Knowledgeable LID Patterning for Ecologically-

Sensitive Developments

A study of LID practices

A thesis submitted in partial fulfillment of the requirements of the Honors Program of the Department of Landscape Architecture in the School of Architecture, University of Arkansas.

Sarah DaBoll Geurtz

Thesis Committee:Chair: Mark Boyer

Member: Carl SmithMember: Kimbell Erdman

Spring 2010

Acknowledgements

I would like to thank my professors and mentors Mark Boyer, Dr. Carl Smith, and Kimball

Erdman for their help and guidance in the research and development of this paper. Special

thanks goes out to Mark Boyer for the time he took to read, reread, and guide my paper

from the beginning - and for getting me on the proper track for graduating with honors.

Table of Contents

Heading Page #

Chapter 1 - Introduction & Site Ecological Specifics 2-21

Thesis Statement 2

Introduction 2

LID Historical Background 4

Map Overlay Technique / Site Fingerprinting 7

Buffer Zones 16

Chapter 2 - Construction 21-35

Street Layout 21

Reducing Soil Disturbance 23

Compacted Soil 28

Preventing Erosion 33

Chapter 3 - Hardscapes 35-60

Stormwater Reduction as it Pertains to Impermeable Surfaces 35

Plastic Geocell Systems 38

Open-Celled Paving Block Systems 42

Open-Jointed Paver Systems 45

Porous Concrete 48

Table of Contents Continued

Heading Page #

Porous Asphalt 50

Installation of Porous Concrete & Porous Asphalt Systems 51

Porous Pavement Systems Compared 52

Chapter 4 - Bioretention 60-86

Bioretention Introduction 60

Soil Chemistry 61

Bioretention Systems 66

Bioswales 67

Raingardens 77

Detention Systems 79

Constructed Wetlands 80

Urban Bioretention Methods 83

Chapter 5 - Roof Stormwater Controls 87-91

Rainbarrels & Cisterns 82

Greenroofs 88

Conclusion 89

Bibliography 92-103

List of Illustrations (Figures)

Figure # and Description Page #

Figure 1. McHarg Map 9

Figure 2. Subsurface Water flows 14

Figure 3. Riparian Zones 17

Figure 4. Loop Layout 22

Figure 5. Loop Layout Close-up 22

Figure 6. Pin Foundation System 26

Figure 7. Pin Foundation System Being Installed 27

Figure 8. Pin Foundation System Installed Over a Stream 27

Figure 9. Compaction Depth by Moisture Levels 29

Figure 10. Compaction Depth Due to Tire/Tracts Used 30

Figure 11. Subsoiler’s Tines 31

Figure 12. Soil as a Sponge 36

Figure 13. Soil Partially Covered 36

Figure 14. Geocell System 38

Figure 15 Geocell Panels Coming Out of the Ground 41

Figure 16. Exposed Geocells 41

Figure 17. Open-Celled Paving Block System 43

List of Illustrations (Figures) Continued

Figure # and Description Page #

Figure 18. Open-Jointed Paver System 45

Figure 19. Porous Concrete 48

Figure 20. Porous Asphalt 50

Figure 21. Cations & Anions in a Soil Profile 62

Figure 22. Trapezoidal Swale 68

Figure 23. Wet Bioswale 73

Figure 24. Compost Blanket and Compost Berm/Sock 75

Figure 25. Constructed Wetland 80

Figure 26. Four Constructed Wetland Sections 82

Figure 27. Bump-Out 83

Figure 28. Bump-Out 83

Figure 29. Flow-Through Planter Being Installed 85

Figure 30. Flow-Through Planter Installed 85

Figure 31. Flow-Through Planter Overflow Pipes 86

List of Illustrations (Tables)

Table # and Description Page #

Table 1. Site elements to be Identified 10

Table 2. Gird Versus Curvilinear Street Layouts 21

Table 3. Cations and Anions 64

Table 4. Bioswale study 65

1

Landscapes of concrete and asphalt

Buildings towering above

Water sliding into hidden pipes

Dumping the filth of the gray city

Far outside the city’s walls,

Into streams, miles away.

And flooding…

“We need nature as much in the city as in the countryside...”

- Ian McHarg

2

Chapter 1 - Introduction & Site Ecological Specifics

Thesis Statement:

Through patterning LID on site-specific land characteristics, LID can be applied to de-

sign in such a manner that environmental sensitivity is addressed, while an area of unique-

ness and character can be created.

Introduction:

Low Impact Development (LID) is increasingly being utilized in place of conventional

stormwater management systems in order to reduce the environmental impacts of develop-

ment. It is recognized as a site development methodology that utilizes site fingerprinting,

vegetation and soil conservation, porous surfaces, and natural soil infiltration of water in

order to handle and treat stormwater runoff through natural processes. Specifically, it uses

soil and vegetation for nutrient removal, water retention, and groundwater recharge, while

providing attractive vegetated areas that add community character and uniqueness. As the

need for protecting our water sources from pollution and increasing water volumes grows

more noticeable in our waterways, municipalities are increasinly requiring the use of LID

methods, hence advancing the knowledge and installation of LID in development.

3

Current environmental issues such as toxic levels of nutrients and heavy metals

found in stormwater and waterways, stream degradation, and lack of greenspace for wild-

life and humans alike reveal a need for LID to be applied on a large scale across America to

reduce the impact development has on the environment. The stormwater management

strategies of LID perform to conserve existing ecologically sensitive sites and to reduce

stormwater runoff in manners that increase soil absorption and reduce runoff’s nutrient/

toxicity levels. The variety of LID techniques available to designers are numerous and the

manner in which they are designed differ according to the individual characteristics of each

site being developed. Therefore, knowledge of LID methods and their benefits and limita-

tions can aid the designer in developing not only successful LID site design, but can result in

developments that are often more saleable due to the ecologically–sensitive label they pos-

sess, and in communities of character and uniqueness that become a reflection of the land

and people who live there.

This paper’s study of LID techniques will look at the many methods of LID, the prag-

matic implications of LID, and at its many well-known benefits, in order for the reader to be

equiped to propose educated and knowledgeable information to a client. This paper will

begin with the historical background of urban water pollution and LID’s beginnings in rela-

tion to the concerns of stormwater pollution and volumes.

4

LID Historical Background:

Traditionally, sewage and stormwater were treated the same - and they were often

times piped straight into bodies of water. When it was discovered that sewage was causing

health problems, society designed to dispose, and later to treat the sewage. We have now

discovered that stormwater harms the environment by carrying pollution and large volumes

of water within stormwater flows. Automotive fluids, heavy metals from brake pads, pesti-

cides, herbicides, fertilizers, and many other pollutants are found in stormwater and pose a

strong environmental concern. Pollutants found in our waterbodies, stream ecology death,

channel erosion, channel degradation, and flooding are all results of not treating stormwater

before it rushes into streams and other bodies of water.

By the nature of development and human living, we build buildings, parking lots,

sidewalks, and driveways. If soil is covered with such materials and cannot absorb rainwa-

ter, where does the stormwater and its collected contaminants go?

……….water sliding into hidden pipes……..

………. into streams, miles away.……..

They pour into small brooks, which become roaring monsters as they rush through their

courses, sweeping away their channel sides and depositing sediment and whatever is in

their paths down-stream. The brooks then go back to “normal,” small streams in canyons of

what they used to be, and flooding ensues downstream.

5

Perhaps it is not surprising how society handles stormwater – you could say it’s al-

most in the human DNA. We have been piping water to urban areas and then straight back

into waterbodies for so long that it has become a way of life. Indeed, as far back as 4000BC

in Babylonia, complex piping networks for handling sewage and stormwater were being

used, and we know well of ancient Rome’s complex water and sewer networks (Sewer His-

tory 2004). Later, by 400BCE, the Romans had built an extensive sewer system to carry both

stormwater and sewage away from Rome. But oh, the dichotomy of it; these sewers took

both sewage and stormwater and emptied them directly into the Roman’s Tiber River where

stench and pollution then reigned (Hough 1995). However, there were other societies that

recognized the need to protect waterbodies and therefore handled their sewage by piping

it to farms for fertilization and irrigation, or to cesspools, instead of to water bodies. Unfor-

tunately, when the Roman Empire fell, society reverted not only in education, but in sanita-

tion (Sewer History 2004). “Garde-loo!” - it was common-place for waste to be dumped

into streets with this warning phrase. Rainwater would later wash the waste to where the

topography was lowest. At those low elevations, a city’s filth stagnated and bred vicious dis-

ease. By the 1700s, the sewer was reinvented; however, these sewers carried stormwater

and human refuge together because human waste was still poured onto the streets where

rain washed it together into stormwater pipes (ibid). Also, such as before, these pipes often

dumped this wastewater into a body of water. Even in the mid to late 1800s, stormwater

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and raw sewage were still piped through the same pipes and often dumped straight into

waterbodies. However, some societies and cities recognized the need to protect waterbod-

ies and therefore handled their sewage by piping it to farms for fertilization and irrigation,

or to cesspools, instead of to water bodies. Thankfully, separate stormwater and sanitary

sewers were eventually built in most cities, and at some point sewage began being treated.

Whereas this was a huge improvement over prior conditions, people still did not recognize

the need for also treating stormwater.

Somewhere after the 1960s, people began noticing downstream flooding, channel

erosion, and fish death; people began questioning the practice of collecting stormwater and

sending it, untreated, directly to streams and rivers (Reese 2001). That’s when the idea of

the detention pond was born (ibid). But, even with detention ponds popping up all over

America, flooding was still occurring and pollutants were still being carried by stormwater

into waterbodies – a need for yet a different approach was recognized.

In the early 1980s, stormwater managers and developers saw the damage develop-

ment had been causing. At the same time, the ability of wetlands to capture, hold, and

treat stormwater was being recognized (Azous 2001). The natural thought progression of

developers was to utilize existing wetlands to “handle” the ecological problems caused by

post development’s stormwater volumes and pollutants (ibid). However, natural resource

specialists were concerned about the ecological health of wetlands if they were to be used

77

essentially as detention or retention ponds. This led to, in early 1986, natural resource and

stormwater managers convening in the Puget Sound area of Washington State to resolve

the separate concerns of the two fields (ibid). This stormwater meeting resulted in research

which lead to techniques for handling stormwater runoff in order to protect wetlands

(Azous 2001). Immediately following this, in 1987, the EPA stepped in with the 1987 Water

Quality Act which regulated stormwater pollutants. Then, in 1989, the Low Impact Design

Center, Inc. was developed in Prince George’s County, Maryland (Weinstein 2008) and the

idea and phrase “Low Impact Development” (LID) was coined. This organization worked to

educate the public about the necessity of LID’s methods of treating and managing stormwa-

ter. Out of necessity was therefore born a movement that, in 1989 had its beginning, and by

the 2000s had gained world-wide attention and following.

Map Overlay Technique / Site Fingerprinting

LID techniques are designed to replicate nature’s system for filtering stormwater be-

fore it reaches bodies of water. These methods encourage the reduction of soil compaction

and increase permeable soil cover, thereby preserving soil’s complex structure. This permits

water percolation into underground aquifers and the filtering out of impurities through the

soil. These actions reduce both the amount of stormwater and the levels of pollution found

in it. “Site fingerprinting” utilizes carefully study of a site’s soil types, hydrological cycles,

and water flows to address natural water flows and preserve ecologically important sites.

8

A large volume of published research continues to provide scientific evidence of LID’s ef-

fectiveness. Some examples of research pertinent to the field are works carried out by B.O.

Brattebo and D.B. Booth, Robert France, and J.S. Tyner and W.C. Wright, et al. Yet, many

LID methods involve not only preserving greenspace, but also planting of vegetation. There-

fore, LID can result in community character and uniqueness where it becomes a reflection of

the people and their care of the land and its ecological health. For all of these reasons, cit-

ies are beginning to adapt LID methods and requiring it of developers as development takes

a more noticeable toll on waterways.

LID involves careful study of a site before any design work is ever begun. The land

helps to guide the eventual layout together with the program. It involves specific and care-

ful site study in order to determine the best manner in which to develop a site. This utilizes

a technique called “map overlaying.” This is not exactly a new concept; it acquired its origin

in 1967 from the well-known Landscape Architect Ian McHarg. Note this date. This was the

time of hippies who petitioned to save the environment; McHarg’s timing was perhaps per-

fect to get people’s attention on how development and planning needed to work with the

natural features of land instead of ignoring them. Note also the coming years – the 1970s.

During this time, the use of detention ponds came about to control stream flooding. While

we now know that detention ponds are not exactly “the answer,” people were beginning

to pay attention to stormwater problems. In his book, Design with Nature, McHarg (1967)

9

explained in detail how to use a

complex mapping technique to

determine the best areas for a

development’s footprint while

preserving ecologically important

land. These maps dealt with

planning issues such as social

benefits, best sites for agriculture

and mining, and best lands for

recreation, among a multitude

of other uses. McHarg would then overlay these with further ecological, developmental,

historical, societal, hazardous, mining, and many other aspects to determine and influence a

property’s development (Figure 1). Likewise, LID methods take the extensive mapping tech-

niques of McHarg and overlay them to indicate land characteristics that would specify where

certain development features should be placed.

This overlay process involves a complex analysis of site inventory information that

studies both on- and off-site conditions to determine how they would impact a site; it looks

at a site’s hydrology, topography, soils, vegetation, water movement patterns, and many

other elements (Hinman 2005). A designer takes this information and applies it to an LID

Figure 1. One of Ian McHarg’s color-coded overlay maps that indi-cated urban suitability areas (McHarg 1967, p. 155).

10

layout to align roads, lots, and structures so as avoid impeding natural stormwater flow

across, or through, the soil profile, to protect natural ecologies, and to increase soil infiltra-

tion of water (Hinman 2005).

Ideally, the location of any of the ecologically-important features indicated in

Table 1, including the species that comprise these areas, and the health of the various ecolo-

gies, should be mapped/researched for every site (Hinman 2005). Then, all the maps should

be studied in compilation for characteristics that can indicate to the designer land character-

istics that might impact a site’s development in order for a design to work with the existing

landscape while trying to avoid sensitive ecologies.

Table 1. This table illustrates some site conditions that ought to be identified prior to development so the designer can make efforts to either protect or design with or in mind of these elements (Hinman 2005; McHarg 1992)

Soils Soil Erodibility Closed Depressions

Topography Bodies of Water Groundwater

Existing Hydrologic Patterns Wetlands Aquifers

Offsite Drainage Springs & Seeps Geology

Habitat Conservation Down-Stream Analysis Bedrock Formations

Vegetation / Forest Preservation Slope Stability / Protection Existing Development

Minor Drainage Features Floodplains Aquifer Recharge Areas

11

Global Imagining Systems (GIS) technology is an important tool available to designers

in the map overlay stage of LID. GIS imagery, such as Landsat, is especially useful when deal-

ing with large acreages where walking an entire site is not reasonable or possible. These

systems can determine compacted soil areas, dying vegetation, extensive imperviousness,

exposed sediment, soil moisture readings, and urban heat islands, among many other land

and site characteristics (Low Impact Development Center, Inc. 2008). Mapping of precipita-

tion data from the National Oceanic and Atmospheric Administration (NOAA) can also be

very beneficial, especially when combined with hydrologic modeling that utilizes topography

maps of two-foot intervals (Mandarano 2010). Through utilizing the two-foot topography

maps instead of the more common ten-foot contours, the hydrology of a site can be un-

derstood much more accurately (ibid). However, computer modeling by itself is not suf-

ficient information to fully understand a site’s hydrology. According to Mandarano (2010),

human observations are an important “fine-tuning” step that should follow the computer

hydrologic pattern setting; these observations can indicate site characteristics such as water

flow constrictions that might not appear in a topographical map. Human observations and

reasoning tie this gathered information into a cohesive map that can be utilized with further

information to create crucial mapping information. This information is then employed by a

LID designer to determine the most applicable areas for either development or preservation.

In addition to the GIS data, LID looks at the soil profiles that comprise a site; this

12

information is used to determine where site conditions are best suited for LID applications.

In conventional engineering and design, a site’s soil characteristics are used to determine

buildable and unbuildable areas. However, LID utilizes detailed soil information in a more

integrated manner for a development’s layout. It looks at not only buildable and unbuild-

able soils, but which soils should be retained for infiltration of stormwater or covered by

impervious surfaces. In order to be able to make these decisions, tests should be conducted

to determine factors such as infiltration rates and the specific soil profile matrix that com-

prises a site (Hinman 2005). Knowing the location and specifics of a site’s features such as

soils, bedrock, and depth to groundwater, and how they affect soil permeability, is impor-

tant for determining where and how to locate LID techniques. Areas with shallow depth to

bedrock or a high water table may indicate areas that will not drain well; therefore, locating

a bioswale or raingarden on these natural features perhaps should not be done. However,

a detention pond or created wetland might be ideally suited for these areas. Also, being

knowledgeable about the extent and location of these soil characteristics would help a de-

signer understand a site’s ecology and might indicate features such as a seasonal wetland or

seep that are important for rain retention on a site.

Further studying of soil profile layers can indicate other important features. Areas

with high clay content and fragipan layers will limit permeability and, if bioretention cells

will be designed over these, soil amendments and ripping of the relatively impermeable lay-

13

ers may need to be carried out (Tyner et al 2009). Soils composed of gravely or sandy soils

may be ideal locations on which to locate bioretention because they should drain well (Hin-

man 2005). However, soils high in gravel and sand must be studied carefully for groundwa-

ter levels, as the high drainage of these soils may mask conditions of seasonally high water

table levels (ibid). Soils such as these may further prove to be problematic due to fragility

and tendency to change in volume. Additionally, areas with low permeability may be ideal

locations for construction of roads, homes, and other impermeable development because

the change in discharge from a C or D hydrologic group soil to that from an impermeable

surface (if so used) is not as great as that from an A or B soil covered with an impermeable

surface. If a high rate of water runoff is due to shallow bedrock, these sites may require

blasting for construction of basements and burying of utilities. Alternatively, if porous roads

will be installed, their success will largely be determined by the permeability of the soils

beneath the roads (United States Environmental Protection Agency & National Pollution Dis-

charge Elimination Services a. 2010). When areas ideal for road and building locations have

been determined, the designer should then turn to the specific patterns of these elements.

A site’s sub-surface water flows should be understood to the best of a designer’s

ability. These flows can indicate where possibly stable or unstable ground exists for devel-

opment, or where a major subsurface water supplies water to a wetland or stream. This in-

formation can be gathered by learning the groundwater depth and through studying a site’s

14

topography. Visual elements such as moist areas or waterbodies such as streams or ponds

can be indicative of subsurface flow routes. Also, groundwater depth and aquifers, when

mapped, can indicate sub-surface flow patterns and can be very useful to designers (Hinman

2005). Because sub-surface flows evolve through groundwater percolation, designing to

enable this natural process to continue post-development is important; however, due to the

very nature of development, this may not be either practical or achievable.

Springs and streams fed with sub-surface flows are created when water is held in soil

pores and flows downwards and horizontally , as well as when water flows through cracks in

bedrock, eventually daylighting (see Figure 2) (France 2002). Actions that cause a reduction

in this sub-base water flow can be: lack of rain, disturbance of the sub-surface water flow

patterns, or a decrease in the amount of water allowed to infiltrate the soil. All of these

actions can cause wetlands, wells, and public water supplies to decrease or stop flowing.

This can be detrimental to fragile ecologies and is sometimes the fault of site design and

construction. A designer should

look at this “invisible” water trail

and truly attempt to understand

it, keeping underground streams

in mind when designing a site. An

example might be attempting to Figure 2. Subsurface water flows. (France 2002)

15

preserve a spring by utilizing porous hardscape materials in the area of its water absorption

shed, or in incorporating bioretention cells to maintain a site’s water percolation in order to

help preserve a wetland’s water supply.

Wetlands in particular are an important natural water purification element that

designers should carefully locate and protect. These delicate natural ecologies must not be

utilized to treat stormwater volumes greater than what they originally managed, due to deg-

radation of the natural wetland’s ecology. Avoiding disturbance of natural wetlands may or

may not be mandated by law. Developments may have what are called “wetland mitigation

banks” which permit developers to drain wetlands and purchase a proportional percentage

of a designated area which has been set aside as a large wetland. Simply from an engineer-

ing point of view, developing on an existing wetland or a very moist site can involve complex

engineering and might easier be left well alone. The ecological and LID approach is for a

designer to utilize his or her creative design skills to create a development that incorporates

wetland areas into the unique design of a site. This is carried out by utilizing more stable

land for development while not building on wetlands, while perhaps incorporating a wet-

land ecology into a site’s design.

If a wetland is thought to occur on a property, a professional should complete a wet-

land determination study. This information will inform the designer of the wetland’s impor-

tance, specifics, and incoming water flow routes. If, for example, there is an underground

16

seep that feeds a wetland, a designer should ensure the soil profile’s hydrologic flows are

not disturbed during or after development (Hinman 2005; Azous and Horner 2001). Also,

the increase and decrease of stormwater level fluctuations into a wetland should be kept to

a minimum: a wetland’s natural dry and wet seasonal periods should remain close to that

of pre-development without large differences due to man-made structures and site distur-

bances if possible (ibid; ibid). Large fluctuations can cause a shift in the ecological balance

that a wetland has evolved to handle and can alter the entire ecosystem of a wetland (Hin-

man 2005).

Buffer Zones

After determining locations of wetlands and other important site characteristics (see

Table 1), designating what site areas should be protected with buffer strips is an important

step. The use of buffer zones, to be called out on construction drawings, should indicate

where fencing would be installed in order to provide a visual and physical barrier for sensi-

tive ecologies during development (see Figure 3 for riparian buffer zones example). These

zones aid in protecting the soils and vegetation from harm during development (Hinman,

Curtis 2005). Additionally, compost berms or compost blankets should be installed in order

to protect sites such as wetlands and water bodies from excess and polluted water flow

entering these sites during development. Signs should also be placed to identify and ex-

plain the measures being taken to protect these areas (ibid). Buffers should, for matters of

17

practicality as well as environmental, be utilized for the 100-year floodplain, wetlands, steep

slopes, and waterbodies (see Table 1) (ibid). Although there is not a universally applicable

set width for buffer zones, several sources offer width suggestions according to the ecologi-

cal site characteristic being protected: the Puget Sound Water Quality Authority’s wetland

guides, or Citations of Recommended Sources of Best Available Science, 2002, or local codes,

as some municipalities have their own buffer width requirements (Azous and Horner 2001;

Hinman 2005).

Technically speaking, buffers are strips of land that surround delicate ecologies and

protect them from high stormwater flows and the sediment and pollution carried by this

water. Within a buffer strip, when stormwater rushes over its vegetation, the vegetation

provides resistance to the flowing stormwater. This results in a transfer of energy from the

stormwater to the vegetation, causing the stormwater velocity to slow before it enters the

Figure 3. Riparian zones such as this are buffer zones located around bodies of water, whereas buffer zones are strips of land that protect either riparian zones or other sensitive ecologies. (image by Sarah DaBoll Geurtz)

18

protected site, resulting in a decrease in water volumes. If the buffer is protecting a mov-

ing waterbody such as a stream, this transfer of energy results in a total reduction of the

stream’s energy. This is important for maintaining more natural and safe velocities of mov-

ing bodies of waters as well as for helping to prevent stream bank erosion. Additionally, ve-

locity reduction results in a decrease of disturbance to the site element being protected by

a buffer zone. Attempts to reduce stormwater’s velocity before it enters buffer zones should

be addressed in order to prevent stormwater flows with considerable velocities. Preventing

these water from flowing undeterred across long surfaces would help to reduce stormwater

flow velocities into a buffer zone (Hinman 2005), and these entering waters should be de-

signed to stay below one foot per second to further prevent harm and allow more time for

the water to be filtered by a buffer zone (ibid). The vegetation of these zones also performs

as a filter and “catch basin” for sediment. At the same time, the soil matrix adsorbs pollut-

ants held in the stormwater, and soil microorganisms consume various pollutants, thereby

preventing some pollutants from reaching the site being protected (Brady and Nyle 1999).

Buffer zones therefore provide crucial ecological barriers for highly fragile, sensitive, and

unique environments.

An additional and often overlooked facet of buffers is the rich ecologies that can ex-

ist within these zones. Simply with the hydrological topography characteristics that occur

within buffers, both permanent and ephemeral ponding and channels may exist that pro-

19

vide habitat for fish and other aquatic life (Hinman 2005). Buffer areas also provide shade

and temperature regulations for waterbodies and are therefore of high importance for the

survival and life cycles of aquatic wildlife (ibid). Also, while buffer strips capture nutrients

and debris from stormwater flows, a certain quantity of these elements is accumulated by

stormwater as it flows through buffer zones and enters a protected ecological site. In small

quantities, this offers an important source of nutrients for a waterbody’s or wetland’s aquat-

ic life, as well as for the soil microbes and all forms of life within the protected areas and

buffer zones (ibid). Therefore, there are delicate ecological aspects that exist within both

the protected element (such as a wetland or a stream), and the buffer zone.

Because these zones are protected and allowed to remain in their natural state, they

become important pockets or corridors for wildlife while providing green, natural environ-

ments within developments and urban areas. Additionally, there are community benefits.

Riparian areas are ideal spaces in which to place community trails. Because designers do

not have to work with existing property lines within riparian zones, these trails can be undu-

lating and interesting, and often have existing tree cover to shade trail users. They also have

the additional benefit of making people more aware of the existence, health, and need for

protecting waterways in their communities. These riparian buffers can also become cor-

ridors and public spaces for communities in which to enjoy nature. However, the ecologi-

cal sensitivity of the areas where the trail would be located must be carefully considered,

20

as well as the material that would be used to construct these trails. A trail might need to

veer away from a stream where a sensitive site needs protection, or boardwalks might be

required when a concrete or asphalt trail would be too harmful for the environment. How-

ever, any type of trail will increase stormwater runoff volumes; this in itself is not ideal for

the environment being protected. Because of this issue, porous surfaces have been sug-

gested as ideal materials for trails within buffer zones. The unfortunate reality with this idea

is that porous pavements must be protected from overland water flow and flooding. If not,

sediment settles into the void spaces and reduces or eliminates water infiltration. A viable

solution might be to relegate trails toward the outer edge of riparian zones and in certain ar-

eas briefly bring the trail toward the inner protected zone. Utilizing buffer strips for societal

benefits can impart significant social functions that can become community-creating ele-

ments of an LID design.

Once the multiple map overlays are compiled and studied, the gathered informa-

tion is analyzed to see how it all works together, and buffer zones have been determined, a

designer can see how a development should be designed within a site’s natural ecological

context. The designer can then move on to the next phase of development in which he or

she determines placement and patterns of elements such as buildings and roads. This will

be the topic of the next chapter. There are two basic street layouts utilized by designers:

grid and curvilinear streets. The grid street network is highly popular in New Urbanist devel-

21

opments, and the curvilinear street layout is often associated with older cul-de-sac develop-

ments. Each has its own benefits and drawbacks.

Chapter 2 - Construction

Street Layout

While the grid layout may lend itself to usage of back alleys and off-street parking,

these alleys can cause twenty to thirty percent more impermeable surfaces from roads than

curvilinear road layouts which often do not have back alleyways (see Table 2). Grid layouts

also may not account for a site’s specific land characteristics and ecologies (Hinman 2005).

However, curvilinear developments tend to result in secluded developments that discour-

age through traffic and walkability. Because both street systems have their benefits and

drawbacks, a mixture of the two can provide for an interesting design alternative that can be

incorporated with LID methods. These hybrid layouts, as seen in Figures 4 and 5 are known

Table 2. Some of the benefits and drawbacks of the grid and curvilinear layouts (Hinman 2005, p. 29).

Road Pattern Impervious Coverage

Site Distur-bance

Biking, Walk-ing, Transit

Auto Efficiency

Grid 27-39% (Center for Housing In-novation, 2000 and CMHC, 2002)

Less Adaptive to site features and topography

Promotes by more direct ac-cess to services and transit de-pending upon grid size

More efficient - disperses traffic through multiple access points

Curvilinear 15-19% (Center for Housing In-novation, 2000 and CMHC, 2002)

More adaptive for avoid-ing natural features, and reducing cut and fill

Tends to discourage through longer, more confus-ing, & less con-nected system

Less efficient-concentrates traffic through fewer access points & inter-sections

22

as “open space,” “hybrid street,” or “loop

layout” plans because they are a compi-

lation of grid, curvilinear, and ecological

layouts (Hinman 2005). This technique

is especially exciting from both a LID and

aesthetic point of view. It minimizes

road coverage per house and provides

two points of ingress and egress (ibid). Also, while incorporating some of the grid layout, it

allows for usage of oddly shaped parcels and “left over” land by utilizing a more curvilinear

arrangement (the “loop”) without the usage of cul-de-sacs which prevent through traffic

(see Figures 4 and 5). In addition, the homes around the “loops” and the more gridded

areas become tiny communities in and of themselves, and can be designed to look out over

potentially beautiful greenspace that

doubles as a bioretention cell (ibid).

Roads can be curved to avoid ecologi-

cally important areas and the grid

network can be applied where a site

warrants its use. Backyard “alleys”,

if wanted by the developer, can be Figure 5. Loop Layout Close-up (Hinman 2005)

Figure 4. Loop Layout (Himan 2005)

23

constructed with vegetated porous pavements to provide for water infiltration while also re-

moving garages from the front streetscape view. Creating narrow parcels would provide for

less road length requirement, and bringing homes close to the front of these lots by reduc-

ing required road set-backs would reduce the amount of driveway required (ibid). Pedes-

trians and bicyclists can be designed for by providing mid block breaks; these breaks would

reduce the distances to travel and would encourage walkability within a community (ibid).

These are all examples of how designers can be creative when working with a specific site’s

character and ecological aspects, while also instilling a sense of place.

Reducing Soil Disturbance

When an LID design has been drawn and construction is the next step, the designer

should designate protection zones, heavy machinery zones, and vehicle paths within the site

as part of the construction plans. Sensitive areas and riparian zones, as mentioned ear-

lier, should be delineated with visible fencing and signage, preferably with a wooden fence

to further inhibit construction vehicles from entering these zones (Murphy 2006; Hinman

2005). A sturdy fence would be less apt to be ridden over by equipment than warning bar-

rier fencing. Areas of special importance can be further protected with signage that not only

details rules for staying out of these areas, but details a monetary fine for anyone who dis-

turbs them and causes harm. Heavy machinery must be kept away from tree roots to pre-

vent soil compaction which would lead to poor water infiltration, poor soil-oxygen exchange,

24

and quite probably would lead to eventual tree death (Brady and Weil 1999). Likewise,

the integrity of soils that drain well should be carefully retained to protect the soil’s natural

drainage characteristics; the future of the entire low impact design will depend upon the

permeability and health of these soils.

To aid construction workers in avoiding sensitive areas and to increase the chances

of these sites being left alone, the designer should designate specific “disturbance enve-

lopes” where construction disturbances are permitted to occur. These disturbance enve-

lopes delineate paths in which vehicles are allowed to travel and park, where material and

equipment may be stored, and where temporary construction crew’s buildings may be sited

(Murphy 2006). To prevent unnecessary soil compaction, disturbance envelopes should

be sited where existing hardscapes or impermeable soils exist, or where future site distur-

bances will occur. Ideal locations are places where buildings, roads, or other soil compact-

ing structures will be built in later phases of the project, where bedrock is close beneath the

soil surface, or where utility easements will be built (ibid). Careful site planning, as well as

providing thorough construction specifications and timely follow-ups in the field during con-

struction, must be carried out by the designer to insure these soil protection strategies are

implemented. Prior to construction beginning on a site, the contractor(s) should be educat-

ed in how they and their workers must follow specific protocols in order to protect the site’s

soils and natural water percolation within the site. The construction crew must be educated

25

about how compacted soil does not infiltrate water well and is therefore detrimental to

an LID design. These education measures will greatly limit unnecessary site disturbances,

which is an integral part of low impact design.

Reducing cut and fill volumes are also LID techniques used by designers to minimize

soil disturbances, thereby permitting a higher degree of water percolation. Careful build-

ing placement can have a large effect on the ecological health and water penetration of a

site. This will also reduce soil compaction from heavy machinery and fill soil. For instance,

flat areas may be ideal for development because they may require little cut and fill. Hilly

topography can be utilized with homes designed to fit into the hillsides where the building’s

floor levels are staggered with the natural topography; this reduces the amount of needed

cut and fill for leveling a flat area for a slab foundation (Hinman 2005). Cut and fill can also

be minimized by orienting a building’s longest axis along contours (ibid). Selection of less

intrusive development measures can not only decrease cut and fill volumes but can lead

to reduced costs for the developer through decreased labor and fill material requirements.

This monetary savings can become a strong selling point to a developer.

26

Large soil disturbances from building construction can be further reduced by elimi-

nating the use of concrete slab foundation systems. A current replacement for slabs called

“minimal excavation foundation systems” offers a promising alternative to concrete founda-

tions and pouring of concrete footings for installation of decks and boardwalks (see Figure

6). This type of new foundation is easy to install and can support many residential building

weights. Unless the site consists of very wet or high freeze-thaw soils, no digging or site

grading is required when installing Pier Foundation Systems (Hinman 2005; Pin Foundations

2010). As can be seen in Figure 7, a structure is built on top of a series of connecting beams

to transfer loads horizontally and then into the piles, then downward into the ground along

the galvanized metal piles (see Fig-

ure 7) (Pin Foundations 2010). When

installed, this system still permits

surface stormwater to pass beneath a

structure and into the ground, without

harming a foundation’s stability if prop-

erly designed (Figure 8) (Pin Founda-

tions Inc. 2009). While well-draining

soils of the A/B hydrologic group are

ideal candidates for minimal founda-Figure 6. Pin Foundation System (Pin Foundation 2010)

27

tion systems, poorly draining C/D soils

can sometimes be supported with this

system as well (ibid). Ecologically-

important and sensitive sites such as

wetlands can therefore have this type

of system installed to diminish soil dis-

turbances. However, the soils must be able to support the piles. Designers should be aware

that soils with a high rate of vertical freeze-thaw or swelling forces may require a larger

size of foundation system (ibid). Sites that experience these forces in extreme amounts

will require still larger-scaled vertical piers with deeper penetrating metal pins in addition

to installment of pea gravel around the piers. It

should be noted here that soils with such condi-

tions are difficult to build concrete slab and verti-

cal pier foundations on, as well (ibid).

Where concrete must be poured on

a site, the following application methods can

result in less soil compaction, thereby maintain-

ing water infiltration. One possible technique

is to use ready-mixed concrete in place of mix-

Figure 7. Example of the installed post, beam, and pin founda-tion system being installed. (Pin Foundation 2010)

Figure 8. Minimal Foundation Systems can some-times be successfully installed in very wet soil conditions (Pin Foundation 2010)

28

ing the concrete on site; concrete trucks eliminate the need for a construction site to hold

large amounts of dry concrete mix bags on-site, which may reduce the disturbance envelope

needed (VanGeem 2010). But, most importantly, a concrete truck with a boom pump can

deliver concrete from a distance, thereby avoiding soil compaction around structures or

in sensitive areas (ibid). Lastly, “self-compacting concrete” is fluid enough that it permits

large areas of concrete to be poured from a single point (ibid). If possible for the designer

to specify for a job, methods such as these are fairly simple and can have a large impact on

reducing a site’s soil compaction .

Compacted Soil

Compaction rates caused by equipment and foot traffic during construction are

strongly affected by a soil’s moisture content. Wet soils can become compacted more easily

than dry soils for a couple of reasons. One is that moisture present in the voids between

the soil particles makes it easier for the particles to slide past one another (University of

Minnesota 2001). Secondly, soils such as clay and silts that consist of small diameter par-

ticles with much surface area emit strong energy forces which hold water by capillary bond-

ing within a soil’s profile; such soils stay moist longer than soils such as sands which have

less surface area due to their large particle size (ibid). Construction during dry times of the

year should therefore be carried out whenever possible to reduce soil compaction (Figure 9

illustrates various compaction depths and how soil moisture content affects these depths).

29

Doing so will also decrease erosion and sedimen-

tation from stormwater flow off the project site

(Hinman 2005).

Compaction depths created from con-

struction equipment are more complex than per-

haps often thought. First, one must look at the

depth levels and causes of these compactions.

Within a soil’s profile, there are three levels in which soil compaction is studied: the top-

soil, upper subsoil, and lower subsoil. Topsoil compaction is determined by ground contact

pressure only (such as from tires), compaction to the upper subsoil is caused by both ground

contact pressure and axle load, and the lower subsoil becomes compacted from axle loads

only (Sjoerd Duiker 2004).1

When heavy machinery must be taken onto soil, there are best management prac-

tices to be followed that will reduce compaction in all three soil levels. First, machinery tire

pressures should never be overinflated; tire inflation should be the correct psi for the tire

used; overinflating will result in higher soil compaction (Duiker 2004). The following ma-

chinery characteristics will also reduce soil compaction: “ultra-flexible” tires that use less air

pressure than normal, flotation tires (which have a wide footprint, and low inflation pres-

sure), double tires (they spread the load weight), tracks, and the use of wider than normal

Figure 9. A soil’s moisture content has a large im-pact on the deepness that compaction can reach in a soil profile (Frisby and Pfost 2010).

30

tires or tracks to spread pressure concentration loads (Figure 10) (Blake 2009; Duiker 2004).

In addition, 4-wheel drive machinery or front wheel assisted vehicles decrease soil compac-

tion because the machinery’s movement force is more evenly distributed across the wheels

or tracks (Duiker 2004; DaBoll 2010). All of these discussed machinery characteristics might

be specified in a contract preamble or in a performance specification to result in a reduc-

tion in machinery’s compaction on soil during construction. However, these soil compaction

reduction methods would greatly increase the

rate of cost for a project and might prove to be

unaffordable.

Regardless of measures taken to reduce soil

compaction, the very nature of development

will result in some soil compaction. Fortunate-

ly, there are methods that can be employed

to repair compacted soil in all three of the soil compaction levels. The most easily allevi-

ated soil compaction is topsoil compaction. Tilling of the upper soil layer with a disc plow,

Figure 10. Compaction depth according to tire/tracks being used (Sjoerd Duiker 2004, p. 5).

1 Duiker, Sjoerd 2004, ‘Soil Compaction’, Penn State University, CAT UC 186, accessed

9 Ocotober 2010, <http://pubs.cas.psu.edu/freepubs/pdfs/uc186.pdf>. (this document of-

fers information on how to determine machinery’s specific axle loads and tire pressure psi)

31

chisel plow, or subsoiler will loosen this layer and allow for water and plant root penetration

(Roa-Espinosa 2010; Multiquip 2010). However, chisel plowing may result in greater water

infiltration and lower soil bulk density measurements in comparison to disk plowing (Caplin,

Minn and Pulley 2008). If a site’s compaction has reached the subsoil layer, chisel plows can

also be used to rip to a depth of 12 inches. A subsoiler (such as that in Figure 11) is needed

to fracture deeply compacted soils in lower subsoil layers, though. However, even with such

a massive machine, compacted clay

soil can be very difficult to rip; there-

fore, compaction should be espe-

cially avoided whenever possible on

these soils (USDA 2008). It should be

noted here that the very act of soil

tillage will result in a certain level of

soil compaction, especially if the soil is moist. Tillage, even subsoiling, will be ineffective on

wet soils; the machine’s weight will compact the wet earth and the tines will compress the

soil particles together as the machinery rips through the soil profile (University of Minnesota

2001). To avoid this, soil should be dry enough that it “shatters” when tines rip through it.

After tilling has been completed, compacting forces must be thereafter especially avoided,

as the soil will have a very low bearing capacity due to the large void spaces created from

Figure 11. A subsoiler’s tines (USDA 2008).

32

all forms of tilling (ibid). It should be noted that subsoiling should only be used where tree

roots are not present, as damage will be caused by the tines at both the deep and shallow

depths they reach. Lastly, keep in mind that tilling is only partially effective for alleviating

subsoil compaction and can be an expensive process (Duiker, 2004). Natural processes such

as water percolation, freeze/thaw actions, root growth, insect action, reptiles, amphibians,

and certain mammals all create further soil aeration and are an important part of creating

porous soils. A naturally well-draining soil is always preferable to a compacted and then

ripped soil; ripping cannot completely return a soil’s infiltration rate to what it was prior to

soil compaction, as the tilling tines rip only a certain percentage of the soil, leaving the rest

still compacted.

Applying a thin layer of sand following tilling and/or subsoiling can help increase fu-

ture drainage (Tyner et al 2009). Also, incorporation of compost can also be used to reduce

soil compaction. 2 Biologically active compost not only increases a soil’s porosity and mois-

ture holding ability, but pollutants and nutrients found in stormwater can be broken down

by microbes living in this compost. To avoid nutrient burning or nutrient binding, all incor-

porated compost must be mature compost. Nutrients from immature compost would not

be immediately available, as microbial activity from early stages of decomposition have the

potential to bind and withhold much-needed nutrients from plants.

33

Preventing Erosion

Preventing soil erosion, nutrient washing, and sedimentation onto and off a devel-

oping site is pertinent for a site’s ecological health and for protecting nearby waterbodies.

Currently, most localities require installation of silt fencing. Unfortunately, these often fail

due to poor knowledge of the installation requirements and to the very nature of sediment

fencing. Compost berms or compost blankets are increasingly gaining in popularity over

silt fences as the benefits of compost-based erosion prevention methods become more

understood and researched (Oregon State DEQ 2001). In fact, the Oregon Department

of Environmental Quality recommends the use of compost berms over silt fencing for the

reduction of total solids, phosphorous, and heavy metals (except zinc) (Oregon State DEQ

2001). Compost berms are typically about 1 foot tall and about 2 feet wide and should be

located to coincide with where other sediment control measures (such as silt fencing or

straw bales) would be placed (ibid). To prevent blowouts of compost berms, a site’s storm-

water flow velocity must be kept low, and the stormwater cannot be permitted to rise taller

2 Hinman, Curtis 2005, ‘Low Impact Development Technical Guidance Manual for

Puget Sound’, Washington State University Pierce County Extension, p. 92 (Source for de-

tailed information on determining specific rates of organic matter needed (and also deter-

mines eventual soil settlement after tillage).

34

than the berm (ibid). Alternatively, compost blankets are a continuous layer of compost

applied by shovel or blown in a dry state across a site where soil has been left bare and

exposed. Compost blankets perform very well on steep slopes (even on slopes as steep as

1:1.4) and should therefore be considered over compost berms in these situations (ibid). As

slope percentages increase, so also should a compost blanket’s depth. Netting may also be

required for very steep slopes to aid in preventing washing of portions of the blankets (ibid;

U.S. Environmental Protection Agency 2010). To avoid blowouts and rilling, stormwater

must flow as sheetflow toward berms or over blankets; sites with concentrated flows are

not suitable for either erosion method (Oregon State DEQ 2001). Temporary check dams

can perhaps be applied to slow channel flow to permit greater usage of compost methods

in these situations. The massive potential for sediment reduction with compost berms and

blankets as stormwater controls was illustrated in a test study reported by Oregon State

DEQ. It was found that, on a thirty-four percent slope with compost berms, total solids were

reduced by 83%, and compost blankets reduced total solids by 99.94% (ibid). Alternatively,

siltation fencing only reduced sediment loss by 39% (ibid). Encouragingly, nutrient leaching

from these berms or blankets has not been shown to be a large problem. In fact, nutrients

leaching from the compost have been reported to actually be less than nutrients leaching

from bare topsoil in areas without compost berms or blankets (U.S. Environmental Protec-

tion Agency 2010). Preventing site erosion through utilizing compost berms and compost

35

blankets can therefore greatly aid in protecting surrounding waterbodies and prevent sedi-

mentation loss from on-site. Unlike current siltation fencing, compost does not requiring

removal; the compost can be left or spread out on the site (as in the case of compost berms)

at a later date. The products then provide organic matter to the site’s soils.

Chapter 3 - Hardscapes

Stormwater Reduction as it Pertains to Impermeable Surfaces

The impermeability and soil compaction generated by road installation alone in

developments is a huge stormwater generator. Indeed, streets and driveways combine to

form the largest source of impervious surfaces in our urban landscapes, followed by build-

ings and parking lots (Kloster, Leybold, and Wilson 2002). The many miles of impermeable

pavements that make up our parking lots, sidewalks, driveways, and streets lead to water

degradation, erosion, sedimentation, flooding, and fish kills due to this water usually being

piped straight to waterbodies and not being filtered by soil and vegetation (Bean et al 2007).

Indeed, in 1996, the United States Environmental Protection Agency found that forty-six

percent of identified cases of estuarine water quality impairment were due to stormwater

runoff; in 2000 they reported that stormwater runoff was among the top three sources of

waterway pollution (ibid).

36

Look past a site’s surface hydrology

for a moment and consider its sub-surface

flows, as discussed earlier in this paper.

Impermeable surfaces have the potential

to greatly damage these flows. Consider

for a moment a sponge. In Figure 12, this

sponge (i.e. the “soil”) is in a natural state

and holds much water from a rain event be-

cause there is a great deal of space within the soil profile in which water can be held. When

this soil reaches its saturation point, the excess water will percolate slowly into groundwater

and sub-surface water flows (Figure 12). However, consider the alternative. In Figure 13,

only half this sponge is available for holding

water because half its storage ability has been

removed due to impervious cover from roads,

buildings, and soil compaction. Due to the

decreased pore volume for holding water, the

half of the water that sheds off impermeable

surfaces flows off the soil surface as stormwa-

ter, or (if handled with bioretention methods)

Figure 13. When soil’s surface is partially covered with impermeable surfaces, there results both increased runoff and groundwater flows (image by Sarah DaBoll Geurtz).

Figure 12. When undeveloped, a large percentage of a soil’s profile is available for water retention and purifica-tion (image by Sarah DaBoll Geurtz).

37

as subsurface water flows. In the later case, subsurface flows could be expected to increase

from what would be naturally occurring for the site. Increased stormwater surface and sub-

surface flows due to development can result in flooding and are therefore of great concern

to developers, ecologists, designers, and municipalities (Reese 2001).

There are a number of design methods that can be utilized to reduce impervious

surface cover. Homes can be clustered and brought closer to the street to reduce the length

of streets and driveways. Also, streets and driveways can be designed with narrower widths

and/or can be constructed of porous materials (San Mateo 2007). These design elements

can have a large effect on the percentage of compacted and degraded soil within a devel-

opment. Additionally, these methods will also increase a soil’s ability to manage larger

volumes of water runoff through providing more exposed soil for infiltration and water

holding. Parking structures also hold potential for decreasing impermeable soil cover, but

are expensive to build. Another available option is mechanized parking lifts which can fit

double or triple the vehicles than that of an open parking lot (ibid). However, soils with high

water table or poor drainage might be incompatible for installation of either underground or

above-ground garages. Likewise, low soil structural stability or karst bedrock might preclude

a parking structure due to weakness of bedrock material. All of these methods should be

studied as possible ways for reducing impermeable covers in order to increase soil’s water

infiltration on a site being developed.

38

Roads, parking, and sidewalks are a necessity; thankfully, there are numerous po-

rous surface types available that hold great potential for aiding in treating and preventing

the stormwater issues created from impermeable surfaces. The five basic porous pavement

systems are: plastic geocells filled with soil and turf or gravel; open-celled paving block

systems; open jointed paver systems; porous concrete; and, porous asphalt. All five of these

basic systems permit water infiltration through incorporation of an aggregate sub-grade

topped with a porous surface material.

Plastic Geocell Systems

Plastic Geocell systems have been in use in America since 1977, when they were

first used to allow water drainage while also allowing the growth of turf (Ferguson 2005).

The mode of action of this system is that a vehicle’s weight rests upon the grid network,

therefore not compacting the underlying gravel and soil beneath (Figure 14). This allows

water to infiltrate instead of washing across an impermeable surface and into stormwater

pipes. Installation of geocell systems

is fairly easy, and if needed, future

removal is also relatively easily car-

ried out because the cells can simply

be pulled out of the ground. There

are two basic forms in which geocells

Figure 14. Note the optional geotextile layer. If used, it helps to keep fine particles from settling into the bedding material (Boddingtons Ltd 2010).

39

are manufactured: square panels or rolls, depending upon the manufacturer and specific

type of geocell system being purchased. Both of these forms work through load weight

being spread across the grid network and along its supportive aggregate base material; this

results in a system that works together as a unit (ibid). There are also two materials with

which the cells can be filled which results in very different appearances and maintenance re-

quirements. The most maintenance-free version involves the geocell voids being filled with

gravel. Alternatively, the other version is filled with soil and planted with turf; this results in

a “field” of turf if correct maintenance is maintained.

Besides the water infiltration benefit of geocells, an additional benefit is that geo-

cells may be a good choice for stabilizing remote bike and all terrain vehicle trails, as well

as remote trails for foot traffic (Ferguson 2005). In situations such as these, the geocells

do not have to be filled with soil or gravel; this makes installation relatively easy in remote

areas where transporting aggregate and/or soil may be difficult or impossible (ibid). Other

places where its use is sometimes promising is on beaches, and areas prone to erosion.

When used on sandy beaches, the geocell matrix provides for a more stable surface than

that of loose sand. This characteristic can be utilized to provide access trails and portions

of beaches for people in wheelchairs and for those who have difficulty walking (ibid). At

the same time, the geocells retain the appearance and experience of a beach. Additionally,

steep slopes and bioswales which will experience high velocity flows can be greatly benefit-

40

ted by geocell installation to prevent soil erosion. The relative ease of installation and the

opportunities for grass coverage and soil stability result in this system being very appealing

in many situations.

There is no standard soil profile requirement for Geocell systems; the specifica-

tions depend upon the make and model of the geocells being installed. However, a typical

installed profile consists of a firm aggregate sub-base that absorbs weight loads while also

facilitating drainage. This is followed with an upper coarse of smaller aggregate to form a

more uniform surface for setting the geocells. Once the geocells are placed on top of this

aggregate layer, metal or plastic pins are installed to hold the cells together and in contact

with the ground; this prevents the cell panels from shifting and popping upwards (Ferguson

2005). A seeded soil mixture, or gravel, is filled into these cell holes. If being filled with

soil, the soil must be watered or vibrated into the cells; if gravel is used, a vibrator plate or a

roller should also be used (ibid). Gravel-filled geocells should be filled with angular gravel,

or the gravel will easily become displaced by vehicular and foot traffic (ibid).

The shallow soil profile of the geocell system poses a problem for turf growth. This

profile essentially required the turf to grown in plastic pots only about two inches deep and

about the same in width. This is asking a lot. Additionally, because the sub-base is com-

pacted and topped with gravel, the sub-base becomes a very uninhabitable root zone. How-

ever, some planting medium can be incorporated into the gravel sub-base to make the soil

41

profile more easily permeable to root growth (Ferguson 2005). In these instances, geotex-

tiles, while sometimes specified over the gravel sub-base to prevent soil fines from clogging

the gravel sub-base, must be omitted to allow root growth to enter the gravel (ibid).

Potential problems with geocell

systems that are not uncommon involve

upheaval, shifting of the panels, and

filling of the cells with sediment. In a

study by Brattebo & Booth (2003), shift-

ing of the cells was found after only six

years. An example of upheaval and shifting can be seen in Figure 15. This photograph was

taken in a medium foot traffic area, with occasional golf cart traffic. Also, thatch buildup

and sediment deposition from adjacent soil will eventually raise the soil layer within the

geocell voids. When this occurs, the soil can become partially compacted because vehicle

tires drive and park upon the soil instead of the ribs of the cells. Also, the rise of soil height

exposes the turf to crushing, resulting in

turf death, or in the very least, sparse and

thin turf cover (see Figure 16). Because

of this problem, there are a number of

recommended methods of preventing and

Figure 15. A buckling geocell system in Fayetteville, Arkan-sas

Figure 16 Soil buildup that results in soil compaction and subsequent turf death is a problem in geocell sys-tems.

42

addressing this problem: removal of clippings after mowing, addition of beneficial bacteria

to aid in breaking down accumulated thatch, dislodgement of thatch with spring tine equip-

ment, using sod removal equipment, or actually burning off the thatch layer (Invisible Struc-

tures 2010). However, none of these methods addresses the problem of actual soil filling of

the voids. The above methods only address methods for slowing the cell-filling process and

in removing thatch so soil accumulation can be slowed.

Open-Celled Paving Block Systems

A different yet very similar porous pavement system is called “open-celled paving

block systems”. They have been around since 1961, when they were first used (planted with

turf), for parking lots in Germany (Ferguson 2005). The material used for these systems is

most often composed of concrete or brick that contains large spaces for water infiltration

(ibid). The grid network created with these “pavers” creates a wider “rib” on which vehicles

drive that is usually apparent even with grass coverage. Even though there are a number of

different patterns available in open-celled paving blocks, the stark exposure of the concrete

they are composed of may appear to some as unattractive.

Like its geocell counterpart, open-celled pavers are installed on an aggregate

sub-base for stability and water retention, and the voids can be filled with either gravel or

soil. As seen in figure 17, the pavers are placed upon a profile of compacted, coarse gravel

topped with finer gravel which acts as a leveler for the paving units (Lampus 2010; Ferguson

43

2005). The pavers are then installed

on top of these layers and the paver’s

grid voids are filled with topsoil and

seeded with grass, or filled with

gravel only (Lampus 2010). Turf can

be established from seed or plugs.

When soils are expected to drain

poorly, installation of an underdrain may be required to carry excess infiltrated stormwater

away from the site. To prevent shifting and buckling of pavers, installation of some form

of edge restraint must be used along edges where vehicles travel or where there is a slope

(Ferguson 2005). If gravel is used to fill the voids, the same compaction steps should be fol-

lowed as those outlined for geocell systems. As mentioned earlier, use of gravel in place of

soil and turf will greatly reduce the amount of maintenance required, but the environmen-

tal benefits of stormwater contact with an upper soil profile and vegetation would be lost.

Also, moss and weeds would eventually grow in the gravel, as they would in geocell gravel

systems; this would result in an unkept appearance unless herbicidal controls were carried

out (ibid). If turf will be grown, the final soil level should be about half to a quarter-inch

lower than the top of the pavers to allow for water absorption and to prevent soil compac-

tion (ibid). Lastly, soil and turf and also gravel might pose pedestrian problems for people

Figure 17. Open-celled paving block system. Note the need for soil compaction for eight inches at the perimeter of Open-Celled Paving Block Systems in order to provide support to the outer blocks (Interlocking Concrete Paver Institute 2010).

COMPACTED TO 8" (200 MM) MIN DEPTH1/2" TO 1 1/2" (13-40 MM) WASHED AGGREGATE

1/16" TO 3/8" (2-10 MM) WASHED AGGREGATE

SIDES OF BASECOVER BOTTOM, AND

ICPI-29

8"(200 MM)

MIN

CONCRETE GRID PAVERS FOR

IN OPENINGS AND 3" (75 MM) THICK UNDER GRIDS

GEOTEXTILE

COMPACTED SUBGRADE

CONCRETE GRID PAVERS CONCRETE PAVERS 3 1/8" (80 MM) MIN THICKNESS

COMPACTED SOIL AT PERIMETER

F.S.

DRAWING NO.

SCALESTORM WATER RUN-OFF CONTROL

NOTES:1. DEPTH OF OPEN GRADED BASE WILL EFFECT RUNOFF STORAGE CAPACITY2. BEARING CAPACITY AND PERMEABILITY OF SOIL SUBGRADE SHOULD BE EVALUATED FOR

SUITABILITY WITH APPLICATION, TRAFFIC & STORM WATER STORAGE CRITERIA.DRAINAGE OF SUBGRADE MAY BE REQUIRED.

44

wearing heels or who walk with the assistance of a cane or walker. Lastly, the same prob-

lems with thatch and soil buildup are of concern as in geocell systems.

A great benefit of this and other porous block pavement systems is that, if CU struc-

tural soil is used, trees can grow relatively well with their roots surrounded with these

systems (Ferguson 2005). However, it is possible that root growth may cause bulging of the

pavement systems. To help alleviate this possibility, a layer of aggregate can be installed

above the structural soil directly beneath the finer settling course of the pavers to aid in

preventing root bulging of the pavement (ibid). The lack of nutrients in this plain aggregate

can prevent roots from reaching into and pushing through this layer. Projects dating from

the early 1980s give encouragement that the use of structural soils as an aggregate base can

encourage the growth of tree roots while providing enough weight that the tree roots will

not cause heaving (ibid). It must be kept in mind that the moisture holding capabilities of

structural soil are different from that of a straight aggregate base and must therefore be cal-

culated differently. A positive result of encouraging tree root growth beneath pavement by

using CU structural soil is that a greater amount of tree roots should result in greater water

uptake rates. At the same time, these trees would be healthier, larger, and longer-lived.

45

Open-Jointed Paver Systems

Another porous system, pavers, has been used for centuries to eliminate streets

and sidewalks from muddiness and potholes. However, it was the damage that occurred to

historic areas of Europe during World War II that led to the creation of the first concrete pav-

ers which were uniform, durable, and would eventually become available in many different

shapes, sizes, and colors (Ferguson 2005). These concrete pavers were created to replicate

the character of Europe’s damaged historic brick streets (ibid). Open Jointed Pavers are

simply pavers which are typically composed of concrete or clay. They are held slightly apart

with protrusions from the sides of the pavers or simply by aggregate held in the void spaces

between the paving units to permit water infiltration (Figure 18). The edges of each paver

are often chamfered to prevent chipping of the surfaces, though some pavers are manufac-

tured to have a “worn” look whereby their edges have been mechanically “distressed” (ibid).

They not only come in numerous styles

and colors, but some companies have

the ability to make custom pavers with

specific patterns and etchings. Besides

the benefits of water infiltration and

nutrient/pollutant capture from water

seeping between the paver spaces, this

Figure 18. Open-jointed paver system. Note the 12” length of geotextile installed at the curb to avoid large amounts of water entering this larger 1” gap (Interlocking Concrete Paver Institute 2010).

46

system is easier to remove in comparison to asphalt or concrete for fixing of underground

utilities and for repairing parts of the paver’s surface (ibid). Figure 18 is a detail graphic

from the Interlocking Concrete Paver Institute which illustrates a product profile with a thin

sub-base layer meant to handle driveway traffic. Heavier and more frequent traffic would

make a thicker sub-base necessary than that illustrated in this figure. A bedding layer for

leveling of the blocks is installed over the sub-base aggregate reservoir. A requirement for

these paver systems is that there must be an edge restraint to prevent horizontal moving of

the pavers and bedding material (Ferguson 2005). If edge restraints are not installed or if

they fail, the surface’s blocks will move, separate, and surface settlement will occur (ibid).

Porous pavers may be designed to provide more structural stability at the expense of

water infiltration or vise-versa (Ferguson 2005). However, water infiltration rates are partly

affected by the type of void material and size of the void openings. Typically, paver units are

installed with openings between the pavers that comprise from five to fifteen percent of the

paver surface area and are filled with sand or aggregate for water infiltration (United States

Environmental Protection Agency d. 2010). Highly structural pavers may have a narrower

void width and deeper sub-base depth than pavers of lower structural ability. The wider

the joints, the larger and more permeable aggregates can be used, such as small gravel.

When, after time, the void material (either aggregate or sand) becomes clogged from fine

particulate matter or experiences crusting of its surface, a vacuum sweeper may be used to

47

increase water infiltration (Ferguson 2005). A clean and dry aggregate should be replaced

after this procedure. Due to the possibility of this aggregate having had accumulated toxici-

ties, the removed particulate matter should be disposed of properly.

Open-jointed paver systems give a stark appearance in comparison to that of turf

systems, but they offer a traditional appearance and character that turf cannot. Addition-

ally, pavers do not require routine turf maintenance and are more durable than geocells

or open-celled systems. They may therefore appear more pleasing to the public due to an

appearance of better quality and upkeep (Brattebo & Booth 2003). However, the sub and

base courses must be installed well or settling will be a problem. Also, loose aggregate

might pose a problem with walking if the voids between the pavers are large and the gravel

has a tendency to pop out. This would pose a maintenance issue. Lastly, pavers can have a

very long lifespan (Ferguson 2005). Even after the sub-base needs replacement or if a por-

tion must be removed, pavers can be removed, set aside, and reused.

48

Porous Concrete

A durable and quickly installed system, porous concrete made its appearance as an

environmental product in the 1970s but had been used for some road applications since

World War II (Ferguson 2005). This concrete product looks like very rough concrete because

it contains much void space through a reduced quantity of fines (such as sand). This allows

water percolation through the pavement and into the sub-grade material. The voids created

leave an average void space of around twenty percent or more than that found in traditional

impermeable concrete (United States Environmental Protection Agency b. 2010). The base

thickness of porous concrete systems is a minimum of six inches thick, with increased thick-

ness required for pavements that will have higher traffic loading, are on a weaker subgrade,

or that need greater water holding capacity (see Figure 19) ( United States Environmental

Protection Agency & National Pollution Discharge Elimination System b. 2010; Ferguson

2005). The sub-base should be extended out from the edge of where the concrete will be

poured in order to prevent edge cracking from vehicles driving over or near the porous

Figure 19. Like many porous systems, porous concrete requires a relatively thick profile in order to hold infiltrated water (United States Environmental Protection Agency d, 2010)

49

concrete edge (Ferguson 2005). Porous concrete must not be troweled or floated because

doing so might seal the surface pores. Instead, a steam roller should be used to compress

the porous concrete mixture before it sets up (ibid).

There are some perhaps little-considered aspects of porous concrete that should

be understood. First, porous concrete may get as hot as asphalt in the sun as a result of

the void spaces. The reason behind this is that porous concrete is designed to allow water

through it and so does not hold water which otherwise would result in a high rate of radiant

cooling of the concrete’s surface (Ferguson 2005). However,this rough surface created by

the absence of fines results in sound absorption from vehicular traffic and could therefore

be excellent for reducing street noise (ibid). There are cases where this surface could be a

slight problem, though; the roughness is unwanted in ball courts and in areas where shoes

aren’t worn such as around pools (ibid). But, in these cases, the concrete can be ground

down and the resulting dust vacuumed from the concrete’s pores (Ferguson 2005). If this

dust is not removed, it would cause clogging of the void spaces. Because these void spaces

result in porous concrete being weaker than traditional concrete, porous concrete might not

be a viable replacement under certain circumstances such as in areas with high freeze-thaw

actions (Ferguson 2005). Also, winter treatments with sand or gravel chips should not be

applied because these materials would clog the poor spaces. Regular maintenance must be

maintained on porous concrete surfaces to preserve the pore spaces. However, the highly

50

permeable nature of porous concrete, and the generally quicker and cheaper installation of

it over some of the other porous systems makes porous concrete a popular pavement op-

tion.

Porous Asphalt

A similar paving system to porous concrete is porous asphalt. This material has been

used to improve drainage and safety since the 1950s as an overlay on highways and air-

plane airfields (Ferguson 2005). However, it wasn’t until 1968 that it was seriously studied

as a stormwater control measure (ibid). Such as with porous concrete, the voids that allow

water infiltration are created through reducing the percentage of fines in the material’s

mixture. This creating of void space combined with the flexible nature of asphalt permits

its usage in areas with heavy freeze-thaw cycles (ibid). It requires installation of a sub-

base topped with a porous asphalt mixture. The thickness of the asphalt varies due to the

amount of weight the asphalt is engineered to handle, the existing soil’s strength, and the

required water holding capacity of the subgrade.

Figure 20. Porous asphalt system - note the similarities of this system to that of the porous con-crete profile (United States Environmental Protection Agency d, 2010)

51

A technical problem with porous asphalt called “drain down” poses a serious prob-

lem with porous asphalt’s use. Drain down is the occurrence of asphalt’s binder flowing

downwards within the asphalt mixture, resulting in clogging of void spaces (Ferguson 2005).

About a half inch below the top surface, this binder has been found to settle; sediment then

gathers and sticks to this layer, further blocking the void’s drainage capabilities (ibid). This

prevents water penetration and lessens the pavement’s structural stability. In addition, the

upper aggregate that losses its binder layer then gets knocked loose (ibid). It has also been

discovered that separation of the materials in porous asphalt mixtures occurs in the truck

during transport to the job site and results in an uneven mixture being laid (ibid). However,

research is being conducted to determine the extent of this problem, and new porous pave-

ment mixtures are being invented in an attempt to eliminate drain-down (ibid).

Installation of Porous Concrete and Porous Asphalt Systems

Due to a general lack of knowledge of installation of both porous concrete and as-

phalt systems, installers must be specifically trained in these systems. Laying these porous

systems is more time-intensive than are traditional asphalt or concrete. This is due to the

mixtures being thicker because less water is used in these blends; this causes the mix to be

stiffer, hence more difficult to handle (United States Environmental Protection Agency &

National Pollution Discharge Pollution Elimination System b. 2010). Also, the porous mixes

must be poured within one hour of mixing (ibid). This could present issues with installers

52

and suppliers; getting the materials mixed and shipped, then poured and readied for curing

takes skill and planning (ibid). Due to the differences in installation and/or the increased

installation time, the cost of installation can often be expected to be greater than would

normally be anticipated for traditional impermeable concrete or asphalt.

Porous Pavement Systems Compared

Porous pavements provide valuable systems for nutrient and contaminant capture

and are pertinent for reducing pollution and water runoff. This is especially true as human

densities increase and flooding become more common. Concerning toxicity capture, a study

was conducted that found toxic concentrations of copper and zinc in ninety-seven percent of

water samples from impermeable asphalt (Brattebo and Booth 2003). Alternatively, seven-

ty-two percent of the water tests for copper, and twenty-two percent of the water tests for

zinc in the infiltrated water samples from porous pavements tested below minimum detec-

tion limits (ibid). Research findings such as this make porous systems very encouraging.

However, designers must understand potential drawbacks of these systems, such as existing

soil conditions and system limitations, in order to use porous pavements successfully.

In addition to infiltration rates, the designer should study how the different porous

pavement systems vary in their ability to capture toxicities from stormwater. An example

of this is that open jointed pavers have been found to be less beneficial in removing heavy

metals than geocells, open celled pavers, and porous concrete (Brattebo & Booth 2003).

53

The biological activity and chemistry of soil may be the reason for this difference. Because

soil adsorbs minerals and nutrients to its surface, elements have a reduced potential of

leaching into groundwater and water bodies. Therefore, the higher soil contact a system

provides, a higher rate of nutrient capture might exist. Likewise, a system such as porous

pavement offers more surface area for stormwater to make contact with than do open-

jointed pavers. This presents a separate issue of a potential for more pollution adsorption.

While there is concern of toxic contaminant levels being held in these upper profiles (to be

discussed in more detail later in this paper), porous systems have the capability of keeping

these toxicities from traveling unhindered to waterways where they pose serious problems

to water sources and aquatic life. It would likely be simpler and cheaper to address an envi-

ronmental threat in a permeable system than in a massive body of water.

An additionally important and often unknown element of permeable systems is the

effect water hardness has on stormwater’s toxicity levels. Copper, lead, and zinc all become

less toxic as water hardness increases (Lenntech 2009). Water hardness is caused by the

amount of either calcium or magnesium in water (Adhikari et al 1999). Encouragingly, an in-

crease in the calcium content of water enables a human’s biologic systems to handle greater

levels of certain toxic metals (Brattebo & Booth 2003). Toxicity to fish, as well, lessens when

water is hardened with calcium (Adhikari 1999). It is important to note that when fish are

introduced to water hardened with the other water hardener, magnesium, the fishes’ ability

54

to handle toxic metal levels is not increased and high fish death is observed (ibid). Porous

pavements that contain limestone in the paving product or sub-base contain an existing

calcium water hardener, as do limestone-based soils. All of this illustrates the great impor-

tance that calcium in our soils and in our choice for porous pavements can have on heavy

metal toxicity levels. It suggests that designers should perhaps consider porous pavement

choice and limestone aggregate depth with consideration and knowledge of this toxicity-

lowering affect of calcium.

The nature of porous pavements – permeability – depends strongly upon the under-

lying soil’s characteristics and the porous pavement being used. Soils well suited to handle

infiltration from porous systems are sandy or sandy loams, and soils without seasonally high

water tables. Therefore, addressing the problem of poorly draining clay soils is important in

areas that contain soils of the C and D hydrologic classifications. However, being aware of

the permeability potentials of the various porous pavement systems is equally important.

Knowledge of existing soils should be combined with the infiltration potentials for the dif-

ferent porous pavements. A soil with a poor infiltration rate might not benefit from a highly

permeable pavement if the underlying soil cannot hold the volume of stormwater flowing

into it. Likewise, in situations such as this, the soil beneath the pavement might need to

be amended or otherwise treated in order to permit stormwater infiltration. Porous pave-

ment that contains larger aggregate in the void spaces and larger void openings generally

55

possesses greater water infiltration rates than their counterparts; however, other factors

such as the soil infiltration rates lying beneath these systems must also be considered. As an

illustration of this, a study conducted in 2003 by Brattebo & Booth tested a variety of porous

systems for water infiltration on well-draining soils; the results were that during rain events,

plastic grid systems with turf failed on five occasions while open-celled pavers and open-

jointed pavers did not. Other issues such as a high water table level or high bedrock may

prevent the use of any type of porous system due to lack of space for the aggregate sub-

base required. In these situations, a traditional paving system may need to be installed. It is

easy to see how a design for a porous system can become quite complicated once a designer

begins looking at a site’s specific qualities compared to those of the various porous systems.

Clay soils present special design and handling requirements over those of well-drain-

ing soils. First, when preparing clay soils for porous pavement installation, it is pertinent to

work in dry soil conditions and when rain is not expected. Machinery and foot traffic will

cause compaction of all soil types, but this compaction rate greatly increases when a soil is

wet. Additionally, clay’s bare surface during construction must be protected from rainfall,

as rain falling upon bare clay causes surface sealing and soil compaction, thereby lessen-

ing final infiltration rates (Tyner et al 2009). Secondly, as discussed earlier, clay soils benefit

greatly from ripping of the clay and an application of a thin layer of sand across and into the

ripped fissures (ibid). This creates veins of sand that, due to the closeness of the large sand

56

particles within this “vein”, holds the clay particles apart within the fissures, and permits

water flow. But, to even further increase a clay soil’s infiltration rate beneath porous pave-

ment, rows should be trenched and filled with aggregate; this has been found to substan-

tially increase drainage (ibid). To give an example of the importance of these techniques,

the following are test results that illustrate how underlying soil treatment can affect water

infiltration:

• 0.8 cm/day (clay control; surface sealing occurred; no alteration made to the clay soil)

• 10.0 cm/day (ripped and sand layer applied)

• 25.8 cm/day (trenched and filled with aggregate plot) (Tyner et al 2009)

This gives an idea of how physically altering a clay soil can improve clay’s infiltration rate and

improve a porous pavement’s success in these situations.

An additional key to successful stormwater design involves the load-bearing and

infiltration capacities of the sub-grade soil, the infiltration capacity, and the storage capacity

of the stone base/sub-base (Porous 2010). Clay soils have a tendency to hold water and be-

come weak as they absorb moisture; the moisture causes the soil’s strength to decrease as

the interstitial water pressure increases (Alfakih et al 1999). Therefore, a soil’s potential for

collapse should be considered when designing porous systems on these soils. The infiltra-

tion rate is affected by factors such as soil work impacts, plant-root effects, and clogging due

to fine particles in the soil’s and aggregate base’s void space (ibid). Also, hydraulic conduc-

57

tivity is affected by groundwater levels (ibid). A high hydraulic conductivity indicates that a

soil infiltrates water well, making installing porous pavements on these soils more successful

than in soils with low hydraulic conductivity, such as clay soils. In order to provide for high-

volume rain events and for instances when numerous rains occur (which can cause filling

of a porous pavement’s void spaces), it is recommended to design an adjacent bioswale to

receive runoff from porous pavement. This also provides for other possible failures such as

clogging of void spaces. In addition, a thicker than required aggregate depth, and possible

installation of perforated underground drain pipes to carry excess water away from the site

beneath permeable pavements may need to be considered. These issues relating to porous

pavements and potential problems cause some municipalities to be reluctant to encourage

porous systems in their cities.

Durability and life span of each of these systems are important considerations for

cities and developers alike. A cheaper system may prove to be a less viable option if faced

with a shorter life span than that of a more expensive but longer-lived material. Take per-

meable pavers: they can be expected to perform for twenty to twenty-five years (Smith

2006). However, eventual failure is usually due to the paver’s sub-base ability to store water

runoff. If the sub-base no longer stores the required amount of water due to clogging,

the pavers must be removed and the base removed and replaced (ibid). This operation is

expensive. However, the same pavers may be reinstalled, unlike concrete which can only be

58

repaired until it is eventually jack-hammered up. However, while porous concrete can be

engineered to handle heavy vehicle loading, surface abrasion from constant traffic on the

coarser surfaces may cause more rapid deterioration of porous concrete than would occur

with traditional concrete (United States Environmental Protection Agency & National Pollu-

tion Discharge Pollution Elimination System b. 2010).

Regular upkeep absolutely must also be maintained on porous systems in order for

them to continue to perform well. Regular sweeping or vacuuming throughout the mate-

rial’s lifespan is required for all five system types. Specifically, fine particles, if not kept

vacuumed out of porous systems, will clog porous pavement’s void spaces and lead to a

decrease in infiltration, resulting in an increase in water runoff. Sediment accumulation in

the top thirteen to eighteen millimeters in void spaces should be removed through regular

maintenance with a vacuum sweeper in order to sustain high surface infiltration rates (Bean

et al 2007). This maintenance should be carried out before fines have become compacted

into the void spaces or have had a chance to migrate to lower, more difficult to maintain

depths within the pavement sub-base (ibid). Another issue concerns porous pavement han-

dling in cold climates. Sand and rock chips cannot be used for traction on porous pavements

in these climates because these traction materials clog the pavement’s void spaces (Porous

Pavements 2010). Also, unlike in traditional parking lots, snow cannot be piled on top of po-

rous systems because high sediment concentration from piles of melting snow will result in

59

rapid void space clogging (Pervious Pavement 2010). Salt may be used, but its usage should

be reduced up to seventy-five percent, or an alternative chemical considered (ibid). Thank-

fully, porous systems do not require as much de-icing as traditional impermeable pavements

because as snow and ice melts, this water percolates through the porous systems instead of

sitting and re-freezing on top.

Future care for the turf systems in particular may be plagued with issues. When

growing conditions are poor, sparse turf coverage can be experienced; this would result in

a reduced amount of water absorption by the turf, muddy cells, and possible sediment loss.

Even on well-draining soil, spotty and sparse turf cover can be found due to poor turf care,

drought, disease, shade, or vehicular chemical leaks (Brattebo & Booth 2003). However,

turf performance is largely related to care and maintenance and therefore results can vary

greatly. Proper installation of these systems is also a strong determining factor in the suc-

cess and future appearance of turf systems. A turf-planted grid system must be maintained

as would a lawn – with watering, fertilization, weed control, trash removal, and mowing. If

any of these components is missing, the turf system may become an eye sore for the public.

Also, the genus and species of turf must be carefully considered. Some turf species require

more water than others, while some handle drought better; others some prefer shade, sun,

or partial sun and shade. As an example, in heavily used parking lots, shading of the grass

from vehicles and trees may likely create a problem. Turf species that can tolerate shade

60

are species that require more moisture; likewise, species that are drought tolerant gener-

ally require more sunlight. Therefore, a site’s specific conditions must be taken into account

when a designer chooses the type of turf to be seeded in turf systems, and careful consider-

ation of each system is imperative in order to choose the best system for any given situation.

Knowledge of all these facets of maintenance will enable a designer to properly specify

components and maintenance requirements, thereby ideally resulting in longer-lived porous

systems that perform as originally intended.

Chapter 4 - Bioretention

Bioretention Introduction

While porous pavements are used to reduce stormwater flow off their own sur-

faces, bioretention stormwater management systems are used to address stormwater flows

before the water can flow off-site. These systems consist of various forms of bioretention,

stormwater conveyance systems, green street stormwater control designs, and methods of

holding water runoff. All of them enable designers to manage stormwater volumes close to

the area of origination, prevent water from flowing off site, decrease high peak flow rates/

volumes into water bodies, reduce/prevent stream bank erosion due to decreased flow ve-

locities, and utilize rich biological soil matrixes to capture and treat stormwater nutrient and

pollutant loads. At the same time, bioretention can provide biodiverse ecologies that also

provide aesthetically pleasing elements into a development.

61

Bioretention design begins with careful knowledge and understanding of a site’s spe-

cific conditions. Factors that will affect the applicability and success of bioretention methods

are: a site’s slope, soil classification, impervious/pervious ground layers, groundwater level,

and the existing soil’s hydrology patterns. A steep slope will result in low water infiltration

and possible soil instability, soils with high clay content or an impermeable soil layer will lim-

it water infiltration, and high bedrock or water table levels may preclude installation of dry

bioretention designs (San Mateo, 2007). All of these factors will be discussed in detail later

and will determine whether bioretention can be installed, the type of bioretention used, and

if the existing soil must be amended to insure water will be able to infiltrate.

Soil Chemistry

In order to understand the complex actions of soil particles upon pollutants in a

bioretention cell, the soil chemistry that causes water and nutrient holding capabilities

within soil should be understood. The most basic level of the soil-water interaction involves

cohesive and adsorptive forces. Cohesion (the attraction of water molecules to one another)

and adsorption (whereby water molecules and elements bind to soil particles) bind water,

nutrients, and pollutants in the soil profile. Both cohesion and adsorption forces are stron-

ger in soils of fine particle size such as clays and silts, and weaker in larger particle-sized soils

such as sand (Brady & Weil 1999). The small size of the finer soil grains, combined with the

resulting closeness of the particles, results in strong capillary action energy forces that hold

62

water tightly and act to pull and tightly hold water toward and within these soil matrixes.

This is why clay and silt soils hold water and sandy soils infiltrate water rapidly. These

forces contain such strong energy that when soil becomes saturated, its water pressure en-

ergy increases and the inter-soil pressure from this causes a soil’s strength to decrease. This

can result in the settlement, slumping, or sliding of soils; hence, municipalities often have

distance requirements from foundations, steep slopes, and hardscapes in order to provide

safe distances to prevent these actions from occurring.

The rate of a soil’s cation exchange capacity (CEC) and anion exchange capacity (AEC)

are important factors in the success of bioretention for removing stormwater contaminants.

A soil’s CEC is the total amount of cations (positively charged ions) that can be adsorbed by

soil particles, whereas AEC is the total amount of anions (negatively charged ions) that can

be adsorbed by soil particles. The higher the

CEC of a given soil, the greater the amount of

positively charged pollutants that can be held

in the soil profile instead of washed into a

downstream water body. Likewise, high rates

of AEC in a soil increase the soil’s adsorption

of negatively charged ions. Therefore, de-

pending upon the contaminants a designer is Figure 21. Example of cation and anions in a soil pro-file (image by Sarah DaBoll Geurtz).

63

trying to control, a soil mix could be altered to have a higher or lower CEC or AEC. However,

soils in the United States of America usually have various rates of CEC capability and rarely

have high AEC rates. Soils with a pH in the range of 6.5 to 8.5 perform CEC capture well,

while a pH of around 2 is required for a high AEC rate (Jurries 2003; Brady & Weil 1999).

This extremely low AEC pH requirement makes designing AEC bioretention perhaps unrea-

sonable. Fortunately, soils are commonly a mixture of various weathered minerals which

contain different charges. This gives most soils a mixture of both cations and anions where

the cations bind negatively charged molecules to their surfaces and the anions bind posi-

tively charged molecules to their surfaces (Figure 21).

This mixture of cations and anions binds differently charged elements to soil par-

ticle’s surfaces where the elements are held for plant uptake and microbial action. Jurries

(2003) reports that a soil’s recommended CEC rate is at least 15 milliequivalent/100 grams of

soil. If a tested soil falls below this level, the addition of organic matter (which is negatively

charged) will greatly improve a soil’s CEC rate, as organic matter’s CEC is from two hundred

to four hundred milliequivalent /100g of soil (ibid). Soils with a high CEC or AEC rating have

the potential for adsorbing high amounts of the elements found in Table 3. For an example

of just how beneficial at pollutant removal a bioretention cell can be, see Table 4 for the

turbidity, sedimentation, and nutrient capture of a bioswale as reported by Oregon State’s

Department of Environmental Quality (2003).

64

Addition of organic matter to

bioretention cells not only increases a

soil’s ability to adsorb minerals to soil

particle’s surfaces, but high organic

matter content creates areas where

microorganisms thrive. These organ-

isms consume nutrients and pollut-

ants as food and result in the breaking

down/consumption of these products.

Additionally, many pollutants enter

the soil in an inorganic form which cannot be utilized by plants; microbial action on inor-

ganic elemental forms can break down these elements into organic forms whereby plants

can then absorb and hold the nutrients on site (Jurries 2003). Because humus, also known

as compost, is a rich source of food for microbial life, it is therefore an important component

of bioretention design for increasing a soil’s microbial levels. Therefore, by incorporating

compost and planting vegetation thickly in bioswales (which will provide leaf litter as com-

post), both the CEC pollution adsorption and microbial rate of a soil is increased. A designer

should keep all of these basic soil actions in mind when designing bioretention facilities

because they will greatly affect the outcome and success of all such designs.

Table 3. Both cations and anions are adsorbed within most soil profiles. However, the rates of adsorption and elements adsorbed can vary greatly depending upon a soil’s composi-tion.

65

It must also be understood that the

very action of adsorption by soil particles,

compost, and mulch can result in elevated

levels of nutrients and pollutants in bioreten-

tion cell profiles (Brady & Weil 1999; Davis et al 2001). This poses a possible problem with

stormwater capture. Indeed, research has shown that the upper soil layers, mulch, and

vegetation in bioretention cells can contain high levels of contaminants; encouragingly, this

level has been found to decrease with increasing soil depth (Weis, et al 2008; Davis et al

2001). This decrease is a hopeful indicator that bioretention cells do a magnificent job at

protecting groundwater sources. However, it also indicates that periodic removal of the up-

per levels of soils (possibly only a matter of some centimeters of soil), mulch, and/or veg-

etation matter may be required. Indeed, municipalities sometimes recommend or require

periodic sediment removal (Weinstein et al 2010). However, removing soil involves much

work, and would include removal and replanting of any vegetation and would, in practice,

Table 4. A bioswale study, as reported by Oregon State Department of Environmental Quality (2003), found significant pollutant removal abilities from bioswales:

Results %

reduction Engineering Specifications Total Suspended Solids 83-92% at least 200 feet long

Turbidity (with 9 minutes of residence)

65%

maximum runoff velocity was 1 1/2 feet/second

Lead 67% water depth of 1-4 inches

Copper 46% grass height of at least 6 inches

Phosphorous 29-80% minimum contact time of 1 1/2 minutes

Aluminum 63%

Total Zinc 63% Dissolved Zinc 30% Oil/Grease 75% Nitrate 39-89%

Results %

reduction Engineering Specifications Total Suspended Solids 83-92% at least 200 feet long

Turbidity (with 9 minutes of residence)

65%

maximum runoff velocity was 1 1/2 feet/second

Lead 67% water depth of 1-4 inches

Copper 46% grass height of at least 6 inches

Phosphorous 29-80% minimum contact time of 1 1/2 minutes

Aluminum 63%

Total Zinc 63% Dissolved Zinc 30% Oil/Grease 75% Nitrate 39-89%

66

most likely not happen. Research has found that far more pollutants are captured and held

in mulch and vegetation than in soil in bioretention cells (Davis 2001). Therefore, retaining

a thick mulch and vegetation cover in bioretention cells not only has a greater potential for

trapping contaminants, but would make removal of contaminants a relatively non-invasive

measure by periodically removing mulch cover and leafy vegetation. A new layer of mulch

would then need to be re-applied. Specific regulating and testing would need to be carried

out to ensure that highly polluted soils were properly remediated. While this might suc-

cessfully address toxicity loading, the problem of siltation accumulation over time results in

reduced volume-holding ability for a bioretention cell and would require eventual re-grad-

ing. These methods would be so time-consuming that they most likely would not be carried

out. Therefore, areas where high pollution levels were suspected (such as around gasoline

stations) might be better candidates for these actions.

Bioretention Systems

Understanding all of these complex soil systems allows a design team to properly de-

velop bioretention cells to work with nature in infiltrating and filtering stormwater. Some of

the bioretention methods utilize these natural systems while others are designed to partially

bypass them. Consider the two main categories by which these different bioretention meth-

ods are characterized: indirect and direct infiltration (San Mateo 2007). Indirect infiltration

is the most commonly known and used method; it uses techniques such as vegetated swales

67

and buffer strips to enable water to percolate into subsurface soils. From there the water

will continue to percolate downward into subsurface water flows and into groundwater

(ibid). Bioretention designs are examples of these and will be discussed in detail in the fol-

lowing pages. Direct Infiltration involves the movement of water directly to subsurface soils,

bypassing surface soils (ibid). Examples of these are infiltration trenches and basins, and

dry well basins. While direct infiltration devices may be beneficially used to infiltrate large

amounts of stormwater quickly, care must be taken as bypassing the purification action of

the upper soil level leaves a high risk for groundwater contamination (ibid). Due to the risk

of groundwater pollution, low levels of microbial activity, and a decrease or lack of veg-

etated infrastructure, direct infiltration will not be further discussed in this paper. However,

information on some of these methods may be found in the San Mateo ‘Stormwater Techni-

cal Guidance Manuel’ (2007) produced by San Mateo’s Water Pollution Prevention Program.

Bioswales

Probably the most well-known and most commonly used type of stormwater biore-

tention method is the vegetated dry swale. These swales are planted thickly with vegetation

or turf and are shallow, open channels that convey water to a designated place. In short,

they are highly engineered ditches. However, a bioswale is designed to use vegetation cover

to slow stormwater’s velocity, trap sediment, infiltrate water, and to remove a percentage

of various nutrients and pollutants in stormwater. A dry bioswale does all of these things

68

within a short time frame so water is not left standing for long periods of time. A wet bio-

swale carries out the same results as a dry bioswale but retains water for longer periods of

time; soils of poor water drainage may be designed with this type of bioswale (King County

2009). There is not a set standard for bioswale design; required dimensions frequently dif-

fer between sources. Therefore, the design specifics given in this paper likely differ across

municipalities. They are, therefore, presented here as approximations based upon various

municipality requirements and recommendations.

There are four basic cross sectional shapes employed in bioswale design: rectangu-

lar, triangular, trapezoidal, and parabolic. Parabolic and trapezoidal shaped bioswales (Fig-

ure 22) are the most highly recommended, partly because of their ease of construction and

maintenance. Trapezoidal swales are in particularly a good design for bioswales, as the flat

bottom of this shape allows for greater water contact with the soil, greater sediment set-

tling, and longer periods of time for nutrients and water to be absorbed/adsorbed from the

stormwater (Jurries 2003). In trapezoidal bioswales, in order to provide for ease of mowing,

Figure 22 A trapezoidal swale (Jurries 2003 p. 14)

69

the minimum bottom dimension is recommended to be two feet wide. If planted with other

vegetation material, the bottom width is not so important, as a specific mowing width is not

needed. The side slopes should have a maximum slope of 3:1 for ease of mowing mainte-

nance and safety on a slope (Jurries 2003). However, if possible, specify side slopes to be

shallower for safety and increased water/soil contact.

When designing the longitudinal slope of a bioswale, designers must keep in mind

that the greater the slope, the less contact time stormwater will have with the soil and

plants for pollutant removal and water infiltration. The chance of erosion is also greater.

The following slope percentages are given here as a general idea of bioswale slopes and

what to expect of them. These numbers differ amongst authorities and depend upon spe-

cific soils and site conditions for a given site. Longitudinal slopes for bioswales are gener-

ally recognized as ideally ranging from around one to six percent, with the ideal slope being

around one to two percent (Jurries 2003; King County 2009). However, low slope percent-

ages can result in pools of standing water, especially if a soil does not drain well. Therefore,

it might be recommended for all hydrological soil groups that have relatively flat slopes to

have an underdrain installed (King County 2005). Very flat slopes could perhaps be consid-

ered as candidates for wet biofiltration swales with weirs or check dams (ibid). Also, when

bioswale slopes reach over six percent, there is a short time period for water-soil-plant

contact, and there will be a larger risk of erosion. Slopes such as these can be addressed

70

by designing the bioswale to follow the contours of the land in order to lessen the slope, or

weirs/check dams can be used along the bioswale channel to slow and hold water for infil-

tration (Jurries 2003). If used, weirs or check dams should be designed with flow spreaders

or a layer of stones at the toe of every vertical drop which must extend the entire width of

the swale in order to prevent water scouring at the toe of these structures.

Water flow in a bioswale is designed to be shallow and slow to enable water infiltra-

tion, sediment drop-out, and nutrient/pollutant removal. If the water depth is not carefully

engineered to specific standards, the water can create rills and gullies due to high volumes

or velocities (King County 2009). Once water depth has been determined, some municipali-

ties require maximum widths for bioswales; designers should therefore be sure to consult

their local municipal regulations (see Jurries 2003, King County 2005; and San Mateo 2007

for further information on this). The allowable bioswale widths can still be quite wide, but

are sometimes not permitted to become extensively wide bioswales flowing across a land-

scape. When a bioswale’s water volume will be too great for a municipality’s requirements

for bioswale width, a longitudinal divider can be designed to split the flow of water into

separated routes in order to decrease the width of moving water (King County 2005). The

divider should begin at the inlet and continue for a minimum distance of three-quarters of

the swale’s length (King County 2005). If multiple dividers will be installed, the inlet water

flow should split so the water flows evenly into each swale (ibid). To further avoid high flow

71

rates and scour, a swale’s water depth should be kept low and never higher than the height

of mown turf or short vegetation (San Mateo 2007). This ensures that water being carried

by a bioswale will be flowing through vegetation so the it can slow and act to filter and ab-

sorb the stormwater.

In addition to water depth, velocity and infiltration rates are important consider-

ations that will determine the final size of a bioswale. The velocity must be retained at a

relatively low flow rate (San Mateo 2007). To slow water flow through a wet or dry bio-

swale, native plants should be planted and, if needed, check dams/weirs should be con-

structed; these can be constructed of formed, poured in place concrete, cut stone, or of

piled aggregate. Weirs and check dams act upon water to slow and hold specific amounts of

water so infiltration can occur. However, the amount of water held in a dry bioswale should

be engineered so the water will not stand longer or in such quantities that water from a sec-

ond storm could not be handled properly and cause flooding. To increase a soil’s infiltration

rate, a bioswale’s soil depth is often recommended to be around one and a half feet deep

of amended soil; however, installation of deeper soil profiles will capture greater amounts

of nutrient and pollutant loading (Hinman 2005). The distance between the bottom of a

bioswale and the seasonal water table also affects the ability for a bioswale to infiltrate wa-

ter. Municipalities may require specific distances between the bottom of a bioswale and the

top of the seasonally high water table; this distance may vary depending upon the volume

72

of water the bioswale will be handling (ibid). The regulations are born out of concern with

groundwater seeping upwards into bioswales (making the bioswales useless for stormwater

filtering and conveyance), and from concern of pollutants entering groundwater easily due

to a shallow soil barrier between the bioswale and the groundwater (Weis et al 2008).

Just as poorly draining soils need special treatment when porous pavements will be

installed upon them, these poorly draining soils need the same attention in design when

bioretention will be placed upon them. In soils of poor water infiltration ability such those

of C or D hydrologic soils, low volumes of water can be expected to be infiltrated by these

soils; any water volumes that these soils cannot handle must be engineered to be transmit-

ted away from the site through an underdrain pipe and through the swale’s longitudinal

drainage. Indeed, we must remember that while bioretention is designed to treat runoff,

bioswales are also designed to safely convey runoff from a hundred year twenty-four hour

storm event; however, different municipalities may require bioswales to manage water for

other year storm events. Bioswales are therefore frequently designed to connect with de-

tention ponds, raingardens, constructed wetlands, or inlet structures. These sites will hold,

infiltrate, or safely release water that bioswales are not able to infiltrate.

An alteration to the more widely known dry bioswale, a wet bioswale (Figure 23)

may be used on sites that do not drain well. Because these bioswales are engineered to

remain moist, no underdrain or low-flow drain is required. Conditions that might instigate

73

their usage are: high bedrock, high water table, low longitudinal slope that results in water

ponding, or poorly draining soil. Constant low water flow that causes saturated soil condi-

tions due to a seep or off-site base flow also makes wet bioswales a viable LID option. The

design requirements of wet swales are very similar to that of dry swales. However, there are

some notable differences. For instance, if the site slope is around two to six percent, ter-

racing of the swale should probably be carried out in order to slow the water velocity (King

County 2005). This can be accomplished through the use of check dams, weirs, or rock pile

dams to collect water where it will stand for slow infiltration (ibid). If high flow rates can be

expected at times, a high-flow bypass should be installed to prevent damage to the vegeta-

tion and swale banks (ibid). The minimum bottom width would still be two feet for ease of

maintenance.

Figure 23. Wet bioswale that handles water being released from a detention pond at a low slope (image by Sarah DaBoll Geurtz).

74

An issue involving both wet and dry bioswales that may occur during construction

is that the soil – both topsoil and subsoil - can become compacted during construction. In

order to alleviate compaction while providing for nutrient and water uptake, plant growth,

and water infiltration, well-rotted compost should be tilled into the existing native soil along

the entire length and sides of a bioswale. An important note on soil amendment here is

that it is best to not add sand in an attempt to increase a clay or silt soil’s drainage. It takes

much sand to increase clay’s water percolation rate (Brady & Weil 1999). If not enough sand

is added, the sand particles held adjacent to the clay particles do not increase drainage and

can actually make the drainage worse (ibid). The action behind these microscopic actions is

that the large-sized sand particles come together and create large void spaces that become

clogged with finely textured soils such as clay or silt due to these soils being small enough

to become trapped and locked tightly into the sand’s void spaces (Brady & Weil, 1999). This

causes very tight and compacted bonds within these spaces that can create a soil that drains

worse than it did before (Brady & Weil, 1999). Fortunately, due to the microscopic form

of clay and its soil chemistry, clay holds a certain amount of humus content so strongly to

its soil particles that this compost will be slow to fully degrade; this causes some humus to

be locked throughout a compost-amended clay profile. This action holds the clay particles

apart and allows for water infiltration between these particles (Brady & Weil, 1999). Once

amendment is completed, another problem faces the designer: erosion. This can be ad-

75

dressed through a number of different methods. The type of vegetation cover used in a

bioswale is a large factor in erosion potential. Bioswales seeded with grass will typically

develop a dense vegetation cover rapidly; however, bioswales planted with broad-leafed

vegetation typically take two years to establish themselves; therefore, these bioswales are

prone to high levels of erosion during this maturation time. Biodegradable geotextile or

biodegradable matting, such as coir matting, is one way to prevent erosion and enable es-

tablishment of seeds and/or plugs. A more permanent erosion solution is geocell paving in-

stalled along the bottom and sides of a bioswale and filled with soil. Geocell grids are more

applicable for seeding of turf in high flow areas than for planting of broad-leafed plants;

this is due to difficulties of planting broad leafed vegetation within geocell’s grid openings.

Mulch may also be used but non-floating ones must be carefully chosen so the mulch does

not become washed away in high rain events.

Therefore, non-floating mulches that “lock”

together, or mulches with a tackifier added

should be specified. Another alternative is us-

ing compost blankets either by themselves or

in combination with compost socks; this offers

promising results for reducing erosion as well

as for providing nutrient capture (Figure 24).

Figure 24. Compost blanket and compost berm/sock being used for erosion and nutrient loose prevention (Land and Water 2009).

76

A designer’s choices for soil amendments, vegetation cover, and erosion prevention mea-

sures, in addition to care and maintenance, will ensure a bioswale will thrive and perform as

designed upon establishment.

While bioswales utilize channel flow to treat and transmit stormwater, buffer strips

transmit water in sheet flow and serve essentially the same purpose. They are utilized to

direct water away from an impermeable surface. The wide expanse of buffer surfaces

decreases stormwater’s velocity and captures sediment and pollutants. A grassy field could

technically be called a vegetated buffer strip; however, when used in an urban context to ad-

dress stormwater flows, specific engineering calculations must be applied so a design team

can be sure the strip will slow and handle a specific amount of water flowing from imperme-

able surfaces. These strips are typically of a minimum fifteen feet wide and around sixty

feet wide, and are sloped no more than fifteen percent (San Mateo 2007); these numbers

vary according to local municipality’s regulations. Depending upon the soil type, slopes as

low as half a percent may require installment of an underdrain to prevent standing water

from occurring. The foliage surface area within the flow of this water will in part determine

the rate at which the entering stormwater will be slowed. Tall vegetation may become laid

down flat during water flow; when this occurs, water may be able to flow over the vegeta-

tion. This reduces the amount of stormwater flow obstruction and is therefore not as ben-

eficial as vegetation three to six inches tall that stands upright (ibid). In this manner, there

77

is more vegetation surface area to offer resistance to stormwater, hence slowing the water’s

velocity. Additionally, the topography of the site should be free of gullies or rills that would

direct water into specific paths, thereby decreasing a buffer strip’s ability to slow water’s

flow (ibid). After stormwater has passed through a vegetated buffer strip, it can then enter

a bioswale, raingarden, or inlet for further treatment or conveyance of what water was not

absorbed by the buffer strip.

Raingardens

A form of bioretention, raingardens (also known as bioretention facilities), are a

closed form of bioretention that manages stormwater flows through water storage, soil infil-

tration, soil actions, and vegetation uptake. They can be installed on residential property or

in public spaces to facilitate draining from impervious surfaces. Water may be directed into a

raingarden through filter strips, bioswales, or disconnected gutter downspouts. The pond-

ing depth required is relatively shallow (as shallow as six inches to as deep as twenty-four

inches) depending upon the amount of water the raingarden must handle (Emanuel 2010).

Raingardens must be able to infiltrate stormwater at a high rate; slow infiltrations risk a rain-

garden becoming a retention pond and harboring mosquito breeding (ibid). To provide for

times when water flow may overtop a raingarden, an overflow channel should be installed

and a catch basin or rock pad should be installed beneath it (King County 2005).

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Raingardens are often richly planted and can be just what their name implies - a

garden. They can be by themselves or be incorporated to be a part of a larger garden. The

plant palette used should be specifically chosen to utilize plant species that can tolerate

periodic times of water inundation as well as those of the existing soil moisture rates. The

moisture gradient that the homeowner or grounds keeper intends to keep the garden at

year-round should also be considered.

Detention Systems

Detention ponds are not designed to hold water for infiltration but are sometimes

needed in LID designs to detain water for a short period of time and permit the water to

slowly discharge through an outlet structure such as a weir or check dam. These temporary

“ponds” are designed to hold water for no more than forty-eight hours and are generally

larger than raingardens. However, there are extended detention ponds that may hold water

for up to five days (San Mateo 2007). For matters of safety and maintenance, side slopes of

detention ponds should be no greater than 3:1, and preferably of a lower slope than this to

protect against construction installation error. As with the already mentioned bioretention

methods, incorporation of compost plays a pertinent role in water storage capacity and wa-

ter infiltration. To provide for this infiltration, at least half of the water volume of a normal

detention pond should be held for at least 24 hours (San Mateo 2007). A forebay should be

installed where water enters a detention pond to capture sediment. Even so, sedimentation

79

buildup will occur as it does in any LID structure that acts to collect stormwater sedimenta-

tion. Sediment buildup is a large concern because it reduces water volume holding capacity

and holds excess nutrients and pollutants. Such as in the case with bioswales, detention

ponds require periodic cleaning to remove sediment accumulation in order to ensure the

pond could hold the volume of water it was designed to manage. Because detention ponds

are designed mainly to hold and release stormwater (not to infiltrate and treat stormwa-

ter), they are not the best solution for handling on-site stormwater. However, they are at

times required and needed. Other treatment methods such as those discussed in this paper

should be utilized whenever possible to reduce the need and size of required detention

ponds.

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Constructed Wetlands

Another form of bioretention is constructed wetlands. These are essentially reten-

tion ponds with topographical variety designed to develop many different ecological wet-

land-like environments. This type of wetland holds stormwater in pools at various depths

where vegetative absorption and microbial action can occur; the variation in topography in

these wetlands results a rich variety of ecologies (Figure 25). Wetland plants that can toler-

ate having their roots wet most or all of the time are planted in these various water levels.

Constructed wetlands can be designed to handle water for sites with both large or small

runoff volumes. Wetlands designed to treat small runoff volumes are often called “pocket

wetlands” (ibid). Also, a site with groundwater base flow, regardless of the contributing

acreage, may be an ideal place for a created wetland. In order to protect the diverse ecolo-

gies found in these wetlands, buffer/filter strips of at least twenty-five feet in width and

should surround these wetlands;

these filter strips serve to protect

the quality and runoff velocity of

stormwater entering the wetland

(ibid). Not all soils are ideally

suited for wetland design without

some form of liner applied; well-Figure 25. Undulating topography of a constructed wetland pro-vides many different ecologies for both fauna and flora.

81

draining soils will infiltrate water too rapidly and will need either a plastic liner or a 6 inch

layer of clay applied (ibid). Alternatively, B, C, and D, soils, if compacted well, could hold

water and experience only small water losses (ibid). However, if there is a high water table,

groundwater contamination might be of concern and a liner might be required regardless of

the existing soil type.

To address entering water’s sediment volumes and high water temperature, a fore-

bay or micropool should be designed to capture entering/exiting water. The forebay acts to

capture sediment before the sediment can reach the wetland, transfers directed water flow

into sheet flow, and permits time for the stormwater’s temperature to drop before reach-

ing the wetland (Metropolitan Council 2001). It must be designed so maintenance vehicles

such as backhoes can reach this area for annual removal of sediment accumulation. Also,

forebays ideally should be four to six feet deep and hold at least ten percent of the entire

wetland’s water storage (ibid). Another use of forebays is for reducing the temperature of

incoming stormwater. The forebay provides an area where incoming stormwater can cool

down before this water enters the fragile ecosystem of the wetland; it protects the wet-

land’s ecosystem from the shock of warm or hot water from open urban spaces. After water

has entered a wetland and snaked throughout it, the water will then enter what is called a

micropool. This feature should generally be of the same size requirements as that of the

forebay. Water that has accumulated sediment on the path through the wetland will have its

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sediment settled out of suspension

here before the water can flow into

a swale or other outlet structure

(ibid).

There are a number of differ-

ent topography grading techniques

for constructed wetlands (Figure

26). They all provide low veloc-

ity and shallow flow patterns. The

water is generally encouraged to

meander in order to slow the water

for increased nutrient capture and

evaporation. A constructed wetland

can have a greatly diverse ecology through designing an undulating wetland soil surface that

will create pockets of wet, moist, and dry ground. Figure 26 illustrates various constructed

wetland examples from the Minnesota Urban Small Sites BMP Manuel ‘Constructed Wet-

lands Stormwater Wetlands’ (2010).

Figure 26. Sections of the four basic types of constructed wetlands, as set forth in Metropolitan Council 2001, p. 3-234.

83

Urban Bioretention Methods

The bioretention methods dis-

cussed so far may require more land for

construction in comparison to that of

traditional piping methods. In urban set-

tings, enough space for elements such as

constructed wetlands, detention ponds,

or even infiltration strips may not be

available. Therefore, designers have developed bioretention methods that can be utilized in

urban sites within the constraints of lack of undeveloped land. Due to the large quantities

of land that impervious streets hold in the urban landscape, the invention of “green streets”

has been an important step in controlling stormwater volumes in cities. Waters flowing off

impervious streets can contain large

amounts of pollutants from vehicles,

thereby making the usage of LID tech-

niques especially beneficial in these

areas.

Figure 28. Bump outs can also collect water from side-walks, and can be used to narrow a wide street. In this manner, bump outs can perform as traffic calming devises.

Figure 27. In this image, water runoff enters both street bump outs as well as flows into and out of tree wells for water absorption by the street trees. Weirs within the bump out slow water to increase water infiltration.

84

One green street method involves making streets narrower than they are typically

built today. As an example, instead of constructing a twelve-foot wide vehicular lane, an

eight or ten foot lane might be constructed. This reduces the amount of soil disturbance

needed to construct the road and reduces the amount of water runoff from the impervious

surface. Porous pavements also come into play in these circumstances whereby they reduce

the amount of water runoff created. Another popular method, bioretention bump-outs,

takes a number of different forms (Figures 27 and 28). Bioretention cells can be installed

periodically within the space of parking stalls. Water is routed from the streets into these

areas by way of curb cuts. Weirs can be installed to slow the water as it flows through these

vegetated areas to hold water and permit water infiltration.

Another method uses bioswales and raingardens in parking lot islands; indeed, there

is great potential for retrofitting existing parking lots to have multiple bioretention cells.

By lowering the soil heights of the islands, amending the soils, thickly planting vegetation,

installing check dams, and performing curb cuts to permit water to flow into the bioreten-

tion cells, a traditional parking lot design can easily begin infiltrating water instead of dispos-

ing of it to water drains. Newly designed parking lots could have underdrains installed in

the parking islands to carry excess water from these areas to other bioretention methods

outside of the parking lot area. These second bioretention cells could accomplish backup

filtering if the bioretention methods could not fully handle the runoff quantities. These

85

parking lots could also be constructed with porous pavement to further reduce the storm-

water runoff levels. Additionally, in these situations, the porous pavement’s aggregate base

could be designed for additional water holding capacity.

Flow-through planters and tree well planters offer other stormwater bioretention

methods designed specifically for urban situations. These planters enable treatment near

buildings and streets in urban situations.

The flow-through planters handle water

from a building’s roof where the water is

directed into a planter box that incorpo-

rates vegetation and well-draining me-

dia. Portland, Oregon has adopted these

planters successfully into sidewalks to

handle water flow from streets and side-

walks (Figures 29 and 30). In both cases,

the planters provide for attractive greens-

paces for citizens and can be designed to

provide seat walls for the public as well.

Both uses of this technique allow plants,

soil, and microorganisms to act upon Figure 30. Planted flow-through planters in Portland, Oregon (Greenworks 2010).

Figure 29. Installation of flow-through planters with side walls to contain stormwater within the planters, and built-in weirs for slowing and holding stormwater (Greenworks 2010).

86

stormwater in urban settings. When the planter boxes are installed above the ground, this

method also eliminates the concern of weakening a soil’s bearing capacity under saturated

soil conditions. The soil mixes must be able to infiltrate at least large rates of water an hour

to prevent overflow onto sidewalks and streets (San Mateo 2007). Even so, to further insure

overflow will not occur, a perforated underdrain pipe should be installed within a layer of

aggregate beneath the mix (Figure 31) (ibid). Above this, the rooting medium should be

well-draining soil around one and a half feet thick to allow ample root growth and water

storage/infiltration (ibid). A mixture of compost and topsoil should be added on top of this

for plant growth (ibid). Additionally, an overflow pipe is often suggested to help ensure that

the planter will not overflow against the building and over a sidewalk or street in high rain

events or from possible clogging of the underdrain system.

Figure 31. Many municipalities require installation of underdrains and overflow pipes to provide for large rainfall events.

87

Chapter 5 - Roof Stormwater Controls

Rainbarrels & Cisterns

In addition to handling stormwater from hardscapes, LID techniques also offer solu-

tions for treating the large volumes of stormwater that pour off roofs. Fortunately, this

water is relatively easy to handle through a number of different methods and is an asset

due to a much lower pollution potential due to a decrease or elimination of the pesticides,

herbicides, fertilizers, and metals from vehicles and yard maintenance that ground storm-

water carries. Rainbarrels are probably the easiest and cheapest method. These are simple

barrels, most often shaped like a wooden whiskey barrel, and placed at strategic locations

around a building’s perimeter where gutter downspouts are located. Other aesthetic

alternatives are barrels covered in trellising so vines can disguise them, and even sleek and

modern rainbarrel designs. To prevent clogging of rainbarrels, a filter screen or sediment

trap should be placed where the water flows from the downspout into the barrel in order to

capture debris. Otherwise, shingle and leaf debris will accumulate in the barrel and lead to

rapid clogging. Water is removed through a spout located toward the bottom of these bar-

rels and allows for attachment of a hose for gravity-fed watering. An alternative container

to rainbarrels is the cistern. These can be designed to hold specific quantities of water and

can be placed underground. If placed underground, a pump is often needed to get water

above ground for irrigation. Rainbarrels and cisterns are both used to not only capture and

88

hold stormwater, but to provide water for later irrigation usage. This saves property own-

ers money while decreasing stormwater runoff and preventing potable water from being

wasted on watering landscapes. If a person does not intend to use water harvested from

their roof, another option is to disconnect the gutter downspouts and direct the water to a

raingarden for slow percolation.

Greenroofs

Another LID technique to handle water runoff from roofs is greenroofs. This storm-

water and energy-saving technique is a trend gaining in popularity as people understand

the ecological benefits as well as the monetary benefits from the insulating characteristics

of greenroofs. This system incorporates a layer of engineered light-weight growing medium

planted with what is often comprised of succulents such as sedums. The plant choices

depend upon the USDA zone where the roof is being installed, and the depth of the grow-

ing medium. There are two main classifications for greenroofs: intensive and extensive.

Intensive Greenroofs contain from eight to ten inches of growing medium and can be as

deep as fifteen feet or more, depending on the load capabilities of the supporting structure

(Miller 2010). The deep growing medium profile of these roofs allows for growth of shrubs

and trees and may look nothing like what most people associate with the appearance of a

greenroof. The more typical greenroof is called an extensive roof. The soil profile is much

shallower on these – from two and a half inches to a maximum of six inches. The growing

89

medium depths of these roofs cannot support the deeply rooting plants of an intensive roof.

Therefore, hardier, low profile, and shallow rooting plants such as sedums, herbs, alpine

plants, and certain grasses are planted on these roofs. These growing media often contain

considerable inorganic soil material such as expanded clay pellets in order to decrease the

weight of the roof on the supporting structures of these roofs. The soil and plants of both

intensive and extensive roofs capture and utilize considerable amounts of water and nutri-

ents. Water not used by the plants will leave by way of drainage layers and downspouts and

can be captured in a rainbarrel, cistern, or raingarden for even further water filtration.

Conclusion:

The greenroof acts like a miniature example of LID, as it, in effect, replaces the per-

meable soil footprint that a home covers. It provides a permeable surface for rainwater to

infiltrate, be held, and utilized by vegetation. This paper has dealt with such issues - how

to limit undue increases in water volumes, and how to preserve as many of nature’s soil-

water interactions as possible. Additionally, LID utilizes these methods to provide vegetated

spaces for humans and wildlife alike.

Now, pause for a moment and consider Ian McHarg’s quote from the beginning of

the paper: “We need nature as much in the city as in the countryside...” Low impact design,

by its very nature, introduces greenery into the urban environment and utilizes nature’s nat-

ural processes to cleanse stormwater. Without plants and a biologically diverse soil matrix,

90

LID simply wouldn’t work. It is the complex relationship between soil chemistry, microor-

ganism life, and plants that enables LID to filter stormwater for reductions in pollution found

in stormwater and waterbodies. Researchers, developers, landscape architects, engineers,

municipalities, governments, ecologists, conservationists, and concerned citizens, do the

rest by supporting the utilization of LID in developments, thereby spreading LID’s usage.

The large amount of vegetation provided in many LID developments results in neigh-

borhoods and urban areas that are cooler, have cleaner air, and that can be more attractive

than concrete-clad cities. LID not only incorporates high levels of vegetation in the form of

turf, perennials, shrubs, and trees, but preserves special ecological areas as well. Through

careful protection of on- and off-site factors which these special ecologies depend upon for

survival, designers of LID work to save these spaces – properly – for both nature and the

enjoyment of people. These preserved areas, in conjunction with greenspace that doubles

as parks and stormwater recharge areas, add life and vitality to a community. Due to the

amount of vegetation encouraged in LID projects, they can provide shade for people to

walk and rest and can attract wildlife. Imagine: a city with sounds of nature. In some cities

people find this only in the surrounding countryside.

A strong sense of place – that element so vital for a community - can also be devel-

oped and grown with careful LID planning. Because LID techniques are each designed to

specific sites, there is an opportunity to develop communities with character. In these green

91

developments, the occupants can take pride in where they live. They are encouraged to

walk within and between their neighborhoods because trails, sidewalks, and parks connect

them. There can be a sense of place and connectivity, through careful site design and care of

nature within low impact development design.

92

Bibliography

Alfakih, E; Barraud, S; and Martinelli, I 1999, ‘A Study of Stormwater Infiltration

System Feasibility and Design”, Water Science and Technology, vol. 30, no. 2, pp.

225-231.

Azous, Amanda; and Horner, Richard 2001, Wetlands and Urbanization Implica-

tions for the Future, Lewis Publishers, Boca Raton.

Bartsch, Leslie; and Jennifer S Raible (n.d.), ‘Bioswales: A guide to Low-Impact Development

Design and Maintenance,’ Biological Engineering University of Arkansas, Fayetteville, AR,

accessed 20 July 2010, < http://arkansaswater.org/319/pdf/05-1100%20Urban%20Low%20

Impact%20Bioswale%20Binder%20Intro.pdf>.

Blake, Cary 2009, ‘Technology helps reduce subsoil compaction while boosting yields’, Agri-

cultural Technology/Farm, p. 10-11, Western Farm Press, Prism Business Media.

93

Boddingtons Ltd 2010, ‘Civils and Construction’, BodPave® 85 Porous Grass Pavers/

Paving Grid, Accessed 30 March 2010, <http://www.boddingtons-ltd.com/civil/im-

ages/BodPave-Typical-Profile_000.jpg>.

Brady, Nyle; Weil, Ray 1999, The Nature and Properties of Soils, 12th ed., Prentice Hall, New

Jersey.

Brattebo, B.O.; and Booth, D.B. 2003, ‘Long-term stormwater quantity and quality

performance of permeable pavement systems’, Water Research, vol. 37, no. 18, pp.

4369-4376, accessed 20 February 2010 from Web of Science database.

Chaplin, Jonathan; Min, Min; and Pulley, Reid 2008, ‘Compaction Remediation for Construc-

tion Sites’, MN/RC 2008-01, Department of Bioproducts and Biosystems Engineering, Minne-

sota Department of Transportation, p. 1-26.

Clar, Michael (ed) 2007, Low Impact Development New and Continuing Applications: pro-

ceedings of the second National Low Impact Development Conference, American Society of

Civil Engineers.

94

Croney, David; and Paul Croney 1997, The Design and Performance of Road Pavements, 3rd

edition, R.R. Donnelley & Sons Co, New York, accessed 2 February 26 2010, <http://books.

google.com>.

DaBoll, Mathew B 2010, personal communication, United Rentals, 31 July 2010.

Davis, Allen P; Shokouhian, Mohammad; Sharma, Himanshu; Minami, Christi 2001, ‘Labora-

tory Study of Biological Retention for Urban Stormwater management’, Water Environment

Research, Vol 73, No. 1, pp 5-14.

Duiker, Sjoerd 2004, Pennsylvania State University, CAT UC 186, accessed 31 July 2010,

<http://pubs.cas.psu.edu/freepubs/pdfs/uc186.pdf>.

Environmental Protection Agency 1998, ‘Handbook of Constructed Wetlands,’ accessed 12

September 2010, <http://water.epa.gov/type/wetlands/restore/cwetlands.cfm>.

Ferguson, Bruce K 2005, Porous Pavements, Taylor & Francis, Boca Raton.

95

France, Robert L (ed) 2002, Handbook of Water Sensitive Planning and Design, Florids: GRC

Press LLC.

Frisby, James; and Pfost Donald 2010, “Soil Compaction: The Silent Thief”, Department of

Agicultural Engineering, University of Missouri Extension Service, accessed 31 July 2010, <

http://extension.missouri.edu/publications/DisplayPub.aspx?P=G1630 >.

Greenworks 2010, accessed September 12, 2010, < http://greenworkspc.wordpress.com/

category/streetscape\>.

Hinman, Curtis 2005, ‘Low Impact Development Technical Guidance Manual for Puget

Sound’, Washington State University Pierce County Extension.

Hough, Michael 1995, Cities and Natural Process, Routledge, London.

Interlocking Concrete Paving Institute (ICPI) 2010, ‘Residential Driveway with Concrete

Edges’, ICPI-09, 2010, accessed 30 March 2010, http://www.icpi.org/sites/default/files/de-

tail_drawings/doc/ICPI-09.pdf.

96

Invisible Structures, Inc. 2010, White papers, accessed 14 August 2010, http://www.invisi-

blestructures.com/white_papers/GP2ThatchBuilding.pdf.

Jurries, Dennis 2003, Biofilters, Bioswales, Vegetative Buffers, & Constructed Wetlands for

Storm Water Discharge Pollution Removal, State of Oregon Department of Environmental

Quality, accessed 10 August 2010, http://www.deq.state.or.us/stormwater/docs/nwr/biofil-

ters.pdf.

Kloster, Tom; Leybold, Ted; and Wilson, Clark 2002, ‘Green Streets: Innovative Solutions for

Stormwater and Stream Crossings,’ Metro.

Lake County Forest Preserves 2003, “Permeable Paver Research Summary”, February, ac-

cessed 20 February 2010, <http://artfiles.org/files/pdf/PermPavers.PDF>.

Lampus Company 2010, “Turfstone Paving Stones”, accessed 19 February 2010, <http://

www.lampus.com>.

97

Land and Water the Magazine of Natural Resource Management and Restoration 2009,

vol 52, no. 2, accessed August 12 2010, <http://landandwater.com/features/vol52no2/

vol52no2_1.html>.

Lenntech 2009, “Lenntech Water Treatment Solutions”, accessed 18 February

2010, <http://www.lenntech.com>.

Lindsey, Andy E 1999, ‘Comparisons of Maintenance’, University of South Ala-

bama, Grounds Management, accessed 20 February 2010, < http://www.invisible-

structures.com/white_papers/USAB-GPGVvsAsphalt.pdf>.

Low Impact Development Center, Inc. 2008, ‘Anacostia - LID Retrofit and Resto-

ration Study for Urban Areas’, Sustainable Design and Water Quality Research,

accessed 19 July 2010, <http://www.lowimpactdevelopment.org/anacostia_rs1.

htm>.

Mandarano, Lynn 2010, ‘Sustainable land-use planning: revitalizing a flood prone office

park’, Journal of Environmental Planning and Management, 53:2, 183-196.

98

McHarg, Ian L 1992, Design with Nature, John Wiley & Sons, Inc., New York.

Metropolitan Council 2001, ‘Constructed Wetlands’, Minnesota Urban Small Site BMP

Manuel, accessed 10 August 2010, http://www.metrocouncil.org/environment/Water/BMP/

manual.htm>.

Multiquip, Inc. 2010, ‘Soil Compaction Handbook’, Concrete-Catalog, accessed 31 July 2010,

<http://www.concrete-catalog.com/soil_compaction.html>.

Murphy, Susan 2006, ‘The Practice of Low Impact Development, Coastal Smart Growth

Program’, Massachusetts Office of Coastal Zone Management and the Executive Office of

Environmental Affairs.

Oregon State DEQ 2001, ‘Restoring Soil Health To Urbanized Lands The Crucial Link between

Waste Prevention, Land Use, Construction, Stormwater Management and Salmon Habitat

Restoration’, accessed 1 August 2010, <http://www.lowimpactdevelopment.org/lid%20ar-

ticles/RestoringSoilHealth_odeq_jul01.pdf>.

99

Pin Foundations Inc. 2009, ‘Heave and Expansion’ 2009, accessed 5 August 2010, <http://

www.pinfoundations.com>.

Pin Foundations Inc. 2010, accessed 10 September 2010, <http://www.pinfoundations.com/

boardwalks.htm>.

Randrup, Thomas; and Lichter, John 2001, ‘Measuring Soil Compaction on Construction

Sites: a Review of Surface Nuclear Gauges and Penetrometers’, Journal of Arboriculture

27(3): May 2001, accessed 31 July 2010, http://www.treelink.org/joa/2001/may/01_MEA-

SURING_SOIL_COMPACTION_randrup.pdf>.

Reese, Andy 2001, ‘Stormwater Paradigms’, Stormwater Journal, July-August.

Roa-Espinosa, A 2010, ‘An Introduction to Soil Compaction and the Subsoiling Practice,’

Dane County Land Conservation Department, accessed 28 July 2010, <www.countyofdane.

com>.

Rousan, Laith M; and Nusier, Osama K 2008, Communications in Soil Science & plant Analy-

sis, Vol. 39 Issue 17/18, p 2637-2647, accessed 31 July 2010 from EBSCO Host.

100

San Mateo 2007, C.3 “Stormwater Technical Guidance for developers, builders and project

applicants”, San Mateo Countywide Water Pollution Prevention Program, San Mateo, Califor-

nia.

Sewer History 2004, ‘Tracking Down the Roots of Our Sanitary Sewers’, accessed 10 August

2010, <http://www.sewerhistory.org>.

Smith, David R 2006, ‘Permeable Interlocking Concrete Pavements’, Interlocking

Concrete Pavement Institute, 3rd edition, pp. 1-62.

University of Minnesota 2001, ‘Soil Compaction Causes, Effects and Control’, Extension Ser

vice publication, accessed 31 July 2010, <http://www.extension.umn.edu/distribution/crop

systems/DC3115.html>.

Tyner, J.S., W.C. Wright, et al 2009, ‘Increasing exfiltration from pervious concrete

and temperature monitoring’, Journal of Environmental Management, February, pp

2636-2641, accessed 19 February 2010 from ScienceDirect Web.

101

UNI-Group USA, 1995, Permeable interlocking Paver installation UNI Group U.S.A.,

accessed 19 February 2010, <http://www.uni-groupusa.org>.

USDA Forest Service 2008, ‘Technology and Development’, accessed 25 July 2010, <http://

www.fs.fed.us/t-d/pubs/htmlpubs/htm08342828/page01.htm>.

U.S. Environmental Protection Agency 2010 , ‘Compost Blankets’, National Pollutant Dis-

charge Elimination System (NPDES), accessed 1 August 2010, < http://cfpub.epa.gov/npdes/

stormwater/menuofbmps/index.cfm?action=factsheet_results&view=specific&bmp=118>.

United States Environmental Protection Agency & National Pollution Discharge Elimi-

nation System a. 2010, “Permeable Interlocking Concrete Pavement”, accessed 19

February 2010, < http://cfpub.epa.gov>.

United States Environmental Protection Agency & National Pollution Discharge Elimi-

nation System b. 2010, ‘Pervious Concrete Pavement’, accessed 19 February 2010,

<http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=browse&

Rbutton=detail&bmp=137&minmeasure=5>.

102

United States Environmental Protection Agency & National Pollution Discharge

Elimination System, c. 2010, ‘Porous Asphalt Pavement’, accessed 19 February 2010,

<http://cfpub.epa.gov>.

University of South Alabama, Grounds management 1999, Comparisons of Main-

tainence, accessed 20 February 2010, <http://www.grasspave.com/white_papers/

USAB-GPGVvsAsphalt>.

VanGeem, Martha 2010, ‘Minimize Site Disruption’, Portland Cement Association , accessed

22, July 2010, <http:/concretethinker.com/solutions/Minimize-Site-Disruption.aspx>.

Weinstein, John E; Kevin D Crawford; Denise M Stranger 2010, ‘Sediment Contamination in

Stormwater Detention Ponds’, Stormwater Journal, July/August 2010.

Weinstein, Neil, ‘Neil’s White Paper’ 2008, The Low Impact Design Center, Inc, accessed 15

August 2010, <http:www.lowimpactdevelopment.org/tenth.htm>.

103

Weiss, Peter T; Greg LeFevere; and Gulliver John S 2008, ‘Contamination of Soil and Ground-

water Due to Stormwater Infiltration Practices,’ Project Report No. 515, University of Minne-

sota Stormwater Assessment Project, accessed 12 August 2010, <www.pca.state.mn.us>.

Woolley, Tom; Sam Kimmins, et al 2001, Green Building Handbook Volume I, London: E & FN

Spon Press.