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Knowledgeable LID Patterning for EcologicallySensitive 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 Smith Member: 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

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Figure 1. McHarg Map

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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

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Figure 9. Compaction Depth by Moisture Levels

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Figure 10. Compaction Depth Due to Tire/Tracts Used

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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

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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

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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


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

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Chapter 1 - Introduction & Site Ecological Specifics

Thesis Statement: Through patterning LID on site-specific land characteristics, LID can be applied to design in such a manner that environmental sensitivity is addressed, while an area of uniqueness 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 development. 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.

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Current environmental issues such as toxic levels of nutrients and heavy metals found in stormwater and waterways, stream degradation, and lack of greenspace for wildlife 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 limitations 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 possess, 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 pragmatic 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 relation to the concerns of stormwater pollution and volumes.

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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, pesticides, 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 rainwater, 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.

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Perhaps it is not surprising how society handles stormwater – you could say it’s almost 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 History 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. Unfortunately, when the Roman Empire fell, society reverted not only in education, but in sanitation (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 disease. 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 waterbodies 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 development 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

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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 stormwater. 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 before 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.

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A large volume of published research continues to provide scientific evidence of LID’s effectiveness. 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. Therefore, 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, cities 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 careful 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 perfect 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)

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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

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

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 techniques 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

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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 infiltration 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 ecologies, 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 characteristics 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

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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 dealing 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 precipitation 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 understood much more accurately (ibid). However, computer modeling by itself is not sufficient 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

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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 unbuildable 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 comprises 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 important 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 designer 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-

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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 (Hinman 2005). However, soils high in gravel and sand must be studied carefully for groundwater 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 Discharge 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 development, or where a major subsurface water supplies water to a wetland or stream. This information can be gathered by learning the groundwater depth and through studying a site’s

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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 Figure 2. Subsurface water flows. (France 2002)

example might be attempting to

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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 degradation 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 engineering 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 wetland ecology into a site’s design. If a wetland is thought to occur on a property, a professional should complete a wetland determination study. This information will inform the designer of the wetland’s importance, specifics, and incoming water flow routes. If, for example, there is an underground

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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 disturbances 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 (Hinman 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 sensitive 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 explain the measures being taken to protect these areas (ibid). Buffers should, for matters of

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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 ecological 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)

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protected site, resulting in a decrease in water volumes. If the buffer is protecting a moving 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 moving bodies of waters as well as for helping to prevent stream bank erosion. Additionally, velocity 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 designed 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 pollutants 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 exist within these zones. Simply with the hydrological topography characteristics that occur within buffers, both permanent and ephemeral ponding and channels may exist that pro-

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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 aquatic 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 environments 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 undulating 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 corridors and public spaces for communities in which to enjoy nature. However, the ecological sensitivity of the areas where the trail would be located must be carefully considered,

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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. However, 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 suggested 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 areas 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 elements of an LID design. Once the multiple map overlays are compiled and studied, the gathered information 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-

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opments, and the curvilinear street layout is often associated with older cul-de-sac developments. 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 discourage 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 Disturbance

Grid

27-39% (Center Less Adaptive Promotes by for Housing In- to site features more direct acnovation, 2000 and topography cess to services and CMHC, and transit de2002) pending upon grid size

More efficient - disperses traffic through multiple access points

Curvilinear

15-19% (Center for Housing Innovation, 2000 and CMHC, 2002)

Less efficientconcentrates traffic through fewer access points & intersections

More adaptive for avoiding natural features, and reducing cut and fill

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Biking, Walking, Transit

Tends to discourage through longer, more confusing, & less connected system

Auto Efficiency


as “open space,” “hybrid street,” or “loop layout” plans because they are a compilation of grid, curvilinear, and ecological layouts (Hinman 2005). This technique is especially exciting from both a LID and aesthetic point of view. It minimizes Figure 4. Loop Layout (Himan 2005)

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 ecologically 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)

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constructed with vegetated porous pavements to provide for water infiltration while also removing 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 reducing required road set-backs would reduce the amount of driveway required (ibid). Pedestrians 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 earlier, 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 barrier 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 disturbs them and causes harm. Heavy machinery must be kept away from tree roots to prevent soil compaction which would lead to poor water infiltration, poor soil-oxygen exchange,

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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 envelopes” where construction disturbances are permitted to occur. These disturbance envelopes 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 disturbances will occur. Ideal locations are places where buildings, roads, or other soil compacting 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 construction, 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 educated 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

24


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 building 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.

25


Large soil disturbances from building construction can be further reduced by eliminating the use of concrete slab foundation systems. A current replacement for slabs called “minimal excavation foundation systems” offers a promising alternative to concrete foundations 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 Figure 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 properly designed (Figure 8) (Pin Foundations Inc. 2009). While well-draining soils of the A/B hydrologic group are Figure 6. Pin Foundation System (Pin Foundation 2010)

26

ideal candidates for minimal founda-


tion systems, poorly draining C/D soils can sometimes be supported with this system as well (ibid). Ecologicallyimportant and sensitive sites such as wetlands can therefore have this type of system installed to diminish soil dis-

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

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 conditions are difficult to build concrete slab and vertical 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-

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

ing water infiltration. One possible technique is to use ready-mixed concrete in place of mix-

27


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 particles with much surface area emit strong energy forces which hold water by capillary bonding 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).

28


Doing so will also decrease erosion and sedimentation from stormwater flow off the project site (Hinman 2005). Compaction depths created from construction equipment are more complex than perhaps often thought. First, one must look at the depth levels and causes of these compactions.

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

Within a soil’s profile, there are three levels in which soil compaction is studied: the topsoil, 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 practices 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 machinery 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 pressure), double tires (they spread the load weight), tracks, and the use of wider than normal

29


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 compaction 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 reduction 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

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

will result in some soil compaction. Fortunately, there are methods that can be employed

to repair compacted soil in all three of the soil compaction levels. The most easily alleviated soil compaction is topsoil compaction. Tilling of the upper soil layer with a disc plow,

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

30


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; therefore, compaction should be especially 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

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

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

31


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 soilâ&#x20AC;&#x2122;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 future 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 soilâ&#x20AC;&#x2122;s porosity and moisture 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 incorporated 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.

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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 stormwater 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 detailed information on determining specific rates of organic matter needed (and also determines eventual soil settlement after tillage).

33


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 blanketâ&#x20AC;&#x2122;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 Protection Agency 2010). Preventing site erosion through utilizing compost berms and compost

34


blankets can therefore greatly aid in protecting surrounding waterbodies and prevent sedimentation 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 siteâ&#x20AC;&#x2122;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 buildings 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).

35


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

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 stormwater, or (if handled with bioretention methods)

36

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).


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 subsurface 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 development. Additionally, these methods will also increase a soilâ&#x20AC;&#x2122;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 soilâ&#x20AC;&#x2122;s water infiltration on a site being developed.

37


Roads, parking, and sidewalks are a necessity; thankfully, there are numerous porous 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 vehicleâ&#x20AC;&#x2122;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 carried out because the cells can simply

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

38

be pulled out of the ground. There are two basic forms in which geocells


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 requirements. 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 â&#x20AC;&#x153;fieldâ&#x20AC;? of turf if correct maintenance is maintained. Besides the water infiltration benefit of geocells, an additional benefit is that geocells 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-

39


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 specifications 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 compacted and topped with gravel, the sub-base becomes a very uninhabitable root zone. However, some planting medium can be incorporated into the gravel sub-base to make the soil

40


profile more easily permeable to root growth (Ferguson 2005). In these instances, geotextiles, 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), shifting of the cells was found after only six

Figure 15. A buckling geocell system in Fayetteville, Arkansas

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 16 Soil buildup that results in soil compaction and subsequent turf death is a problem in geocell systems.

41


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 equipment, using sod removal equipment, or actually burning off the thatch layer (Invisible Structures 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

42


COMPACTED SOIL AT PERIMETER

2005). The pavers are then installed

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

on top of these layers and the paverâ&#x20AC;&#x2122;s

1/16" TO 3/8" (2-10 MM) WASHED AGGREGATE IN OPENINGS AND 3" (75 MM) THICK UNDER GRIDS 8" (200 MM) MIN

grid voids are filled with topsoil and

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

GEOTEXTILE COVER BOTTOM, AND SIDES OF BASE

seeded with grass, or filled with

COMPACTED SUBGRADE

gravel only (Lampus 2010). Turf can be established from seed or plugs.

Figure 17. Open-celled paving block system. Note the need for soil compaction for eight inches at the perimeter of OpenCelled Paving Block Systems in order to provide support to the NOTES: 1. DEPTH OF OPEN GRADED BASE WILL EFFECT RUNOFF STORAGE CAPACITY (Interlocking Paver 2. outer BEARINGblocks CAPACITY AND PERMEABILITY OFConcrete SOIL SUBGRADE SHOULDInstitute BE EVALUATED2010). FOR SUITABILITY WITH APPLICATION, TRAFFIC & STORM WATER STORAGE CRITERIA. DRAINAGE OF SUBGRADE MAY BE REQUIRED.

When soils are expected to drain

DRAWING NO.

CONCRETE GRID PAVERS FOR STORM WATER RUN-OFF CONTROL

SCALE

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

43

ICPI-29 F.S.


wearing heels or who walk with the assistance of a cane or walker. Lastly, the same problems 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 structural 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 calculated 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.

44


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 pavers 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 manufactured 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).

45


system is easier to remove in comparison to asphalt or concrete for fixing of underground utilities and for repairing parts of the paverâ&#x20AC;&#x2122;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 surfaceâ&#x20AC;&#x2122;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

46


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 toxicities, 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. Additionally, 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 portion must be removed, pavers can be removed, set aside, and reused.

47


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 thickness 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)

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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 concreteâ&#x20AC;&#x2122;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 arenâ&#x20AC;&#x2122;t worn such as around pools (ibid). But, in these cases, the concrete can be ground down and the resulting dust vacuumed from the concreteâ&#x20AC;&#x2122;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

49


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 option. 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 airplane 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 subbase 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 concrete profile (United States Environmental Protection Agency d, 2010)

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

51


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, seventy-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 detection 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).

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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 openjointed 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 environmental 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 stormwaterâ&#x20AC;&#x2122;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 increase in the calcium content of water enables a humanâ&#x20AC;&#x2122;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 fishesâ&#x20AC;&#x2122; ability

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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 importance 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 toxicitylowering affect of calcium. The nature of porous pavements â&#x20AC;&#x201C; permeability â&#x20AC;&#x201C; depends strongly upon the underlying soilâ&#x20AC;&#x2122;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 different 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 pavement that contains larger aggregate in the void spaces and larger void openings generally

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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 openjointed 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 subbase 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 siteâ&#x20AC;&#x2122;s specific qualities compared to those of the various porous systems. Clay soils present special design and handling requirements over those of well-draining 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, clayâ&#x20AC;&#x2122;s bare surface during construction must be protected from rainfall, as rain falling upon bare clay causes surface sealing and soil compaction, thereby lessening 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

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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 pavement, rows should be trenched and filled with aggregate; this has been found to substantially 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 become 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 infiltration 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-

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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 highvolume rain events and for instances when numerous rains occur (which can cause filling of a porous pavementâ&#x20AC;&#x2122;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 permeable pavers: they can be expected to perform for twenty to twenty-five years (Smith 2006). However, eventual failure is usually due to the paverâ&#x20AC;&#x2122;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

57


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 Pollution 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 materialâ&#x20AC;&#x2122;s lifespan is required for all five system types. Specifically, fine particles, if not kept vacuumed out of porous systems, will clog porous pavementâ&#x20AC;&#x2122;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 handling in cold climates. Sand and rock chips cannot be used for traction on porous pavements in these climates because these traction materials clog the pavementâ&#x20AC;&#x2122;s void spaces (Porous Pavements 2010). Also, unlike in traditional parking lots, snow cannot be piled on top of porous systems because high sediment concentration from piles of melting snow will result in

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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). Thankfully, 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 success and future appearance of turf systems. A turf-planted grid system must be maintained as would a lawn â&#x20AC;&#x201C; 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

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are species that require more moisture; likewise, species that are drought tolerant generally require more sunlight. Therefore, a siteâ&#x20AC;&#x2122;s specific conditions must be taken into account when a designer chooses the type of turf to be seeded in turf systems, and careful consideration 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 surfaces, 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 velocities, 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.

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Bioretention design begins with careful knowledge and understanding of a siteâ&#x20AC;&#x2122;s specific conditions. Factors that will affect the applicability and success of bioretention methods are: a siteâ&#x20AC;&#x2122;s slope, soil classification, impervious/pervious ground layers, groundwater level, and the existing soilâ&#x20AC;&#x2122;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 limit 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 stronger 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

61


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 energy 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, deFigure 21. Example of cation and anions in a soil profile (image by Sarah DaBoll Geurtz).

pending upon the contaminants a designer is

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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 unreasonable. 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 positively 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).

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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 composition.

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 organisms consume nutrients and pollutants 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 inorganic 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 compost), 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.

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Table 4. A bioswale study, as reported by Oregon State Department of Environmental Quality (2003), found significant pollutant removal abilities from bioswales:

Results Results Total Suspended Total Suspended Solids Solids Turbidity 9 minutes of Turbidity (with 9(with minutes of residence) residence) Lead Lead CopperCopper Phosphorous Phosphorous

% % reductionEngineering Engineering Specifications reduction Specifications least 200 feet long 83-92%83-92% at leastat200 feet long 65%

65%

67%

67%

46%

46%

water of depth of 1-4 inches water depth 1-4 inches grass height of at6 least 6 inches grass height of at least inches

minimum 1 1/2 minutes 29-80%29-80% minimum contactcontact time oftime 1 1/2ofminutes

Aluminum Aluminum

63%

63%

Total Zinc Total Zinc

63%

63%

Dissolved Dissolved Zinc Zinc

30%

30%

Oil/Grease Oil/Grease

75%

75%

NitrateNitrate

maximum was 1 1/2 maximum runoff runoff velocityvelocity was 1 1/2 feet/second feet/second

It must also be understood that the very action of adsorption by soil particles,

39-89%39-89%

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 upper levels of soils (possibly only a matter of some centimeters of soil), mulch, and/or vegetation 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,

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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 successfully 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-grading. 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 develop 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 methods 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

66


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 following 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 vegetated 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 Technical 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 bioretention 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

67


within a short time frame so water is not left standing for long periods of time. A wet bioswale 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 differ 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: rectangular, triangular, trapezoidal, and parabolic. Parabolic and trapezoidal shaped bioswales (Figure 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 settling, 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)

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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 maintenance 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 specific soils and site conditions for a given site. Longitudinal slopes for bioswales are generally 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 percentages 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 considered 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

69


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 infiltration (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 infiltration, 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 municipalities 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 landscape. 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

70


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 absorb the stormwater. In addition to water depth, velocity and infiltration rates are important considerations 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 bioswale, native plants should be planted and, if needed, check dams/weirs should be constructed; 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 second 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 water. 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

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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 transmitted away from the site through an underdrain pipe and through the swaleâ&#x20AC;&#x2122;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 detention 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

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Figure 23. Wet bioswale that handles water being released from a detention pond at a low slope (image by Sarah DaBoll Geurtz).

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 conditions 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, terracing 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 vegetation and swale banks (ibid). The minimum bottom width would still be two feet for ease of maintenance.

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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-

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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 establishment of seeds and/or plugs. A more permanent erosion solution is geocell paving installed 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 geocellâ&#x20AC;&#x2122;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 â&#x20AC;&#x153;lockâ&#x20AC;? together, or mulches with a tackifier added should be specified. Another alternative is using 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).

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Figure 24. Compost blanket and compost berm/sock being used for erosion and nutrient loose prevention (Land and Water 2009).


A designerâ&#x20AC;&#x2122;s choices for soil amendments, vegetation cover, and erosion prevention measures, 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 stormwaterâ&#x20AC;&#x2122;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 address 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 impermeable 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 municipalityâ&#x20AC;&#x2122;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 vegetation. This reduces the amount of stormwater flow obstruction and is therefore not as beneficial as vegetation three to six inches tall that stands upright (ibid). In this manner, there

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is more vegetation surface area to offer resistance to stormwater, hence slowing the waterâ&#x20AC;&#x2122;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 stripâ&#x20AC;&#x2122;s ability to slow waterâ&#x20AC;&#x2122;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 infiltration, 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 ponding 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 raingarden 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 â&#x20AC;&#x153;pondsâ&#x20AC;? 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 water 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

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buildup will occur as it does in any LID structure that acts to collect stormwater sedimentation. 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 stormwater), 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 retention ponds with topographical variety designed to develop many different ecological wetland-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 tolerate 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 â&#x20AC;&#x153;pocket wetlandsâ&#x20AC;? (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 ecologies 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 Figure 25. Undulating topography of a constructed wetland provides many different ecologies for both fauna and flora.

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some form of liner applied; well-


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 forebay 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 reaching 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 wetland’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 velocity and shallow flow patterns. The water is generally encouraged to meander in order to slow the water Figure 26. Sections of the four basic types of constructed wetlands, as set forth in Metropolitan Council 2001, p. 3-234.

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 â&#x20AC;&#x2DC;Constructed Wetlands Stormwater Wetlandsâ&#x20AC;&#x2122; (2010).

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Urban Bioretention Methods The bioretention methods discussed so far may require more land for construction in comparison to that of traditional piping methods. In urban settings, enough space for elements such as constructed wetlands, detention ponds, or even infiltration strips may not be

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.

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 â&#x20AC;&#x153;green streetsâ&#x20AC;? 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 techniques especially beneficial in these areas.

Figure 28. Bump outs can also collect water from sidewalks, and can be used to narrow a wide street. In this manner, bump outs can perform as traffic calming devises.

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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 bioretention cells, a traditional parking lot design can easily begin infiltrating water instead of disposing 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

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parking lots could also be constructed with porous pavement to further reduce the stormwater runoff levels. Additionally, in these situations, the porous pavementâ&#x20AC;&#x2122;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 buildingâ&#x20AC;&#x2122;s roof where the water is directed into a planter box that incorporates vegetation and well-draining media. Portland, Oregon has adopted these planters successfully into sidewalks to

Figure 29. Installation of flow-through planters with side walls to contain stormwater within the planters, and builtin weirs for slowing and holding stormwater (Greenworks 2010).

handle water flow from streets and sidewalks (Figures 29 and 30). In both cases, the planters provide for attractive greenspaces 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).

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stormwater in urban settings. When the planter boxes are installed above the ground, this method also eliminates the concern of weakening a soilâ&#x20AC;&#x2122;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.

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Chapter 5 - Roof Stormwater Controls Rainbarrels & Cisterns In addition to handling stormwater from hardscapes, LID techniques also offer solutions 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 stormwater 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 buildingâ&#x20AC;&#x2122;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 barrels 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

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hold stormwater, but to provide water for later irrigation usage. This saves property owners 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 stormwater 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 growing 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 â&#x20AC;&#x201C; from two and a half inches to a maximum of six inches. The growing

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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 nutrients. 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 permeable 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 soilwater 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 natural processes to cleanse stormwater. Without plants and a biologically diverse soil matrix,

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LID simply wouldn’t work. It is the complex relationship between soil chemistry, microorganism 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 neighborhoods 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 developed 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

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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.

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Knowledgeable LID Patterning for Ecologically-Sensitive Developments  

Undergraduate thesis paper written for partial fulfillment of the requirements of the Honors Program of the Department of Landscape Architec...

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