PROJECT AREA:

NEWBURGH

HUDSON VALLEY REGIONAL COUNCIL

3 Washington Center, Newburgh NY 12550 http://www.hudsonvalleyregionalcouncil.com/

GREEN INFRASTRUCTURE CONCEPT PLAN FOR NEWBURGH ARMORY UNITY CENTER

Project type: Retrofits for multipurpose community center DECEMBER 2011 Proposed practices: 1- Permeable paving 2- Rainwater harvesting

3- Green Roof 4- Rain garden 5- Tree planting

Google 2011

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The following report describes a schematic landscape design proposal using green infrastructure practices for stormwater management. This plan is intended to give practical guidance for the owner, design professionals, contractors, and other interested parties to use in developing a final design. It is not intended to be used as a final design or for construction documents.

OVERVIEW

The City of Newburgh acquired the Newburgh Armory site and in early 2011 finalized an agreement with a new non-profit organization, the Newburgh Armory Unity Center, to lease the site for use as a community center, with the primary focus being programs and facilities for inner city youth. The 12.1 acre site houses a building with a number of different sections totaling 73,000 square feet, including classrooms, a gymnasium that has been newly renovated, a large old drill hall, and other space. A number of athletic programs and a community garden are operating at the center, and additional programming is planned. The director, Deirdre Glenn, has expressed interest in developing demonstration and training projects incorporating green infrastructure. This green infrastructure concept plan has been prepared in consultation with Ms. Glenn and with Alexandra Church, the author of a Site and Land Use Plan (Site Plan) finalized in August 2011. The Site Plan recognizes the potential for demonstrating green infrastructure (GI) on the grounds and this GI concept plan was developed to fit with the Site Plan’s recommendations. The Site Plan indicates several areas where green infrastructure could be introduced, and the concept plan presented here includes these practices and several others. Green infrastructure practices at this site can provide an important demonstration opportunity for the community as the City of Newburgh develops plans for remediating significant combined sewer overflows from the City’s sewer system. Ian MacDougall, Acting Director of Planning and Economic Development, City of Newburgh, and Chris Hawkins, P.E., who has provided pro-bono services to help the Center plan and manage building and site upgrades, also provided information and assistance for development of this concept plan, and we appreciate the interest and support of Ms. Glenn, Ms. Church, Mr. MacDougall and Mr. Hawkins.

This report includes four green infrastructure plans for the Newburgh Armory Unity Center site, each focusing on one set of practices proposed for one or more locations on the site: Green roofs, downspout disconnection, trees and other plantings, and permeable paving.

ROOFS Downspout disconnection Green Roofs

PLANTINGS Courtyard garden at new east entrance Tree plantings in locations along the proposed parking areas and along the street Rain gardens at front entrance and new east entrance

PAVING Permeable paving on driveways and courtyard pathways

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LOCATION

STREET ADDRESS: 321 South William Street, Newburgh, New York 12550

SECTION 43, BLOCK 1, LOT 13

OWNERSHIP

City of Newburgh

EXISTING CONDITIONS

SOILS AND TOPOGRAPHY Most of the site is flat. The ground slopes down more steeply on the southwest corner. The soils on the site are classified as Dumps and Urban Land. According to information gathered for the Site Plan, much of the rear portion of the site is covered in a deep layer of compacted gravel. Based on observations by site occupants, stormwater drains into the soil very rapidly. The lawn and landscaped portion of the property at the front of the building, however, are not believed to have this gravel base.

SOLAR AND WIND EXPOSURE The site is generally exposed. Wooded areas on the southwest and southeast, the street trees, and the trees in the landscaped areas provide some shade. The building casts shade as noted where relevant in the following sections.

EXISTING VEGETATION The street trees are in very poor condition, having been pruned severely away from the overhead wires. The condition of the trees on the landscaped portion of the site varies. The trees should be assessed by a certified arborist before developing a final plan for conservation and new planting.

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Figure 1 Site Plan illustration from Newburgh Armory Unity Center Site and Land Use Plan prepared by Alexandra Church in August 2011.) This illustration includes proposed new trees and locations and numbers of proposed parking spaces.

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Figure 2 Green Infrastructure Concept Plan (An 11x17 version of the plan is included at the end of the report.)

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GREEN INFRASTRUCTURE SIZING AND DESIGN

The green infrastructure practices included in these plans are among those considered acceptable for runoff reduction in the New York State Stormwater Management Design Manual 2010 (Design Manual). These are practices that:

reduce calculated runoff from contributing areas capture the required water quality volume.

The Water Quality Volume (denoted as the WQv) is designed to improve water quality sizing to capture and treat 90% of the average annual stormwater runoff volume. For Kingston this 90% rainfall number is 1.1 inches. The WQv is directly related to the amount of impervious cover created at a site. The following equation can be used to determine the water quality storage volume WQv (in acre-feet of storage):

WQv = (P) (Rv)(A)/12 where: WQv = water quality volume (in acre-feet) P = 90% Rainfall Event Number Rv = 0.05 + 0.009(I), where I is percent impervious cover A = site area in acres (Contributing area) A minimum Rv of 0.2 will be applied to regulated sites.

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PROPOSED GREEN INFRASTRUCTURE PRACTICES DOWNSPOUT DISCONNECTION

Downspout disconnection is a relatively low-cost, simple GI retrofit opportunity for many buildings and is especially important as a GI practice that can be widely implemented over time for retrofitting urban areas like Newburgh that have wastewater overflow problems. The Armory can serve as a useful public demonstration site, and potentially a location for providing educational information and training workshops for building owners, contractors and others on this and other GI practices. At the Armory site, where roofs currently drain to an outside corner of the building where a new scupper and exterior downspouts could be installed, the runoff would be channeled to locations away from the foundation to an area where it would infiltrate. Because a large part of this site is known to be especially well-drained, it is believed that there’s very good potential for infiltrating runoff if downspouts are disconnected. According to the Design Manual this practice can be incorporated into the final plans for the green space shown on the plan, where:

“The contributing area of rooftop to each disconnected discharge shall be 500 square feet or less; larger roof areas up to 2,000 square feet may be acceptable with a suitable flow dispersion technique such as a level spreader;

The disconnected, contributing impervious area shall drain through a vegetated channel, swale, or filter strip (filtration/infiltration areas) for a distance equal to or greater than the disconnected, contributing impervious area length (page 5-70).

An example of this is the proposed new scupper and downspout at the corner of the rooftop garden , draining to Rain Garden B, is discussed in more detail in the Planting section below.

GREEN ROOFS

From the Design Manual: Green roofs reduce stormwater runoff volumes and attenuate peak flows by capturing rainwater and allowing evaporation and evapotranspiration. Green roof systems designed to cover the rooftop materials provide extra insulation that reduces energy costs, absorbs noise and protects the rooftop materials, and extends the life of the roof. Additional benefits include cooling the surrounding area and reducing the urban heat island effect, improving air quality, and providing habitat for birds and butterflies.

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The first step for greening an existing roof is an assessment of the structure to determine the load bearing capacity. This GI Concept Plan did not include evaluation of the building’s structure and the load bearing capacity of the existing roof structure is unknown. The roofs must be evaluated by professionals with the appropriate expertise and credentials as part of the final planning and design for any green roof project. While all the roofs could possibly be converted to extensive green roofs over time, when individual roof areas need to be re-roofed the easiest to convert would be the three flat ones. One roof that is accessible from the second floor and suitable for outdoor gatherings is shown on the Site Plan as a roof garden that could combine several green roof systems.

Stormwater treatment in green roofs occurs via evaporation, transpiration, and filtration, so the deeper the storage media and denser the plant the material the greater the benefits, and the heavier the weight load. Green roofs are broadly classified into two categories – extensive green roofs, which typically have planting media from 3-6” deep; and intensive green roofs, which have planting media deeper than 6”. Extensive green roofs systems typically range from 8 to 24 pounds saturated weight per square foot. They consist of a layered system including light weight soil and drainage media a root barrier and low growing, drought tolerant plants. They come in variety of prefabricated modules as trays, mats or rolls or

1 Design Manual, 5-87.

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can be custom installed. In contrast, intensive green roofs include larger plantings and soil volumes and usually used as roof gardens with furnishings. Besides the load bearing capacity of the roof, other factors to consider in developing a final design include how it will handle stormwater, local wind and solar exposure, and aesthetic goals. The roof proposed as a roof garden on the Site Plan is shaded by the building walls on the south and west, a factor which will need to be considered in developing a plan for plantings.

EXTENSIVE GREEN ROOFS The Armory has several flat roofs that could be converted to extensive green roofs. One example is the approximately 7,500 square foot roof on the front of the building. The sizing computations below are for an extensive green roof on this front section of the building, with a 3 inch soil layer and 2” drainage layer are given below. This calculation indicates that the storage volume of this green roof plan would exceed the water quality volume (WQv), a key design threshold for protecting water quality as detailed in Design Manual (and described above on page 6).

SIZING CALCULATIONS EXAMPLE FOR EXTENTSIVE GREEN ROOF

Roof area 7,500 sf

WQv = (P)(Rv)(A)/12

where:

P = 90% rainfall number = 1 in

Rv = 0.05+0.009 (I) = 0.05+0.009(100) = 0.95 1

I = the percentage of impervious area draining to site = 100% 1

A = area draining to practice = 7500 ft2

WQv = 653 ft3

Step 2: Calculate the drainage layer and soil media storage volume:

where:

AGR = green roof surface area = 7500 ft2

DSM = depth soil media = 0 ft

DDL = depth drainage layer = 0 ft

PSM = porosity of soil media = 0

PDL = porosity of drainage layer = 0

VSM = AGR x DSM x PSM 375

VDL = AGR x DDL x PDL 319

DP = ponding depth = 0.5 inches = 0.04 ft 0 ft

Storage Volume =VSM+VDL+(DP x AGR) = 994 ft3

Figure 3 Low-growing, drought tolerant sedums on extensive green roof

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INTENSIVE GREEN ROOF—ROOFTOP GARDEN An intensive green roof is proposed in the Site Plan for one roof on the northern side of the garage, which is accessible via a door from the former officer’s bar above the garage. A modified version of this approach is shown in this GI Concept Plan. The 2,500 square foot roof could be used as an outdoor meeting area that would include plantings in containers. Its use as a children’s garden with raised beds for edible plants was suggested. It is shaded by the building on the south and west so this limits the potential for certain plants. Shade loving plants could certainly be grown in containers and these are shown on the concept plan along with an area of low green roof surface at the east end. The plan shows a new scupper at this corner and an alternative approach channeling roof runoff to a rain garden (discussed in a later section below.) The area of low plantings at the end of the roof could be custom designed with some gentle topography, grasses and stones. It would be designed with a passage to allow runoff from the impervious roof surface to move through it to the roof drain. The depth of soil in this area might range from 12”-18”.

Figure 4 Example green roof profiles -- roll out system (left); modular tray system (right).

Figure 5 Roof Garden Plan

Alternative

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MATERIALS The general components of any green roof system include:

a roof structure capable of supporting the weight of a green roof system a waterproofing barrier layer designed to protect the building and roof structure a drainage layer consisting of a porous media capable of water storage for plant uptake and

storm buffering a geosynthetic layer to prevent fine soil media from clogging the porous media soil with

appropriate characteristics to support selected green roof plants growing media plants with appropriate tolerance for regional climate variation, harsh rooftop conditions and

shallow rooting depths (Design Manual 5-86)

CONSTRUCTION STEPS For any system, the first steps would be to inspect the underlying roof components and install edging as required. The specific construction steps would be determined by the final design. The steps shown below are for a roll type green roof system. The basic installation steps for a mat or roll type green roof system would be as follows Install:

Root barrier Drain Mat Retention Fleece Growing Medium Vegetation Mat Fill in or redistribute displaced growing medium Water thoroughly Install ballast or paver along edge

MAINTENANCE CONSIDERATIONS Green roof maintenance may include watering, fertilizing and weeding and is typically greatest in the first two years as plants become established. Roof drains should be cleared when soil substrate, vegetation or debris clog the drain inlet. Maintenance largely depends on the type of green roof system installed and the type of vegetation planted. Maintenance requirements in intensive systems are generally more costly and continuous, compared to extensive systems. The use of native vegetation is recommended to reduce plant maintenance in both extensive and intensive systems. A green roof should be monitored after completion for plant establishment, leaks and other functional or structural concerns. Vegetation should be monitored for establishment and viability, particularly in the first two years. Irrigation and fertilization is typically only a consideration during the first year before plants are established. After the first year, maintenance consists of two visits per year for weeding of invasive species, and safety and membrane inspections (Magco, 2003).2

2 In Design Manual pages 5-94-5-95.

Figure 6 Mixed planting on green roof at Chicago City Hall (Photo: Christopher Macsurak, accessed 2011 Flicker.com)

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COST Two Sources of Cost Data For installation, maintenance costs and lifespan data for the practices discussed here, the Cost Sheet developed by the Center for Neighborhood Technology (CNT) in collaboration with the US EPA Office of Wetlands, Oceans, and Watersheds (OWOW), Assessment and Watershed Protection Division, Non-Point Source Branch, provides useful information based on examples from various locations. It may be found at their website. http://greenvalues.cnt.org/national/cost_detail.php Another useful source of cost data can be found in the Center of Watershed Protection's Urban Subwatershed Restoration Manual Series. Manual 3: Urban Stormwater Retrofit Practices, pages E-1 though 14, includes a discussion of costs in terms of the amount of stormwater treated. The information was compiled in 2006, so an increase about 10 percent should be factored in to account of cost of living increases. http://www.cwp.org/categoryblog/92-urban-subwatershed-restoration-manual-series.html

One green roof has recently been installed in Newburgh at the new Kaplan Hall at the SUNY-Orange campus. The designers and installers of this project may be useful sources of information.

RAIN GARDENS

Rain gardens capture and treat runoff on site. They are slightly depressed below the surrounding grade and allow runoff to pond temporarily, providing detention, and infiltration and pollutant removal benefits. Water above the ponding limit exits through and overflow device. Two rain gardens are shown on the plan. Rain Garden A is proposed for the main entrance circle at the front of the building to receive runoff from part of the paving. Rain Garden B would be would be part of the new planting and entrance court on the east side of the building and would receive runoff from the roof garden through a new downspout. It could include native shrubs, grasses and colorful flowering perennials. All of these would be tolerant of the wet and dry conditions of the rain garden.

RAIN GARDEN A

Runoff from the paving in the front driveway that currently flows to the inlet would be channeled to the rain garden instead. A depressed concrete collar around the inlet would allow the runoff to flow into stone runnels that would be installed on each side of the flag pole. The garden could include a rain garden zone combined with more conventional planting since the area available is larger than necessary to capture the targeted amount of runoff from the driveway drainage area that would carry most of the pollutants (the WQv . An underdrain would be connected back to the catch basin if needed.

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Figure 7 A slightly depressed collar around the inlet and stone runnels would channel the runoff to the garden.

Figure 8 Existing view towards the proposed garden site

Figure 9 Visualization of garden, elimination of wires, and new paving in the crosswalks.

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RAIN GARDEN B

If a new scupper is installed to replace the drain at the east end of the rooftop garden the exterior downspout could discharge through a runnel to the Rain Garden B. This would be an alternative to the partial green roof plan. It would be designed to overflow to adjacent lawn.

CONSTRUCTION STEPS Excavate to a depth of 24” Back fill with a 6-10” layer of clean washed gravel (approximately 1.5-2.0 inch diameter rock) Fill to rain garden bed depth with certified soil mix Install planting Top dress with 3” layer shredded hardwood mulch

MATERIALS Plants would be selected that are adaptable to wet and dry conditions, easy to maintain, and make an attractive contribution to the garden area overall. Recommended native shrubs, grasses, and herbaceous plants for rain garden are listed on page 5-86 of the Design Manual. Plants with well-established root systems would be required in order to establish the gardens quickly and effectively. Mulch -- 3” layer of shredded hardwood mulch

MAINTENANCE CONSIDERATIONS Rain gardens are intended to be relatively low maintenance. Weeding and watering are essential the first year, and can be minimized with the use of a weed free mulch layer. Rain gardens should be treated as a component of the landscaping, with routine maintenance including the occasional replacement of plants, mulching, weeding and thinning to maintain the desired appearance (Adapted from the Design Manual page 5-84)

Figure 10 Plan of Rain Garden B

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Rain Garden A

As noted above, the area available for a rain garden in front of the building is much larger than needed for a rain garden that would capture the water quality volume (WQv). According to the calculations below a rain garden designed with a surface area of 600 square feet would be more than adequate for the estimated 5,000 square foot drainage area.

Total Drainage Area 5000 Ft2

Step 1: Calculate Water Quality Volume (WQv)

WQv = (P) (Rv) (A) / 12

P = 90% rainfall number = 1.1 inches

Rv = 0.05+0.009 (I), if Rv < 20%, use Rv = 20% 95%

I = percent impervious of area draining to practice = 100%

% of Total area that drains to practice 100%

A = Area draining to practice = 5000 Ft2

WQv = 435 Ft3

Step 2: Calculate for drainage layer and soil media volume:

VSM = ARG x DSM X nSM

VDL = ARG x DDL X nDL

ARG = proposed rain garden surface area = 600 ft2

DSM = depth soil media = 2 ft

DDL = depth drainage layer = 0.5 ft

nSM = porosity of soil media = 0.2

nDL = porosity of drainage layer = 0.4

VSM = volume of soil media = ARG * DSM * nSM 240 Ft3

VDL = volume of gravel drainage layer = ARG * DDL * nDL 120 Ft3

DP = ponding depth above surface = 0.5 ft

WQv less or equal to the soil volume + the gravel volume + the

olu e of the po ded ater, hich is ≤ VSM+VDL+ DP x ARG ? 660 ft2

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Rain Garden B

The rain garden would receive the runoff from and estimated 1,000 square feet of the roof, and as shown in this example, a shallow garden with and 18” would be more than adequate.

Total Drainage Area 1000 Ft2

Step 1: Calculate Water Quality Volume (WQv)

WQv = (P) (Rv) (A) / 12

P = 90% rainfall number = 1.1 inches

Rv = 0.05+0.009 (I), if Rv < 20%, use Rv = 20% 95%

I = percent impervious of area draining to practice = 100%

% of Total area that drains to practice 100%

A = Area draining to practice = 1000 Ft2

WQv = 87 Ft3

Step 2: Calculate for drainage layer and soil media volume:

VSM = ARG x DSM X nSM

VDL = ARG x DDL X nDL

ARG = proposed rain garden surface area = 150 ft2

DSM = depth soil media = 1.5 ft

DDL = depth drainage layer = 0.5 ft

nSM = porosity of soil media = 0.2

nDL = porosity of drainage layer = 0.4

VSM = volume of soil media = ARG * DSM * nSM 45 Ft3

VDL = volume of gravel drainage layer = ARG * DDL * nDL 30 Ft3

DP = ponding depth above surface = 0.5 ft

WQv less or equal to the soil volume + the gravel volume + the volume of

the po ded ater, hich is ≤ VSM+VDL+ DP x ARG ? 150 ft2

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TREE PLANTINGS

Tree plantings reduce runoff in several ways, including intercepting rainfall in the canopy, holding water in surrounding soils, and taking water up through the roots and releasing it through evapotranspiration. Street tree pits with good quality, uncompacted soil will enhance the infiltration of runoff, and tree roots and leaf litter enhance the soil conditions for infiltration. In addition to these stormwater management functions, trees can provide many other benefits including shading and cooling, buffering wind and noise, purifying air and beautification.

STREET TREES A plan for the replacement of trees on the site frontage and all along South William Street should be developed. If the overhead wires were placed below ground, new trees could be planted in the green strip along the curb. The strip is narrow, but the roots of the trees would grow in the adjacent lawn area. This would not be the case in front of the building to the east of the Armory site – in that area new tree plantings should include replacing a portion of the sidewalk, and installing structural soil below it. If the utility wires are not placed underground new trees planted along the street would be smaller species and would be planted in available locations set back from the wires. Trees would be selected according to aesthetic and functional criteria. Monoculture planting should be avoided. (See the resources at the end of this section for information about visually compatible trees.)

Figure 11 Existing street trees (view west) Figure 12 Existing street tree (view east)

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TREES IN EXISTING LAWN AND NEW GREEN SPACES.

New trees are shown in existing lawn and in areas that are currently paved. For all areas, chemical, biological, drain, percolation, and infiltration tests should be conducted prior to the development of the final design. Site preparation would be based on soil conditions revealed in the assessment, including drainage, pH range, compaction levels, texture and other factors. New planting bed installation for the paved areas will require removing the deep gravel layer and providing adequate soil volume for the long term healthy growth of the trees. The large trees shown on the plan would have mature canopies in the range of fifty feet or more. Soil Volume and Tree Size

Soil volume calculations should take into account a variety of specific factors including the soil type, whether the tree is growing in an open space or surrounded by paving, local climate conditions such as reflected heat and from cars, and other factors revealed in the complete site assessment. For the purpose of this plan, a good quality loam soil 3 feet deep is assumed, and healthy, large canopy trees are the goal. The chart below, developed by James Urban, shows that the soil volumes exceeding 1600 cubic feet would be required for trees with an ultimate crown projection over 1200 square feet, or about 40 feet in diameter. A general rule of thumb is a ratio of 2 CF of soil to 1 SF of mature crown spread. (Grabosky and others, 1999; Urban, 1999). 3 Another factor to consider is the positive effect of extended pits for multiple trees -- when trees share soil, the volume of soil per tree is reduced.

3 In Urban Watershed Forestry Manual Part 3: Urban Tree Planting Guide, United States Department of

Agriculture Forest Service Northeastern Area State and Private Forestry NA-TP-01-06,September 2006, page 26.

Figure 14 The soil volume required for various size trees assumes a soil depth of 3 feet. (Source: James Urban) in Urban Watershed Forestry Manual - Part 3 page 26.)

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Tree planting with Silva Cells or Structural Soil According to the soil volume calculation procedure above, the large canopy trees shown on the plan would need approximately 2,000 cubic feet of soil. Where trees are shown in the proposed parking area on the east side of the building the rooting zone would be extended under the paving to provide the necessary soil volume using structural soil or Silva Cells.

Silva cells have a frame and deck that would be filled with good quality soil. Structural soil, combines crushed stone, clay loam and a hydrogel agent to support the paving and extend the rooting zone. B.4

CONSTRUCTION STEPS Prepare tree pits according to the final design, including soil amendment and structural support

installation Plant trees according to approved specification prepared by a qualified design professional Apply mulch Plant ground cover or turf as required

MATERIALS Soil and Soil Amendments: as required in final design Structural support for paved areas: Silva cells or structural soil Trees Mulch: Three inch layer in area at least 5 feet in diameter around the base of the tree (below the

root flare).

MAINTENANCE CONSIDERATIONS Well-prepared planting areas designed with appropriate plants and soils require routine maintenance. During the establishment period just after planting the new tree plantings would be watered using water bags and spot watering with a clear understanding of the requirements of the trees to avoid over- or under-watering. Instructions for watering and for monitoring for disease or damage and removing stakes are included in the Appendix. Ongoing maintenance would include occasional pruning and replacements, twice yearly clean up and yearly application of mulch.

4 For more information on Silva Cells, see DeepRoot at http://www.deeproot.com/products/silva-cell/cost.html.

For information on structural soil, the following visit the website of the Urban Horticulture Institute of Cornell University: http://www.hort.cornell.edu/uhi/index.html .

Figure 13 Silva Cells below permeable paving Copyright 2008. Casey Trees, Washington D.C..

Figure 14 Structural soil below sidewalk Copyright 2008. Casey Trees. Washington D.C.

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COST INFORMATION

Silva Cells The following information is provided by DeepRoot, the manufacturer of Silva Cells (http://www.deeproot.com/products/silva-cell/cost.html: Accessed 11/5/2011)

Each Silva Cell installation is unique to existing site requirements. Costs will vary based on characteristics of the site, the quantity of Silva Cells required for the project, the tree size and stormwater treatment goals, and the design objectives. Remember that each frame is 48'' (1200 mm) long x 24'' (600 mm) wide x 16'' (400 mm) high and holds about 10 ft3 (.28 m3) of soil. According to bid tabulations from projects across North America, the Silva Cell system generally costs $14 - $18 per cubic foot installed (that estimate includes everything except the base course, the final paving and the tree itself.)

Structural Soil According to Nina Bassuk of Cornell University’s Urban Horticulture Institute (in CU Structural Soil: An Update after More than a Decade of Use in the Urban Environment, 2008), structural soil costs in the range of $35-42 per ton.

RESOURCES The following resources on site assessment and tree selection are recommended: From Urban Horticulture Institute of Cornell University at http://www.hort.cornell.edu/uhi/:

Recommended Urban Trees: Site Assessment and Tree Selection for Urban Tolerance. Urban Horticulture Institute, Department of Horticulture, Cornell University, Ithaca, NY.

Visual Similarity and Biological Diversity: Street Tree Selection and Design. Bassuk, Nina,.

Trowbridge, Peter. Grohs, Carol. From the Center for Watershed Protection http://www.cwp.org/documents/cat_view/69-urban-watershed-forestry-manual-series.html

Urban Watershed Forestry Manual,Part 3:.Urban Tree Planting Guide. Cappiella, Schueler, Tomlinson, Wright. Center for Watershed Protection and USDA Forest Service, Sept 2006.

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PERMEABLE PAVING The GI concept plan shows two types of permeable paving – small permeable concrete interlocking pavers for pedestrian areas and larger PaveDrain® concrete pavers for driveways and parking. The new east entrance courtyard would include pedestrian areas that could be paved with permeable concrete interlocking pavers. The area is currently paved with asphalt. The plan shows two paths and a plaza that could be designed for gathering and seating as well.

The driveway and parking areas could all utilize permeable paving. The site appears to present an unusual opportunity for this since a deep base course of gravel already exists. Testing of the base for infiltration would be required, and if it is suitable the additional cost of excavation and stone usually required for permeable paving might be drastically reduced.

PaveDrain® is a permeable concrete paver that has recently been developed and is proposed here. Other options include poured in place concrete or asphalt.

Figure 17 Existing asphalt on east side Figure 18 Existing compacted gravel

Figure 16 Example of permeable concrete pavers http://www.pavestone.com/blog/project-of-the-month-spotlight-on-permeable-paving

Figure 15 Location of proposed new east entrance court

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Figure 19 PaveDrain® arched design (left) and installation of wired mats.

The PaveDrain system is a recent introduction. Like the typical permeable concrete unit paver system, it allows runoff to flow through cracks between the units into the gravel base course, but the arched design allows for the elimination of the finer setting bed aggregate,which may reduce the likelihood of clogging. The units are assembled into mats for easy installation, and the size of the mat can be customized. This would allow for a design where the mat is intended to be lifted periodically for cleaning a filter zone below.

TYPICAL CONSTRUCTION STEPS The constructions steps would follow specifications developed by a qualified professional and would vary with according to the type of permeable paver used. Typical construction steps are as follows:

Excavate to proposed depth and level the bottom of infiltration bed. Place geotextile if required Place sub base as required by final design Install edge restraint Place permeable interlocking pavers or paving mats Place joint aggregate if required

MATERIALS Typical manufacture’s specifications for permeable interlocking concrete pavers require the following materials, and as previously mentioned, the PaveDrain system would not require the granular base (setting bed).

Concrete pavers Granular subbase Granular base Bedding and void opening aggregates Edge restraints Underdrain if required Geotextile fabric (optional)

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MAINTENANCE CONSIDERATIONS The paving should be kept clean of debris. Vacuum sweep as needed. Upland and adjacent areas should be kept mowed and bare areas should be seeded. The need for sweeping is reduced or even eliminated in areas where there is little opportunity for debris to fall onto the surface and get ground down into the spaces between the pavers. Two excellent fact sheets on permeable and porous paving are available from the NC State University Stormwater Engineering Group at http://www.bae.ncsu.edu/stormwater/pubs.htm: 5

Research Update and Design Implications

Maintaining Permeable Pavements

COST Generally permeable surface materials are comparable in cost to their impervious counterparts, but the added excavation and gravel base increases the cost. However, permeable paving also can reduce or eliminate other costs for conventional stormwater for pipes, basins, and additional land. According to the PaveDrain® website: Depending on location and project size a conservative installed cost of the PaveDrain System is $10-11 per SF. This typically includes an installed 6 - 12" layer of clear stone (#3, #57 (TBD) and 1-inch of #8). The installation of the PaveDrain blocks or mats will be around $2.00- $2.50 per SF. The materials cost will be $5.00-$6.00/SF. Delivery will add $0.50-$1.00 per SF depending on the distance to the jobsite. Color blocks adds ± $1 per SF.6

SIZING COMPUTATIONS FOR PERMEABLE PAVING The underlying conditions already include a deep gravel base with dimensions and characteristics that may be appropriate for permeable surfaces. Sizing computations are based on the depth and permeability of the base, which would need to be determined.

Report and plan by Simon Gruber and Marcy Denker. Thanks to Deirdre Glenn, Alexandra Church, Chris Hawkins, Ian MacDougall and Matthew Ryan for their support and contributions.

5 Urban Waterways, NC State University and A&T State University Cooperative Extension.2011. 6 http://www.pavedrain.com/faqs.php (accessed 11/9/2011).