residencies in the tree roong
zone may interfere with tree establishment, growth, health, and stability of the
roong system.
16 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Figure 8. The top illustraon shows a diversion mound system as used on a
roadway. The photo to the le shows the installaon of diversion mounds for an
access road. The diversion mounds are circled in red. In the photo to the right you
can see the mounds during the structural soil installaon process.
Figure by Joe Dove. Photos by Susan Day.
Use addional drainage as necessary to decrease ooding and inundaon from
extreme storm events. Although structural soil is highly porous, ooding will
occur if the rate of water leaving via inltraon is slower than the rate that
water enters the system via rain and runo (see Reservoir Sizing above).
CU-Soil specicaons require that the mineral soil component of the mix be
heavy clay loam or loam with a minimum of 20% clay, because of its greater
water- and nutrient-holding capacity. Carolina Stalite structural soil mixes
specify a sandy loam since the porous structural parcles also hold water,
but soils with a ner texture (i.e. more clay) can also be used. Structural soil
should also have organic maer content ranging from 2-5% to ensure nutrient
and water holding while encouraging benecial microbial acvity.
Chapter 2— System Design to Meet Site Requirements 17
Level and Unlevel Sites
Does the reservoir need to be level? A level or nearly level reservoir will


can be designed in two ways. First the subsoil can be excavated in a series of
terraces. This is appropriate for a slightly sloped parking area, for example.
Alternately, diversion mounds (Figure 8) can be used to direct water under
pavement on a slope. This technique was employed at an access road
installation in Blacksburg, Virginia. Runoff collected in roadside swales and
was then directed under the road pavement with diversion mounds that
intersected the swales. In such cases, hydrostatic buildup under the pavement
must be prevented by appropriate drainage. Because the reservoir will allow

be minimal.

A good, well drained topsoil may be used around the newly installed tree if the
pavement opening allows. If this is not practical, structural soil can be used
right up to the tree root ball. In drier climates, establishing some tree species
directly in structural soil may require frequent irrigation because of the high
porosity of the soil. Tree roots need to establish good root-soil contact before

sensitive to drought during establishment (e.g. swamp white oak (Quercus
bicolor
after planting. Because structural soil gives tree roots a larger volume of soil,
irrigation may not be necessary after establishment. Again, this is climate
dependent and the expertise of a plant professional with local knowledge
should be sought.
While structural soils may have less total moisture on a per volume basis than
in conventional soil (around 16% versus a normal 25% in a agricultural soil),
the plant available moisture within the structural soil matrix is actually quite
comparable to a normal landscape soil (in the range of 8-11% by volume).
Traditional planting designs in paved areas surround the planting hole with
materials which restrict root penetration and growth. Because the use of
structural soils expands total rooting volume, trees have access to greater
water resources and can usually be managed very similarly to trees planted in
landscape soils. Similar to trees in the landscape, supplemental water should
be provided until the tree is established and then irrigation practices should
follow local climatic requirements.
18 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Geotextiles
By Joseph E. Dove
Geotextiles are part of the broad class of materials called Geosynthetics, which
are synthetic polymer materials that are used in a wide range of geotechnical
engineering applications such as reinforcement, erosion control, separation,

geosynthetics are available from the International Geosynthetics Society
(http://www.geosyntheticssociety.org/guideance.htm).

the general appearance of a cloth fabric. They are typically manufactured
from polypropylene or polyester and are categorized as either woven or
nonwoven. Woven geotextiles are produced by interweaving two orthogonal
sets of yarns. They typically have high tensile strength and resistance to
elongation. Non-woven geotextiles are manufactured by extruding individual

are then interlocked through needle punching or heat bonding processes.
Needlepunched geotextiles typically have high permeability; whereas heat
bonded non-woven geotextiles have higher tensile strength characteristics.
In the structural soil system, possible locations for a geotextile include
(Figure 8): 1) between the top of the natural (subgrade) soil and the base of
the structural soil, and/or 2) below the aggregate base soil supporting the
pavement or other surface treatment and the top of the structural soil. In

separation functions. However in the second case, the geotextile provides a
separation function only. The reinforcing function arises when the subgrade

resulting in rutting at the ground surface. This function typically requires
geotextiles with high tensile strength. A civil engineer can determine if
a reinforcing geotextile is required and recommend tensile strengths for
selecting candidate materials, if needed. The separation function in the
second case arises to prevent the aggregate base from commingling with
the structural soil below. This downward migration can result in decreased
pavement performance and a separation geotextile may be warranted as a
mitigation measure. A check can be made to assess if the aggregate base soil

soil to fall into the voids between the underlying structural soil particles.
Fortunately, migration of aggregate base soil has not proved to be a problem in
other installations. Geotextiles are not be required if the above consequences

Chapter 2— System Design to Meet Site Requirements 19
Figure 9. Enlarged view of woven and nonwoven geotexles.
Photosfrom“IGSGeosynthecsinDrainageandFiltraonbyJ.P.
GourcandE.M.Palmeira.
Selection of a geotextile is made after the required material properties are
estimated from design computations performed by a civil engineer (for

geotextile for this application is the reduction in mechanical performance



Applications” provides guidance geotextile selection. This standard is
intended for geotextiles used in subsurface drainage, separation, stabilization


each function. Selection of the minimum geotextile material properties

selection of locally available candidate geotextile products with the required
engineering properties is made from information published by manufacturers.
Most manufacturers of geotextiles provide the M288-00 survivability class for
each of their products.
It has been found that the woven geotextiles tested in the structural soil
system do not prevent tree root penetration, a summary of this research is in

Citation

Upper Saddle River, NJ.
20 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Trees and Other Plants
Trees are an integral component of this stormwater system and must grow
well in order to realize maximum stormwater mitigation. By enlarging the
rooting volume typically available to trees in paved areas, canopy size has
the potential to increase faster and trees may ultimately reach a greater size.
Rainfall interception, storage, and ultimately evapotranspiration from leaf
surfaces, are directly related to canopy size. In addition, rainfall captured by
tree canopies is often directed down limbs and trunks into the soil at the base
of the tree—effectively bypassing the pavement.
Trees are living organisms and have certain requirements in order to grow


stormwater system. However, tree selection should never be undertaken

arborist, or related professional). Pest resistance, urban forest diversity,
regional climate factors, growth form, invasive potential and numerous other

Soil Chemistry
Structural soils can have very different pHs than local mineral soils. Structural
soils with a limestone base will typically have high pH. A structural soil with a
granite base may have lower pH. The soil pH determines nutrient availability
among other things. A pH of 7 is neutral, with lower pH being acid and a
higher pH, basic or alkaline. The ideal pH for most trees is about 5 to 6.5, but
urban soils are typically very basic (pH 7.5 to 8.5) because of disturbance,
including concrete and limestone debris mixed into the soil. A typical

yellowing, of the leaves (Figure 10). If the structural soil used in the system
has a high pH, then a “pH tolerant” tree species should be used. These include
many elms and ashes and certain maples and oaks as well as a variety of other
species (see the tree guide sources at the end of this chapter). The key is to
test the structural soil pH and select trees that tolerate it.
Figure 10. Visual comparison of a healthy
pin oak leaf (le) and a chloroc leaf
(right). This chlorosis ulmately interferes
with carbohydrate producon in the plant
and is a result of nutrient deciencies
stemming from elevated soil pH.
Photo by Susan Day.
Chapter 2— System Design to Meet Site Requirements 21
Soil Volume
Trees need enough room to grow—for their roots as well as their canopy.
Tree pits (a.k.a. cutouts, planters) should be as large as possible—but how
large is that? The key to designing sites that support large trees is to have
essentially unlimited rooting space. A typical 4 × 4 ft. cutout with no access
to surrounding soil limits tree growth almost immediately. A 25× 25 ft. cutout
limits growth very little until the tree is quite large. The usable rooting space
provided by any cutout can be expanded by a continuous structural soil bed
under pavement. Some species are more adept at exploiting weakness in
pavement, penetrating compacted soils, or reaching nearby open spaces.
However, the system should be designed to support the tree fully without
infrastructure damage. Structural soils have been shown to support

should supply rooting space without compromising structural integrity.
Again, species selection and site conditions must be compatible so a plant
professional should be consulted. Always consider local regulations and
permitting requirements.
Innovave Soluon: High Shipping Costs of Structural Soils for Western
States
High shipping costs can make using
Carolina Stalite, produced in North
Carolina, prohibively expensive
in Western states. The University
of California at Davis designed an
engineered soil from local, inexpensive
volcanic rock and gave it the name
of Davis Soil. This soil has been
successfully used to increase drainage
in open areas adjacent to parking lots
and in certain turf applicaons. Davis
Soil is not considered a structural
soil because it cannot support
the weight of pavement, cars and
other structures. It can maintain
perviousness under foot trac and
supports healthy tree growth. It is very porous (40 % porosity), and so it is
able to store stormwater which can be then be used by trees. In addion,
its large surface area with many nooks and crannies act to trap common
stormwater pollutants. Contact Qingfu Xiao at qxiao@ucdavis.edu for
more informaon on obtaining Davis Soil.
Figure 11. Davis Soil, a non-
loadbearing soil (i.e. not a
structural soil) with high
inltraon rate and high potenal
for water storage.
PhotobyQingfuXiao.
22 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.



(see Chapter 4, Tree Development in Structural Soils at Different Drainage
Rates), root systems developed best when water was retained in the rooting

white oak (Quercus bicolor) or American elm (Ulmus americana) can survive





turf or groundcovers can be used if climate permits. See Chapter 3 for more
information on surface treatments.
Although high water tables may limit tree rooting depth, when species
selection and site design allow trees to root into lower soil regions and
penetrate through impervious zones, they may be an effective tool to increase


in highly restrictive soils. To ease establishment, trees should ideally be
established in mineral topsoil, with the structural soil components being
reserved for under the pavement. However, establishing trees directly in
structural soil can simplify installation. If trees will be irrigated regularly
during establishment and climatic conditions are appropriate, this approach
can be used.
Tree root systems are wide spreading. For maximum tree growth, provide
rooting area about twice the diameter of the ultimate canopy for which you
are designing.
Chapter 2— System Design to Meet Site Requirements 23
General tree guide sources:
Dirr, Michael. Woody Landscape Plants.
PLANTS Database, hp://www.plants.usda.gov/
Northern Trees, hp://orb.at.u.edu/TREES/index.html
Tree guide sources for the Eastern United States:
Appleton, B. 2001. New York / Mid Atlanc Gardener’s Book of Lists. Taylor
Publishing Company, Dallas.
Bassuk, N.L. Cornell Department of Horculture Woody Plant Database, hp://
hosts.cce.cornell.edu/woody_plants/
Bassuk, N.L., J. Grabosky, and P. Trowbridge, 2005. Using CU-Structural Soil in
the Urban Environment, hp://www.hort.cornell.edu/uhi/outreach/csc/index.
html
Day, S.D. Virginia Urban Tree Selector, hp://www.cnr.vt.edu/dendro/
treeselector/
Trowbridge, P.J. and N.L. Bassuk. 2004. Trees in the Urban Landscape: Site
Assessment, Design, and Installaon. Wiley and Sons, New York.
Tree guide sources for the Western United States:
McPherson, E.G., J.R. Simpson, P.J. Peper, Q. Xiao, D.R. Pienger and D.R.
Hodel. 2001. Tree Guidelines for Inland Empire Communies. Sacramento, CA:
Local Government Commission
McPherson, E.G., J.R. Simpson, P.J. Peper, K.I. Sco and Q. Xiao. 2000. Tree
Guidelines for Coastal Southern California Communies. Sacramento, CA: Local
Government Commission
McPherson, E.G., J.R. Simpson, P.J. Peper and Q. Xiao. 1999. Tree Guidelines
for San Joaquin Valley Communies. Sacramento, CA: Local Government
Commission
24 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 25
Special Concerns
Soil Migraon
The excavaon of a seven-year-old tradional installaon of a London plane
(Platanus x acerifolia) tree in CU-Soil with a pervious surface did not show
any aggregate migraon. The pores between stones in the structural soils are
mostly lled with soil so there are few empty spaces for soil to migrate to.
Frost Heave
By design, structural soils are gap-graded to provide rapid drainage, and limits
the silt fracon to be consistent with very low frost heave suscepbility as
dened by the US Corp of Engineers Cold Weather Research Laboratories.
However, two important issues are related to this queson. First, if the design
system is installed as a trench under the pavement, there needs to be an
awareness of the depths of layers in each pavement layer prole, and their
dierent frost heave potenals. The designer needs to be sure there is not a
major dierence in frost heave potenal at the interface of the two systems
or else the pavement surface will move and crack as the total layered systems
will behave dierently. Secondarily, frost concerns also suggest snow removal
concerns, so the placement of trees in the system and the needs of snow
removal and storage on site need to be addressed with the maintenance
authority to prevent the loss of the trees or damage to the system.
Observaon of structural soil throughout the US and Canada shows that the
depth of the reservoir negates any heaving due to consequent freezing and
thawing. Addionally, there have been no observed instances of freeze/thaw
damage in any structural soil installaons in the een plus years since its
incepon.
Chapter 3— Surface Treatments 25
Chapter 3— Surface Treatments
This section describes two surface treatments that can be used with this
system: turf and porous pavement. The sections in this chapter are
summaries from manuals published by the Urban Horticulture Institute
(Cornell University). A citation to the complete manual is provided at the end
of each section.
Structural Soils and Turf
By Nina Bassuk, Ted Haffner,
Jason Grabosky, and Peter
Trowbridge
Introduction
Turf is primarily used as a
ground cover in residential
lawns, parks, playgrounds and

providing a sense of open space
and as a protective surface for
recreation. If turf is properly
installed, it can have additional

lanes, and parking lots. In these
instances, turf can contribute to
a sense of open green space and
reduce temperatures in urban
settings that may otherwise be
paved.
When turf is used for these
applications, however, it is

will compact the soil. These
situations also limit drainage,
healthy root growth, and the
ability of turf to grow at all.
Cornell Developments in Turf
Use
Cornell University has
combined turf with structural
soil to create a healthy growing
medium for the grass that

Figure 12. Area of park used for a weekly farmers
market in Chicago. Compacon from foot and
vehicle trac has denuded the grass in this
secon of the park.
PhotobyTedHaner.
Figure 13. Photo simulaon of turf-covered
perimeter parking at a big box lot in Ithaca, NY.
For best results, turf should be only placed in
parking stalls and not in driving lanes of the
parking lot.
PhotosimulaonbyTedHaner.
Chapter 3— Surface Treatments 2626 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 27
to be virtually maintenance free, and can be used in areas that receive high




both people, cars and temporary structures to safely use a turf covered surface

the turf surface and hold it in a reservoir underneath the grass. Increased
water and air within the structural soil media not only allows for healthier
root and shoot growth for the grass, but also allows rainwater and runoff to
be collected and held within the reservoir in large amounts until it can slowly

system infrastructure and also recharges the groundwater levels over time.
This combination, then, not only serves the environment from a water quality
standpoint, but also adds a “sustainably green” component to highly urbanized
areas.
Figure 14. Aerial view of structural soil and turf experimental plots at Cornell University
in Ithaca, NY. Surface Treatments: PA= Porous Asphalt, Z= Zoysia Grass, F= Tall Fescus,
C= Tradional Asphalt.
GraphicsbyTedHaner.UnderlyingphotobyGoogleEarth.
Chapter 3— Surface Treatments 26
Chapter 3— Surface Treatments 27
Figure 15. Construcon detail for turfgrass
and structural soil prole. Note that the 24”
reservoir depth was based on local rainfall data
and will vary by region according to the local
rainfall data and/or ancipated runo amounts.
FigurebyTedHaner.
• Minimize vehicular wear on the turf as much as possible. To do this, place
turf only in parking stalls and not the driving lanes of the lot.
• Angle parking stalls to minimize turning from automobile wheels. Excessive
turning causes the turf grass leaf blades to tear and can create bare patches in
the turf. Research indicates that turf can recover from this damage but it takes
extra me.
• Use turf only in overow parking areas on the outskirts of large parking lots.
• Use inset stonework between stalls, or posts to demark parking stalls. This
design maneuver may cost more upfront to install, but will save me and
money during post-installaon maintenance.
• Specify proper post-installaon maintenance regimes. Mowing every 10
days is necessary, as is the applicaon of annual fall ferlizaon with proper
applicaon rates.
• Never snow plow the turf poron of the parking lot. The blades from the
plow will damage the turf surface, removing the turf and necessitang costly
replacement.
Chapter 3— Surface Treatments 2828 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 29
Designing and Working with Turf and Structural Soil

achieve. With many different factors involved in the process, it is not as simple
as spreading seed or unfurling a roll of sod. Proper decision making at every
step of the planning, design, installation, and post-installation process are
absolutely necessary.
Working with turf and structural soil requires a change in the way that
designers and contractors go about their work. Rather than just installing
sod or seeding grass directly onto existing soil, entire areas will need to be
excavated to a depth of at least 18” to 24” (to accomodate stormwater- see

soil. Once the structural soil mix is in place it must be compacted with a
vibratory or rolling compactor. Once compacted, the sod should be installed
directly onto the structural soil and then irrigated for a number of weeks
until established. Once established, research indicates that maintenance
requirements are minimal, other than regular mowing and periodic
fertilization.
With the previous guidelines, a few simple construction details will provide
the bulk of information needed for bidding and installation of a construction
project. While a few simple drawings are helpful, keep in mind that every
design is different and will necessitate the level of detail appropriate for
each different design scenario. Additional details will be needed for, ADA
compliance curbing, tree planting and staking, hydrant water supply, signage
FAQs
What type of maintenance is needed for a turfgrass and structural soil system?
Our research was performed with the idea of the most basic maintenance
regime in mind. Test plots on the Cornell campus received no maintenance
other than roune mowing once every 7 to 10 days during the growing season.
Addional annual ferlizaon in the fall is recommended with the proper
applicaon rates.
What happens when neighboring tree roots expand in structural soil?
There will come a me when the roots will likely displace the stone because
there are no pavement layers above the structural soil, but if the roots are,
as we have observed, deep down in the prole, the pressure they generate
during expansion would be spread over a larger surface area. We have seen
roots move around the stone and actually surround some stones in older
installaons, rather than displace the stones.
Chapter 3— Surface Treatments 28
Chapter 3— Surface Treatments 29
Case Study
Turf on CU-Soil has been successfully used at a Mercedes dealership (Crown
Automobile) in Alabama. At this installaon, the soil in an enre median
was excavated and replaced with CU-Soil and then sod was placed on top.
The median can now properly withstand the compacon from the weight
of the cars and serves as a exible open space for the dealership, providing
impromptu space to display inventory, or as overow parking for the
dealership. Aer three years, this installaon is maintenance free and as
healthy as the day it was installed.
Figure 16. In winter
when the sod is
dormant, the median
serves as addional
storage and display
space for the
dealership inventory.
This exibility is
invaluable to the
dealership.
PhotobyBillIsaacs.
Citation:
Haffner, E.C. 2008. Porous asphalt and turf: exploring new applications
through hydrological characterization of CU Structural Soil and Carolina
Stalite Structural Soil. Master’s Thesis. Department of Horticulture, Cornell
University.
Chapter 3— Surface Treatments 3030 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 31
Using Porous Pavement on Structural Soils
By Ted Haffner, Nina Bassuk, Jason Grabosky, and Peter Trowbridge


into the subgrade below, naturally recharging groundwater levels.
Porous asphalt is similar to traditional asphalt in every way but the mix




porous pavements should be limited to 1-6%.
Figure 17. The le gure shows rain on a tradional asphalt parking lot- aer it hits the
surface it typically runs o into a storm sewer system. The right gure shows rain on
a porous asphalt parking lot- aer it its the surface it inltrates through the pavement
into the structural soil reservoir below. Water then inltrates into the ground,
recharging the groundwater over me.
BothguresbyTedHaner.
Structural soil and porous asphalt are a new combination of 15- and 30-year-

in Ithaca, NY and was installed in 2005. Porous asphalt parking lots are
numerous and the oldest include the Walden Pond Reservation in Concord,
MA, the Morris Arboretum in Philadelphia, PA, as well as an ever expanding
list of corporations and universities across the United States. Structural
soil has been used extensively without porous asphalt pavement and the
(Gleditsia
triacanthos) planting at the Staten Island Esplanade Project in New York City,
the second is a London planetree (Platanus acerifolia) planting on Ho Plaza
on the Cornell campus, Ithaca, NY. There are now hundreds of installations of
various sizes across the United States and Canada.
Chapter 3— Surface Treatments 30
Chapter 3— Surface Treatments 31
Figure 18. A comparison of tradional asphalt (le) and
porous asphalt (right) when wet. The gaps created by
leaving out the ner parcles in porous asphalt allow
water to inltrate pavement and into the structural soil
reservoir below. As a result, porous asphalt appears
dull when wet, because water runs through and does
not pond, which creates a high fricon surface.
PhotobyTedHaner.
Concerns of Clogging
The best maintenance for any type of porous pavement is a vacuum treatment

pavement, although the oldest installations have never been vacuumed and
show little effects of clogging. Porous asphalt systems should not be pressure
washed since this treatment further embeds sediment within the surface.
Additionally, porous asphalt systems should never be sealed. Once a sealant is
applied, the system will not work ever again.
Porous Bituminous Asphalt Specification
Ithaca, NY Porous Asphalt Medium Duty Parking Lot
1. Bituminous surface course for porous paving shall be two and one-half (2.5)
inches thick with a bituminous mix of 5.5% to 6% by weight dry aggregate.
In accordance with ASTM D6390, draindown of the binder shall be no greater
than 0.3%. If more absorptive aggregates, such as limestone, are used in the
mix then the amount of bitumen is to be based on the testing procedures
outlined in the National Asphalt Pavement Association’s Information Series
131 – “Porous Asphalt Pavements” (2003) or NYSDOT equivalent.
Chapter 3— Surface Treatments 3232 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.

binder meeting the requirements of PG 76-22. The elastomeric polymer shall
be styrene-butadiene-styrene (SBS), or approved equal, applied at a rate of
3% by total weight of the binder. The composite materials shall be thoroughly


storage stable.
3. Aggregate in the asphalt mix shall be minimum 90% crushed material and
have a gradation of:
U.S. Standard
Sieve Size Percent Passing
½” (12.5mm) 100
3/8” (9.5mm) 92-98
4 (4.75mm) 32-38
8 (2.36mm) 12-18
16 (1.18mm) 7-13
30 (600 mm) 0-5
200 (75 mm) 0-3
4. Add hydrated lime at a dosage rate of 1.0% by weight of the total dry
aggregate to mixes containing granite. Hydrated lime shall meet the
requirements of ASTM C 977. The additive must be able to prevent the
separation of the asphalt binder from the aggregate and achieve a required
tensile strength ratio (TSR) of at least 80% of the asphalt mix.
The asphaltic mix shall be tested for its resistance to stripping by water in
accordance with ASTM D-3625. If the estimated coating area is not above 95
percent, anti-stripping agents shall be added to the asphalt.
Citation:
Haffner, T., Bassuk, N.L., Grabosky, J., and P. Trowbridge. 2007. Using Porous
Asphalt and CU-Structural Soil. http://www.hort.cornell.edu/uhi/outreach/
csc/index.html Urban Horticulture Institute, Cornell University, Ithaca, NY.
Chapter 3— Surface Treatments 32Chapter 4— Research and Recommendations 33
Chapter 4— Research and
Recommendations
Tree Root Penetration into Compacted Soils Increases
Inltration
Based on Research by Julia Bartens, Susan Day, Joseph E. Dove, J. Roger
Harris, and Theresa Wynn, Virginia Tech
Research Summary
A container experiment with
recently transplanted black
oak (Quercus velutina) and red
maple (Acer rubrum) tested
whether roots can penetrate into
compacted soil and once they
penetrate, if they can increase

species were grown in pine bark
and surrounded on all sides
and the bottom with compacted
soils. Within 12 weeks, both tree
species were able to penetrate
into compacted soil and increase


by 153%. There was no
difference in performance
between black oak (coarse roots)

In a second container experiment, green ash (Fraxinus pennsylvanica)
were grown in CU-Soil and were separated from the compacted subsoil by
geotextile. Roots were able to penetrate into compacted subsoil and increase

Next Steps/Research Needs

applies to larger scale trees in the ground needs to be done. Tree species with
different requirements should also be observed.
Citation
Bartens, J., S. D. Day, J. R. Harris, J. E. Dove, and T. M. Wynn. 2008. Can urban

management? Journal of Environmental Quality, 37 (6):2048-2057.
Figure 19. Ash roots penetrang geotexle aer
compacted subsoil has been washed away.
Roots increased inltraon by a factor of 27.
Photo by Susan Day.
34 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Tree Development in Structural Soils at Different Drainage
Rates
Based on Research by Julia Bartens, Susan Day, J. Roger Harris, Joseph E.
Dove, and Theresa Wynn, Virginia Tech
Research Summary
A container experiment involving 2 tree species (swamp white oak (Quercus
bicolor), and green ash (Fraxinus pennsylvanica), 3 drainage rates (slow,
medium, rapid), and 2 structural soils (CU-Soil and Carolina Stalite)
evaluated the optimal reservoir detention times for tree root development
and water uptake from the reservoir. Structural soils had an impact on root
distribution— tree roots grew wider in Carolina Stalite than with CU-Soil.
Drainage rate also had an impact on tree growth; Root:shoot ratios for swamp
white oak were much higher for the slow drainage treatment and trees were

and no difference in Root:shoot ratios for the different drainage rates was
observed but roots did grow deepest in the rapidly draining treatment.
Recommendations based on this research
In general, water should drain from the parking lot within 2 days so adequate
root systems can develop. For water uptake from the reservoir it is clearly

inundation can prevent this deeper root exploration, depending upon species.
Transpiration rates were varied but similar to trees grown in traditional
landscapes. Of course, size of tree canopy is important in determining
amount of water that can be removed. In general, the largest trees with the
best developed root systems removed the greatest amount of water from the
stormwater reservoirs.
Next Steps/Research Needs
Temperatures of the structural soils could be compared in future experiments
because this could also be affecting the root growth and maybe of interest if


information about lateral root growth (which was limited in this experiment

tolerances can be expected to respond similarly, more species trials would be
useful.
Chapter 4— Research and Recommendations 35
Citation
Bartens, J., J. R. Harris, S. D. Day, J. E. Dove, and T. M. Wynn. 2008 Ecologically
integrated stormwater distribution using urban trees and structural soils. (in
review)
Drainage Rate at the Mini Parking Lot Demonstration Site in
Blacksburg, VA
Based on Research by Mona Dollins, Virginia Tech
Research Summary
A Mini Parking Lot demonstration site which had a Carolina Stalite structural

allowed to naturally drain into the clay textured subsoil beneath. The water
levels were checked from 15 observation wells every 5 minutes (during the

determine the speed of drainage and lateral water movement through the
system.
Within 2.5 hours, the water had completely drained from the reservoir. Lateral
water movement within the reservoir was very rapid through the structural
soil media traveling over 18 feet in a matter of minutes.
Next Steps/Research Needs
Drainage data from larger systems, at varying depths, and different types of
subsoils should be tested to gain better understanding of the systems behavior
in different conditions.


recommended (see the blue box on page 15).
36 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
System Effects on Water Quality
Based on Research by Qingfu Xiao, University of California at Davis
Research Summary
Research shows that 97.9-99% of the hydrocarbons found in pollutants

suspension, microorganisms biodegrade the hydrocarbons into their
constituent parts of simple chemical components which cease to exist as
pollutants and render them harmless to the environment.
Surface runoff from four types of parking lots was collected (commercial,
older institutional (>10 years), newer institutional (<3 years), and
residential). Pollutant removal (nutrients, heavy metals, soil column tests) by
3 types of substrates (CU-Soil, Davis Soil, and Carolina Stalite) were compared.
Tests: single event test, multiple events test and synthetic runoff test.
All three engineered soils were effective at removing nutrients and materials
in polluted surface runoff. Pollutant removal rates were strongly related to the
type and size of the rainfall event.
Next Steps/Research Needs
Research that determines the pollutant saturation point for these soils should

Once tree roots explore the reservoir it is expected that they would enhance
pollutant removal— but research is needed to accurately evaluate these
effects.
How effective the system is at removing/degrading nutrients and pollutants
with trees in the system.

other BMPs need to be used for pre-treating the surface runoff.
Chapter 4— Research and Recommendations 37
Table 3. Pollutant removal of single storm event. CU= CU Soil, CS= Carolina Stalite, and
DS= Davis Soil.
TablebyQingfuXiao.
Table 4. Pollutant removal of mulple storm events. CU= CU Soil, CS= Carolina Stalite,
and DS= Davis Soil.
TablebyQingfuXiao.
38 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
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Appendices 43
Appendices
Note:
to the reader and are presented “as is” from resources provided by the

an endorsement of or warranty of these products by Virginia Tech, Cornell
University or University of California at Davis or any of their employees.
CU-Soil is a patented material and must be purchased from a licensed supplier.
Amereq (http://www.amereq.com/) licenses the manufacturing of CU-Soil to
ensure quality control of installations.
Carolina Stalite is composed primarily of a manufactured component available
from Carolina Stalite Company (Salisbury, NC). It is available through the
horticultural division of Carolina Stalite (www.permatill.com).
44 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
CU-Soil Specication and Mixing Procedure
CU-Soil is a patented material and must be purchased from a licensed
supplier. Amereq (http://www.amereq.com/) licenses the manufacturing
of CU-Soil to ensure quality control of installations.
Appendices 45
46 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 47
48 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 49
50 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 51
Carolina Stalite Structural Soil Specication
52 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 53
54 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Carolina Stalite Mixing Specication
Appendices 55
... Highly porous ESM mixes provide ample infiltration and pore space for temporary storage of surface runoff. Also, they support tree growth by providing more water and aeration to roots than compacted native soil alone [13]. ESMs can reduce conflicts between surface roots and sidewalks by promoting deeper rooting systems [25,26]. ...
... Importantly, decentralized green infrastructure strategies control runoff and contaminants at their source [9,10]. Vegetation is a green infrastructure strategy that can play an important role in surface runoff management [11][12][13]. Large-scale tree planting programs have been established in many cities to mitigate the urban heat the annual precipitation (446 mm) occurs between November and April. ...
... The pH ranged from 7.90 to 8.24 for the control site and 8.07 to 8.20 for the treatment site. The relatively high pH values indicate that alkalinity was not elevated by the ESM used in this bioswale [13]. The high pH and alkalinity results from the relatively poor quality of the irrigation water originating from groundwater. ...
Article
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This study evaluated the effectiveness of two bioswales eight years after construction in Davis, California. The treatment bioswale measured 9 m × 1 m × 1 m (L × W × D). Engineered soil mix (75% native lava rock and 25% loam soil) replaced the native loam soil. Four Red Tip Photinia (Photinia × fraseri Dress) trees and two Blueberry Muffin Hawthorn (Rhaphiolepis umbellata (Thunb.) Makino) shrubs were planted in the bioswale. Runoff flowed into the bioswale from an adjacent 171 m2 panel of turf grass. An identically sized control bioswale consisting of non-disturbed native soil was located adjacent to the treatment bioswale. Surface runoff quantity and quality were measured during three experiments with different pollutant loads. When compared to the control, the treatment bioswale reduced surface runoff by 99.4%, and reduced nitrogen, phosphate, and total organic carbon loading by 99.1%, 99.5%, and 99.4%, respectively. After eight years, tree growth characteristics were similar across both sites.
... In order to achieve optimal tree growth, and thus ecosystem service performance, consideration of favourable soil type for different species is also important (Day and Dickinson, 2008). The same authors suggest the largest trees with the best developed root systems remove the greatest volume of water from stormwater reservoirs. ...
... This is not a significant conclusion in other primary studies, but it does indicate potential for the integrated use of trees in urban environments. Tree performance can be hindered by lack of consideration to the planting area of the tree (Day and Dickinson, 2008;Rahman et al., 2019). ...
Article
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Sustainable urban water management initiatives are increasingly required to combat rapid urbanisation and climate pressures. Initiatives include the role of tree planting for which there is need for strong evidence of benefits and drawbacks to support effective future planning. We report robustness of evidence from an assimilated database of urban hydrological impact studies which often had differing primary purposes. Consistent impacts were found at local level, with trees reducing runoff and infiltration. Despite the consistency of evidence, much is undermined by being somewhat lacking in robustness and scientific rigour. Many studies lack adequate controls, and models are often not strongly tested against observations. Moreover, evidence of impact at larger scales is lacking. Effects of tree characteristics were also investigated; such as maturity and species for which evidence is consistent and detailed, and arrangement for which there is less evidence. Realising the full potential of trees in urban water management decision-making would benefit from more-rigorous evidence.
... Effective management of storm-water is a multidisciplinary and an integrated approach. Urbanization disrupts natural soil profiles, increases impervious surfaces as "asphalt is the land's last crop" and decreases vegetative cover [1,2]. These disruptions increase stormwater runoff at the expense of groundwater recharge, degrading water quality and impairing aquatic habitats. ...
... Inevitably, area under roads and parking lots will continue to increase to keep up with the pace of population growth and its needs. In the other hand, the huge area under CT shows at least 40% compaction in the United States mainly due to use of machinery and several other reasons [1,6]. Ideally, in other word, 44% (174 mill ha or 430 mill acre) of the total can be estimated as impervious surface area in the United States which would be depicted to have the same impact as roads and parking lots in stormwater management and hydrological cycle. ...
... While comparing the pervious runoff, the base flow represented a minor percentage of total runoff. Compact soil with low infiltration rate is quite common in urban areas, so compact soil improvement, such as structural soil [54] could directly increase infiltration rate and volume. Another approach is to decrease impervious surface runoff by adding canopy cover above the impervious cover [6]. ...
Article
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This paper reveals the role of urban green space (UGS) in regulating runoff and hence on urban hydrological balance. The modeling software i-Tree Hydro was used to quantify the effects of UGS on surface runoff regulation and canopy interception capacity in four simulated land-cover scenarios. The results showed that the existing UGS could mitigate 15,871,900 m3 volume of runoff (accounting for 9.85% of total runoff) and intercept approximately 9.69% of total rainfall by the vegetation canopy. UGS in midterm goal and final goal scenarios could retain about 10.74% and 10.89% of total rainfall that falls onto the canopy layer, respectively. The existing UGS in the Luohe urban area had a positive but limited contribution in runoff regulation, with similar responses in future scenarios with increased UGS coverage. UGS rainfall interception volume changed seasonally along with changing leaf area index (LAI) and precipitation, and the interception efficiency was distinctly different under various rain intensities and durations. The UGS had a relatively high interception performance under light and long duration rain events but performed poorly under heavy and short rain events due to limited surface storage capacities. Our study will assist urban planners and policy-makers regarding UGS size and functionality in future planning in Luohe, particularly regarding future runoff management and Sponge City projects.
... Mickelbart et al. (2012) showed that height and width of all Spiraea alba were reduced when grown in soils with a high alkaline soil pH. The ideal pH range for most trees is about 5 to 6.5 (Day and Dickinson 2008). Indeed, most urban soils are too alkaline for optimal plant nutrient availability (Kelsey and Hootmann 1988). ...
Article
Full-text available
Tree growth is sensitive to soil pH in urban areas and is often higher than in rural forest. However, there are knowledge gaps on how soil pH and alkalization are affected by urban environments and seasonal climate as well as the cascading effects on tree species. In order to fulfill these gaps, we analyzed the soil pHCaCl of four common native deciduous tree species: Acer platanoides, Tilia cordata, Quercus robur and Betula pendula in five different types of urban green spaces in Kaunas city municipality (Lithuania). The results show that soil pH in urban environments with Betula pendula sites were most alkaline (pH 7.04), whereas the soil pH of urban environments with Acer platanoides (pH 6.7) and Tilia cordata (pH 6.8) were most acidic. The soil pH of street tree greeneries was alkaline, while soils of peri-urban forests and large urban parks were acidic. Differently to natural conditions in peri urban forests the soil pH level drop down by 0.5 is observed during spring-autumn period in broad street greeneries with largest urban pressure. The variation in soil pH of the different types of green space and tree species shows that city planner should consider the unique conditions of all green space to maximize their potential for human well-being.
... Mickelbart et al. (2012) showed that height and width of all Spiraea alba were reduced when grown in soils with a high alkaline soil pH. The ideal pH range for most trees is about 5 to 6.5 (Day and Dickinson 2008). Indeed, most urban soils are too alkaline for optimal plant nutrient availability (Kelsey and Hootmann 1988). ...
Article
Full-text available
Tree growth is sensitive to soil pH in urban areas and is often higher than in rural forest. However, there are knowledge gaps on how soil pH and alkalization are affected by urban environments and seasonal climate as well as the cascading effects on tree species. In order to fulfil these gaps, we analyzed the topsoil pHCaCl of four common native deciduous tree species: Acer platanoides, Tilia cordata, Quercus robur and Betula pendula in five different types of urban green spaces in Kaunas city municipality (Lithuania). The results show that topsoil pH in urban environments with Betula pendula sites were most alkaline (pH 7.04), whereas the topsoil pH of urban environments with Acer platanoides (pH 6.7) and Tilia cordata (pH 6.8) were most acidic. The topsoil pH of street tree greeneries were alkaline, while the topsoil profiles of the peri-urban forests and large urban parks were acidic. Differently to natural conditions in peri-urban forests, the topsoil pH level drop down by 0.5 is observed during spring-autumn period in broad street greeneries with largest urban pressure. The variation in soil pH of the different types of green space and tree species shows that city planner should consider the unique conditions of all green space to maximize their potential for human well-being.
... Urban greening is a fundamental ele- ment in guaranteeing life quality in urban contexts ( Gill et al., 2007;Newell et al., 2013;Demuzere et al., 2014;Norton et al., 2015). In this regard, urban forestry plays a central role (Rowntree and Nowak, 1991;Dwyer et al., 1992;Nowak et al., 1996;Coder, 1996;Day and Dickinson, 2008;Gillner et al., 2015), since trees can have a wide effect on water cycling, thermal balances, carbon sequestration and many other ecosystem services ( Lafortezza et al., 2009;Pataki et al., 2011;approximately 92 g (0.2 pounds) of PM10 per year" (City of Portland, 2010). Nevertheless, the above-mentioned "effects are only possible if the urban tree stock is vital and unaffected by pests and diseases" (Sjöman et al., 2012, p. 31). ...
Article
The management and care of urban greenery is essential to ensure the effective delivery of the ecosystem services it is capable of supplying. A sufficient and adequate care for urban greenery and in particular for urban forestry is an on-going challenge due to economic hardship of public administrations, to a lack of qualified personnel, and to a lack of a culture of valorization of public goods. To identify an opportunity to reuse any by-products resulting from pruning operations could signify economic benefits that could be invested in a better maintenance of urban arboreal patrimony, following a circular economy approach. This paper is the first step of a wider research that has the goal to delineate a strategy for the utilization of the pruning waste of the urban trees as thermal insulation materials. Particularly, in this paper is studied one of the most common tree species in urban greening and forestry: Tilia sp. Three tiles with different densities obtained mixing wood wool and PVA glue are realized and tested. The resulting thermal conductivity and specific heat varying respectively from 8.30 ± 0.54 to 8.60 ± 1.40 10-2 W/m*K, and from 2.26 ± 0.51 to 2.80 ± 0.65105 J/m3*K. Using these values, the paper developed a thermal simulation model, regarding the insulation of the roof of a residential building. The model aimed at comparing the thermal performance of the studied tiles, with two commercial materials produced with wood residues from the timber industry, and with the fibers of two vegetable crops, widely used as insulating materials. The linden tiles entailed an energy saving of 55% respect to the model without insulation, the other materials between 51 to 62%. The results show as the Tilia sp.’s pruning waste could be a good source material to create panels for thermal insulation.
... The image below speaks to a lack of water and soil volume, but also the issue of soil compaction. Figure 13: Trees planted at the same time in tree pits (left) and in a grassy area (right); Philadelphia Avenue, Washington, DC; photo by Nina Bassuk (Day & Dickinson, 2008) In these types of hardscape areas, one must provide a hospitable environment for tree growth. ...
Research
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While collecting resources from around the United States on the subject of parking lots and "urban" forestry (for lack of a better term), I set out to understand New England tree infrastructure. Research entailed communicating with urban foresters. I also contacted planners in Burlington, VT, Concord, NH, Fitchburg, MA, Lewiston, ME, Norwich, CT, and Woonsocket, RI to see if they could identify meaningful tree projects in the parking environment. None surfaced. Plymouth State University facilitated this research by giving me a graduate teaching assistantship. Faculty members Diana Jolles and Mary Ann McGarry offered guidance. John Parry of the US Forest Service also participated on my committee. The primary tool that I utilized in data collection for each city was i-Tree. This document gives the reader an explanation of how i-Tree can be used to identify parking areas in a particular city as well as estimate run-off potential. In the second half of the report I provide several recommendations for adding tree canopy that will benefit a town or city--including local businesses.
... The water trapped in this way does not burden the sewage system at the time, but seeps into the soil gradually and thus more efficiently. This in turn improves the quality of otherwise poor urban soils (Day and Dickinson, 2008). ...
Article
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In urban areas vegetation (especially woody vegetation) is of utmost importance, since it affects the ecological conditions of the city. Urban trees play an important role in improving urban climate both at the local (city, district) and the micro-level (e.g. in public squares). Establishing and maintaining advanced and detailed information systems necessary for the management of urban tree stands is an important task of environmental and climate-conscious city management. Despite that, few of the Hungarian municipalities have a regularly updated tree database. The city of Szeged started efficient green space management in autumn 2013, when we started the creation of a detailed and up-to-date tree register for the public areas, which has been continuously expanded ever since. The survey of the present study covers the period of the growing season, from late spring to early autumn of 2013. All the trees are included in the survey and quite a number of data are recorded for each individual (e.g. species, age, size parameters, exact location, health status, etc.). The recorded data are paper-based, however they are included in a GIS-based green space inventory software, Greenformatic, where coordinates are associated to each object, while information on the state of the tree, its location and handling can be found in the attribute table. The trees included are mostly concentrated in the inner city of Szeged, but the surveys will gradually cover ever larger areas of the city. The results highlight the fact that the structural attributes of the different species’ populations are formed by the integrated effect of the species’ urban tolerance and planting policies of the past decades. The current database already allows highly complex analysis, which contributes to the well-being of city residents.
... The retention of water in soils is a valuable, ecological service that mimicks the functioning occurring within forest ecosystems. This is due to the great potential of trees to develop massive root systems, which greatly reduce soil loss,thus enabling soils to increase their water retention capacity and almost eliminate soil erosion (Day and Dickinson, 2008). In addition to this, storm water capture exerted by trees at WSU reduced non-point source pollution by water run-off and soil erosion, saving the university $28,832 that otherwise could have been spent to dispose of water excesses due to rain and ice melting during spring (March-April). ...
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Trees possess intrinsic and extrinsic attributes that contribute to enhancing high environmental standards in a majority of landscapes, also while improving quality of life for human communities and other biota. Winona State University was recognized recently as a Tree Campus USA, the only institution of higher education within the Minnesota State Colleges and Universities (MnSCU) system to achieve this distinct honor. To retain this honorable recognition, a tree inventory was conducted during fall 2015 to evaluate, more tangibly, the economic benefits of its campus tree community and also to develop a data base that could assist with future management practices of this newly established Arboretum. Assessing tree density, size (DBH), diversity, and overall condition of this community (n=1,482) allowed the researchers to estimate a monetary value of the more specific ecological services that are provided by trees. In this study, we considered the potential economic value in reference to: energy savings, storm water capture, esthetic value, air quality, and carbon sequestration. The total annual value provided by trees on our campus, based on the five variables, was estimated to be $90,974, whereas the total annual cost for maintaining (e.g., pruning, replacement) the tree community was $18,553. This suggests that for every $1.00 spent on trees in 2015, Winona State University gained $4.90 in return. This study demonstrates that there are tangible economic benefits in maintaining a healthy arboretum at our institution, besides enhancing educational and research endeavors for students and more educational opportunities for the Winona community.
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Three tree species (Tilia cordata Mill. ‘Olympic’, Acer campestre L., and Malus sp. Mill. ‘Adirondack’) were grown in a standard sidewalk pavement profile, an experimental sidewalk profile (SSM), and in the field. Root systems in the paved treatments were harvested after three years to analyze root length, root density, and profile distribution. SSM tree foliage quality (measured by SPAD 502) and shoot extension measured in the second and third years were not different than those of the field control trees. Tree foliage quality and shoot extension were reduced in the standard sidewalk profile. There was an increase in root length of Acer and Tilia in the SSM profile versus the standard sidewalk profile and an increase in depth of the root zone for all species. The results indicate several advantages in root and canopy growth for street trees grown in the experimental profile compared to the standard sidewalk pavement profile.
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Inadvertent soil compaction at the urban lot scale is a process that reduces infiltration rates, which can lead to increased stormwater runoff. This is particularly important in low impact development strategies where stormwater is intended to infiltrate rather than flow through a traditional stormwater network to a detention basin. The effect of compaction on infiltration rates on sandy soils in North Central Florida was measured with a double ring infiltrometer on urban construction sites and across various levels of compaction. Average non-compacted infiltration rates ranged from 377 to 634 mm hr1 (14.8 to 25.0 in hr-1) for natural forest, from 637 to 652 mm hr-1 (25.1 to 25.7 in hr-1) for planted forest, and 225 mm hr-1 (8.9 in hr1) for pasture sites. Average infiltration rates on compacted soils ranged 8-175 mm hr1 (0.3-6.9 in hr1), 160 to 188 mm hr1 (6.3 to 7.4 in hr1), and 23 mm hr1 (0.9 in hr1) for the same respective sites. Although there was wide variability in infiltration rates across both compacted and non-compacted sites, construction activity or compaction treatments reduced infiltration rates 70 to 99 percent. Maximum compaction as measured with a cone penetrometer occurred in the 20 to 30 cm (7.9 to 11.8 in) depth range. When studying the effect of different levels of compaction due to light and heavy construction equipment, it was not as important how heavy the equipment was but whether compaction occurred at all. Infiltration rates on compacted soils were generally much lower than the design storm infiltration rate of 254 mm hr 1 (10.0 inches hr1) for the ioo-yr, 24-hr storm used in the region. This implies that construction activity in this region increases the potential for runoff and the need for large stormwater conveyance networks not only due to the increase in impervious area associated with development but also because the compacted pervious area effectively approaches the infiltration behavior of an impervious surface.
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Current surveys have dramatically documented the plight of struggling and dying urban trees. Inadequate soil rooting volume is an important cause of this premature mortality. The soil acts as a vital reservoir, holding and then supplying water as the tree demands it. A weather-based methodology has been developed that enables the arboriculturist to size a tree pit or container based on a tree's daily expected water requirements, thereby reducing or eliminating water stress over a growing season. For use as a general estimate, a soil volume of 5 m for a medium sized tree is recommended.
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The latest tests on permeable road surfacing demonstrate the rate at which clogging occurs, according to the type of surface, and the best methods for recovering micro-pollutants. In the first year there is little change in underseepage rate but thereafter decline is rapid, reaching 50% of the original rate after 2 or 3 years. For highly permeable surfacing containing many spaces, absorption capacity remains high even after this period, but methods of preventive maintenance, such as suction, need to be used in order to avoid the use of even more costly methods, such as high pressure water jet with simultaneous suction. However, permeable surfacing does enable the storage and recovery of most micro-pollutants, thus preventing such particles being washed off into the ground.
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A porous pavement is one that allows pavement to drain all the way through the pavement structure. When properly designed and installed, porous asphalt pavement areas can provide cost-effective, attractive parking lots with a long life span, and at the same time, provide storm water management systems that promote infiltration, improve water quality, and eliminate the need for a detention basin. With the proper information, most asphalt plants can easily prepare the mix and most paving contractors can install it.
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Five species of trees were measured in 12- to 27-year-old parking lots in north-central Florida, U.S. Tree species evaluated were Chinese elm (Ulmus parvifolia Jacq.), live oak (Quercus virginiana Mill.), sycamore (Platanus occidentalis L.), shumard oak (Q. shumardii Britton), and laurel oak (Q. laurifolia Michx.). Tree growth in the parking lot planting zones was reduced as the surface area of nonpaved surface was reduced for all trees except live oak. Regression models yielded significant relationships with prediction value between parking lot detail space openings and tree size, measured by diameter at breast height or by canopy radius. Normalization of the data within the site allowed for multiple site analysis. A method for generating similar data is introduced for development of regionally species-specific growth reduction multipliers. Those multipliers can be used to better meet long-term, canopy expectations or to advocate for larger planting zones.