Green Parking Lot Resource Guide

1. Green Parking Lot Resource Guide

2. TABLE OF CONTENTS
INTRODUCTION ............................................................................... 1
CHAPTER 1: IMPACTS OF PARKING LOTS ........................................... 2
Environmental Impacts of Parking Lots .....................................................................................................2
Costs of Parking Lots.......................................................................................................................................5
CHAPTER 2: “GREEN” PARKING LOT TECHNIQUES............................. 7
Planning Aspects .............................................................................................................................................7
On-Site Stormwater Management ..............................................................................................................8
Parking Surface Material Selection .............................................................................................................9
Landscaping and Irrigation ...........................................................................................................................9
CHAPTER 3: PLANNING ASPECTS ..................................................... 10
Municipal Parking Requirements ............................................................................................................. 10
Parking Lot Placement and Aesthetics.................................................................................................... 12
Linking Parking to Smart Growth ............................................................................................................. 13
CHAPTER 4: STORMWATER MANAGEMENT....................................... 14
Green Parking Lot Stormwater Management Techniques ................................................................. 14
BMP Pollutant Removal and Effectiveness ............................................................................................. 17
BMP Cost Considerations............................................................................................................................ 18
Case Study 1: Stormwater Best Management (BMP)
—Bloedel Donovan Park, Bellingham, Washington .................................................................. 20
CHAPTER 5: ALTERNATIVE PARKING SURFACE MATERIALS ............... 22
Porous Pavement .......................................................................................................................................... 22
Alternative Pavers ......................................................................................................................................... 23
Design and Installation Considerations .................................................................................................. 24
Maintenance of Permeable Pavement .................................................................................................... 25
Infiltration & Pollutant Removal Effectiveness of Permeable Pavements ...................................... 27
Cost Considerations ..................................................................................................................................... 29
Case Study 2: Parking Surface Alternatives—Heifer International, Little Rock, Arkansas 31
Case Study 3: Parking Surface Alternatives—University of Rhode Island, Kingston,
Rhode Island ........................................................................................................................................ 33
Green Parking Lot Resource Guide—February 2008 i

3. CHAPTER 6: LANDSCAPING AND IRRIGATION .................................... 35
Overview of Natural Landscaping and Irrigation ................................................................................. 35
Environmental Benefits of Using Natural Landscaping and Associated Irrigation ...................... 38
Cost Effectiveness of Using Natural Landscaping ................................................................................ 40
Case Study 4: Landscaping and Irrigation—Heifer International, Little Rock, Arkansas .. 42
CHAPTER 7: REDUCED INFRASTRUCTURE BURDEN ............................. 44
Regional Stormwater and Wastewater Impacts.................................................................................... 45
Cost Effectiveness ......................................................................................................................................... 45
Case Study 5: Reduced Infrastructure Burden
—Green Streets Program, Portland, Oregon ............................................................................... 47
KEY RESOURCES .................................................................................... 49
United States
Environmental Protection Agency
Office of Solid Waste and Emergenc Response (5101T)
EPA-510-B-08-001
February 2008
http://epa.gov/oswer/iwg/
Recycled/Recyclable—Printed with vegetable oil based inks on 100% postconsumer,
process chlorine free recycled paper.
Table of Contents ii

4. INTRODUCTION
G
■
“ reen” parking lot is a term increas­ Chapter 2 provides an overview of the
ingly used to describe parking lots benefits of green parking lot development
that may incorporate a variety of techniques, briefly describing major plan­
environmentally preferable features, includ­ ning, design, and material considerations.
ing a minimized footprint and/or impervi­
■ Chapters 3 through 6 provide detailed
ous surfaces, stormwater best management
information on specific elements of sus­
practices (BMPs), and alternative parking
tainable parking lot approaches including
surface materials. To date, however, informa­
planning and design approaches (Chapter
tion on green parking lots has been scattered
3), sustainable stormwater management
across planning, construction, stormwater,
techniques (Chapter 4), alternatives to
engineering, and landscaping resources. The
asphalt parking surfaces (Chapter 5), and
goal of this resource guide is to present the
water efficient landscaping and irrigation
fundamental planning and design concepts
(Chapter 6).
of a green parking lot and connect readers to
existing resources on the environmental ben­ ■ Chapter 7 discusses how green parking
efits and cost effectiveness of green parking lots can help municipalities reduce future
approaches. This document is expected to be stormwater infrastructure and utility
particularly useful for local government of­ maintenance costs.
ficials involved in planning and development
Case studies are included throughout the
activities, as well as construction industry
guide to provide real world examples of
professionals (developers, project managers,
green parking lot techniques.
facility managers and other decision makers)
interested in green parking lot technologies. Key resources consulted in developing this
guide are listed in the back of the document.
The guide is organized into seven chapters:
■ Chapter 1 describes the environmental
and cost impacts associated with conven­
tional parking lots.
Green Parking Lot Resource Guide—February 2008 1

5. CHAPTER 1
IMPACTS OF PARKING LOTS
P
arking lots are a ubiquitous feature high rate and volume, negatively impacting
of the American landscape. Perhaps the surrounding ecosystem. Hence, parking
because they are so commonplace, lots degrade water quality, strain stormwa­
the significant environmental and cost im­ ter management systems, consume large
pacts associated with parking lots are often amounts of land and resources, and enable
overlooked. In this chapter, we provide an urban sprawl. Furthermore, materials used
overview of these impacts. to construct parking lots have a variety
of impacts on air, water, and biodiversity
ENVIRONMENTAL IMPACTS OF throughout their life cycle. Some of the major
PARKING LOTS environmental impacts of traditional parking
The prevailing low-density American devel­ lots are described below.
opment pattern (i.e., urban sprawl) necessi­
Water Quality Impacts
tates reliance on automobiles, along with the
construction of parking lots to accommodate, Parking lot runoff is a major contributor to
and many times overaccomodate, demand non-point source pollution of our waterways.
for parking. As parking lots have become a Conventional parking lots quickly move
dominant feature of urban and suburban stormwater into receiving water bodies. As
landscapes, their environmental impacts it flows across pavement, the water picks up
have also become increasingly apparent. pollutants from the surface. This results in
large volumes of polluted runoff entering
Most parking lots are made of pavement—a
surface water and groundwater resources,
combination of asphalt concrete, the most
negatively affecting water quality.
widely used paving material in the United
States, and aggregates such as sand, gravel, Contaminants in parking lot runoff can
or crushed stone. Pavement is an impervi­ originate from a variety of sources, includ­
ous, heat absorbing material that collects ing the paving materials used to build them.
stormwater on its surface and does not allow Recently, the U.S. Geological Survey (USGS)
it to filter into the soil, inhibiting the natural pinpointed parking lot sealants as a signifi­
water cycle. With this in mind, parking lots cant source of non-point source pollution,
have traditionally been built with the primary specifically polycyclic aromatic hydrocarbons
goal of channeling stormwater into receiving (PAHs), a known carcinogen that can be toxic
water bodies as quickly as possible, via means to fish and wildlife.1 Automobiles are also a
such as gutters, drains, and pipes. As a result, major source of pollutants in parking lot run­
runoff that is contaminated with many types off, including antifreeze, oil, hydrocarbons,
of petroleum residues, fertilizers, pesticides, metals from wearing brake linings, rubber
and other pollutants from parking surfaces particles from tires, nitrous oxide from car
enters receiving waters at an unnaturally exhausts, and grease.
CHAPTER 1—Impacts of Parking Lots 2

6. Water Supply Impacts ide (CO), volatile organic compound (VOCs),
polycyclic aromatic hydrocarbons (PAHs), and
Conventional parking lots consist of large ar­
carbon dioxide (CO2) during the manufactur­
eas of impervious surfaces that do not permit
ing process. The activities associated with
the infiltration of water into the soil. Unlike
the construction and maintenance of park­
natural conditions where rainwater filters
ing lots also generate emissions, typically in
into the ground, impervious surfaces halt
the form of dust, fumes, and equipment and
this process, inhibiting a watershed’s natural
vehicle exhaust. For example, the use of hot
hydrological cycle and preventing ground­
mix asphalt, a common process where the
water recharge. As a result, water tables are
asphalt is heated to extremely high tempera­
lowered, reducing streamflow during dry
tures prior to application, can cause health
periods, depleting water supplies, and exac­
problems for workers including headache,
erbating the negative impacts of droughts.
skin rash, fatigue, throat and eye irritation,
Stormwater Management breathing problems, and coughing. Diesel
Impacts emissions from on-site equipment can also
cause similar health effects.2 In addition, the
According to the USGS, an impervious, typical after effects of parking lot construc­
man-made surface will generate two to six tion, such as fewer trees and less vegetation
times more runoff than a natural surface. In due to clearing, as well as heat island effect
addition to the direct impact of paving, con­ (see below), also lead to higher amounts of
ventional parking lots also typically include CO2 in the air.
pipes, curbing, gutters, and drains to help
speed water off of parking surfaces. These Heat Island Effect
systems cause runoff to move even faster
Heat island effect (HIE) occurs in urban areas
downstream, increasing the risk of stream
where materials that have heat-absorbing
flooding. Sewer systems often become over­
properties, such as asphalt, are prevalent.
whelmed by the rapid runoff of stormwater,
In urban areas, the combined effect of such
causing them to overflow and, in the case of
surfaces can cause a change in the energy
combined sewer and stormwater systems,
(temperature) balance, leading to hotter air
discharge raw sewage into receiving water­
and surface temperatures. Recent research
ways. In addition to the human health risks
indicates that urban areas are 2 to 8ºF hotter
related to combined sewer overflows, these
in summer due to this increased absorbed
discharges can cause algal blooms to form,
heat.3
depleting aquatic oxygen levels and altering
a waterbody’s habitat. Parking lots contribute significantly to HIE.
Asphalt, one of the most common paving
Air Emission Impacts materials used in parking lots, is a dark, heat
Pollutant air emissions occur throughout absorbing material.4 When asphalt cools at
the lifecycle of a parking lot. Asphalt cement night, all the heat it has absorbed during
plants emit particulate matter, nitrogen ox­ the day is released into the air, slowing the
ides (NOX), sulfur oxides (SOX), carbon monox­ rate of nighttime cooling. This hot surface,
Green Parking Lot Resource Guide—February 2008 3

7. combined with stormwater runoff from the fauna. The velocity and volume of runoff from
parking lot also affects surrounding water- parking lots can damage plant, fish and inver­
bodies. When water is forced to flow quickly tebrate habitat. During storm events, runoff
off the lot’s surface, not enough time is al­ can erode stream banks and alter the natural
lowed for evaporation to occur, again limiting shape of a waterway. Stream edge habitat
natural cooling of the air. In addition, the land and stream channel protection removed
clearing needed to create space for parking during the construction of the parking lot
lots diminishes tree cover and other natural increases the potential for erosion. Sediments
vegetation that can help shade land and entering the waterway as a result of erosion
moderate temperatures. can smother habitat and stress aquatic organ­
isms. The turbidity created from the sedi­
The environmental impacts of the HIE are
mentation can disrupt an aquatic ecosystem
varied. Hotter temperatures can lead to
by diminishing light transmission, reducing
more CO2 emissions due to increased energy
plant growth, altering food supplies, interfer­
demand to cool neighboring buildings.5 HIE
ing with navigation, decreasing spawning
can also increase smog, and subsequently
habitat, and reducing shelter.
exacerbate pulmonary and cardiovascular
health problems. During rain events, paved The contaminants in parking lot runoff also
surfaces can transfer heat to runoff, increas­ pose a risk to wildlife. Toxic substances from
ing the temperature of receiving waters. This contaminated ground and surface water sup­
warmer water can be detrimental to the natu­ plies have the potential to bioaccumulate in
ral habitats of fish and other aquatic life. the tissue of fish and other organisms in the
wildlife food chain. They can also accumulate
Waste Impacts in sediments, posing risks to bottom feeding
The traditional production and application organisms and their predators.
of asphalt relies heavily on the use of virgin The impact of parking lots on water supplies
stone and aggregate and non-renewable, affects local ecology. Unnaturally low stream
petroleum-based materials. Use of fresh flows as a result of decreased infiltration
asphalt in parking lot construction creates a can negatively impact deep water and swift
lost opportunity for reusing waste products, flowing habitats. Impaired water quality, and
such as recycled asphalt, which would reduce increased volume and velocity of runoff, can
the amount of material sent to landfills lead to habitat loss, stress aquatic species,
and increase the amount of virgin materi­ and have an overall negative effect on bio­
als conserved. The use of recycled asphalt is logical diversity in abutting areas.
common in the construction of roads, but
has yet to become prevalent in parking lot Decrease In Greenspace
construction.
Greenspace is a finite resource with a wide
Disturbance of Habitat and Local range of intrinsic values, including conserva­
Ecology tion, recreation, and agricultural purposes, as
well as its scenic qualities and contribution to
Traditional parking lots can have a host of the overall character of a city or town. Proper
negative impacts on adjacent habitat and
CHAPTER 1—Impacts of Parking Lots 4

8. management of greenspace is essential to ing walking and bicycling, and encourages
achieving and maintaining sustainable com­ automobile travel, disconnecting communi­
munities. Nevertheless, greenspace areas are ties and decreasing the habitability of cities
commonly paved to accommodate demand and towns. The resulting increase in vehicle
for parking. For example, it is estimated that miles traveled and the associated high levels
30 to 40 percent of a typical American down­ of mobile source air emissions exacerbate
town is used for parking spaces.6 air quality issues, and contribute to global
climate change.
Ineffective local government zoning restric­
tions also result in the creation of larger areas COSTS OF PARKING LOTS
of paved surface than necessary to meet the
parking demand. Many municipalities require Beyond their environmental impacts, parking
a minimum number of parking spaces per lots have economic and social costs related
development project, often forcing devel­ to their construction—costs that are often
opers to build more spaces than needed much higher than consumers realize. More­
to meet actual demand. For instance, com­ over, parking costs are shouldered by many
mercial parking lots frequently have 60 to stakeholders, including developers, local
70 percent vacancy rates.7 Parking stall sizes governments, parking users, and community
required by zoning can also be larger than members. Below we describe the types of
necessary, eliminating opportunities to alter costs related to parking lot construction, as
parking lot configuration designs to achieve well as who pays.
higher car capacity and minimize impervious
On-site Costs
surface area.
On-site costs include the construction, opera­
Conventional parking lots are often viewed as
tion, maintenance, and disposal of materials
unattractive, hostile, and sometimes unsafe
needed to develop and maintain parking lots,
areas. In contrast, green parking lots with
including paving materials and infrastructure
urban greenscaping provide aesthetic ben­
such as gutters and curb cuts. In addition,
efits, including privacy and noise reduction,
on-site costs include the cost of parking lot
to landowners and to communities. These
landscaping that, depending on the shrubs,
benefits are lost when conventional parking
trees, and turf chosen, vary in their need for
lot construction and paving techniques are
mowing, pruning, and irrigation. These costs
used.
are typically paid by developers, although
Urban Sprawl local governments sometimes subsidize
infrastructure costs. HIE can add to parking
Urban sprawl and prevailing low-density lot user costs, by decreasing an automobile’s
development patterns characterized by free, value by quickening the deterioration of the
plentiful parking reinforce dependence on vehicle’s paint, plastics, and tires while on
automobiles for commuting to work, shop­ the lot. HIE can also shorten the life of the
ping, and social activities. Thus, conven­ pavement, causing it to become brittle and
tionally designed parking is an enabler of weak (a cost to parking lot owners); and can
urban sprawl. Conventional parking creates increase the energy costs of adjacent build-
barriers to alternative transportation, includ­
Green Parking Lot Resource Guide—February 2008 5

9. ings due to the hotter air temperatures (a Distributional Issues
cost to the building owner and potentially to
Parking lots provide a value to consumers
third parties).
who use them, but result in negative im­
Infrastructure Costs pacts for neighbors and other community
members who do not use them. Community
Local governments bear the brunt of infra­ members would be better served by almost
structure costs related to parking. The high any other land use, particularly in cases of
volume and velocity of polluted run-off from excessive sizing of paved areas, which can
parking lots can stress stormwater man­ reduce adjacent property values.
agement systems and hasten the need for
repairs, upgrades, and expansions to handle Community Development Costs
water flow and treat runoff. Flooding caused
Parking lots and associated sprawl decrease a
by runoff can also degrade bridges, roads,
community’s habitability, livability, and sense
and other parts of a city’s infrastructure.
of identity, a cost to all community members.
Additionally, groundwater shortages due to
Unattractive expanses of pavement placed
disruption of the water cycle can increase
in front of buildings create voids and discon­
the frequency, and thus cost, of pumping
nectedness, discouraging pedestrian-friendly
groundwater.
communities and alternative methods of
Opportunity Costs transport. The presence of multiple conven­
tional parking lots can also signal develop­
Parking lots consume large areas of open ers that a community accepts urban sprawl
space that could otherwise be used for development. This signal can create a cyclical
alternative, higher value purposes, such as effect on a community’s future development
parks, wildlife habitat, recreation, agriculture, patterns. Subsequent developments in these
housing or other businesses. Building park­ areas are far more likely to have a similar pat­
ing instead of other types of development tern of urban sprawl, further disconnecting
could reduce the property tax base, a cost the link with any older non-sprawl develop­
to local governments and local taxpayers. ment, and eroding or precluding unique
Enforced minimum parking requirements characteristics that establish a community’s
do not benefit developers either. They limit sense of place.
the development potential of land; the more
parking spaces that are required, the less land
available for more profitable uses. This can be
costly because parking is relatively expensive
to construct and yields little return, or no
return where parking is free.
CHAPTER 1—Impacts of Parking Lots 6

10. CHAPTER 2
“GREEN” PARKING LOT TECHNIQUES
I
nnovative approaches to planning and of transport, through company support or
design can greatly mitigate many of the subsidies. Another alternative is for mu­
negative impacts of parking lots, includ­ nicipalities to institute an optional fee that
ing diminished recharge of groundwater, developers can pay towards an appropriate
high rates of stormwater runoff, and non- municipal fund, such as a traffic mitigation
point source pollution, by decreasing imper­ fund, in lieu of meeting minimum parking
vious surface area, protecting water quality, requirements.8
reducing stormwater management and
Depending on the site, developers may not
maintenance costs, and increasing aesthetic
opt for constructing less parking because it
value. Below, we introduce green parking lot
may make a site less marketable. A technique
techniques, many of which are described in
applicable in this case would be to set park­
detail in subsequent chapters.
ing maximums and/or area wide parking
PLANNING ASPECTS restrictions, which would limit the number
of spaces allowed across a larger area, eve­
Local planners regularly reinforce car depen­ ning the playing field for the marketability of
dence through zoning bylaws that, although sites in the area.
meant to meet a community’s parking needs,
can result in an oversupply of parking. As a Beyond reducing the number of parking
result, cities and towns are increasingly trying spaces required, municipalities and develop­
new approaches to parking management ers can also encourage practices that reduce
that allow for greater flexibility and adapt­ stall dimensions by creating more compact
ability by determining parking space num­ car spaces and realistic stall size require­
bers on a project-specific basis, rather than ments. Some local zoning laws currently
through a one-size-fits-all regulation. require unnecessarily large stall dimensions
that are bigger than even the largest SUV.9
One such technique is to reduce minimum In many cases smaller, more realistic, stall
parking requirements based on project sizes would be sufficient while reducing the
location or demographics. For example, local amount of disturbed land and impervious
governments can encourage projects that are surface associated with a project.
located near public transportation to reduce
the demand for parking spaces. Adaptations Improving the aesthetic of the parking lot is
of this technique include municipalities also a central technique in green parking lots.
allowing a reduction in the minimum park­ For instance, placing a parking lot behind a
ing requirements in return for a developer/ building rather than in front of it creates a
employer agreeing to implement a transpor­ more inviting and pedestrian-friendly envi­
tation demand management program to en­ ronment. Reducing the number of curb cuts
courage employees to use alternative modes also decreases the frequency of pedestrian/
Green Parking Lot Resource Guide—February 2008 7

11. traffic interaction, thus making for a more
pedestrian-accessible area. These practices
aim to improve the character of the develop­
ment while maintaining accessibility to the
lot. Additionally, parking lots can be divided
into two or more parking areas, again project­
ing a more visually welcoming appearance. Strategically sloped vegetated strips are a better option
than conventional grassy parking islands for collecting and
filtering runoff.
The impact of locating a parking lot at the
front of a building can be mitigated by by 25-30 percent compared to conventional
providing ample space between the lot and approaches.10
the road, and then creating a buffer with
Stormwater BMPs include structural controls
landscaping, fencing, or a wall. Landscaping
and bioengineering techniques designed to
inside the parking lot is also an important
facilitate natural water cycling processes (i.e.
technique. Beyond making the parking lot
evaporation, transpiration, and groundwater
more visually pleasing, vegetation and land­
recharge) by capturing, filtering, infiltrating,
scaping (including trees) around and inside
and/or storing stormwater. Components
the parking lot reduce HIE and help to absorb
of these soil- and plant-based systems can
CO2 emissions. Landscaping is discussed
carry out one or more of the aforementioned
below.
functions, including some that store water for
Chapter 3 provides detailed information on various durations (from 24 hours to perma­
green parking planning. nent storage). Examples of BMPs include
swales, vegetated buffer strips, and bioreten­
ON-SITE STORMWATER tion areas.
MANAGEMENT
Unlike traditional stormwater management
Innovative stormwater management strate­ systems designed only for efficiency in storm-
gies are increasingly being incorporated into water removal, which can lead to negative
parking lot design as part of the overarching downstream effects, BMPs represent a shift
concept of Low Impact Development (LID). towards a sustainable approach to storm-
LID stormwater techniques (also known as water management. Thus, in the context of
Best Management Practices, or BMPs) man­ parking lots, BMPs add value by minimizing
age stormwater on-site, reducing negative environmental impacts of runoff, and often
impacts on receiving waters and municipal lower site development costs while improv­
stormwater management systems, and ing aesthetics.
decreasing the need for costly infrastruc­
ture such as pipes, gutters, and curbs. Done Chapter 4 provides detailed information on
on a small-scale, these controls attempt to greener stormwater management and BMPs.
mimic the pre-development ecological and
hydrological processes of an area and can
reduce stormwater and site development
design, construction, and maintenance costs
CHAPTER 2—”Green” Parking Lot Techniques 8

12. PARKING SURFACE MATERIAL economical for developers than incurring
SELECTION the rising costs in some states for disposal of
construction, demolition, and clearing debris
The negative impacts associated with large
in landfills.
impervious surface areas in parking lots can
be reduced through the use of new perme­ Chapter 5 provides detailed information on
able materials as substitutes for pavement. greener choices for parking surface materials.
A number of paving substitutions have been
developed to reduce the range of environ­ LANDSCAPING AND IRRIGATION
mental impacts associated with the use of Green parking lot techniques work to mini­
pavement. Types of permeable and semi­ mize the amount of land cleared for construc­
permeable alternative pavers include gravel, tion, conserving as much of a site’s natural
cobble, concrete, wood mulch, brick, open vegetation and open space as possible,
jointed pavers filled with turf or aggregate, and retaining habit for local wildlife. When
turf blocks, natural stone, and pervious designing a parking lot area, landscapers
concrete. can use native trees and shrubs rather than
Based on a site’s characteristics (i.e. traffic non-indigenous species, which are more suit­
volume, soil type, climate etc.), alternative able to local climates and, therefore, require
pavers may not be an option for the entire less irrigation. The benefits of increasing the
surface of primary parking areas.11 However, amount of greenscape in and around park­
in many cases, the aisles and driveways can ing areas include reduction of CO2 in the air;
be constructed using conventional pave­ improved stormwater runoff management
ment, while alternative pavers can be used in including water storage; increases aquifer
parking stalls, crosswalks, and overflow lots. recharge and flood protection; and increased
Alternative pavers slow the flow of runoff, human comfort through mitigation of HIEs.
allowing it to filter into the soil, sustaining an Wetlands preservation or creation is particu­
area’s natural hydrological cycle, and in some larly beneficial, as they can act as natural
cases, allowing microbes to break down con­ bioretention basins, providing water quality
taminants before entering the soil layer. improvements, flood protection, and ero­
sion control. Wetlands also provide excellent
Opportunities for materials recycling ex­ habitat for local avian and fish species, and
ist in the management and construction of are invaluable for water storage; one acre of
parking lots. For example, the use of recycled wetlands can store over million gallons of
asphalt in parking lot construction is not only water.12
environmentally beneficial, but can make
economic sense. Other environmentally pref­ Chapter 6 provides detailed information on
erable materials, such as recycled rubberized green parking lot landscaping and irrigation.
asphalt, may also be used in parking lot con­
struction. Recycling materials can be more
Green Parking Lot Resource Guide—February 2008 9

13. CHAPTER 3
PLANNING ASPECTS
P
arking lot design and parking avail­ quantity of spaces in a parking lot. It is these
ability are vital to transportation regulations that manage a community’s park­
management throughout the United ing capacity, and thus a large amount of its
States. Parking availability may determine a impervious surface area.
customer’s willingness to visit a business, and
Zoning requirements for developers to
it is often a sought after feature in urban resi­
provide off-street parking first began in the
dential areas. However, parking lots should
1930s as a solution to an on-street parking
be designed efficiently so that spaces are
shortage. Over the years, off-street parking
used frequently and not left empty a majority
requirements expanded in response to the
of the time. When developing a parking lot, a
population’s dependence on automobiles.
number of factors combine to determine the
Today, according to the U.S. Department of
lot’s size, layout, and design. These decisions,
Transportation, 87 percent of trips of less
made during the planning stages of a devel­
than 50 miles are made by personal motor
opment, can transform a parking lot from a
vehicles.13 Americans have become accus­
sparsely landscaped expanse of impervious
tomed to the availability of free parking and
paving to a space that is more aesthetically
automobile travel, rather than public transit
pleasing, land efficient, and community and
or other alternative methods, even for very
environmentally friendly.
short distance trips. Increased parking avail­
Local governments can use better park­ ability encourages more driving, more driving
ing planning as a tool to promote infill and requires more parking, and so on.
smart growth developments while reducing
One of the most important local parking
the direct environmental impact of park­
ordinances addresses minimum space re­
ing. In many cases, revisions to zoning and
quirements, or parking ratios. Typically, local
other parking ordinances may be needed to
governments require developers to construct
achieve better parking planning. This chap­
the minimum number of parking spaces
ter provides a summary of parking planning
needed to satisfy peak demand. These mini­
considerations that have environmental
mum parking regulations often result in an
implications, including municipal parking
oversupply of parking. One study found that
lot regulations, parking lot aesthetics and
the average parking supply at worksites is 30
design, and the connection between parking
percent greater than peak parking demand.14
and smart growth.
In many instances, minimum parking require­
MUNICIPAL PARKING ments are inflexible to adaptation or vari­
REQUIREMENTS ances. Also, the methods to determine these
minimum parking requirements are often
In most urban and suburban areas, a num­ excessive and over-generalized, leading to an
ber of zoning laws govern the layout and oversupply of parking.15 In addition, although
CHAPTER 3—Planning Aspects 10

14. municipalities regulate the minimum number City of San Francisco, where city planners
of parking spaces, they typically do not put eliminated minimum parking require­
a cap on the maximum. Thus, developers ments for development within a half mile
can frequently construct even more than the of train stations and one-quarter mile of
required minimum, which is often the case major public transit routes.17
at large retail developments, leading to a
Municipalities can also consider the land
further surplus in supply.
uses in the surrounding area. For instance,
In addition to requirements for the number it is possible that existing nearby develop­
of spaces in a parking lot, regulations for the ment and parking may already provide
size of each space are also common. Some some of the parking necessary to sup­
local zoning laws require unnecessarily large port a new development. Mixed used
stall dimensions that are bigger than even developments often have natural parking
the largest SUV.16 In many cases, smaller flexibility; an office where peak parking
stall sizes would satisfy parking needs while demand occurs during the day can share
reducing impervious surface, and the entire the same parking spaces with restaurants,
footprint, of the parking lot. entertainment venues, or residential units
that have peak parking demands at night
Re-thinking Municipal Parking and on weekends. Shared parking is also
Requirements an option for single use developments in
There are a number of planning alternatives mixed-use areas.18
to minimum parking requirements that lead­ • Maximum Limits on Parking—The
ing local governments throughout the United opposite of parking minimums, parking
States are implementing to minimize land maximums limit the number of spaces
dedicated to parking. These include reducing that a developer can construct, which is
minimum parking requirements; assessing often determined by the development’s
parking needs on an individual project basis square footage. Portland, Oregon is one
rather than using a generic formula; en­ city that has successfully implemented
couraging shared parking; and establishing the use of parking maximums. Benefits
parking maximums, area wide parking caps, of such a policy include open space
in-lieu parking fees, and reduced parking preservation, reduction in impervious
space dimensions. surface area, traffic congestion reduction,
• Reduced minimum parking require­ promotion of alternative transport, and
ments—Parking requirements should the development of pedestrian-friendly
be determined on a project-by-project urban design. For developers, such limits
basis instead of by formula, taking into mean lower parking lot construction
consideration how a project’s location can costs.19 Similar policies include setting
shape parking needs. This approach may both a parking minimum and maximum,
decrease the required parking capac­ or determining a median parking ratio.
ity where there is accessibility to public • Area wide parking caps—Municipalities
transportation and/or a high level of foot can control the amount of parking by
and bike traffic. Such was the case for the
Green Parking Lot Resource Guide—February 2008 11

15. setting limits on the total amount of park­ stalls to achieve the greatest car capacity,
ing spaces allowed in a certain area. This again reducing the amount of land neces­
strategy is being used in major U.S. cities sary for the lot.
including Boston and San Francisco. Such
regulations require greater research and PARKING LOT PLACEMENT AND
planning efforts by the city or town to AESTHETICS
ensure that the parking cap is appropriate Parking lots have been described as “sterile,
and reasonable, but if done properly, it unattractive environments that deaden city
can be very successful in minimizing the and suburban streets alike, further isolate
land area used for parking and encourag­ users and preclude lively pedestrian-friendly
ing use of public transportation. This op­ streets.”24 Although all parking lots do not
tion is appropriate for areas with adequate match this description, many are eyesores
access to public and alternative transpor­ that inhibit the usability and walkability of
tation, as well as desirable location that an area. Several techniques can be incorpo­
would outweigh the perceived drawbacks rated into the design and layout of a parking
of more limited parking.20 lot to improve aesthetics and help connect
• In-Lieu Parking Fees—Towns such as parking lots to community design. This not
Berkeley, California, Lake Forest, Illinois only benefits the user, but also the organi­
and Orlando, Florida incorporated systems zation or business adjacent to the lot, as a
of in-lieu parking fees. This optional fee more pleasing atmosphere will help draw in
is offered to developers by municipalities the public. Plantings around the perimeter,
in-lieu of meeting minimum parking re­ especially trees and shrubs, can screen the lot
quirements. This fee is typically allocated from passer-bys and break-up the otherwise
to an appropriate municipal fund, such as continuous strip of asphalt and cars from
a traffic mitigation fund.21 An alternative the street to the parking lot. This can also be
under the in-lieu system is that in return achieved through the use of fencing or a wall.
for the developer’s fee, the city provides Vegetation can also be used to divide one
existing centralized, off-site parking to the large lot into two or more smaller lots, again
new development’s tenants and visitors.22 increasing the site’s visual appeal. Equally
important, landscaping within the lot pro­
• Reduced stall size requirements— vides an environmental benefit by decreasing
Adjusting a local government’s stall size dust, wind, noise, glare and air pollution; and
requirements may reduce impervious sur­ minimizing heat island effect.25
face coverage as well. Alternatives include
creating a certain number of compact The placement of a parking lot is a simple,
car spaces and/or limiting stall dimen­ yet fundamental feature that can improve a
sions to feasible sizes. For example, in the development’s attractiveness. A majority of
town of Needham, Massachusetts, up to parking lots are placed in the front of build­
50 percent of off-street parking can be ings, between buildings and streets, requiring
reduced dimension spaces designed for pedestrians and bicyclists to cross expanses
compact cars.23 If possible, developers can of parking in order to enter a building. Alter­
also adapt the layout and angle of parking natively, parking lots could be placed at the
CHAPTER 3—Planning Aspects 12

16. rear of a building, increasing the intercon- States, and promoting sustainable land use
nectedness between pedestrians and the patterns. With many cities designed around
built environment. This simple zoning change use of the automobile, planners are often
is incredibly effective in shifting the orienta­ presented with the conflicting challenge of
tion of a streetscape from cars to pedestrians. promoting smart growth development while
This also helps give the community a greater supporting the parking needs of a popula­
sense of place and interconnectedness. In tion. Green parking planning approaches
recognition of such benefits, the City of Fort support smart growth by creating more
Collins, Colorado requires that no more than sustainable land use patterns and decreasing
50 percent of the parking for a retail devel­ the environmental impacts of conventional
opment be located between the principle parking lot development. By promoting and
building and the primary abutting street.26 supporting alternative transport and com­
Limiting the number of curb cuts also makes muting, local governments may reduce the
a parking lot more pedestrian friendly and parking needs.
inviting. Furthermore, by minimizing the
A concept linked to smart growth is “transit­
number of vehicular entries to parking areas,
oriented development,” defined as develop­
pedestrian mobility is improved, and pedes­
ment placed within close proximity of public
trian/traffic is minimized.
transportation, designed to create walkable
LINKING PARKING TO communities and alleviate traffic conges­
SMART GROWTH tion and environmental impacts caused by
urban sprawl. When building parking lots,
Smart Growth is a state and local government local governments can encourage or require
planning movement aimed at improving the developers to incorporate features that help
long-term habitability and sustainability of reduce automobile reliance, such as bicycle
cities and towns by minimizing environmen­ racks. Employers can support use of alterna­
tal impacts, improving human health, build­ tive transport options by subsidizing the cost
ing a sense of community, creating walkable of public transit, encouraging participation
neighborhoods, promoting traditional and in a commuting program, and/or providing
alternative transport, and preserving open shower facilities on-site so that staff can bike
space. Most fundamentally, smart growth to work.
entails moving away from the urban sprawl
development pattern common in the United
Green Parking Lot Resource Guide—February 2008 13

17. CHAPTER 4
STORMWATER MANAGEMENT
A
pivotal component of green park­ project alone can be minimal, but multiplied
ing lots is the inclusion of innova­ by the current, and growing, number of
tive stormwater management commercial and residential parking lots, the
techniques, often referred to as stormwater combined effect of stormwater runoff has be­
“best management practices” (BMPs). BMPs come the leading cause of non-point source
are practices, techniques, and measures pollution to our waterbodies.29
that prevent or reduce water pollution from
As discussed in Chapter 2, the environmen­
non-point sources (i.e. runoff ) using the most
tal effects of increased volume and velocity
effective and practicable means available.27
of stormwater include not only diminished
Stormwater management BMPs often include
water quality in surrounding waterbodies,
engineered, on-site systems that, when
but also:
coupled with reduction of impervious surface
area, can help significantly reduce detrimen­ • Degradation of stream channels resulting
tal environmental effects and infrastructure erosion and sedimentation;
burden from stormwater runoff.
• Minimized groundwater recharge, which
Increased development and conventional can diminish water flow in the dry weath­
stormwater systems have significantly er, and lead to poorer water quality during
changed the characteristics of stormwater low flows;
flow from land into receiving waters. Accord­
• Higher water temperatures, which
ing to the Natural Resources Defense Council,
negatively impact aquatic organisms and
the amount of rain converted to runoff under
plants; and
natural conditions is less than ten percent of
the rainfall volume.28 However as more devel­ • More frequent and severe flooding.30
opment occurs, rainwater or snow melt that
This chapter provides an overview of green
would have infiltrated into the soil, evapo­
parking lot stormwater management BMPs
rated into the air, or been absorbed by plants,
that can help mitigate these impacts, in­
instead flows quickly off of the pavement as
cluding information on pollutant removal
stormwater runoff. Moreover, conventional
efficiency and cost considerations.
stormwater management exacerbates this
problem. Conventional parking lot stormwa­ GREEN PARKING LOT STORMWATER
ter management typically consists of costly MANAGEMENT TECHNIQUES
systems of man-made drains, pipes, gutters,
storm ponds, and paved channels that direct Green parking lots offset environmental im­
runoff from impervious lots into storm drains pacts of parking by using on-site stormwater
and neighboring waterbodies. The environ­ infrastructure that more closely mimics the
mental ramifications of one development natural water cycle, and manages stormwater
CHAPTER 4—Stormwater Management 14

18. through effective rainfall retention, pollutant ter runoff, and often are incorporated to
removal, and water infiltration. Although still pre-treat and remove sediment before
in the early stages of wide-spread implemen­ water enters infiltration devices such as
tation, cities and towns are recognizing the bioretention areas.34 Other benefits in­
benefits of stormwater BMPs, and many have clude protection of riparian areas, habitat
introduced both voluntary and mandatory creation, and streambank stability.
policies for their inclusion in development
Vegetated filter strips are frequently used
projects.31
in combination with riparian buffers, an­
Some of the most commonly used structural other common BMP, to increase pollutant
BMPs are described below. It also should be removal effectiveness. Riparian buffers
noted that incorporating BMPs is not lim­ are vegetated strips along waterways that
ited to new development. As illustrated by trap and filter contaminants, encourage
the case study of building a rain garden at infiltration, and slow stormwater flow.
Bloedel Donovan Park in this chapter, exist­ They also help to preserve streambank
ing parking lots can be retrofitted to include stability.
them.
• Bioretention Areas (Rain Gardens)
• Swales
One of the more well-know BMPs, biore­
Swales are open channels or depressions tention treatment areas (a.k.a., rain
with dense vegetation used to transport, gardens) consist of a grass buffer strip,
decelerate, and treat runoff. In parking shallow ponding area, organic layer, plant­
lots, they are designed to help direct ing soil, and vegetation. These areas are
water into bioretention areas. Swales can typically used in parking lot islands. Unlike
come in the form of a grassed channel, dry swales, bioretention areas are well-suited
swale, or wet swale. They can be used in for parking lots in denser, urban areas
most climatic regions of the United States, with less available open space.
but may be unsuitable for densely urban
• Dry Detention Basins
areas as they require a large amount of
pervious surface area.32 A dry detention basin is a vegetated
basin with controlled outlets, designed to
• Vegetated Filter Strips/Riparian
detain runoff (lowering flows and reduc­
Buffers
ing velocity) for a short amount of time
Vegetated filter strips are flat pieces of (e.g. 24 hours or less), partially removing
land with low slopes, which are designed pollutants before the water is discharged.
to encourage natural sheet flow of storm- This helps limit flooding and other storm-
water as opposed to channeled runoff. water impacts, such as stream channel
Vegetated filter strips are well suited for erosion and wildlife habitat destruction.
low-density development or areas with Dry extended detention basins are better
less concentrated amounts of runoff.33 suited for pollutant removal than standard
They function by using soil and vegeta­ dry detention basins because they retain
tion to remove pollutants from stormwa­ the water for an “extended” period of time
Green Parking Lot Resource Guide—February 2008 15

19. (i.e., up to 48 hours). They are effective and filled with stone to form a subsurface
at treating certain runoff contaminants, basin, where water is stored until it infil­
particularly those contained in spring trates into the soil. This system greatly re­
and winter runoff in colder climate areas. duces the volume of runoff, and is particu­
However, because water temperature larly good for groundwater recharge as it
increases while in this type of system, dry allows a significant amount of rainwater to
detention basins discharge warmer than infiltrate. Both of these BMPs are consid­
natural water into waterbodies, which ered effective for pollutant removal when
should be taking into consideration. Both used in conjunction with a pre-treatment
dry detention and dry extended detention BMP such as a swale. However, potential
basins are normally dry between storm drawbacks include higher failure rates
events, thus giving them their name.35 due to improper design and maintenance,
limited site applicability, and increased
• Wet Retention Basins
sediment clogging.38
Wet retention basins are designed to cap­
Porous pavement is another type of infil­
ture, filter, store, and infiltrate storwmater,
tration technique used in green parking
and have storage capacity adequate for
lots; as it is also an asphalt alternative, it
flood volumes of water. Because they have
is discussed in Chapter 5: Parking Surface
the capacity to store a permanent pool of
Materials.
water, wet basins can be very effective for
water control, and can provide the bene­ • Constructed Wetlands
fits of aesthetic value and wildlife habitat,
Constructed wetlands are designed to
both terrestrial and aquatic. Although not
capture, filter, and store stormwater simi­
suitable for smaller areas because of their
lar to a wet retention basin. However, they
size, when applicable, retention basins are
also contain a large quantity of wetland
a very effective BMP.36
vegetation and have wetland channels.
• Infiltration Systems Although they are not built to replicate all
of the ecological functions of wetlands,
Infiltration systems are designed to
constructed wetlands help simulate the
capture and retain stormwater runoff,
natural water cycle, recharge groundwa­
allowing water to gradually infiltrate into
ter, remove pollutants, reduce erosion,
the ground over a period of hours or days,
and provide wildlife habitat. They are
depending on the design.37 Two common
considered to be a very effective pollutant
infiltration systems used in green parking
removal option.39 Constructed wetlands
lots are infiltration basins and infiltration
have a few limitations; they are not ap­
trenches. An infiltration basin is an open
plicable in arid climates and, due to their
depression that covers a relatively large
large size, they are not suitable for dense
area. It is constructed to work in conjunc­
urban areas.
tion with filter strips or swales, which help
direct runoff from a parking surface into It is not necessary for developers to in­
the basin. Infiltration trenches are shallow corporate all available green stormwater
excavated ditches lined with filter strips techniques into a project; rather, they should
CHAPTER 4—Stormwater Management 16

20. determine those useful for specific site condi­ source of pollutants in parking lot runoff, in­
tions. Considerations should include all fac­ cluding antifreeze, oil, hydrocarbons, metals
tors that affect the amount, speed, and pol­ from wearing break linings, rubber particles
lutant loadings of runoff: soil type, the slope from tires, nitrous oxide from car exhausts,
and landscape of the site, amount of impervi­ and grease. Other polluting materials include
ous surface, local precipitation patterns, and pesticides, fertilizers, litter, pet waste, dirt,
rainfall surface retention.40 Carefully choos­ and sand.45
ing the appropriate BMP(s) is important to
One of the main goals of a green parking lot
avoid any secondary environmental impacts
is to decrease pollutant levels in stormwater
caused by the use of an inappropriate BMP.
runoff as much as possible before it enters a
BMPs should address peak discharge, runoff
waterbody. Exhibit 1 shows a range of pol­
volume, infiltration capacity, base flow levels,
lutant removal efficiencies for selected BMPs.
ground water recharge, and maintenance of
Understanding the effectiveness of each BMP
water quality, so that they are ideally man­
for pollutant removal is a complex undertak­
aged in the pre-development stormwater
ing because pollutant removal is affected by
filtration conditions of the site.41
a large number of variables. Fundamentally,
It should be noted that BMPs are helping to removal effectiveness depends on: 1) BMP
meet the Clean Water Act’s mandate to “re­ type, 2) the quantity of runoff treated, and
store and maintain the chemical, physical and 3) the type of pollutant being removed.46
biological integrity of the Nation’s waters”.42 Variation in one of these factors can affect
By 2025 the U.S. population is predicted to a BMP’s efficiency. For example, infiltration
grow 22 percent, which could mean an ad­ trenches show a high pollutant removal ef­
ditional 68 million acres of development, a ficiency for pathogens, but much lower for
good fraction of which will be dedicated to phosphorus. However, these effectiveness
parking.43 Thus, BMPs may play a larger role ranges can vary based on the climate, soil,
in the future to mitigate non-point water and land type of a particular site. Infiltra­
pollution. tion trenches may be less effective in colder
climates when surface waters freeze and can­
BMP POLLUTANT REMOVAL AND not allow runoff to flow into them, a limita­
EFFECTIVENESS tion that can be partially remedied through
Stormwater can carry a number of harmful proper design and maintenance, but may still
pollutants, and is the prime contributor to reduce pollutant removal effectiveness.47
non-point source pollution. Runoff contami­ As seen in Exhibit 1, not all BMPs have a high
nants can originate from a variety of sources, level of pollutant removal effectiveness.
including the paving materials used to build Instead, they serve other roles in control­
the parking lots. Recently, the USGS pin­ ling the impacts of runoff. This is the case for
pointed parking lot sealants as a large source dry detention basins, which serve to reduce
of non-point source pollution, specifically peak discharges of stormwater to neighbor­
polycyclic aromatic hydrocarbons (PAHs), a ing waterbodies, as well as limit erosion and
known carcinogen that can be toxic to fish downstream flooding.
and wildlife.44 Automobiles are also a major
Green Parking Lot Resource Guide—February 2008 17

21. EXHIBIT 1: BMP EFFECTIVENESS
Typical Pollutant Removal Efficiency (percentage)
BMP Type Suspended
Nitrogen Phosphorus Pathogens Metals
Solids
Dry Detention Basins 30-65 13-45 15-45 <30 15-45
Retention Basins 50-80 30-65 30-65 <30 50-80
Constructed Wetlands 50-80 <30 15-45 <30 50-80
Infiltration Basins 50-80 50-80 50-80 65-100 50-80
Infiltration Trenches/
50-80 50-80 15-45 65-100 50-80
Dry Wells
Grassed Swales 30-65 15-45 15-45 <30 15-45
Vegetated Filter Strips 50-80 50-80 50-80 <30 30-65
Source: U.S. EPA, 1993, Handbook Urban Runoff and Pollution Prevention Planning, EPA-625-R-93-004, taken from Purdue Uni­
versity Engineering Department’s Long-Term Hydrologic Impact Assessment (L-THIA): http://cobweb.ecn.purdue.edu/~sprawl/
LTHIA7/lthia/lthia_index.htm.
BMP COST CONSIDERATIONS constructing a BMP considerably because of
excavation costs.
Innovative structural stormwater BMPs are
more effective than conventional storm- Another significant variable in the compara­
water management in removing pollutants tive cost of BMPs is the value of land; in areas
and maintaining the environmental quality where real estate prices are high, construct­
of a site. However, because some of these ing a BMP may take up too much space to be
techniques are relatively new and have not cost effective.49 BMPs operation and mainte­
achieved market penetration, it is not clear nance costs can also be significant. The long-
their costs compare to conventional storm- term cost to maintain certain, more complex,
water management approaches.48 Calculating stormwater BMPs over a 20-25 year period
the cost-effectiveness of a stormwater BMP can be close to its initial construction cost.50
is a very site-specific endeavor, and current However, some BMPs, such as swales and
cost information is limited and inconsistent. bioretention areas, are less expensive to build
The main factors affecting the relative costs than their conventional counterparts of pipe
of BMPs include the cost of land, engineering and gutter systems. These BMPs can decrease
and design, permitting, construction, and development costs by reducing or eliminat­
operation and maintenance. These costs can ing the high cost of conventional stormwater
vary greatly due to individual site characteris­ infrastructure such as piping, gutters, and
tics such as climate, topography, government drains, as well as reduced long-term mainte­
regulations, soil type, time of year of con­ nance costs for such systems. Furthermore,
struction, drainage, accessibility of equip­ some BMPs, such as constructed wetlands,
ment, and economics of scale. For instance, may increase the property value by creat­
very rocky soils may increase the cost of ing a water feature and vegetation that has
CHAPTER 4—Stormwater Management 18

22. high aesthetic value. Developers may also protect the health of waterbodies, but also
gain from local government incentives that because they can avoid long-term costs.
encourage incorporating structural stormwa­ Without stormwater BMPs, many waterbod­
ter BMPs. For instance, the City of Portland, ies and water infrastructure may deteriorate.
Oregon will give up to a 35 percent discount Taxpayers bear the cost burden to slow
off its stormwater utility fee to properties or repair damage caused by downstream
with on-site stormwater management.51 In flooding, stream and aquatic habitat dete­
addition, some costs are tax deductible, and rioration, and repairs and upgrades to worn
operating costs may be fully deductible as town stormwater infrastructure systems, all
expenses in the year they are incurred.52 of which are very expensive and time-con­
suming.53 Infrastructure costs associated with
Although the costs of BMPs vary by site and
stormwater management and how green
type, they are almost always a good invest­
parking can help mitigate these costs are
ment from the perspective of local govern­
discussed further in Chapter 7.
ments and taxpayers, not only because they
Green Parking Lot Resource Guide—February 2008 19

23. CASE STUDY 1: STORMWATER BEST MANAGEMENT PRACTICES (BMP)
BLOEDEL DONOVAN PARK, BELLINGHAM, WASHINGTON54
Stormwater runoff in Bellingham, Washing­
ton, like much of the U.S., is a foremost water
quality issue. The Washington Department of
Ecology estimates that roughly one-third of
the state water bodies with pollution related
problems are impaired because of stormwa­
ter runoff impacts. In an effort to protect the
receiving waters of nearby Lake Whitcom
from such impacts, City of Bellingham of­
ficials chose to retrofit stormwater manage­
ment at the heavily used Bloedel Donovan
Park parking lot. Rather than choosing a
conventional technique, they elected to build
an innovative rain garden to manage storm- The raingarden in Bloedel Donovan Park helps protect the water quality in nearby
Lake Whatcom, and recharge groundwater supplies.
water on-site.
of drain rock, and topped with a layer of
DESIGN AND CONSTRUCTION fabric to constrain the sand and restrict
Designed on a 550 square-foot section of the any plants from growing through. An
parking lot near the catch basin, the park’s 18- to 24-inch layer of sand composed of
rain garden supports runoff from 80 parking twenty percent organic materials is the
spaces and two parking lanes. To meet water top layer .
quality guidelines, the rain garden was also • Landscaping—For landscaping, the city
designed to treat 91 percent of the runoff chose native plants that could survive the
from a 50-year storm event. Aspects of its year-round climatic conditions of the site.
construction included: This included plants that prefer wet soil,
• Site excavation—From site topography
but could also tolerate drought.
and soils logs, the city determined the
maximum allowable depth for water to EXHIBIT 2: CASE STUDY INITIAL
pond in the rain garden. Under a 50-year COST COMPARISON
storm event, the depth should be no more
Conventional stormwater
than six-feet. Thus, the site was excavated technique $52,800
three to four feet. (4,400 ft3 wet vault)
• Layering of materials—The rain garden Rain Garden $12,820
is composed of three layers of non-woven Cost Savings $39,980
geotextile fabric alternated with six inches
CHAPTER 4—Stormwater Management 20

24. COST AND POLLUTANTS REMOVAL in-ground storage and treatment stormwa­
EFFECTIVENESS ter system (see Exhibits 2 and 3). This was
achieved through reduced construction and
The benefits from incorporating this rain
equipment costs, as well as reduced labor
garden are numerous. It adds aesthetic value
costs from the relative ease of installation,
to the site, increases wildlife habitat, and is a
some of which was accomplished by volun­
highly effective BMP for treating stormwater
teer landscaping help. These costs savings do
runoff. According to officials at the Belling­
not include future regular maintenance costs.
ham Public Works Department’s, monitoring
shows that approximately 80 percent of total A more detailed case study of the city of
runoff is captured by the rain garden, with Bellingham’s rain garden can be found
overflows running through media filtration on the Puget Sound Action Team’s Web
and then another infiltration bed. Further­ site at www.psat.wa.gov/Publications/
more, Bellingham saved 70 percent in initial Rain_Garden_book.pdf.
costs compared to installing a conventional
EXHIBIT 3: COST FOR BLOEDEL DONOVAN PARK RAINGARDEN
Labor $3,600
Vehicle use 1,900
Amended soil 1,650
Concrete 1,200
Asphalt 1,200
PVC/grates/catch basins/fabric/other misc. 1,000
Washed rock 805
Excavator rental (1.5 days) 500
Plants 400
Debris Removal 300
WCC crew planting time 265
Total Cost $12,820
Green Parking Lot Resource Guide—February 2008 21

25. CHAPTER 5
ALTERNATIVE PARKING SURFACE MATERIALS
T
he majority of parking lots are made Permeable pavements provide a sustainable
of a combination of asphalt concrete, alternative to the conventional asphalt and
the most widely used paving material concrete parking materials widely used today.
in the United States, and aggregates such as Permeable pavements are a broadly defined
sand, gravel, or crushed stone. Conventional group of pervious paving options that allow
pavement is an impervious, heat absorb­ natural infiltration rates of stormwater into
ing material that collects stormwater on its the soil through certain design techniques
surface, and does not allow it to filter into the and material substitutions.58 For this reason,
soil, inhibiting the natural water cycle. As a re­ like many of the techniques mentioned in
sult, parking lots must be designed to quickly Chapter 4, permeable pavements are consid­
remove the water that gathers during storms ered a best management practice (BMP) for
by channeling it off the lot via means such stormwater management. However, perme­
as gutters, drains, and pipes. The stormwater able pavement should be used in combina­
is directed into receiving water bodies at tion with other BMP techniques to magnify
unnaturally high rates, causing a number of benefits and provide back-up systems in case
adverse impacts including increased down­ of BMP failure.59 Two basic types of perme­
stream flooding, combined sewer overflow able paving designs exist: 1) porous pave­
events, diminished groundwater supplies, ment and 2) alternative pavers. This chapter
streambank erosion, and non-point source describes these permeable pavement alterna­
water pollution from runoff contaminated by tives, considering their functionality, infiltra­
vehicular residues and other pollutants. tion and pollutant removal effectiveness, and
cost implications.
To combat several of the negative impacts
of conventional parking lot paving, develop­ POROUS PAVEMENT
ers are increasingly incorporating modest
changes, such as using light colored concrete Porous pavement is a permeable pavement
instead of asphalt to reduce heat-island surface, often built with an underlying stone
effect, or using recycled rather than virgin reservoir, which temporarily stores storm-
asphalt to reduce emissions and natural re­ water before it infiltrates into the underlying
source consumption. For example, 80 percent soil.60 Porous pavement works by eliminating
of asphalt pavement removed each year the finer aggregates typically used in con­
during widening and resurfacing projects is ventional paving, and binding the remain­
reused, with contractors typically incorpo­ ing aggregates together with an asphalt or
rating up to 20 percent recycled material in Portland cement binder. By eliminating finer
concrete mixes.55, 56 However, these changes aggregates, a less dense material is created
do not address the fundamental problem of that allows stormwater to seep through. The
parking lot impermeability.57 underlying stone bed is designed with an
overflow control structure, helping to ensure
CHAPTER 5—Alternative Parking Surface Materials 22

26. that water does not rise to the pavement significant downstream benefits.66 Although
level. Stormwater settles in the empty spaces porous pavement looks very similar to con­
of the storage bed, infiltrating over time into ventional pavement, it is a far more sustain­
the subgrade soils—a process similar to an able alternative, considered by experts to be
infiltration basin.61 the most effective and affordable technique
for addressing stormwater management
The most common types of porous pavement
from development.67
are porous asphalt and pervious concrete,
which are very similar in their design and Porous pavements typically have a greater
applicability. spectrum of uses than alternative pavers
(discussed below), as porous pavement
• Porous Asphalt—Developed by the
can be applied to both low vehicular traffic
Franklin Institute in the 1970s, porous
areas and some medium traffic areas. Porous
asphalt consists of an open-grade coarse
pavements also have been used in a few high
aggregate, bonded together by a typical
traffic areas, including some highway applica­
asphalt cement in which fine aggregates
tions, because the product can provide better
have been reduced or eliminated, allow­
traction than conventional pavement and
ing water to move through the small voids
reduce hydroplaning.68 Ongoing research is
created.62 Porous asphalt can be used in
working to improve its highway applicability
all climates where conventional asphalt is
through the use of additives and binders.69
suitable.63
In addition, porous asphalt may help reduce
• Pervious Concrete—Pervious concrete noise levels from tires on pavement. In a
was developed by the Florida Concrete study measuring acoustical properties of
Association. It typically contains a mixture pavement types, porous asphalt was shown
of Portland cement; uniform, open-graded to have lower noise levels than conventional
coarse aggregate; and water. There is at hot mix asphalt.70
least 15 percent more void space in pervi­
ous concrete compared to conventional ALTERNATIVE PAVERS
pavements.64 Pervious concrete can be Alternative pavers, also known as perme­
more durable than porous asphalt, par­ able pavers or unit pavers, are interlocking
ticularly in hot weather. However, the State concrete blocks or synthetic fibrous grids
of Pennsylvania’s Department of Environ­ with open areas filled with grass, sand, or
mental Protection has noted that in colder gravel. Unlike concrete or asphalt poured-in­
northern and mid-Atlantic climates, porous place paving surfaces, alternative pavers are
concrete parking lots should always be separate units laid out on a prepared base.71
designed with a stone subbase for storm- When built with a storage bed infiltration
water management, and should not be system, alternative pavers function similarly
placed directly onto a soil subbase.65 to porous paving systems. The voids between
The manufacturing process for porous pave­ the interlocked pavers allow stormwater from
ment has the same environmental and health a parking lot’s surface to collect and then
impacts as the process for conventional pav­ seep into the storage bed, which is made of
ing materials, but porous pavement exhibits sand or crushed stone. The water then gradu-
Green Parking Lot Resource Guide—February 2008 23

27. ally infiltrates over time into the subgrade last 15 to 20 years, a length similar to con­
soils. In addition to stormwater management, ventional asphalt concrete pavement, which
the storage bed also provides added struc­ requires resurfacing after 20 years on aver­
tural support to the pavers.72 As with porous age.78 However, a number of factors need to
pavements, the most beneficial element of be assessed when determining whether a site
alternative pavers is the reduction or elimina­ is suitable for a permeable paving system, in­
tion of stormwater impacts.73 cluding: slope, traffic volume, subgrade, land
use, soil, infiltration and drainage characteris­
A number of alternative paver options are on
tics, and groundwater conditions.79
the market, including but not limited to: Turf-
stone®, UNI Eco-Stone®, Checkerbox®, Grass­ Compared to conventional asphalt surface
pave2®, and Gravelpave2®. Of the alternative installation and design, features such as sub-
paver options, grass paving systems are the grade, soil type, and installation requirements
most permeable. However, they have more are more complicated for permeable paving
limited applicability because grass cannot systems.80 For example, soil, including its
survive daily traffic; thus, grass-based systems type, porosity, and stability, is considered one
are typically used for emergency fire lanes or of the most important factors to determine
temporary overflow parking areas.74 Pavers site suitability. According to the New York
should be filled with fine gravel or other per­ State Stormwater Design Manual, developers
meable materials when more frequent park­ must ensure that soils are permeable enough
ing is expected.75 It should also be noted that to carry out adequate infiltration by consider­
certain types of alternative pavers, including ing the natural qualities of a soil type as well
block, grid pavers, and gravel, are not always as past land uses, because previous grading,
suitable for handicap accessible areas.76 filling, compaction, and other disturbances
of the land can alter soil infiltration qualities.
DESIGN AND INSTALLATION Underlying soils should have a minimum infil­
CONSIDERATIONS tration rate of 0.5 inches per hour to accom­
A number of uses for permeable pavement modate stormwater volumes, and knowledge
exist beyond new, whole parking lot con­ of the organic matter content of the soil is
struction projects. One option for high traffic also important in determining its pollutant
parking lots is to design a hybrid parking lot removal capabilities.81
combining permeable pavement parking Permeable pavement is meant to treat small
spots with more conventional paving in the storm events, which can range from 0.5 to 1.5
aisles.77 In addition, permeable pavements inches. A site must be designed with an ad­
can be used during parking lot retrofits and equate ratio of infiltration area to impervious
replacements. area, and the soil should have a permeability
According to the U.S. Department of Trans­ of between 0.5 and 3.0 inches per hour in
portation, permeable pavements must be order to adequately handle stormwater.82 Oc­
properly sited, designed, and installed in casionally, exceptions can be made to allow
order to function fully over their life span. If for permeable paving when sites do not meet
planned correctly, permeable pavements can certain criteria. For instance, permeable pave-
CHAPTER 5—Alternative Parking Surface Materials 24

28. ment can be used in soils with low porosity that if properly installed, success rates for a
if a discharge pipe is installed to run from a permeable paving system, particularly po­
storage area to a conventional stormwater rous asphalt, can be much higher than earlier
system. This modified system will still treat installations using these materials.88
stormwater from small and medium storms,
but also will prevent flooding during large MAINTENANCE OF PERMEABLE
storm events.83 PAVEMENT
Porous pavement and alternative pavers In the past, studies indicated that permeable
alone are not an appropriate BMP to combat pavement applications had a high failure
extreme flooding events in channels and rate, due not only to improper siting, but
riverbanks. It is recommended that a BMP de­ also poor maintenance. Failure of a perme­
signed specifically to control high waterflows, able paving system means that the surface
such as a dry detention pond, should be used becomes impervious and behaves like con­
in conjunction with porous pavement. This ventional asphalt, yet typically without the
approach is required by some local govern­ fully developed system of piping and gutters
ments as part of flood protection design used to manage runoff on conventional park­
criteria.84 ing surfaces. However, with correct mainte­
nance, permeable pavement can retain its
Permeable pavement should not be used permeability, and be a successful stormwater
in parking lot areas with high volumes of management option.89
sediment-laden runoff, high traffic volume,
high dust areas, and/or heavy equipment The level of maintenance necessary to
traffic.85 Clogging is the main cause of a maintain permeable pavement lots varies.
system malfunction that can result from poor Alternative pavers such as concrete grid pav­
siting of the permeable pavement system. ers and plastic modular blocks will require
During construction, developers can prepare less maintenance because they do not clog
for possible clogging by installing a perim­ as easily as porous asphalt and permeable
eter trench connected to the stone reservoir concrete. Location also impacts the amount
to treat overflow should the surface clog.86 of maintenance, as areas receiving more
Other common problems to avoid include: sediment will require more maintenance. For
example, a parking lot with higher traffic vol­
• Compaction of underlying soil, such as umes will tend to require more maintenance
through the use of heavy equipment. because of the resulting increased quantities
• Contamination of stone sub-base with of soil and particulates brought onto the lot.
sediment. Although the new soil alone will not neces­
sarily clog the pavement’s voids, if ground in
• Tracking of sediment onto pavement.87 repeatedly by tires, clogging can occur.90,91
Like other best management practices, when Regular maintenance can avoid clogging of
permeable paving systems fail, it is frequently permeable paving systems. Facilities manag­
due to mistakes made during the design and ers are generally advised to high pressure
construction process. Recent studies note hose and then vacuum porous pavement a
Green Parking Lot Resource Guide—February 2008 25