PROJECT AREA: KINGSTON
HUDSON VALLEY REGIONAL COUNCIL
3 Washington Center, Newburgh NY 12550 http://www.hudsonvalleyregionalcouncil
Google 2011
GREEN INFRASTRUCTURE CONCEPT PLAN FOR GEORGE WASHINGTON ELEMENTARY SCHOOL
Project type: School campus retrofits
Proposed practices: 1- Parking area reduction 2 - Tree plantings
3 - Permeable paving
4 - Bioretention Areas
DECEMBER 2011
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The following draft report describes a schematic landscape design proposal using green
infrastructure practices for stormwater management. The illustrated plan and report are intended
to give practical guidance for the owner, design professionals, contractors, and other interested
parties to use in developing a final design. They are not intended to be used as final design and
construction documents.
OVERVIEW
The George Washington Elementary School is collaborating with the Kingston Land Trust in creating a
natural playscape outside of classrooms for the younger grades, known as the the Children’s House. A
natural playscape is an outdoor play environment featuring elements like logs, boulders, trees, plants,
and water, encouraging active play and challenging children to investigate the physical world around
them. To help guide the planning, design, and construction of this project, a Natural Playscape
Committee has been formed, guided by Ann Loeding at the Kingston Land Trust.1 Jennifer Schwartz
Berky, Deputy Director of the Ulster County Planning Department, recommended the site to the green
infrastructure planning team. Principal Valerie Hannum expressed an interest in softer, more porous,
environmentally friendly surfaces not only for the playscape, but throughout the campus, which includes a
large amount of impervious paving.
The menu of options presented here concerns the west and south portions of the site -- in the three
parking areas, the existing playground, and the proposed playscape.
The area for the proposed playscape is on the south side of the building and is accessible from the
classrooms. It is currently 100 percent impervious, consisting of asphalt and concrete. A concrete channel
extending along the edge of the area conveys parking lot runoff to a storm drain. A portion of the parking
lot next to the proposed playscape is wider than required and could be narrowed to create a substantial
green buffer between the play area and the cars. The concrete channel could be removed if the
stormwater is handled in other practices. The final plan could include new ways to celebrate the water in
the playscape, like a small meandering dry stream with a pebble surface and shallow ponding areas
where the children could play.
On the west side of the side building a large expanse of asphalt paving that is currently used for
playground activities and school buses is larger than necessary as well. Several alternatives are
proposed for adding green infrastructure there --- new tree plantings, a bioretention garden, and areas of
permeable paving.
Taken one by one these gi practices would contribute to improving Kingston’s water quality. Some of the
practices are interdependent pieces of a whole design strategy, and others could be considered
separately.
The following practices are proposed:
1- Tree plantings
2- Permeable paving
3- Bioretention Areas
([email protected] or visit www.kingstonlandtrust.org)
http://kingstoncitizens.org/2011/03/22/the-kingston-land-trust-forms-a-natural-playscape-committeeand-partners-with-the-gw-elementary-school/
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LOCATION
STREET ADDRESS:
67 Wall Street, Kingston, NY 12401
SECTION, BLOCK, LOT: 56.124-1-1
Figure 1 Parcel Map Ulster County Web Map
http://gis.co.ulster.ny.us/ (10/2011)
OWNERSHIP
Kingston Public Schools
EXISTING CONDITIONS
SURFACE COVER/CONTRIBUTING AREA
The surface cover distributed approximately as follows:
Roof .75 acres
Lawn and landscaped 2.2 acres
Paving 1.75 acres
SOILS AND TOPOGRAPHY
The plan concerns the west and south sides of the site. The large parking lot on the left side of the rear
entrance slopes very gently towards the west and south. Part of the runoff drains to the playground and
the part flows down the driveway leading to the lower level parking, where it flows into an inlet labeled
Inlet A on the plan.
The Web Soil Survey Report of the area under consideration indicates one soil type:
RvB Riverhead fine sandy loam, 3 to 8 percent.2 This is a well drained soil well suited to infiltration
practices.
SOLAR AND WIND EXPOSURE
The rear of the site is open to the sun in areas proposed for retrofits on the west side of the site. On the
south, the building provides some shade and blocks the wind to varying degrees, and the opportunities
and constraints arising from this should be considered in the final planting design.
VEGETATION
There are large shade trees on the west side of the playground, which includes an area of turf. Part of the
strip between the lower parking lot and the concrete channel is turf as well.
2
Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Web Soil
Survey. Available online at http://websoilsurvey.nrcs.usda.gov/. Accessed May 14, 2011.
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Figure 2 View north
Figure 3 View northeast
Figure 4 Playground
Figure 5 Paved gutter from small parking lot
Figure 6
Aerial view of upper level
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INLET A
Figure 7 Inlet A drains to paved channel
Figure 8 Impervious area on south side of building
Figure 9 Grassy area along the eastern end of the channel
Figure 10 Paved channel slopes to drain here
Inlet B
Inlet A
Figure 11 Aerial view of lower level. Concrete channel shown as blue arrows. Runoff from the paving on the west and
east is channeled to Inlet B.
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Figure 12 Concept plan (11x17 plan is included at the end of the report)
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THE CONCEPT
The following discussion introduces the elements and explains how the parts on the illustrated
masterplan relate to each other and briefly describes each of the practices. The detailed
discussion of design issues, materials, maintenance and costs is provided in the last section.
Upper Level
On this site the large amount of impervious area on the upper level can be reduced by adding an
extended tree pit, expanding the lawn and landscaped portion of the playground and adding permeable
paving. Each of the practices would be designed to capture and treat runoff from an impervious drainage
area. Drainage areas are shown as blue lines on the plan.
The plan would not only reduce impervious surface on this part of the site but would also redirect runoff
so that less of it flows down the hill and into the inlet (Inlet A) leading into the proposed playscape. A
small “water bar”, like a low speed bump, or a trench drain would be installed at the end of the entrance
drive so that some of the runoff that currently flows from the large lot down into Inlet A would instead flow
into the Bioretention Garden A by the playground. A row of parking stalls in the small lot on the right of
the entrance would be replaced with Bioretention Garden B, and the asphalt chute that currently drains
this lot would be eliminated. Runoff would instead flow first into the bioretention garden and overflow
would discharge through a drainpipe.
Areas of permeable paving along the edge of the playground turf would define a play lot that could be
used for snow storage in the winter and a row of parking stalls to replace those eliminated from the small
lot. A portion of the paving along the edge of the playground could be converted to permeable pavement
to capture and treat runoff from the adjacent impervious surface.
Lower Level
On the lower level, the west side of the parking lot could be narrowed to the standard 60 foot width, and
new green space could be created instead. Removing this strip of impervious surface would eliminate
most of the runoff that currently flows into the channel from the west end of the lower lot. The asphalt in
the proposed playscape would be converted to pervious surfaces – lawn and planting beds. The paths
could be permeable as well, or alternatively, impervious materials that would shed runoff to adjacent
landscaped areas to infiltrate could be used.
Once most of this area is made permeable it would no longer contribute significant runoff. With the
conversion of a portion of the parking lot and the paved yard to impervious surface, the channel would no
longer be needed. The new greenspace would include a bioretention garden and would be graded to
channel surface runoff to the existing inlet (B). To the east of this inlet, the playscape design could
incorporate a little meandering water feature with small weirs to capture and pond water if desired.
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Green Infrastructure Practices
1- Tree plantings
Tree plantings intercept rainfall in the canopy and release it through evapotranspiration. Tree pits with
good quality, uncompacted soil will infiltrate 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.
The extended tree pit shown in the upper parking lot would be designed with adequate soil volume to
support healthy growth of large shade trees, maximizing stormwater benefits and reducing the urban heat
island effect. The underlying soils in this area of Kingston are very well drained, and, if the site
assessment confirmed an adequate infiltration rate, parking lot runoff could be allowed to drain into the
pits through curb cuts. Areas between the trees could be planted with low maintenance shrubs, grasses
or other groundcovers. Native plants would be recommended.
Evergreen and deciduous trees would be added in the proposed green space along the existing
playground. Deciduous trees are shown along the south side of the building, where they would provide
shade in the warm months and allow sunlight in during the winter.
2-Pervious paving
One area of pervious asphalt paving is shown on the plan in the upper parking lot. Runoff from the
adjacent impervious asphalt would flow through small openings in the surface of the pervious paving and
into a stone base, then infiltrate into the underlying soil. Because this kind of paving drains rapidly, it is
less prone to freezing in the winter, reducing the need for applying salt, which pollutes our waterways.
Installations in the local area and in settings in areas farther north, with heavy snowfall and frequent
plowing have established the ability of porous asphalt to function well in all seasons. (Note that other
types of permeable paving would be possible here, and concrete pavers would be good option.)
Figure 14 Runoff from conventional asphalt on left
flows to porous asphalt parking stalls on right (Cahill
Figure 13 Trees in extended tree pits in Battery
Park, NYC. Photo:Urban Horticulture Institute, Cornell
Associates, Morris Arboretum)
University
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2- Bioretention Areas
Three bioretention areas are shown on the plan. These are gardens that capture and treat runoff on site.
They are slightly depressed below the surrounding grade and allow runoff to pond temporarily, providing
detention and pollutant removal benefits. Depending on the underlying soil type, water can infiltrate or exit
the system through an underdrain to the storm sewer. The well drained soils in this part of Kingston are
well suited for infiltration practices like bioretention garden. Plantings would consist of low maintenance
shrubs, perennials, and grasses that are well adapted to the alternating wet and dry conditions. An
interested palette of native plants that provide wildlife habitat and beautify the playground and parking
areas could be used.
Figure 15 Native planting (Photo: NRCS
http://www.ia.nrcs.usda.gov/features/urbanphotos.html
Figure 16 Bioretention area (Photo: NRCS
http://www.ia.nrcs.usda.gov/features/urbanphotos.html
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A WORD ON COSTS
Green infrastructure costs for retrofits are hard to state accurately. In new construction there is often
considerably lower cost up front using and green infrastructure practices and planning versus
conventional, big pipe systems. But where that “gray infrastructure” is already in place, assessing the
value of adding a gi practice requires a fuller accounting. A recent report by the Center for Clean Air
Policy states:
The value of green infrastructure actions is calculated by comparison to the cost of “hard”
infrastructure alternatives, the value of avoided damages, or market preferences that enhance
value (e.g. property value). Green infrastructure benefits generally can be divided into five
categories of environmental protection:
(1) Land-value,
(2) Quality of life,
(3) Public health,
(4) Hazard mitigation, and
(5) Regulatory compliance.
The report sites, for example, New York City’s 2010 Green Infrastructure Plan, “which aims to reduce the
city’s sewer management costs by $2.4 billion over 20 years. The plan estimates that every fully
vegetated acre of green infrastructure would provide total annual benefits of $8,522 in reduced energy
demand, $166 in reduced CO2 emissions, $1,044 in improved air quality, and $4,725 in increased
property value. It estimates that the city can reduce CSO volumes by 2 billion gallons by 2030, using
green practicesat a total cost of $1.5 billion less than traditional methods. 3
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, NonPoint 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 E1 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-manualseries.html
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The Value of Green Infrastructure for Urban Climate Adaptation. Center for Clean Air Policy. Josh Foster, Ashley Lowe, Steve
Winkelman. February 2011.
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DESIGN, CONSTRUCTION, AND MAINTENANCE
The following section provides details about the specific design, materials, construction and
maintenance considerations and the sizing calculations for each practice.
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 Department of Environmental Conservation Stormwater
Management Design Manual 2010. The green infrastructure techniques include 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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1- TREE PLANTING
DESIGN
The discussion of tree planting here focuses on the tree pit proposed for the upper parking lot. The tree
pit edge would be designed to allow water to flow into it. The area between trees could be planted with
turf, groundcovers, low maintenance shrubs, or a meadow mix to be maintained with occasional mowing.
Three large canopy trees are shown in a tree pit 15 ’x 110’. As shown in the chart below this would
provide adequate soil volume to support the healthy growth of large trees over the long term. Assuming a
3’ depth there would be 1,650 cubic feet of soil available for each tree. Trees with mature canopies over
50 feet (crown projection of about 2,000 square feet) could be selected.
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.
Figure 17 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.)
To maximize stormwater management functions, tree species with a large mature canopy size and a high
Leaf Area Index (LAI) should be selected. “The LAI of a tree represents the relative surface area of
leaves and branches. The LAI is important in terms of potential for trapping small rainfall events and thus
potential for reduction of storm water runoff. LAI is also an important factor in a tree’s ability to yield
various benefits of air pollution reduction. Values for LAI for various common tree species are currently
under development.”4
4
Urban Watershed Forestry Manual,Part 3:.Urban Tree Planting Guide. Cappiella, Schueler, Tomlinson, Wright.
Center for Watershed Protection and USDA Forest Service, Sept 2006, page 17.
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CONSTRUCTION STEPS
Prepare tree pits according to the final design, including soil amendment. Site preparation would
be based on soil conditions revealed in an initial assessment, including drainage, pH range,
compaction levels, texture and other factors.
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
Deciduous Trees
Evergreen 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.
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-urbanwatershed-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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2- POROUS ASPHALT PAVING
DESIGN
Pervious asphalt with an open graded sub base would be installed at the edge of the existing playground.
The size of the area is limited on the plan to what would be required to capture the WQv. Increasing the
area would allow more runoff to infiltrate into the soil below and kept out of the sewage system.
Figure 18 Cross section (National Asphalt Pavement
Association http://www.asphaltpavement.org accessed
11/2011
CONSTRUCTION STEPS
The gravel base layer for pervious paving must be protected from sedimentation during construction.
The construction steps would follow specification developed by a qualified professional.
Typical construction steps are as follows:
Excavate to required depth.
Protect subgrade from compaction or decompact as required.
Install geotextile
Install stone recharge bed
Install choker course
Install porous asphalt
MATERIALS
Non woven geotextile
Crushed stone with 40% voids
Choker course – ½ “ crushed aggregate
Open graded asphalt mix --18% voids
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MAINTENANCE CONSIDERATIONS
Two excellent fact sheets on permeable and porous paving are available from the NC State University
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Stormwater Engineering Group at http://www.bae.ncsu.edu/stormwater/pubs.htm:
Research Update and Design Implications
Maintaining Permeable Pavements
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
COSTS
The cost of porous asphalt is slightly higher than conventional asphalt, 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.
SIZING COMPUTATIONS
As shown in the calculations below, with a 2 foot deep stone reservoir below, 1,400 square feet of paving
would be adequate to capture the WQv of 1089 cubic feet from the 12,500 square foot drainage area.
12500 Ft2
Total Drainage Area
1400 Ft2
Available Surface Area
Step 1: Calculate Water Quality Volume (WQv)
WQv = (P) (Rv) (A) / 12
P = 90% rainfall number =
Rv = 0.05+0.009 (I), if Rv < 20%, use Rv = 20%
I = percent impervious of area draining to practice =
% of Total area that drains to practice
1.1 inches
95%
100%
100%
A = Area draining to practice =
12500 Ft2
WQv =
Step 2: Calculate required surface area for pavement:
1089 Ft3
Ap = WQv / n x dt
0.4
2 ft
where n = assumed porosity
dt =trench depth
1361 Ft2
AP= Required surface area of practice =
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Urban Waterways, NC State University and A&T State University Cooperative Extension.2011.
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1- BIORETENTION AREA DESIGN
The three garden areas shown on the plan would be designed in similar fashion. The following discussion
focuses on Bioretention Area B, at the lower parking lot.
DESIGN
Soil infiltration tests would be provided prior to developing a final design. The soils in the area where the
gardens are proposed are expected to be well drained, based on the Web Soil Survey. The bioretention
areas would be designed without underdrains, and with a ponding area between 4-6 inches deep that
would drain within 24 to 48 hours. The practice would be designed with a gravel diaphragm along the
edge of the paving to capture sediments, and a grass strip and mulch layer would provide additional
filtration.
Figure 19 Typical bioretention design.
Design Manual, page 6-49.
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MATERIALS
Plants Masses of low maintenance native grasses and shrubs would be used for the bioretention
plantings. A list of plants appropriate for the wet and dry environment in bioretention areas is provided in
Appendix H of the Design Manual.
Soil and mulch
Components and proportions would be specified in the final design. Recommendations in the Design
Manual for bioretention area soils and mulch are as follows:
Planting Soil Bed Characteristics The characteristics of the soil for the bioretention facility are
perhaps as important as the facility
location, size, and treatment volume. The soil must be permeable enough to allow runoff to filter
through the media, while having characteristics suitable to promote and sustain a robust
vegetative cover crop. In addition, much of the nutrient pollutant uptake (nitrogen and
phosphorus) is accomplished through adsorption and microbial activity within the soil profile.
Therefore, the soils must balance soil chemistry and physical properties to support biotic
communities above and below ground. The planting soil should be a sandy loam, loamy sand,
loam (USDA), or a loam/sand mix (should contain a minimum 35 to 60% sand, by volume). The
clay content for these soils should by less than 25% by volume. Soils should fall within the SM, or
ML classifications of the Unified Soil Classification System (USCS). A permeability of at least 1.0
feet per day (0.5"/hr) is required (a conservative value of 0.5 feet per day is used for design). The
soil should be free of stones, stumps, roots, or other woody material over 1" in diameter. Brush or
seeds from noxious weeds. Placement of the planting soil should be in lifts of 12 to 18", loosely
compacted (tamped lightly with a dozer or backhoe bucket).
Mulch Layer
The mulch layer plays an important role in the performance of the bioretention system. The mulch
layer helps maintain soil moisture and avoid surface sealing which reduces permeability. Mulch
helps prevent erosion, and provides a micro-environment suitable for soil biota at the mulch/soil
interface. It also serves as a pretreatment layer, trapping the finer sediments which remain
suspended after the primary pretreatment. The mulch layer should be standard landscape style,
single or double, shredded hardwood mulch or chips. The mulch layer should be well aged
(stockpiled or stored for at least 12 months), uniform in color, and free of other materials, such as
weed seeds, soil, roots, etc. The mulch should be applied to a maximum depth of three inches.
Grass clippings should not be used as a mulch material (Appendix H, page 6).
Other Materials
Stone for diaphragm and inflow path as required in final design
CONSTRUCTION STEPS
Excavate to the depth required by the final design
Backfill with layer of clean washed gravel
Construct stone diaphragm and open curb if required
Fill to required depth with amended garden soil
Install plantings
Apply mulch and stones
MAINTENANCE CONSIDERATIONS
The bioretention garden would be designed for low maintenance. Weeding and watering are essential in
the first year and can be minimized with the use of a weed free mulch layer. Routine maintenance would
include the occasional replacement of plants, mulching, weeding and thinning to maintain the desired
appearance. The gravel diaphragm would require regular maintenance to clean sediments and debris.
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SIZING CALCULATIONS FOR BIORETENTION AREA
The sizing for the bioretention areas would be based on the Water Quality Volume (WQv).The following
calculations are for Bioretention Area B. Approximately 6500 sf of the asphalt paving drains to the
bioretention garden at that location, which would have a surface area of approximately 700sf. Given a
depth of 4 feet, the gardens would exceed the required surface area of 533 sf.
2
Garden Surface area
700
ft
Total Drainage Area
6500
Ft2
WQv = (P) (Rv) (A) / 12
P = 90% rainfall number =
Rv = 0.05+0.009 (I), if Rv < 20%, use Rv = 20%
I = percent impervious of area draining to planter =
% of Total area that drains to planter
1.1
95%
100%
100%
inches
A = Area draining to practice =
6500
Ft2
566
Ft3
4
0.5
ft
ft/day
1: Calculate Water Quality Volume (WQv)
WQv =
2 Bioretention Details
WQv*df/k(hf+df)(tf)
df = depth of soil medium =
k = Coefficient of permeability of planting soils
hf = Average ponding depth (max depth/2
tf = filter time (days) =
0.25
2
ft
days
3. Calculation Af = Required surface area for bioretention
533
Ft
Concept Plan by Marcy Denker
Project Outreach by Victor-Pierre Melendez
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