1. Chapter 6Structural BMPs
      1. _
        1. _
          1. _
    2. Introduction
      1. _
        1. _
          1. _
    3. Positive Overflow: Inlet
    4. Maintenance Issues
    5. Cost Issues
    6. Specifications
    7. Description
      1. Variations
      2. Design Considerations
      3. ?
      4. Maintenance Issues
      5. Specifications
    8. BMP 6.5.2: Runoff Capture & Reuse
      1. ?
      2. Maintenance Issues
        1. _
          1. Variations
        2. Applications
        3. Design Considerations
        4. Detailed Stormwater Functions
        5. Construction Sequence
        6. Maintenance Issues
        7. Weed and Invasive Plant Control
        8. Herbicides
        9. Cost Issues
        10. Specifications
        11. References
        12. Construction Sequence
        13. Cost Issues

Pennsylvania Stormwater
Best Management Practices
Manual
Chapter 6
Structural BMPs
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
Chapter 6 Structural BMPs
6.1
Introduction………………………………………………………………………………………………1
6.2
Groupings of Structural BMPs………………………………………………………………………..1
6.3
Manufactured Products………………………………………………………………………………..2
6.4
Volume/Peak Rate Reduction by Infiltration BMPs……………………………………………….5
BMP 6.4.1
Pervious Pavement with Infiltration Bed…………………………….……7
BMP 6.4.2
Infiltration Basin……………………………………………………………..27
BMP 6.4.3
Subsurface Infiltration Bed………………………………………………...33
BMP 6.4.4
Infiltration Trench………………………………………………………….…41
BMP 6.4.5
Rain Garden / Bioretention………………………………………………...49
BMP 6.4.6
Dry Well / Seepage Pit……………………………………………………….61
BMP 6.4.7
Constructed Filter………………………………………………………….…71
BMP 6.4.8
Vegetated Swale………………………………………………………….…..83
BMP 6.4.9
Vegetated Filter Strip………………………………………………….……..99
BMP 6.4.10 Infiltration Berm & Retentive Grading…………………………….…….113
.
6.5
Volume/Peak Rate Reduction BMPs……………………………………………………….……...123
BMP 6.5.1
Vegetated Roof……………………………………………………….……..125
BMP 6.5.2
Runoff Capture & Reuse……………………………….………………….139
6.6
Runoff Quality/Peak Rate BMPs…………………………………………………………….……...149
BMP 6.6.1
Constructed Wetland…………………………………………………….…151
BMP 6.6.2
Wet Pond/ Retention Basin………………………………………………..163
BMP 6.6.3
Dry Extended Detention Basin……………………………………….…...173
BMP 6.6.4
Water Quality Filters & Hydrodynamic Devices…….……………..…..183
6.7
Restoration BMPs…………………………………………………………………………………..…189
BMP 6.7.1
Riparian Buffer Restoration. ……………………………………………...191
BMP 6.7.2
Landscape Restoration…………………………………………………..…211
BMP 6.7.3
Soils Amendment & Restoration…………………………………….…....221
BMP 6.7.4
Floodplain Restoration……………………………………………….…..…231
6.8
Other BMPs and Related Structural Measures……………………………………………..….…241
BMP 6.8.1
Level Spreader………………………………………………………..….…..243
BMP 6.8.2
Special Detention Areas – Parking Lot, Rooftop…………..……….….253
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6.1 Introduction
Twenty-one Structural BMPs are listed and described in this chapter. As indicated in both Chapters 4
and 5, many of these “structures” are natural system-based and include vegetation and soils
mechanisms as part of their functioning. More conventional “bricks and mortar” structures are also
included in this chapter.
Several of the BMPs presented in this chapter lead to variations on a central them. The vegetated
swale is a good example of a core BMP that fosters numerous others. These variations have been
included in this chapter with some explanation and reference made as to how and when such variations
can be successfully applied. As lengthy as the list of Structural BMPs might be , many more BMPs are
expected to emerge as stormwater management practices continue to evolve and mature.
Each BMP is outlined using approximately the same structure or outline as has been applied to the
Non-Structural BMPs.
6.2 Groupings of Structural BMPs
Structural BMPs are grouped according to the primary, though not exclusive, stormwater functions, as
follows:
Volume/Peak Rate Reduction by Infiltration BMPs
Volume/Peak Rate Reduction BMPs
Runoff Quality/Peak Rate BMPs
Restoration BMPs
Other BMPs
In all cases, these stormwater functions are linked to the Recommended Site Control Guidelines
presented in Chapter 3. Most of the Structural BMPs fall into the category of Volume/Peak Rate
Reduction. Some of these BMPs also possess excellent water quality protection capabilities as well.
Volume and Peak Rate functions also can be provided by a smaller group of increasingly important
Structural BMPs such as Vegetated Roofs and Roof Capture/Reuse (e.g., rain barrels and cisterns).
Certain BMPs provide water quality and peak rate control functions, without any significant control of
volume. The Restoration BMPs and Other BMP categories provide a mix of stormwater functions.
Although these BMPs have not been frequently used in the past, they can offer real potential for many
Pennsylvania municipalities in the future.
Lastly, two special lists of instructions, or Protocols, have been developed specifically for use with all
infiltration-oriented structural BMPs and are presented in Appendix C.
Protocol 1: Site Evaluation and Soil Infiltration Testing
Protocol 2: Infiltration Systems Design and Construction Guidelines
These Protocols should be followed whenever infiltration-oriented BMPs are being developed. The
Protocols set forth a variety of actions common to all infiltration BMPs. These actions should be taken
to ensure that proper site conditions and constraints are being addressed, proper design considerations
are being taken, and proper construction specifications are being integrated into the overall design of
the BMP. An especially important aspect of these instructions focuses on full and careful testing of the
soil, thereby necessitating a separate Protocol that addresses soil testing and analysis. If these
Protocols are followed, the risk of failed infiltration BMPs will be minimized, if not eliminated.
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One of the most challenging technical issues considered in this manual involves the selection of BMPs
with a high degree of pollutant reduction or removal efficiency. The Non-Structural BMPs described in
Chapter 5 and the Structural BMPs presented in Chapter 6 are all rated in terms of their pollutant
removal performance or effectiveness. The initial BMP selection process analyzes the final site plan
and estimates the potential pollutant load, using Appendix A. The targeted reduction percentage for
representative pollutants (such as 85% reduction in TSS and TP load and 50% reduction in the solute
load) is achieved by a suitable combination of Non-Structural and Structural BMPs. This process is
described in more detail in Chapter 8.
6.3 Manufactured Products
A variety of product suppliers, distributors, and manufacturers have provided extensive product
information to PADEP during the preparation of this manual. Many of these products can be used in
conjunction with the Non-Structural BMPs set forth in Chapter 5 as well as the Structural BMPs
presented in this chapter. The proper application and use of many of these manufactured products can
further the stormwater management goals and objectives of this manual. It should be noted that
Pennsylvania does not have an established product review and testing function. The interested
reader/user is directed to the following sources to learn about the performance of a specific product or
technology:
The Technology Acceptance Reciprocity Partnership (TARP) – A partnership of the states of
California, Illinois, Maryland, Massachusetts, New Jersey, New York, Pennsylvania and Virginia
that establishes standardized methods to guide the collection and evaluation of new and
innovative technology performance across the states. Information is available at:
www.dep.state.pa.us/dep/deputate/pollprev/techservices/tarp/index.htm
Environmental Technology Evaluation Center (EvTEC) of The Civil Engineering Research
Foundation (CERF), including their Stormwater Best Management Practices (BMPs) Verification
Program - information available at http://www.cerf.org/evtec/index.htm
&
http://www.cerf.org/evtec/eval/wsdot_qr.htm
U.S. EPA's Environmental Technology Verification Program (ETV) - information available at
http://www.epa.gov/etv/
The University of New Hampshire's Center for Stormwater Technology Evaluation and
Verification (CSTEV) - information available at http://www.unh.edu/erg/cstev/index.htm#
The Chesapeake Bay Program's Innovative Technology Task Force (ITTF) - information about
the program as well as many useful links to other programs available at
http://www.chesapeakebay.net/info/innov_tech.cfm
New Jersey's Energy and Environmental Technology Verification Program - results available
through the New Jersey Corporation for Advanced Technology (NJCAT) at http://www.njcat.org/
Disclaimer: The technology descriptions contained in this document including, but not limited to,
information on technology applications, performance, limitations, benefits, and cost, have been
provided by vendors. No attempt was made to examine, screen or verify company or technology
information. The Pennsylvania Department of Environmental Protection has not confirmed the
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accuracy or legal adequacy of any disclosures, product performance, or other information
provided by the companies appearing here. The inclusion of specific products in this document
does not constitute or imply their endorsement or recommendation by the Pennsylvania
Department of Environmental Protection.
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6.4 Volume/Peak Rate Reduction by Infiltration BMPs
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BMP 6.4.1: Pervious Pavement with Infiltration Bed
Pervious pavement consists of a permeable surface
course underlain by a uniformly-graded stone bed
which provides temporary storage for peak rate
control and promotes infiltration. The surface
course may consist of porous asphalt, porous
concrete, or various porous structural pavers laid on
uncompacted soil.
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Limited
Yes
Yes
Yes
Yes
Limited
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Medium
Medium
Medium
Medium
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
30%
.
Almost entirely for peak rate control
.
Water quality and quantity are not addressed
.
Short duration storage; rapid restoration of primary uses
.
Minimize safety risks, potential property damage, and user
inconvenience
.
Emergency overflows
.
Maximum ponding depths
.
Flow control structures
.
Adequate surface slope to outlet
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C
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Description
A pervious pavement bed consists of a pervious surface course underlain by a stone bed of uniformly
graded and clean-washed coarse aggregate, 1-1/2 to 2-1/2 inches in size, with a void space of at least
40%. The pervious pavement may consist of pervious asphalt, pervious concrete, or pervious
pavement units. Stormwater drains through the
surface, is temporarily held in the voids of the stone
bed, and then slowly drains into the underlying,
uncompacted soil mantle. The stone bed can be
designed with an overflow control structure so that
during large storm events peak rates are controlled,
and at no time does the water level rise to the
pavement level. A layer of geotextile filter fabric
separates the aggregate from the underlying soil,
preventing the migration of fines into the bed. The bed
bottoms should be level and uncompacted. If new fill is
required, it should consist of additional stone and not
compacted soil.
Pervious pavement is well suited for parking lots, walking paths, sidewalks, playgrounds, plazas, tennis
courts, and other similar uses. Pervious pavement can be used in driveways if the homeowner is
aware of the stormwater functions of the pavement. Pervious pavement roadways have seen wider
application in Europe and Japan than in the U.S., although at least one U.S. system has been
constructed . In Japan and the U.S., the application of an open-graded asphalt pavement of 1” or less
on roadways has been used to provide lateral surface drainage and prevent hydroplaning, but these
are applied over impervious pavement on compacted sub-grade. This application is not pervious
pavement.
Properly installed and maintained pervious pavement has a significant life-span, and existing systems
that are more than twenty years in age continue to function. Because water drains through the surface
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course and into the subsurface bed, freeze-thaw cycles do not tend to adversely affect pervious
pavement.
Pervious pavement is most susceptible to failure difficulties during construction, and therefore it is
important that the construction be undertaken in such as way as to
prevent
:
• Compaction of underlying soil
• Contamination of stone subbase with sediment and fines
• Tracking of sediment onto pavement
• Drainage of sediment laden waters onto pervious surface or into constructed bed
Staging, construction practices, and erosion and sediment control must all be taken into consideration
when using pervious pavements.
Studies have shown that pervious systems have been very effective in reducing contaminants such as
total suspended solids, metals, and oil and grease. When designed, constructed, and maintained
according to the following guidelines, pervious
pavement with underlying infiltration systems
can dramatically reduce both the rate and
volume of runoff, recharge the groundwater,
and improve water quality.
In northern climates, pervious pavements have
less of a tendency to form black ice and often
require less plowing. Winter maintenance is
described on page 17. Pervious asphalt and
concrete surfaces provide better traction for
walking paths in rain or snow conditions.
Variations
Pervious Bituminous Asphalt
Early work on pervious asphalt pavement was conducted in the early 1970’s by the Franklin Institute in
Philadelphia and consists of standard bituminous asphalt in which the fines have been screened and
reduced, allowing water to pass through small voids. Pervious asphalt is placed directly on the stone
subbase in a single 3 ½ inch lift that is lightly rolled to a finish depth of 2 ½ inches.
Because pervious asphalt is standard asphalt with
reduced fines, it is similar in appearance to standard
asphalt. Recent research in open-graded mixes for
highway application has led to additional improvements
in pervious asphalt through the use of additives and
higher-grade binders. Pervious asphalt is suitable for
use in any climate where standard asphalt is
appropriate.
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Pervious Concrete
Pervious Portland Cement Concrete, or pervious concrete, was developed by the Florida Concrete
Association and has seen the most widespread application in Florida and southern areas. Like
pervious asphalt, pervious concrete is produced by substantially reducing the number of fines in the mix
in order to establish voids for drainage. In northern and mid-Atlantic climates such as Pennsylvania,
pervious concrete should always be underlain by a stone subbase designed for stormwater
management and should never be placed directly onto a soil subbase.
While pervious asphalt is very similar in appearance to standard asphalt, pervious concrete has a
coarser appearance than its conventional counterpart. Care must be taken during placement to avoid
working the surface and creating an impervious layer. Pervious concrete has been proven to be an
effective stormwater management BMP. Additional information pertaining to pervious concrete,
including specifications, is available from the Florida Concrete Association and the National Ready Mix
Association.
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Pervious Paver Blocks
Pervious Paver Blocks consist of interlocking units (often concrete) that
provide some portion of surface area that may be filled with a pervious
material such as gravel. These units are often very attractive and are
especially well suited to plazas, patios, small parking areas, etc. A
number of manufactured products are available, including (but not limited
to):
• Turfstone; UNI Eco-stone; Checkerblock; EcoPaver
As products are always being developed, the designer is encouraged to evaluate the benefits of various
products with respect to the specific application. Many paver products recommend compaction of the
soil and do not include a drainage/storage area, and therefore, they do not provide optimal stormwater
management benefits. A system with a compacted subgrade will not provide significant infiltration.
Reinforced Turf and Gravel Filled Grids
Reinforced Turf consists of interlocking structural units that contain voids or areas for turf grass growth
and are suitable for traffic loads and parking. Reinforced turf units may consist of concrete or plastic
and are underlain by a stone and/or sand drainage system for stormwater management There are also
products available that provide a fully permeable surface through the use of plastic rings/grids filled with
gravel..
Reinforced Turf applications are excellent for Fire Access Roads, overflow parking, occasional use
parking (such as at religious facilities and athletic facilities). Reinforced turf is also an excellent
application to reduce the required standard pavement width of paths and driveways that must
occasionally provide for emergency vehicle access.
While both plastic and concrete units perform well for stormwater management and traffic needs,
plastic units tend to provide better turf establishment and longevity, largely because the plastic will not
absorb water and diminish soil moisture conditions. A number of products (e.g.
Grasspave, Geoblock,
GravelPave, Grassy Pave, Geoweb) are available and the designer is encouraged to evaluate and
select a product suitable to the design in question.
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Applications
Parking
Walkways
Pervious Pavement Walkways
Pervious pavement has also been used in walkways and sidewalks. These installations
typically consist of a shallow (8 in. minimum) aggregate trench that is sloped to follow the
surface slope of the path. In the case of relatively mild surface slopes, the aggregate
infiltration trench may be “terraced” into level reaches in order to maximize the infiltration
capacity, at the expense of additional aggregate.
Playgrounds
Alleys
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Roof drainage; Direct connection of roof leaders and/or inlets
Limited use for roads and highways
Design Considerations
1. Protocol 1, Site Evaluation and Soil Infiltration Testing required (see Appendix C).
2. Protocol 2, Infiltration Systems Guidelines must be met (see Appendix C).
3. The overall site should be evaluated for potential pervious pavement / infiltration areas early
in
the design process, as effective pervious pavement design requires consideration of grading.
4. Orientation of the parking bays along the existing contours will significantly reduce the need for
cut and fill.
5. Pervious pavement and infiltration beds
should not be placed on areas of recent fill
or
compacted fill. Any grade adjust requiring fill should be done using the stone subbase material.
Areas of historical fill (>5 years) may be considered for pervious pavement.
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6. The bed bottom should not be compacted, however the stone subbase should be placed in lifts
and lightly rolled according to the specifications.
7. During construction, the excavated bed may serve as a temporary sediment basin or trap. This
will reduce overall site disturbance. The bed should be excavated to within twelve (12) inches
of the final bed bottom elevation for use as a sediment trap or basin. Following construction and
site stabilization, sediment should be removed and final grades established.
8. Bed bottoms should be level or nearly level
. Sloping bed bottoms will lead to areas of
ponding and reduced distribution.
9. All systems should be designed with an overflow system
. Water within the subsurface
stone bed should never rise to the level of the pavement surface. Inlet boxes can be used for
cost-effective overflow structures. All beds should empty to meet the criteria in Chapter 3.
10. While infiltration beds are typically sized to handle the increased volume from a storm, they
should also be able to convey and mitigate the peak of the less-frequent, more intense storms
(such as the 100-yr). Control in the beds is usually provided in the form of an outlet control
structure. A modified inlet box with an internal weir and low-flow orifice is a common type of
control structure. The specific design of these structures may vary, depending on factors such
as rate and storage requirements, but it always should include positive overflow from the
system.
11. The subsurface bed and overflow may be designed and evaluated in the same manner as a
detention basin to demonstrate the mitigation of peak flow rates. In this manner, the need for a
detention basin may be eliminated or reduced in size.
12. A weir plate or weir within an inlet or overflow control structure may be used to maximize the
water level in the stone bed while providing sufficient cover for overflow pipes.
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13. Perforated pipes along the bottom of the bed may be used to evenly distribute runoff over the
entire bed bottom. Continuously perforated pipes should connect structures (such as cleanouts
and inlet boxes). Pipes should lay flat along the bed bottom and provide for uniform distribution
of water. Depending on size, these pipes may provide additional storage volume.
14. Roof leaders and area inlets may be connected to convey runoff water to the bed. Water
Quality Inserts or Sump Inlets should be used to prevent the conveyance of sediment and
debris into the bed.
15. Infiltration areas should be located within the immediate project area in order to control runoff at
its source. Expected use and traffic demands should also be considered in pervious pavement
placement.
16. Control of sediment is critical. Rigorous installation and maintenance of erosion and sediment
control measures should be provided to prevent sediment deposition on the pavement surface
or within the stone bed. Nonwoven geotextile may be folded over the edge of the pavement
until the site is stabilized. The Designer should consider the placement of pervious pavement to
reduce the likelihood of sediment deposition. Surface sediment should be removed by a
vacuum sweeper and should not be power-washed into the bed.
17. Infiltration beds may be placed on a slope by
benching or terracing parking bays
.
Orienting
parking bays along existing contours will reduce
site disturbance and cut/fill requirements.
18. The underlying infiltration bed is typically 12-36
inches deep and comprised of clean, uniformly
graded aggregate with approximately 40% void
space. AASHTO No.3, which ranges 1.5-2.5
inches in gradation, is often used. Depending on
local aggregate availability, both larger and smaller s
requirements are that the aggregate be uniformly graded, clean washed, and contain a
significant void content. The depth of the bed is a function of stormwater storage requirements,
frost depth considerations, site grading, and anticipated loading. Infiltration beds are typically
sized to mitigate the increased runoff volume from a 2-yr design storm.
M
ize aggregate has been used. The critical
19. ost pervious pavement installations are underlain by an aggregate bed; alternative subsurface
20. ll pervious pavement installations should have a
ails
e
.
s
compromised.
storage products may also be employed. These include a variety of proprietary, interlocking
plastic units that contain much greater storage capacity than aggregate, at an increased cost.
A
backup method for water to enter the stone
storage bed in the event that the pavement f
or is altered. In uncurbed lots, this backup
drainage may consist of an unpaved 2 ft wid
stone edge drain connected directly to the bed
In curbed lots, inlets with water quality devices
may be required at low spots. Backup drainage
elements will ensure the functionality of the
infiltration system, if the pervious pavement i
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1. In areas with poorly draining soils, infiltration beds below pervious pavement may be designed
22. those areas where the threat of spills and groundwater contamination is likely, pretreatment
nt
23. he use of pervious pavement must be carefully considered in areas where the pavement may
ntional
2
to slowly discharge to adjacent wetlands or bioretention areas. Only in extreme cases (i.e.
industrial sites with contaminated soils) will the aggregate bed need to be lined to prevent
infiltration.
In
systems, such as filters and wetlands, may be required before any infiltration occurs. In hot
spot areas, such as truck stops, and fueling stations, the appropriateness of pervious paveme
must be carefully considered. A stone infiltration bed located beneath standard pavement,
preceded by spill control and water quality treatment, may be more appropriate.
T
be seal coated or paved over due to lack of awareness, such as individual home driveways. In
those situations, a system that is not easily altered by the property owner may be more
appropriate. An example would include an infiltration system constructed under a conve
driveway. Educational signage at pervious pavement installations may guarantee its prolonged
use in some areas.
Detailed Stormwater Functions
olume Reduction Calculations
rea (sf) x Void Space
vent, depending on the drainage area and
Infiltration Volume = Bed Bottom Area (sf) x Infiltration design rate (in/hr)
receiving runoff and capable of infiltrating at the design rate.
V
Volume = Depth* (ft) x A
*Depth is the depth of the water stored during a storm e
conveyance to the bed.
x Infiltration period* (hr) x (1/12)
*Infiltration Period is the time when bed is
Not to exceed 72 hours.
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Peak Rate Mitigation
eak Rate Mitigation methodology that addresses link between volume reduction
ater Quality Improvement
uality methodology that addresses pollutant removal effectiveness of this
onstruction Sequence
1. Due to the nature of construction sites, pervious pavement and other infiltration measures
nder
ade
. The existing subgrade under the bed areas should NOT
See in Chapter 8 for P
and peak rate control.
W
See in Chapter 8 for Water Q
BMP.
C
should by installed toward the end of the construction period, if possible. Infiltration beds u
pervious pavement may be used as temporary sediment basins or traps provided that they are
not excavated to within 12 inches of the designated bed bottom elevation. Once the site is
stabilized and sediment storage is no longer required, the bed is excavated to the its final gr
and the pervious pavement system is installed.
2
be compacted or subject to excessive
3. Where erosion of subgrade has caused accumulation of fine materials and/or surface ponding,
ng
4.
arthen berms (if used) between infiltration
n.
. Geotextile and bed aggregate should be placed
in
d
should overlap a minimum of 16 in. It should
us
. Clean (washed) uniformly graded aggregate is placed in the bed in 8-inch lifts. Each layer
ch
to
construction equipment traffic prior to geotextile and stone bed placement.
this material shall be removed with light equipment and the underlying soils scarified to a
minimum depth of 6 inches with a York rake (or equivalent) and light tractor. All fine gradi
shall be done by hand. All bed bottoms should
be at a level grade.
E
beds should be left in place during excavatio
These berms do not require compaction if
proven stable during construction.
5
immediately after approval of subgrade
preparation. Geotextile should be placed
accordance with manufacturer’s standards an
recommendations. Adjacent strips of geotextile
also be secured at least 4 ft. outside of bed in order to prevent any runoff or sediment from
entering the storage bed. This edge strip should remain in place until all bare soils contiguo
to beds are stabilized and vegetated. As the site is fully stabilized, excess geotextile along bed
edges can be cut back to bed edge.
6
should be lightly compacted, with the construction equipment kept off the bed bottom as mu
as possible. Once bed aggregate is installed to the desired grade, a +/- 1 in. layer of choker
base course (AASHTO #57) aggregate should be installed uniformly over the surface in order
provide an even surface for paving.
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7. The pervious pavement should be installed in accordance with current standards. Further
information can be obtained from the appropriate Association.
The full permeability of the pavement surface should be tested by application of clean water at the rate
of at least 5 gpm over the surface, using a hose or other distribution devise. All applied water should
infiltrate directly without puddle formation or surface runoff.
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Maintenance Issues
The primary goal of pervious pavement maintenance is to prevent the pavement surface and/or
underlying infiltration bed from being clogged with fine sediments. To keep the system clean
throughout the year and prolong its life span, the pavement surface should be vacuumed biannually
with a commercial cleaning unit
. Pavement washing systems or compressed air units are not
recommended.
All inlet structures within or draining to the infiltration beds should also be cleaned out
biannually.
Planted areas adjacent to pervious pavement should be well maintained to prevent soil washout onto
the pavement. If any washout does occur it should be cleaned off the pavement immediately to prevent
further clogging of the pores. Furthermore, if any bare spots or eroded areas are observed within the
planted areas, they should be replanted and/or stabilized at once. Planted areas should be inspected
on a semiannual basis. All trash and other litter that is observed during these inspections should be
removed.
Superficial dirt does not necessarily clog the pavement voids. However, dirt that is ground in
repeatedly by tires can lead to clogging. Therefore, trucks or other heavy vehicles should be prevented
from tracking or spilling dirt onto the pavement. Furthermore, all construction or hazardous materials
carriers should be prohibited from entering a pervious pavement lot.
Special Maintenance Considerations:
• Prevent Clogging of Pavement Surface with Sediment
°
Vacuum pavement 2 or 3 times per year
°
Maintain planted areas adjacent to pavement
°
Immediately clean any soil deposited on pavement
°
Do not allow construction staging, soil/mulch storage, etc. on unprotected pavement
surface
°
Clean inlets draining to the subsurface bed twice per year
Winter Maintenance
Winter maintenance for a pervious parking lot may be necessary but is usually less intensive
than that required for a standard impervious surface. By its very nature, a pervious pavement
system with subsurface aggregate bed has superior snow melting characteristics than standard
pavement. The underlying stone bed tends to absorb and retain heat so that freezing rain and
snow melt faster on pervious pavement. Therefore, ice and light snow accumulation are
generally not as problematic. However, snow will accumulate during heavier storms. Abrasives
such as sand or cinders should not be applied on or adjacent to the pervious pavement. Snow
plowing is fine, provided it is done carefully (i.e. by setting the blade slightly higher than usual,
about an inch). Salt is acceptable for use as a deicer on the pervious pavement, though
nontoxic, organic deicers, applied either as blended, magnesium chloride-based liquid products
or as pretreated salt, are preferable.
Repairs
Potholes in the pervious pavement are unlikely; though settling might occur if a soft spot in the
subgrade is not removed during construction. For damaged areas of less than 50 square feet, a
declivity could be patched by any means suitable with standard pavement, with the loss of
porosity of that area being insignificant. The declivity can also be filled with pervious mix. If an
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area greater than 50 sq. ft. is in need of repair, approval of patch type should be sought from
either the engineer or owner. Under no circumstance should the pavement surface ever be seal
coated. Any required repair of drainage structures should be done promptly to ensure
continued proper functioning of the system.
Cost Issues
Pervious asphalt, with additives, is generally 10% to 20% higher (2005) in cost than
standard asphalt on a unit area basis.
Pervious concrete as a material is generally more expensive than asphalt and requires
more labor and experience for installation due to specific material constraints.
Permeable interlocking concrete pavement blocks vary in cost depending on type and
manufacturer.
The added cost of a pervious pavement/infiltration system lies in the underlying stone bed, which is
generally deeper than a conventional subbase and wrapped in geotextile. However, this additional cost
is often offset by the significant reduction in the required number of inlets and pipes. Also, since
pervious pavement areas are often incorporated into the natural topography of a site, there generally is
less earthwork and/or deep excavations involved. Furthermore, pervious pavement areas with
subsurface infiltration beds often eliminate the need (and associated costs, space, etc.) for detention
basins. When all of these factors are considered, pervious pavement with infiltration has proven itself
less expensive than the impervious pavement with associated stormwater management. Recent
(2005) installations have averaged between $2000 and $2500 per parking space, for the pavement and
stormwater management.
Specifications
The following specifications are provided for informational purposes only. These specifications include
information on acceptable materials for typical applications, but are by no means exclusive or limiting.
The designer is responsible for developing detailed specifications for individual design projects in
accordance with the project conditions.
1. Stone
for infiltration beds shall be 2-inch to 1-inch uniformly graded coarse aggregate, with a
wash loss of no more than 0.5%, AASHTO size number 3 per AASHTO Specifications, Part I,
19th Ed., 1998, or later and shall have voids 40% as measured by ASTM-C29. Choker base
course aggregate for beds shall be 3/8 inch to 3/4 inch uniformly graded coarse aggregate
AASHTO size number 57 per Table 4, AASHTO Specifications, Part I, 13th Ed., 1998 (p. 47).
2. Non-Woven Geotextile
shall consist of needled nonwoven polypropylene fibers and meet the
following properties:
a.
Grab Tensile Strength (ASTM-D4632)
120 lbs
b.
Mullen Burst Strength (ASTM-D3786)
225 psi
c.
Flow Rate (ASTM-D4491)
95 gal/min/ft
2
d.
UV Resistance after 500 hrs (ASTM-D4355)
70%
e.
Heat-set or heat-calendared fabrics are not permitted.
Acceptable types include Mirafi 140N, Amoco 4547, Geotex 451, or approved others.
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3. Pipe
shall be continuously perforated, smooth interior, with a minimum inside diameter of 6-
inches. High-density polyethylene (HDPE) pipe shall meet AASHTO M252, Type S or AASHTO
M294, Type S.
4. Storm Drain Inlets and Structures
a.
Concrete Construction: Concrete construction shall be in accordance with PennDOT
Pub. 4082003 including current supplements or latest edition.
b.
Precast concrete iInlets and manholes: Precast concrete inlets may be substituted for
cast-in-place structures and shall be constructed as specified for cast-in-place. Standard
inlet boxes will be modified to provide minimum 12" sump storage and bottom leaching
basins, open to gravel sumps in sub-grade, when situated in the recharge bed.
c.
All PVC Catch Basins/Cleanouts/Inline Drains shall have H-10 or H-20 rated grates,
depending on their placement (H-20 if vehicular loading).
d.
Steel reinforcing bars over the top of the outlet structure shall conform to ASTM A615,
grades 60 and 40.
e.
Permanent turf reinforcement matting shall be installed according to manufacturers’
specifications.
5. Pervious Bituminous Asphalt
Bituminous surface course for
pervious paving
should be two and one-half (2.5) inches thick
with a bituminous mix of 5.75% to 6% by weight dry aggregate.
In accordance with ASTM
D6390, drain down of the binder shall be no greater than 0.3%
. If more absorptive
aggregates, such as limestone, are used in the mix, then the amount of bitumen is to be based
on the testing procedures outlined in the National Asphalt Pavement Association’s Information
Series 131 – “Pervious Asphalt Pavements” (2003) or PennDOT equivalent.
Use neat asphalt binder modified with an elastomeric polymer to produce a binder meeting the
requirements of PG 76-22 as specified in AASHTO MP-1. The elastomer polymer shall be
styrene-butadiene-styrene (SBS), or approved equal, applied at a rate of 3% by weight of the
total binder. The composite materials shall be thoroughly blended at the asphalt refinery or
terminal prior to being loaded into the transport vehicle. The polymer modified asphalt binder
shall be heat and storage stable.
Aggregate shall be minimum 90% crushed material and have a gradation of:
U.S. Standard Sieve Size
Percent Passing
½ (12.5 mm)
100
3/8 (9.5 mm)
92-98
4 (4.75 mm)
34-40
8 (2.36 mm)
14-20
16 (1.18 mm)
7-13
30 (0.60 mm)
0-4
200 (0.075mm)
0-2
Add hydrated lime at a dosage rate of 1.0% by weight of the total dry aggregate to mixes
containing granite. Hydrated lime shall meet the requirements of ASTM C 977. The additive
must be able to prevent the separation of the asphalt binder from the aggregate and achieve a
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required tensile strength ratio (TSR) of at least 80% on the asphalt mix when tested in
accordance with AASHTO T 283. The asphaltic mix shall be tested for its resistance to stripping
by water in accordance with ASTM D-1664. If the estimated coating area is not above 95
percent, anti-stripping agents shall be added to the asphalt.
Pervious pavement shall not be installed on wet surfaces or when the ambient air temperature
is 50 degrees Fahrenheit or lower. The temperature of the bituminous mix shall be between
300 degrees Fahrenheit and 350 degrees Fahrenheit (based on the recommendations of the
asphalt supplier).
6. Pervious Concrete
GENERAL
Weather Limitations
: Do not place Portland cement pervious pavement mixtures when the
ambient temperature is 40 degrees Fahrenheit or lower or 90 degrees Fahrenheit or higher,
unless otherwise permitted in writing by the Engineer.
Test Panels
: Regardless of qualification, Contractor is to place, joint and cure at least two test
panels, each to be a minimum of 225 sq. ft. at the required project thickness to demonstrate to
the Engineer’s satisfaction that in-place unit weights can be achieved and a satisfactory
pavement can be installed at the site location.
Test panels may be placed at any of the specified Portland Cement pervious locations. Test
panels shall be tested for thickness in accordance with ASTM C 42; void structure in
accordance with ASTM C 138; and for core unit weight in accordance with ASTM C 140,
paragraph 6.3.
Satisfactory performance of the test panels will be determined by:
Compacted thickness no less than ¼” of specified thickness.
Void Structure
: 15% minimum; 21% maximum. Unit weight plus or minus 5 pcf of the design unit
weight.
If measured void structure falls below 15% or if measured thickness is greater than ¼” less than
the specified thickness of if measured weight falls less than 5 pcf below unit weight, the test
panel shall be removed at the contractor’s expense and disposed of in an approved landfill.
If the test panel meets the above-mentioned requirements, it can be left in-place and included in
the completed work.
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CONCRETE MIX DESIGN
Contractor shall furnish a proposed mix design with proportions of materials to the Engineer
prior to commencement of work. The data shall include unit weights determined in accordance
with ASTM C29 paragraph 11, jigging procedure.
MATERIALS
Cement: Portland Cement Type I or II conforming to ASTM C 150 or Portland Cement Type IP
or IS conforming to ASTM C 595.
Aggregate
: Use No 8 coarse aggregate (3/8 to No. 16) per ASTM C 33 or No. 89 coarse
aggregate (3/8 to No. 50) per ASTM D 448. If other gradation of aggregate is to be used,
submit data on proposed material to owner for approval.
Air Entraining Agent
: Shall comply with ASTM C 260 and shall be used to improve resistance to
freeze/thaw cycles.
Admixtures
: The following admixtures shall be used:
Type D Water Reducing/Retarding – ASTM C 494.
A hydration stabilizer that also meets the requirements of ASTM C 494 Type B Retarding or
Type D Water Reducing/Retarding admixtures. This stabilizer suspends cement hydration by
forming a protective barrier around the cementitious particles, which delays the particles from
achieving initial set.
Water
: Potable shall be used.
Proportions
:
Cement Content:
For pavements subjected to vehicular traffic loading, the total cementitious
material shall not be less than 600 lbs. Per cy.
Aggregate Content: the volume of aggregate per cu. yd. shall be equal to 27 cu.ft. when
calculated as a function of the unit weight determined in accordance with ASTM C 29 jigging
procedure. Fine aggregate, if used, should not exceed 3 cu. ft. and shall be included in the total
aggregate volume.
Admixtures
: Shall be used in accordance with the manufacturer’s instructions and
recommendations.
Mix Water
: Mix water shall be such that the cement paste displays a wet metallic sheen without
causing the paste to flow from the aggregate. (Mix water yielding a cement paste with a dull-dry
appearance has insufficient water for hydration).
Insufficient water results in inconsistency in the mix and poor bond strength.
High water content results in the paste sealing the void system primarily at the bottom
and poor surface bond.
An aggregate/cement (A/C) ratio range of 4:1 to 4.5:1 and a water/cement (W/C) ratio
range of 0.34 to 0.40 should produce pervious pavement of satisfactory properties in
regard to permeability, load carrying capacity, and durability characteristics
.
INSTALLATION
Portland Cement Pervious Pavement Concrete Mixing, Hauling and Placing:
Mix Time: Truck mixers shall be operated at the speed designated as mixing speed by the
manufacturer for 75 to 100 revolutions of the drum.
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Transportation
: The Portland Cement aggregate mixture may be transported or mixed on site
and should be used within one (1) hour of the introduction of mix water, unless otherwise
approved by an engineer. This time can be increased to 90 minutes when utilizing the specified
hydration stabilizer. Each truck should not haul more than two (2) loads before being cycled to
another type concrete. Prior to placing concrete, the subbase shall be moistened and in a wet
condition. Failure to provide a moist subbase will result in a reduction in strength of the
pavement.
Discharge
: Each mixer truck will be inspected for appearance of concrete uniformity according
to this specification. Water may be added to obtain the required mix consistency. A minimum
of 20 revolutions at the manufacturer’s designated mixing speed shall be required following any
addition of water to the mix. Discharge shall be a continuous operation and shall be completed
as quickly as possible. Concrete shall be deposited as close to its final position as practicable
and such that fresh concrete enters the mass of previously placed concrete. The practice of
discharging onto subgrade and pulling or shoveling to final placement is not allowed.
Placing and Finishing Equipment
: Unless otherwise approved by the Owner or Engineer in
writing, the Contractor shall provide mechanical equipment of either slipform or form riding with
a following compactive unit that will provide a minimum of 10 psi vertical force. The pervious
concrete pavement will be placed to the required cross section and shall not deviate more than
+/- 3/8 inch in 10 feet from profile grade. If placing equipment does not provide the minimum
specified vertical force, a full width roller or other full width compaction device that provides
sufficient compactive effort shall be used immediately following the strike-off operation. After
mechanical or other approved strike-off and compaction operation, no other finishing operation
will be allowed. If vibration, internal or surface applied, is used, it shall be shut off immediately
when forward progress is halted for any reason. The Contractor will be restricted to pavement
placement widths of a maximum of fifteen (15’) feet unless the Contractor can demonstrate
competence to provide pavement placement widths greater than that to the satisfaction of the
Engineer.
Curing
: Curing procedures shall begin within 20 minutes after the final placement operations.
The pavement surface shall be covered with a minimum six-(6) mil thick polyethylene sheet or
other approved covering material. Prior to covering, a fog or light mist shall be sprayed above
the surface when required due to ambient conditions (high temperature, high wind, and low
humidity). The cover shall overlap all exposed edges and shall be secured (without using dirt)
to prevent dislocation due to winds or adjacent traffic conditions.
Cure Time
:
1. Portland Cement Type I, II, or IS – 7 days minimum.
2. No truck traffic shall be allowed for 10 days (no passenger car/light trucks for 7 days).
Jointing
: Control (contraction) joints shall be installed at 20-foot intervals. They shall be
installed at a depth of the 1/ 4 the thickness of the pavement. These joints can be installed in
the plastic concrete or saw cut. If saw cut, the procedure should begin as soon as the
pavement has hardened sufficiently to prevent raveling and uncontrolled cracking (normally
after curing). Transverse constructions joints shall be installed whenever placing is suspended
a sufficient length of time that concrete may begin to harden. In order to assure aggregate bond
at construction joints, a bonding agent suitable for bonding fresh concrete shall be brushed,
tolled, or sprayed on the existing pavement surface edge. Isolation (expansion) joints will not be
used except when pavement is abutting slabs or other adjoining structures.
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TESTING, INSPECTION, AND ACCEPTANCE
Laboratory Testing
:
The owner will retain an independent testing laboratory. The testing laboratory shall conform to
the applicable requirements of ASTM E 329 “Standard Recommended Practice for Inspection
and Testing Agencies for Concrete, Steel, and Bituminous Materials as Used in Construction”
and ASTM C 1077 “Standard Practice for Testing Concrete and Concrete Aggregates for use in
Construction, and Criteria for Laboratory Evaluation” and shall be inspected and accredited by
the Construction Materials Engineering Council, Inc. or by an equivalent recognized national
authority.
The Agent of the testing laboratory performing field sampling and testing of concrete shall be
certified by the American Concrete Institute as a Concrete Field Testing Technician Grade I, or
by a recognized state or national authority for an equivalent level of competence.
Testing and Acceptance
:
A minimum of 1 gradation test of the subgrade is required every 5000 square feet to determine
percent passing the No. 200 sieve per ASTM C 117.
A minimum of one test for each day’s placement of pervious concrete in accordance with ASTM
C 172 and ASTM C 29 to verify unit weight shall be conducted. Delivered unit weights are to be
determined in accordance with ASTM C 29 using a 0.25 cubic foot cylindrical metal measure.
The measure is to be filled and compacted in accordance with ASTM C 29 paragraph 11, jigging
procedure. The unit weight of the delivered concrete shall be +/- 5 pcf of the design unit weight.
Test panels shall have two cores taken from each panel in accordance with ASTM 42 at a
minimum of seven (7) days after placement of the pervious concrete. The cores shall be
measured for thickness, void structure, and unit weight. Untrimmed, hardened core samples
shall be used to determine placement thickness. The average of all production cores shall not
be less than the specified thickness with no individual core being more than ½ inch less than the
specified thickness. After thickness determination, the cores shall be trimmed and measured for
unit weight in the saturated condition as described in paragraph 6.3.1 of ‘Saturation’ of ASTM C
140 “Standard Methods of Sampling and Testing Concrete Masonry Units.” The trimmed cores
shall be immersed in water for 24 hours, allowed to drain for one (1) minute, surface water
removed with a damp cloth, then weighed immediately. Range of satisfactory unit weight values
are +/- 5 pcf of the design unit weight.
After a minimum of 7 days following each placement, three cores shall be taken in accordance
with ASTM C 42. The cores shall be measured for thickness and unit weight determined as
described above for test panels. Core holes shall be filled with concrete meeting the pervious
mix design.
References and Additional Sources
Adams, Michele (2003). Porous Asphalt Pavement with Recharge Beds: 20 Years & Still Working,
Stormwater
4, 24-32.
Backstrom, Magnus (1999).
Porous Pavement in a Cold Climate
, Licentiate Thesis, Lulea, Sweden:
Lulea University of Technology (http://epubl.luth.se
).
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Cahill, Thomas (1993).
Porous Pavement with Underground Recharge Beds, Engineering Design
Manual
, West Chester Pennsylvania: Cahill Associates.
Cahill, Thomas (1994). A Second Look at Porous Pavement/Underground Recharge,
Watershed
Protection Techniques
, 1, 76-78.
Cahill, Thomas, Michele Adams, and Courtney Marm (2003). Porous Asphalt: The Right Choice for
Porous Pavements,
Hot Mix Asphalt Technology
September-October.
Ferguson, Bruce (2005).
Porous Pavements
, Boca Raton, Florida: CRC Press.
Florida Concrete and Products Association (no date).
Construction of a Portland Cement Pervious
Pavement
, Orlando: Florida Concrete and Products Association.
Hossain, Mustaque, Larry A. Scofield, and W.R. Meier, Jr. (1992). Porous Pavement for Control of
Highway Runoff in Arizona: Performance to Date,
Transportation Research Record
1354, 45-54.
Jackson, Newt (2003).
Porous Asphalt Pavements
, Information Series 131, Lanham, Maryland:
National Asphalt Pavement Association.
Kandhal, Prithvi S. (2002).
Design, Construction, and Maintenance of Open-Graded Asphalt Friction
Courses,
Information Series 115, Lanham, Maryland: National Asphalt Pavement Association.
Kandhal, Prithvi S., and Rajib B. Mallick (1998).
Open-Graded Asphalt Friction Course: State of the
Practice
, Report No. 98-7, Auburn, Alabama: Auburn University National Center for Asphalt
Technology.
Kandhal, Prithvi S., and Rajib B. Mallick (1999).
Design of New-Generation Open-Graded Friction
Courses
, Report No. 99-2, Auburn, Alabama: Auburn University National Center for Asphalt
Technology.
Mallick, Rajib B., Prithvi S. Kandhal, L. Allen Cooley Jr., and Donald E. Watson (2000).
Design,
Construction and Performance of New-Generation Open-Graded Friction Courses
, Report No. 2000-01,
Auburn, Alabama: Auburn University National Center for Asphalt Technology.
Paine, John E. (1990).
Stormwater Design Guide, Portland Cement Pervious Pavement
, Orlando:
Florida Concrete and Products Association.
Smith, David R. (2001). Permeable Interlocking Concrete Pavements: Selection, Design, Construction,
Maintenance, 2
nd
ed., Washington: Interlocking Concrete Pavement Institute.
Tappeiner, Walter J. (1993).
Open-Graded Asphalt Friction Course
, Information Series 115, Lanham,
Maryland: National Asphalt Pavement Association.
Thelen, E. and Howe, L.F. (1978).
Porous Pavement
, Philadelphia: Franklin Institute Press.
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Chapter 6
BMP 6.4.2: Infiltration Basin
An Infiltration Basin is a shallow impoundment that
stores and infiltrates runoff over a level, uncompacted,
(preferably undisturbed area) with relatively permeable
soils.
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Yes
*
Yes
Limited
*
Applicable with specific consideration to
design.
Stormwater Functions
Key Design Elements
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
High
High
Med./High
High
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
30%
.
Maintain a minimum 2-foot separation to bedrock and seasonally
high water table, provide distributed infiltration area (5:1
impervious area to infiltration area - maximum), site on natural,
uncompacted soils with acceptable infiltration capacity, and follow
other guidelines described in Protocol 2: Infiltration Systems
Guidelines
.
Uncompacted sub-grade
.
Infiltration Guidelines and Soil Testing Protocols apply
.
Preserve existing vegetation, if possible
.
Design to hold/infiltrate volume difference in 2-yr storm or 1.5”
storm
.
Provide positive stormwater overflow through engineered outlet
structure.
.
Do not install on recently placed fill (<5 years).
.
Allow 2 ft buffer between bed bottom and seasonal high
groundwater table and 2 ft buffer for rock.
.
When possible, place on upland soils.
.
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C
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Description
Infiltration Basins are shallow, impounded areas designed to temporarily store and infiltrate stormwater
runoff. The size and shape can vary from one large basin to multiple, smaller basins throughout a site.
Ideally, the basin should avoid disturbance of existing vegetation. If disturbance is unavoidable,
replanting and landscaping may be necessary and should integrate the existing landscape as subtly as
possible and compaction of the soil must be prevented (see Infiltration Guidelines). Infiltration Basins
use the existing soil mantle to reduce the volume of stormwater runoff by infiltration and
evapotranspiration. The quality of the runoff is also improved by the natural cleansing processes of the
existing soil mantle and also by the vegetation planted in the basins. The key to promoting infiltration is
to provide enough surface area for the volume of runoff to be absorbed to meet the criteria in Chapter
3. An engineered overflow structure should be provided for the larger storms.
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Variations
Re-Vegetation
For existing unvegetated areas or for infiltration basins that require excavation, vegetation may
be added. Planting in the infiltration area will improve water quality, encourage infiltration, and
promote evapotranspiration. This vegetation may range from a meadow mix to more substantial
woodland species. The planting plan should be sensitive to hydrologic variability anticipated in
the basin, as well as to larger issues of native plants and habitat, aesthetics, and other planting
objectives.
The use of turf grass is discouraged
due to soil compaction from the required
frequent mowing and maintenance requirements.
Usable Surface
An Infiltration Basin can be used for recreation (usually informal) in dry periods. Heavy
machinery and/or vehicular traffic of any type should be avoided so as not to compact the
infiltration area.
Soils with Poor Infiltration Rates
A layer of sand (6”) or gravel can be placed on the bottom of the Infiltration Basin, or the soil can
be amended to increase the surface permeability of the basin. (See Soil Amendment &
Restoration BMP 6.7.3 for details.)
Applications
New Development
Infiltration Basins can be incorporated into new development. Ideally, existing vegetation can
be preserved and utilized as the infiltration area. Runoff from adjacent buildings and impervious
surfaces can be directed into this area, which will “water” the vegetation, thereby increasing
evapotranspiration in addition to encouraging infiltration.
Retrofitting existing “lawns” and “open space”
Existing grassed areas can be converted to infiltration basins. If the soil and infiltration capacity
is determined to be sufficient, the area can be enclosed through creation of a berm and runoff
can be directed to it without excavation. Otherwise, excavation can be performed as described
below.
Other Applications
Other applications of Infiltration Basins may be determined by the Design Professional as
appropriate.
Design Considerations
1. Soil Investigation and Infiltration Testing is required; site selection for this BMP should take soil
and infiltration capacity into consideration.
2. Guidelines for Infiltration Systems should be met (i.e., depth to water table, setbacks, Loading
Rates, etc.)
3. Basin designs that do not remove existing soil and/or vegetation are preferred.
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4. The slope of the Infiltration Basin bottom should be level or with a slope no greater than 1%. A
level bottom assures even water distribution and infiltration.
5. Basins may be constructed where impermeable soils on the surface are removed and where
more permeable underlying soils then are used for the base of the bed; care must be taken in
the excavation process to make sure that soil compaction does not occur.
6. The discharge or overflow from the Infiltration Basin should be properly designed for anticipated
flows. Large infiltration basins may require multiple outlet control devices to effectively overflow
water during the larger storms. See BMP 6.3.3 for more information on overflows and berms.
7. The berms surrounding the basin should be compacted earth with a slope of not steeper than
3:1(H:V), and a top width of at least 2 feet.
8. At least one foot of freeboard above the 100-year storm water elevation should be maintained.
9. Infiltration basins can be planted with natural grasses, meadow mix, or other “woody” mixes,
such as trees or shrubs. These plants have longer roots than traditional grass and increase soil
permeability. Native plants should be used wherever possible.
10. Use of fertilizer should be avoided.
11. The surface should be compacted as little as possible to allow for surface percolation through
the soil layer.
12. When directing runoff from roadway areas into the basin, measures to reduce sediment should
be used.
13. The inlets into the basin should have erosion protection.
14. Contributing inlets (up gradient) may have a sediment trap or water quality insert to prevent
large particles from clogging the system based on the quality of the runoff.
15. Use of a backup underdrain or low-flow orifice may be considered in the event that the water in
the basin does not drain within the criteria in Chapter 3. This underdrain valve should remain in
the shut position unless the basin does not drain.
Detailed Stormwater Functions
Infiltration Area
The loading rate guidelines in Appendix C should be consulted
The Infiltration Area is the bottom area of the bed.
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Volume Reduction Calculations
Volume = Depth* (ft) x Area (sf)
*Depth is the depth of the water stored during a storm event, depending on the drainage area and
conveyance to the bed.
Infiltration Volume = Bed Bottom Area (sf) x Infiltration design rate (in/hr)
x Infiltration period* (hr) x (1/12)
*Infiltration Period is equal to 2 hours or tne time of concentration, whichever is larger.
Peak Rate Mitigation Calculations:
See Chapter 8 for Peak Rate Mitigation methodology which
addresses link between volume reduction and peak rate control.
Water Quality Improvement:
See Chapter 8 for Water Quality Improvement methodology, which
addresses pollutant removal effectiveness of this BMP.
Construction Sequence
1. Protect Infiltration basin area from compaction prior to installation.
2. If possible, install Infiltration basin during later phases of site construction to prevent
sedimentation and/or damage from construction activity. After installation, prevent sediment-
laden water from entering inlets and pipes.
3. Install and maintain proper Erosion and Sediment Control Measures during construction.
4. If necessary, excavate Infiltration basin bottom to an uncompacted subgrade free from rocks
and debris. Do NOT compact subgrade.
5. Install Outlet Control Structures.
6. Seed and stabilize topsoil. (Vegetate if appropriate with native plantings.)
7. Do not remove Inlet Protection or other Erosion and Sediment Control measures until site is fully
stabilized.
Maintenance and Inspection Issues
Catch Basins and Inlets (upgradient of infiltration basin) should be inspected and cleaned at
least two times per year and after runoff events.
The vegetation along the surface of the Infiltration basin should be maintained in good condition,
and any bare spots revegetated as soon as possible.
Vehicles should not be parked or driven on an Infiltration Basin, and care should be taken to
avoid excessive compaction by mowers.
Inspect the basin after runoff events and make sure that runoff drains down within 72 hours.
Mosquito’s should not be a problem if the water drains in 72 hours. Mosquitoes require a
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considerably long breeding period with relatively static water levels.
Also inspect for accumulation of sediment, damage to outlet control structures, erosion control
measures, signs of water contamination/spills, and slope stability in the berms.
Mow only as appropriate for vegetative cover species.
Remove accumulated sediment from basin as required. Restore original cross section and
infiltration rate. Properly dispose of sediment.
Cost Issues
The construction cost of Infiltration Basins can vary greatly depending on the configuration, location,
site-specific conditions, etc.
Excavation (if necessary) - varies
Plantings - Meadow mix $2500 - $3500 / acre (2005)
Pipe Configuration – varies with stormwater configuration, may need to redirect pipes into the infiltration
basin.
Specifications
The following specifications are provided for information purposes only. These specifications include
information on acceptable materials for typical applications, but are by no means exclusive or limiting.
The designer is responsible for developing detailed specifications for individual design projects in
accordance with the project conditions.
1. Topsoil
amend with compost if necessary or desired. (See Soil Amendment & Restoration BMP
6.7.2)
2. Vegetation
See Native Plant List available locally, and/or see Appendix B.
References
Michigan Department of Environmental Quality.
Index of Individual BMPs
. 2004. State of Michigan. <
http://www.michigan.gov/deq/1,1607,7-135-3313_3682_3714-13186—,00.html>
Young, et. al., "Evaluation and Management of Highway Runoff Water Quality," Federal Highway
Administration, 1996
California Stormwater Quality Association.
California Stormwater Best Management Practices
Handbook: New Development and Redevelopment
. 2003.
Metropolitan Council Environmental Services.
Minnesota Urban Small Sites BMP Manual.
2001.
New Jersey Department of Environmental Protection.
New Jersey Stormwater Best Management
Practices Manual
. 2004.
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BMP 6.4.3: Subsurface Infiltration Bed
Subsurface Infiltration Beds provide temporary storage
and infiltration of stormwater runoff by placing storage
media of varying types beneath the proposed surface
grade. Vegetation will help to increase the amount of
evapotranspiration taking place.
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Yes
Yes
Limited
Key Design Elements
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
High
High
Med./High
High
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
30%
.
Maintain a minimum 2-foot separation to bedrock and seasonally
high water table, provide distributed infiltration area (5:1
impervious area to infiltration area - maximum), site on natural,
uncompacted soils with acceptable infiltration capacity, and follow
other guidelines described in Protocol 2: Infiltration Systems
Guidelines
.
Beds filled with stone (or alternative) as needed to increase void
space
.
Wrapped in nonwoven geotextile
.
Level or nearly level bed bottoms
.
Provide positive stormwater overflow from beds
.
Protect from sedimentation during construction
.
Provide perforated pipe network along bed bottom for distribution
as necessary
.
Open-graded, clean stone with minimum 40% void space
.
Do not place bed bottom on compacted fill
• Allow 2 ft. buffer between bed bottom and seasonal high
groundwater table and 2 ft. for bedrock.
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C
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Description
A Subsurface Infiltration Bed generally consists of a vegetated, highly pervious soil media underlain by
a uniformly graded aggregate (or alternative) bed for temporary storage and infiltration of stormwater
runoff. Subsurface Infiltration beds are ideally suited for expansive, generally flat open spaces, such as
lawns, meadows, and playfields, which are located downhill from nearby impervious areas. Subsurface
Infiltration Beds can be stepped or terraced down sloping terrain provided that the base of the bed
remains level. Stormwater runoff from nearby impervious areas (including rooftops, parking lots, roads,
walkways, etc.) can be conveyed to the subsurface storage media, where it is then distributed via a
network of perforated piping.
The storage media for subsurface infiltration beds typically consists of clean-washed, uniformly graded
aggregate. However, other storage media alternatives are available. These alternatives are generally
variations on plastic cells that can more than double the storage capacity of aggregate beds, at a
substantially increased cost. Storage media alternatives are ideally suited for sites where potential
infiltration area is limited.
If designed, constructed, and maintained as per the following guidelines, Subsurface Infiltration features
can stand-alone as significant stormwater runoff volume, rate, and quality control practices. These
systems can also maintain aquifer recharge, while preserving or creating valuable open space and
recreation areas. They have the added benefit of functioning year-round, given that the infiltration
surface is typically below the frost line.
Variations
As its name suggests, Subsurface Infiltration is generally employed for temporary storage and
infiltration of runoff in subsurface storage media. However, in some cases, runoff may be temporarily
stored on the surface (to depths less than 6 inches) to enhance volume capacity of the system. The
overall system design should ensure that within the criteria in Chapter 3, the bed is completely empty.
Applications
Connection of Roof Leaders
Runoff from nearby roofs may be directly conveyed to subsurface beds via roof leader connections to
perforated piping. Roof runoff generally has relatively low sediment levels, making it ideally suited for
connection to an infiltration bed. However, cleanout(s) with a sediment sump are still recommended
between the building and infiltration bed.
Connection of Inlets
Catch Basins, inlets, and area drains may be connected to
Subsurface Infiltration beds. However, sediment and
debris removal should be provided. Storm structures
should therefore include sediment trap areas below the
inverts of discharge pipes to trap solids and debris. In
areas of high traffic or excessive generation of sediment,
litter, and other similar materials, a water quality insert or
other pretreatment device may be needed.
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Under Recreational Fields
Subsurface Infiltration is very well suited below
playfields and other recreational areas. Special
consideration should be given to the engineered
soil mix in those cases.
Under Open Space
Subsurface Infiltration is also appropriate in either
existing or proposed open space areas. Ideally,
these areas are vegetated with native grasses
and/or vegetation to enhance site aesthetics and
landscaping. Aside from occasional clean-outs or
outlet structures, Subsurface Infiltration systems
are essentially hidden stormwater management
features, making them ideal for open space locations (deed-restricted open space locations are
especially desirable because such locations minimize the chance that Subsurface Infiltration systems
will be disturbed or disrupted accidentally in the future).
Other Applications
Other applications of Subsurface Infiltration beds may be determined by the Design Professional as
appropriate.
Design Considerations
1. Soil Investigation and Infiltration Testing is needed (Appendix C).
2. Guidelines for Infiltration Systems should be met (Appendix C).
3. The overall site should be evaluated for potential Subsurface Infiltration areas early in the
design process, as effective design requires consideration of existing site characteristics
(topography, natural features/drainage ways, soils, geology, etc.).
4. Control of Sediment is critical. Rigorous installation and maintenance of erosion and sediment
control measures is needed to prevent sediment deposition within the stone bed. Nonwoven
geotextile may be folded over the edge of the bed until the site is stabilized.
5. The Infiltration bed should be
wrapped in non-woven geotextile
filter fabric.
6. Subsurface Infiltration areas
should not be placed on areas of
recent fill or compacted fill. Any
grade adjustments requiring fill
should be done using the stone
subbase material, or alternative.
Areas of historical fill (>5 years)
may be considered if other
criteria are met.
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7. The subsurface infiltration bed is typically comprised of a 12 to 36 inch section of aggregate,
such as AASHTO No.3, which ranges 1-2 inches in gradation. Depending on local aggregate
availability, both larger and smaller size aggregate has been used. The critical requirements
are that the aggregate be uniformly graded, clean-washed, and contain at least 40% void space.
The depth of the bed is a function of stormwater storage requirements, frost depth
considerations, and site grading. Infiltration beds are typically sized to mitigate the increased
runoff volume from the design storm.
8. Water Quality Inlet or Catch Basin with Sump is needed for all surface inlets, should be
designed to avoid standing water for periods greater than the criteria in Chapter 3.
9. Infiltration beds may be placed on a slope by benching or terracing infiltration levels. The slope
of the infiltration bed bottom should be level or with a slope no greater than 1%. A level bottom
assures even water distribution and infiltration.
10. Perforated pipes along the bottom of the bed can be used to evenly distribute runoff over the
entire bed bottom. Continuously perforated pipes may connect structures (such as cleanouts
and inlet boxes). Pipes should lay flat along the bed bottom and provide for uniform distribution
of water. Depending on size, these pipes may provide additional storage volume.
11. Cleanouts or inlets should be installed at a few locations within the bed and at appropriate
intervals to allow access to the perforated piping network and or storage media.
12. All infiltration beds should be designed with an overflow for extreme storm events. Control in the
beds is usually provided in the form of an outlet control structure. A modified inlet box with an
internal concrete weir (or weir plate) and low-flow orifice is a common type of control structure.
The specific design of these structures may vary, depending on factors such as rate and
storage requirements, but it must always include positive overflow from the system. The
overflow structure is used to maximize the water level in the stone bed, while providing sufficient
cover for overflow pipes. Generally, the top of the outlet pipe should be 4 inches below the top
of the aggregate to prevent saturated soil conditions in remote areas of the bed. As with all
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infiltration practices, multiple discharge points are recommended. These may discharge to the
surface or a storm sewer system.
13. Adequate soil cover (generally 12 - 18 inches) should be maintained above the infiltration bed to
allow for a healthy vegetative cover.
14. Open space overlying infiltration beds can be vegetated with native grasses, meadow mix, or
other low-growing, dense vegetation. These plants have longer roots than traditional grass and
will likely benefit from the moisture in the infiltration bed, improving the growth of these plantings
and, potentially increasing evapotranspiration.
15. Fertilizer use should be minimized.
16. The surface (above the stone bed) should be compacted as minimally as possible to allow for
surface percolation through the engineered soil layer and into the stone bed.
17. When directing runoff from roadway areas into the beds, measures to reduce sediment should
be used.
18. Surface grading should be relatively flat, although a relatively mild slope between 1% and 3% is
recommended to facilitate drainage.
19. In those areas where the threat of spills and groundwater contamination exists, pretreatment
systems, such as filters and wetlands, may be needed before any infiltration occurs. In Hot
Spot areas, such as truck stops and fueling stations, the suitability of Subsurface Infiltration
must be considered.
20. In areas with poorly-draining soils, Subsurface Infiltration areas may be designed to slowly
discharge to adjacent wetlands or bioretention areas.
21. While most Subsurface Infiltration areas consist of an aggregate storage bed, alternative
subsurface storage products may also be employed. These include a variety of proprietary,
interlocking plastic units that contain much greater storage capacity than aggregate, at an
increased cost.
22. The subsurface bed and overflow may be designed and evaluated in the same manner as a
detention basin to demonstrate the mitigation of peak flow rates. In this manner, detention
basins may be eliminated or significantly reduced in size.
23. During Construction, the excavated bed may serve as a Temporary Sediment Basin or Trap.
This can reduce overall site disturbance. The bed should be excavated to at least 1 foot above
the final bed bottom elevation for use as a sediment trap or basin. Following construction and
site stabilization, sediment should be removed and final grades established. In BMPs that will
be used for infiltration in the future, use of construction equipment should be limited as much as
possible.
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Detailed Stormwater Functions
Infiltration Area
Loading rate quidelines in Appendix C should be consulted
.
The Infiltration Area is the bottom area of the bed, defined as:
Length of bed x Width of bed = Infiltration Area (if rectangular)
Volume Reduction Calculations
Volume = Depth* (ft) x Area (sf) x Void Space
*Depth is the depth of water stored during a storm event, depending on the drainage area and
conveyance to the bed.
Infiltration Volume = Bed Bottom Area (sf) x Infiltration design rate (in/hr)
x Infiltration period* (hr) x (1/12)
*Infiltration Period is equal to 2 hours or the time of concentration, whichever is larger.
Additional storage/volume reduction can be calculated for the overlying soil as appropriate.
Peak Rate Mitigation Calculations
See in Chapter 8 for Peak Rate Mitigation methodology which addresses link between volume
reduction and peak rate control.
Water Quality Improvement:
See in Chapter 8 for Water Quality Improvement methodology, which
addresses pollutant removal effectiveness of this BMP.
Construction Sequence
1. Due to the nature of construction sites, Subsurface Infiltration should be installed toward the end
of the construction period, if possible. (Infiltration beds may be used as temporary sediment
basins or traps as discussed above).
2. Install and maintain adequate Erosion and Sediment Control Measures (as per the
Pennsylvania Erosion and Sedimentation Control Program Manual) during construction.
3. The existing subgrade under the bed areas should NOT
be compacted or subject to excessive
construction equipment traffic prior to geotextile and stone bed placement.
4. Where erosion of subgrade has caused accumulation of fine materials and/or surface ponding,
this material should be removed with light equipment and the underlying soils scarified to a
minimum depth of 6 inches with a York rake (or equivalent) and light tractor. All fine grading
should be done by hand. All bed bottoms should be at level grade.
5. Earthen berms (if used) between infiltration beds should be left in place during excavation.
These berms do not require compaction if proven stable during construction.
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6. Install upstream and downstream control structures, cleanouts, perforated piping, and all other
necessary stormwater structures.
7. Geotextile and bed aggregate should be placed immediately after approval of subgrade
preparation and installation of structures. Geotextile should be placed in accordance with
manufacturer’s standards and recommendations. Adjacent strips of geotextile should overlap a
minimum of 16 inches. It should also be secured at least 4 feet outside of bed in order to
prevent any runoff or sediment from entering the storage bed. This edge strip should remain in
place until all bare soils contiguous to beds are stabilized and vegetated. As the site is fully
stabilized, excess geotextile along bed edges can be cut back to the edge of the bed.
8. Clean-washed, uniformly graded aggregate should be placed in the bed in maximum 8-inch lifts.
Each layer should be lightly compacted, with construction equipment kept off the bed bottom as
much as possible.
9. Approved soil media should be placed over infiltration bed in maximum 6-inch lifts.
10. Seed and stabilize topsoil.
11. Do not remove inlet protection or other Erosion and Sediment Control measures until site is fully
stabilized.
Maintenance Issues
Subsurface Infiltration is generally less maintenance intensive than other practices of its type.
Generally speaking, vegetation associated with Subsurface Infiltration practices is less substantial than
practices such as Recharge Gardens and Vegetated Swales and therefore requires less maintenance.
Maintenance activities required for the subsurface bed are similar to those of any infiltration system and
focus on regular sediment and debris removal. The following represents the recommended
maintenance efforts:
All Catch Basins and Inlets should be inspected and cleaned at least 2 times per year.
The overlying vegetation of Subsurface Infiltration features should be maintained in good
condition, and any bare spots revegetated as soon as possible.
Vehicular access on Subsurface Infiltration areas should be prohibited, and care should be
taken to avoid excessive compaction by mowers. If access is needed, use of permeable, turf
reinforcement should be considered.
Cost Issues
The construction cost of Subsurface Infiltration can vary greatly depending on design variations,
configuration, location, desired storage volume, and site-specific conditions, among other factors.
Typical construction costs are about $5.70 per square foot, which includes excavation, aggregate (2.0
feet assumed), non-woven geotextile, pipes and plantings.
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Specifications
The following specifications are provided for information purposes only. These specifications include
information on acceptable materials for typical applications, but are by no means exclusive or limiting.
The designer is responsible for developing detailed specifications for individual design projects in
accordance with the project conditions.
1. Stone
for infiltration beds shall be 2-inch to 1-inch uniformly graded coarse aggregate, with a
wash loss of no more than 0.5%, AASHTO size number 3 per AASHTO Specifications, Part I,
19th Ed., 1998, or later and shall have voids 40% as measured by ASTM-C29.
2. Non-Woven Geotextile
shall consist of needled non-woven polypropylene fibers and meet the
following properties:
a. Grab Tensile Strength (ASTM-D4632)
120 lbs
b. Mullen Burst Strength (ASTM-D3786)
225 psi
c. Flow Rate (ASTM-D4491)
95 gal/min/ft
2
d. UV Resistance after 500 hrs (ASTM-D4355) 70%
e. Heat-set or heat-calendared fabrics are not permitted
Acceptable types include Mirafi 140N, Amoco 4547, and Geotex 451.
3. Topsoil
may be amended with compost (See soil restoration BMP 6.7.2)
4. Pipe
shall be continuously perforated, smooth interior, with a minimum inside diameter of 6-
inches. High-density polyethylene (HDPE) pipe shall meet AASHTO M252, Type S or AASHTO
M294, Type S.
5. Storm Drain Inlets and Structures
a. Concrete Construction: Concrete construction shall be in accordance with Section 1001,
PennDOT Specifications, 1990 or latest edition.
b. Precast Concrete Inlets and Manholes: Precast concrete inlets may be substituted for
cast-in-place structures and shall be constructed as specified for cast-in-place.
Precast structures may be used in only those areas where there is no conflict with
existing underground structures that may necessitate revision of inverts. Type M
standard PennDOT inlet boxes will be modified to provide minimum 12 inch sump
storage and bottom leaching basins, open to gravel sumps in sub-grade, when situated
in the recharge bed.
c. All PVC Catch Basins/Cleanouts/Inline Drains shall have H-10 or H-20 rated grates,
depending on their placement (H-20 if vehicular loading).
d. Steel reinforcing bars over the top of the outlet structure shall conform to ASTM A615,
grades 60 and 40.
e. Permanent turf reinforcement matting shall be installed according to manufacturers’
specifications.
6. Alternative storage media:
Follow appropriate Manufacturers’ specifications.
7. Vegetation
see Local Native Plant List and Appendix B.
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BMP 6.4.4: Infiltration Trench
An Infiltration Trench is a “leaky” pipe in a stone filled
trench with a level bottom. An Infiltration Trench may be
used as part of a larger storm sewer system, such as a
relatively flat section of storm sewer, or it may serve as a
portion of a stormwater system for a small area, such as a
portion of a roof or a single catch basin. In all cases, an
Infiltration Trench should be designed with a positive
overflow.
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
30%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Medium
High
Medium
High
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Yes
Yes
.
Continuously perforated pipe set at a minimum slope in a stone
Yes
filled, level-bottomed trench
.
Limited in width (3 to 8 feet) and depth of stone (6 feet max.
recommended)
.
Trench is wrapped in nonwoven geotextile (top, sides, and
bottom)
.
Placed on uncompacted soils
.
Minimum cover over pipe is as per manufacturer.
.
A minimum of 6" of topsoil is placed over trench and vegetated
.
Positive Overflow always provided
Deed restrictions recommended
Not for use in hot spot areas without pretreatment
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C
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Description
An Infiltration Trench is a linear stormwater BMP consisting of a continuously perforated pipe at a
minimum slope in a stone-filled trench (Figure 6.4-1). Usually an Infiltration Trench is part of a
conveyance system
and is designed so that large storm events are conveyed through the pipe with
some runoff volume reduction. During small storm events, volume reduction may be significant and
there may be little or no discharge. All Infiltration Trenches are designed with a
positive overflow
(Figure 6.4-2).
An Infiltration Trench differs from an Infiltration Bed in that it may be constructed without heavy
equipment entering the trench. It is also intended to convey some portion of runoff in many storm
events.
Figure 6.4-1
Figure 6.4-2
All Infiltration Trenches should be designed in accordance with Appendix C. Although the width and
depth can vary, it is recommended that Infiltration Trenches be limited in depth to not more than six (6)
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feet of stone. This is due to both construction issues and Loading Rate issues (as described in the
Guidelines for Infiltration Systems). The designer should consider the appropriate depth.
Variations
Infiltration Trenches generally have a vegetated (grassed) or gravel surface. Infiltration Trenches also
may be located alongside or adjacent to roadways or impervious paved areas with proper design. The
subsurface drainage direction should be to the downhill side (away from subbase of pavement), or
located lower than the impervious subbase layer. Proper measures should be taken to prevent water
infiltrating into the subbase of impervious pavement.
Infiltration Trenches may also be located down a mild slope by “stepping” the sections between control
structures as shown in Figure 6.4-3. A level or nearly level bottom is recommended for even
distribution.
Figure 6.4-3
Applications
Connection of Roof Leaders
Roof leaders may be connected to Infiltration Trenches.
Roof runoff generally has lower sediment levels and often is
ideally suited for discharge through an Infiltration Trench. A
cleanout with sediment sump should be provided between
the building and Infiltration Trench.
Connection of Inlets
Catch Basins, inlets and area drains may be connected to
Infiltration Trenches, however sediment and debris removal
should be addressed. Structures should include a sediment
trap area below the invert of the pipe for solids and debris.
In areas of high traffic or areas where excessive sediment,
litter, and other similar materials may be generated, a water
quality insert or other pretreatment device is needed.
In Combination with Vegetative Filters
An Infiltration Trench may be preceded by or used in
combination with a Vegetative Filter, Grassed Swale, or
other vegetative element used to reduce sediment levels
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from areas such as high traffic roadways. Design should ensure proper functioning of vegetative
system.
Other Applications
Other applications of Infiltration Trenches may be determined by the design professional as
appropriate.
Design Considerations
1. Soil Investigation and Percolation Testing is required (see Appendix C, Protocol 2)
2. Guidelines for Infiltration Systems should be met (i.e., depth to water table, setbacks, Loading
Rates, etc. See Appendix C, Protocol 1)
3. Water Quality Inlet or Catch Basin with Sump (see Section 6.6.4) recommended for all surface
inlets, designed to avoid standing water for periods greater than the criteria in Chapter 3.
4. A continuously perforated pipe should extend the length of the trench and have a positive flow
connection designed to allow high flows to be conveyed through the Infiltration Trench.
5. The slope of the Infiltration Trench bottom should be level or with a slope no greater than 1%.
The Trench may be constructed as a series of “steps” if necessary. A level bottom assures
even water distribution and infiltration.
6. Cleanouts or inlets should be installed at both ends of the Infiltration Trench and at appropriate
intervals to allow access to the perforated pipe.
7. The discharge or overflow from the Infiltration Trench should be properly designed for
anticipated flows.
Detailed Stormwater Functions
Infiltration Area
The Infiltration Area is the bottom area of the Trench*, defined as:
Length of Trench x Width of Trench = Infiltration Area (Bottom Area)
This is the area to be considered when evaluating the Loading Rate to the Infiltration Trench.
* Some credit can be taken for the side area that is frequently inundated as appropriate.
Volume Reduction Calculations
Volume = Depth* (ft) x Area (sf) x Void Space
*Depth is the depth of the water surface during a storm event, depending on the drainage area and
conveyance to the bed.
Infiltration Volume = Bed Bottom Area (sf) x Infiltration design rate (in/hr)
x Infiltration period* (hr) x (1/12)
*Infiltration Period is the time when bed is receiving runoff and capable of infiltration. Not to exceed 72
hours.
The void ratio in stone is approximately 40% for AASTO No 3. If the conveyance pipe is within the
Storage Volume area, the volume of the pipe may also be included. All Infiltration Trenches should be
designed to infiltrate or empty within 72 hours.
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Peak Rate Mitigation Calculations
See Chapter 8 for Peak Rate Mitigation methodology which addresses link between volume reduction
and peak rate control.
Water Quality Improvement
See Chapter 8 for Water Quality Improvement methodology which addresses pollutant removal
effectiveness of this BMP.
Construction Sequence
1. Protect Infiltration Trench area from compaction prior to installation.
2. If possible, install Infiltration Trench during later phases of site construction to prevent
sedimentation and/or damage from construction activity. After installation, prevent sediment
laden water from entering inlets and pipes.
3. Install and maintain proper Erosion and Sediment Control Measures during construction.
4. Excavate Infiltration Trench bottom to a uniform, level uncompacted subgrade free from rocks
and debris. Do NOT compact subgrade.
5. Place nonwoven geotextile along bottom and sides of trench*. Nonwoven geotextile rolls should
overlap by a minimum of 16 inches within the trench. Fold back and secure excess geotextile
during stone placement.
6. Install upstream and downstream Control Structures, cleanouts, etc.
7. Place uniformly graded, clean-washed aggregate in 8-inch lifts, lightly compacting between lifts.
8. Install Continuously Perforated Pipe as indicated on plans. Backfill with uniformly graded,
clean-washed aggregate in 8-inch lifts, lightly compacting between lifts.
9. Fold and secure nonwoven geotextile over Infiltration Trench, with minimum overlap of 16-
inches.
10. Place 6-inch lift of approved Topsoil over Infiltration Trench, as indicated on plans.
11. Seed and stabilize topsoil.
12. Do not remove Inlet Protection or other Erosion and Sediment Control measures until site is fully
stabilized.
13. Any sediment that enters inlets during construction is to be removed within 24 hours.
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(from left to right) Installation of Inlets and Control Structure; Non-woven Geotextile is folded over Infiltration
Trench; Stabilized Site
(Clockwise from top left) Infiltration Trench is on downhill side of roadway; Infiltration Trench is installed;
Infiltration Trench is paved with standard pavement material
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Maintenance and Inspection Issues
• Catch Basins and Inlets should be inspected and cleaned at least 2 times per year.
• The vegetation along the surface of the Infiltration Trench should be maintained in good
condition, and any bare spots revegetated as soon as possible.
• Vehicles should not be parked or driven on a vegetated Infiltration Trench, and care should be
taken to avoid excessive compaction by mowers.
Cost Issues
The construction cost of infiltration trenches can vary greatly depending on the configuration, location,
site-specific conditions, etc. Typical construction costs in 2003 dollars range from $4 - $9 per cubic foot
of storage provided (SWRPC, 1991; Brown and Schueler, 1997). Annual maintenance costs have
been reported to be approximately 5 to 10 percent of the capital costs (Schueler, 1987).
Specifications
The following specifications are provided for information purposes only. These specifications include
information on acceptable materials for typical applications, but are by no means exclusive or limiting.
The designer is responsible for developing detailed specifications for individual design projects in
accordance with the project conditions.
1. Stone
for infiltration trenches shall be 2-inch to 1-inch uniformly graded coarse aggregate, with a
wash loss of no more than 0.5%, AASHTO size number 3 per AASHTO Specifications, Part I,
19th Ed., 1998, or later and shall have voids 40% as measured by ASTM-C29.
2. Non-Woven Geotextile
shall consist of needled nonwoven polypropylene fibers and meet the
following properties:
a. Grab Tensile Strength (ASTM-D4632)
b. Mullen Burst Strength (ASTM-D3786)
c. Flow Rate (ASTM-D4491)
d. UV Resistance after 500 hrs (ASTM-D4355) 70%
e. Heat-set or heat-calendared fabrics are not permitted
Acceptable types include Mirafi 140N, Amoco 4547, and Geotex 451.
3. Pipe
shall be continuously perforated, smooth interior, with a minimum inside diameter of 8-
inches. High-density polyethylene (HDPE) pipe shall meet AASHTO M252, Type S or AASHTO
M294, Type S.
References
Brown and Schueler,
Stormwater Management Fact Sheet: Infiltration Trench.
1997.
Schueler, T., 1987.
Controlling urban runoff: a practical manual for planning and designing urban
BMPs
, Metropolitan Washington Council of Governments, Washington, DC
SWRPC, The Use of of Best Management Practices (BMPs) in Urban Watersheds, US Environmental
Protection Agency,1991.
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BMP 6.4.5: Rain Garden/Bioretention
A Rain Garden (also called
Bioretention) is an excavated shallow
surface depression planted with
specially selected native vegetation to
treat and capture runoff.
Water Quality Functions
TSS:
TP:
NO3:
85% 85%
30%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Medium
Med./High
Low/Med.
Med./High
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial: Ultra
Urban: Industrial:
Retrofit:
Highway/Road:
Yes Yes
Yes
Yes Yes
Yes
.
Flexible in terms of size and infiltration
.
Ponding depths generally limited to 12 inches or less for
aesthetics, safety, and rapid draw down. Certain situations may
allow deeper ponding depths.
.
Deep rooted perennials and trees encouraged
.
Native vegetation that is tolerant of hydrologic variability, salts and
environmental stress
.
Modify soil with compost.
.
Stable inflow/outflow conditions
.
Provide positive overflow
.
Maintenance to ensure long-term functionality
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C
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Description
Bioretention is a method of treating stormwater by pooling water on the surface and allowing filtering
and settling of suspended solids and sediment at the mulch layer, prior to entering the
plant/soil/microbe complex media for infiltration and pollutant removal. Bioretention techniques are
used to accomplish water quality improvement and water quantity reduction. Prince George’s County,
Maryland, and Alexandria, Virginia have used this BMP since 1992 with success in many urban and
suburban settings.
Bioretention can be integrated into a site with a high degree of flexibility and can balance nicely with
other structural management systems, including porous asphalt parking lots, infiltration trenches, as
well as non-structural stormwater BMPs described in Chapter 5.
The vegetation serves to filter (water quality) and transpire (water quantity) runoff, and the root systems
can enhance infiltration. The plants take up pollutants; the soil medium filters out pollutants and allows
storage and infiltration of stormwater runoff; and the bed provides additional volume control. Properly
designed bioretention techniques mimic natural ecosystems through species diversity, density and
distribution of vegetation, and the use of native species, resulting in a system that is resistant to insects,
disease, pollution, and climatic stresses.
Rain Gardens / Bioretention function to:
Reduce runoff volume
Filter pollutants, through both soil particles (which trap pollutants) and plant material (which take
up pollutants)
Recharge groundwater by infiltration
Reduce stormwater temperature impacts
Enhance evapotranspiration
Enhance aesthetics
Provide habitat
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Primary Components of a Rain Garden/Bioretention System
The primary components (and subcomponents) of a rain garden/bioretention system are:
Pretreatment (optional)
Sheet flow through a vegetated buffer strip, cleanout, water quality inlet, etc. prior to entry into
the Rain Garden
Flow entrance
Varies with site use (e.g., parking island versus residential lot applications)
Water may enter via an inlet (e.g., flared end section)
Sheet flow into the facility over grassed areas
Curb cuts with grading for sheet flow entrance
Roof leaders with direct surface connection
Trench drain
Entering velocities should be non-erosive.
Ponding area
Provides temporary surface storage of runoff
Provides evaporation for a portion of runoff
Design depths allow sediment to settle
Limited in depth for aesthetics and safety
Plant material
Evapotranspiration of stormwater
Root development and rhizome community create pathways for infiltration
Bacteria community resides within the root system creating healthy soil structure with water
quality benefits
Improves aesthetics for site
Provides habitat for animals and insects
Reinforces long-term performance of subsurface infiltration
Should be tolerant of salts if in a location that would receive snow melt chemicals
Organic layer or mulch
Acts as a filter for pollutants in runoff
Protects underlying soil from drying and eroding
Simulates leaf litter by providing environment for microorganisms to degrade organic material
Provides a medium for biological growth, decomposition of organic material, adsorption and
bonding of heavy metals
Wood mulch should be shredded - compost or leaf mulch is preferred.
Planting soil/volume storage bed
Provides water/nutrients to plants
Enhances biological activity and encourages root growth
Provides storage of stormwater by the voids within the soil particles
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Positive overflow
Will discharge runoff during large storm events when the storage capacity is exceeded.
Examples include domed riser, inlet, weir structure, etc.
An underdrain can be included in areas where infiltration is not possible or appropriate.
Variations
Generally, a Rain Garden/Bioretention system is a vegetated surface depression that provides for the
infiltration of relatively small volumes of stormwater runoff, often managing stormwater on a lot-by-lot
basis (versus the total development site). If greater volumes of runoff need to be managed or stored,
the system can be designed with an expanded subsurface infiltration bed or the Bioretention area can
be increased in size.
The design of a Rain Garden can vary in complexity depending on the quantity of runoff volume to be
managed, as well as the pollutant reduction objectives for the entire site. Variations exist both in the
components of the systems, which are a function of the land use surrounding the Bioretention system.
The most common variation includes a gravel or sand bed underneath the planting bed. The original
intent of this design, however, was to perform as a filter BMP utilizing an under drain and subsequent
discharge. When a designer decides to use a gravel or sand bed for volume storage under the planting
bed, then additional design elements and changes in the vegetation plantings should be provided.
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Flow Entrance: Curbs and Curb Cuts
Flow Entrance: Trench Drain
Positive Overflow: Domed Riser
Positive Overflow: Inlet
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Applications
Bioretention areas can be used in a variety of applications: from small areas in residential lawns to
extensive systems in large parking lots (incorporated into parking islands and/or perimeter areas).
Residential On-lot
Rain Garden (Prince George’s County)
Simple design that incorporates a planting bed in the low portion of the site
Tree and Shrub Pits
tormwater management
chnique that intercepts runoff
nd provides shallow ponding in
a dished mulched area around
the tree or shrub.
Extend the mulched area to the
tree dripline
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a
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Roads and highways
Parking Lots
Parking Lot Island Bioretention
Commercial/Industrial/Institutional
In commercial, industrial, and institutional situations, stormwater management and
greenspace areas are limited, and in these situations, Rain Gardens for stormwater
management and landscaping provide multifunctional options.
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• Curbless (Curb cuts) Parking Lot Perimeter Bioretention
The Rain Garden is located adjacent to a parking area with no curb or curb cuts ,
allowing stormwater to sheet flow over the parking lot directly into the Rain Garden.
Shallow grades should direct runoff at reasonable velocities; this design can be used in
conjunction with depression storage for stormwater quantity control.
• Curbed Parking Lot Perimeter Bioretention
• Roof leader connection from adjacent building
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Design Considerations
Rain Gardens are flexible in design and can vary in complexity according to water quality objectives
and runoff volume requirements. Though Rain Gardens are a structural BMP, the initial siting of
bioretention areas should respect the Integrating Site Design Procedures described in Chapter 4 and
integrated with the preventive non-structural BMPs.
It is important to note that bioretention areas are not to be confused with constructed wetlands or wet
ponds which permanently pond water. Bioretention is best suited for areas with at least moderate
infiltration rates (more than 0.1 inches per hour). In extreme situations where permeability is less than
0.1 inches per hour, special variants may apply, including under drains, or even constructed wetlands.
Rain Gardens are often very useful in retrofit projects and can be integrated into already developed lots
and sites. An important concern for all Rain Garden applications is their long-term protection and
maintenance, especially if undertaken in multiple residential lots where individual homeowners provide
maintenan .
rt of management that insures their
long-term functioning (deed restrictions, covenants, and so forth).
1.
Sizing criteria
a. Surface area
is dependent upon storage volume requirements but should generally not
exceed a maximum loading ratio of 5:1 (impervious drainage area to infiltration area; see
Protocol 2. Infiltration Systems Guidelines (Appendix C) for additional guidance on loading
rates.)
b. Surface Side slopes
should be gradual. For most areas, maximum 3:1 side slopes are
recommended, however where space is limited, 2:1 side slopes may be acceptable.
c. Surface Ponding depth
should not exceed 6 inches in most cases and should empty within
72 hours.
d. Ponding area
should provide sufficient surface area to meet required storage volume without
t least 18” where only herbaceous plant species
will be utilized. If trees and woody shrubs will be used, soil media depth may be increased,
combined with 20-30% organic material (compost), and 70-80% soil base (preferably topsoil).
toxic substances and unwanted plant
material and have a 5 –10% organic matter content. Additional organic matter can be added to
ase water holding capacity (tests should be conducted to determine volume
storage capacity of amended soils).
ce In such situations, it is important to provide some so
exceeding the design ponding depth. The subsurface storage/infiltration bed is used to
supplement surface storage where feasible.
e. Planting soil depth
should generally be a
depending on plant species.
2.
Planting Soil
should be a loam soil capable of supporting a healthy vegetative cover. Soils
should be amended with a composted organic material. A typical organic amended soil is
Planting soil should be approximately 4 inches deeper than the bottom of the largest root ball.
3.
Volume Storage Soils
should also have a pH of between 5.5 and 6.5 (better pollutant
adsorption and microbial activity), a clay content less than 10% (a small amount of clay is
beneficial to adsorb pollutants and retain water), be free of
the soil to incre
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4.
floo
If
shr
spe
hrub and tree should be planted at a rate of approximately 700 shrubs and 300
ees per acre (shrub to tree ratio should be 2:1 to 3:1). An experienced landscape architect is
5. Planting periods
will vary, but in general trees and shrubs should be planted from mid-March
6. A maximum of 2 to 3 inches of shredded
mulch
or leaf compost (or other comparable product)
enh
uld
be
Mulch / compost layer should not
exceed 3” in depth so as not to restrict oxygen flow to roots.
7. Mus
con
t the
Detailed Stormwater Functions
Infiltra
Volum
The sto
defined as the sum total of 1. and the smaller of 2a or 2b
below. The surface storage volume should account for at least 50% of the total storage. Inter-media
void
) x infiltration design rate (in/hr) x infiltration
period (hr) x 1/12.
Peak R
See Chapter 8 for Peak Rate Mitigation methodology, which addresses link between volume reduction
and
Proper
plant selection
is essential for bioretention areas to be effective. Typically, native
dplain plant species are best suited to the variable environmental conditions encountered.
ubs and trees are included in a bioretention area (which is recommended), at least three
cies of s
tr
recommended to design native planting layout.
through the end of June, or mid-September through mid-November
should be uniformly applied immediately after shrubs and trees are planted to prevent erosion,
ance metal removals, and simulate leaf litter in a natural forest system. Wood chips sho
avoided as they tend to float during inundation periods.
t be designed carefully in areas with
steeper slopes
and should be aligned parallel to
tours to minimize earthwork.
8. Under drains should not be used except where in-situ soils fail to drain surface water to mee
criteria in Chapter 3.
tion Area
e Reduction Calculations
rage volume of a Bioretention area is
volumes may vary considerably based on design variations.
1. Surface Storage Volume (CF) = Bed Area (ft
2
) x Average Design Water Depth
2a. Infiltration Volume = Bed Bottom area (sq ft
2b. Volume = Bed Bottom area (sq ft) x soil mix bed depth x void space.
ate Mitigation
peak rate control.
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Wa
ee Chapter 8 for Water Quality Improvement methodology, which addresses pollutant removal
ffectiveness of this BMP.
onstruction Sequence
he following is a typical construction sequence; however, alterations might be necessary depending
r other inflow entrance but provide
protection so that drainage is prohibited from entering construction area.
3.
ain
garden bed areas may be used as temporary sediment traps provided that the proposed finish
4.
il surfaces. Do not
t in-situ soils.
5. Backfill Rain Garden with amended soil as shown on plans and specifications. Overfilling is
recommended to account for settlement. Light hand tamping is acceptable if necessary.
ting soil prior to planting
vegetation to aid in settlement.
rading to achieve
proposed design elevations, leaving
space for upper layer of compost, mulch
or topsoil as specified on plans.
8. Plant vegetation according to planting
9. Mulch and install erosion protection at
es where necessary.
ter Quality Improvement
S
e
C
T
on design variations.
1. Install temporary sediment control BMPs as shown on the plans.
2. Complete site grading. If applicable, construct curb cuts o
Stabilize grading within the limit of disturbance except within the Rain Garden area. R
elevation of the bed is 12 inches lower than the bottom elevation of the sediment trap.
Excavate Rain Garden to proposed invert depth and scarify the existing so
compac
6. Presoak the plan
7. Complete final g
plan.
surface flow entranc
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Ma
Properly designed and installed Bioretention areas require some regular maintenance.
ry year. Perennial plantings may be cut down at the
rosion is evident and be replenished as needed. Once every
ire mulch replacement.
ected at least two times per year for sediment buildup,
rosion, vegetative conditions, etc.
periods of extended drought, Bioretention areas may require watering.
e per year to evaluate health.
o Issues
ain Gardens often replace areas that would have been landscaped and are maintenance-intensive so
that the net cost can be considerably less than the actual construction cost. In addition, the use of Rain
Gardens can decrease the cost for stormwater conveyance systems at a site. Rain Gardens cost
approximately $5 to $7 (2005) per cubic foot of storage to construct.
Specifications
The following specifications are provided for informational purposes only. These specifications include
information on acceptable materials for typical applications, but are by no means exclusive or limiting.
The designer is responsible for developing detailed specifications for individual design projects in
accordance with the project conditions.
1Vegetation
- See Appendix B
2 Execution
a. Subgrade preparation
1.
Existing sub-grade in Bioretention areas shall NOT
intenance Issues
While vegetation is being established, pruning and weeding may be required.
Detritus may also need to be removed eve
end of the growing season.
Mulch should be re-spread when e
2 to 3 years the entire area may requ
Bioretention areas should be insp
e
During
Trees and shrubs should be inspected twic
C st
R
be compacted or subject to
excessive construction equipment traffic.
2.
Initial excavation can be performed during rough site grading but shall not be
carried to within one feet of the final bottom elevation. Final excavation should
not take place until all disturbed areas in the drainage area have been stabilized.
3.
Where erosion of sub-grade has caused accumulation of fine materials and/or
surface ponding in the graded bottom, this material shall be removed with light
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equipment and the underlying soils scarified to a minimum depth of 6 inches with
a York rake or equivalent by light tractor.
indicated. Fill
and lightly regrade any areas damaged by erosion, ponding, or traffic
ttom.
5.
Halt excavation and notify engineer immediately if evidence of sinkhole activity or
b. Rain Garden Installation
er shall be notified and shall
inspect at his/her discretion before proceeding with bioretention installation.
ld be
o the specified depth.
3.
Planting soil shall be placed immediately after approval of sub-grade
ent that takes
place after approval of sub-grade shall be removed prior to installation of planting
um lifts and lightly
compact (tamp with backhoe bucket or by hand). Keep equipment movement
over planting soil to a minimum –
do not over compact
. Install planting soil to
age 6 months) or compost
mulch evenly as shown on plans. Do not apply mulch in areas where ground
cover is to be grass or where cover will be established by seeding.
Protect Rain Gardens from sediment at all times during construction. Hay bales,
4.
Bring sub-grade of bioretention area to line, grade, and elevations
compaction. All bioretention areas shall be level grade on the bo
pinnacles of carbonate bedrock are encountered in the bioretention area.
1.
Upon completion of sub-grade work, the Engine
2.
For the subsurface storage/infiltration bed installation, amended soils shou
placed on the bottom t
preparation/bed installation. Any accumulation of debris or sedim
soil at no extra cost to the Owner.
4.
Install planting soil (exceeding all criteria) in 18-inch maxim
grades indicated on the drawings.
5.
Plant trees and shrubs according to supplier’s recommendations and only from
mid-March through the end of June or from mid-September through mid-
November.
6.
Install 2-3” shredded hardwood mulch (minimum
7.
diversion berms and/or other appropriate measures shall be used at the toe of
slopes that are adjacent to Rain Gardens to prevent sediment from washing into
these areas during site development.
8.
When the site is fully vegetated
and the soil mantle stabilized the plan desig
shall be notified an
ner
d shall inspect the Rain Garden drainage area at his/her
discretion before the area is brought online and sediment control devices
removed.
at the end of each day for two weeks after planting is
Contractor should provide a one-year 80% care and replacement warranty for all planting beginning
after in all
plants.
9.
Water vegetation
completed.
st ation and inspection of all
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BMP 6.4.6: Dry Well / Seepage Pit
A Dry Well, or Seepage Pit, is a variation on an Infiltration
system that is designed to temporarily store and infiltrate
rooftop runoff.
Water Quality Functions
TSS:
TP:
NO3:
85% 85%
30%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Medium
High
Medium
Medium
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Limited
Yes
.
Fllow Infiltration System Guidelines in Appendix C
No
.
Maintain minimum distance from building foundation (typically 10
feet)
.
Provide adequate overflow outlet for large storms
.
Depth of Dry Well aggregate should be between 18 and 48
inches
.
At least one observation well; clean out is recommended
.
Wrap aggregate with nonwoven geotextile
.
Maintenance will require periodic removal of sediment and leaves
from sumps and cleanouts
.
Provide pretreatment for some situations
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C

Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
Description
A Dry Well, sometimes called a Seepage Pit, is a subsurface storage facility that temporarily stores and
infiltrates stormwater runoff from the roofs of structures. Roof leaders connect directly into the Dry
Well, which may be either an excavated pit filled with uniformly graded aggregate wrapped in geotextile
or a prefabricated storage chamber or pipe segment. Dry Wells discharge the stored runoff via
infiltration into the surrounding soils. In the event that the Dry Well is overwhelmed in an intense storm
event, an overflow mechanism (surcharge
that additional runoff is safely conveyed do
By capturing runoff at the source, Dry Wells can dramatically reduce the increased volume of
stormwater generated by the roofs of structures. Though roofs are generally not a significant source of
runoff pollution, they are still one of the most important sources of new or increased runoff volume from
developed areas. By decreasing the volume of stormwater runoff, Dry Wells can also reduce runoff
rate and improve water quality. As with other infiltration practices, Dry Wells may not be appropriate for
“hot spots” or other areas where high pollutant or sediment loading is expected without additional
design considerations. Dry Wells are not recommended within a specified distance to structures or
subsurface sewage disposal systems. (see Appendix C, Protocol 2)
pipe, connection to larger infiltration area, etc.) will ensure
wnstream.
Variations
Intermediate “Sump” Box
– Water can flow through an intermediate box with an outflow higher to
allow the sediments to settle out. Water would then flow through a mesh screen and into the dry well.
Drain Without Gutters
– For structures without gutters or downspouts, runoff is designed to sheetflow
off a pitched roof surface and onto a stabilized ground cover (surface aggregate, pavement, or other
means). Runoff is then directed toward a Dry Well via stormwater pipes or swales.
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Prefabricated Dry Well –
There are a variety of prefabricated,
predominantly plastic subsurface storage chambers on the market
today that can replace aggregate Dry Wells. Since these systems
have significantly greater storage capacity than aggregate, space
requirements are reduced and associated costs may be defrayed.
Provided the following design guidelines are followed and infiltration is
still encouraged, prefabricated chambers can prove just as effective
as standard aggregate Dry Wells.
Applications
Any roof or impervious area with relatively low sediment loading
Design Considerations
1. Dry Wells are sized to temporarily retain and infiltrate stormwater runoff from roofs of structures.
A dry well usually provides stormwater management for a limited roof area. Care should be
not to hydraulically overload a dry well based on bottom area and drainage area. (See
dix C, Protocol 2 for guidance)
ding soil. At least
12 inches of soil is then placed over the Dry Well. An alternative form of Dry Well is a
subsurface, prefabricated chamber. A variety of prefabricated Dry Wells are currently available
on the market.
taken
Appen
2. Dry Wells should drain-down within the guidelines set in Chapter 3. Longer drain-down times
reduce Dry Well efficiency and can lead to anaerobic conditions, odor and other problems.
3. Dry Wells typically consist of 18 to 48 inches of clean washed, uniformly graded aggregate with
40% void capacity (AASHTO No. 3, or similar). Dry Well aggregate is wrapped in a nonwoven
geotextile, which provides separation between the aggregate and the surroun
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4. Dry Wells are not recommended when their installation would create a significant risk for
basement seepage or flooding. In general, 10 feet of separation is recommended between Dry
Wells and building foundations. However, this distance may be shortened at the discretion of
the designer. Shorter separation distances may warrant an impermeable liner to be installed on
the building side of the Dry Well.
5. All Dry Wells should be able to convey system overflows to downstream drainage systems.
System overflows can be incorporated either as surcharge (or overflow) pipes extending from
roof leaders or via connections to more substantial infiltration areas.
6. The design depth of a Dry Well should take into account frost depth to prevent frost heave.
7. A removable filter with a screened bottom should be installed in the roof leader below the
surcharge pipe in order to screen out leaves and other debris.
8. Adequate inspection and maintenance access to the Well should be provided. Observation
wells not only provide the necessary access to the Well, but they also provide a conduit through
which pumping of stored runoff can be accomplished in case of slowed infiltration.
9. Though roofs are generally not a significant source of runoff pollution, they can still be source
of particulates and organic matter, as well as sediment and debris during construction.
Measures such as roof gutter guards, roof leader clean-out with sump, or an intermediate sump
box can provide pretreatment for Dry Wells by minimizing the amount of sediment and other
a
particulates that may enter it.
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De
Volume
The sto
equatio
proximate storage volume of an aggregate Dry Well:
Dry
Infiltrat
tion according to design.
Pea
See Chapter 8 for corresponding peak rate reduction.
Water
See Chapter 8
Const
2.
. Install and maintain proper Erosion and Sediment Control Measures during construction as per
the Pennsylvania Erosion and Sediment Pollution Control Program Manual (March 2000, or
latest edition).
4. Excavate Dry Well bottom to a uniform, level uncompacted subgrade free from rocks and
debris. Do NOT compact subgrade. To the greatest extent possible, excavation should be
performed with the lightest practical equipment. Excavation equipment should be placed
outside the limits of the Dry Well.
5. Completely wrap Dry Well with nonwoven geotextile. (If sediment and/or debris have
accumulated in Dry Well bottom, remove prior to geotextile placement.) Geotextile rolls should
overlap by a minimum of 24 inches within the trench. Fold back and secure excess geotextile
during stone placement.
6. Install continuously perforated pipe, observation wells, and all other Dry Well structures.
Connect roof leaders to structures as indicated on plans.
7. Place uniformly graded, clean-washed aggregate in 6-inch lifts, lightly compacting between lifts.
8. Fold and secure nonwoven geotextile over trench, with minimum overlap of 12-inches.
9. Place 12-inch lift of approved Topsoil over trench, as indicated on plans.
10. Seed and stabilize topsoil.
11. Connect surcharge pipe to roof leader and position over splashboard.
tailed Stormwater Functions
Reduction Calculations
rage volume of a Dry Well is defined as the volume beneath the discharge invert. The following
n can be used to determine the ap
Well Volume = Dry well area (sf) x Dry well water depth (ft) x 40% (if stone filled)
ion Area: A dry well may consider both bottom and side (lateral) infiltra
k Rate Mitigation Calculations
Quality Improvement
ruction Sequence
1. Protect infiltration area from compaction prior to installation.
If possible, install Dry Wells during later phases of site construction to prevent sedimentation
and/or damage from construction activity.
3
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12. Do not remove Erosion and Sediment Control measures until site is fully stabilized.
s with all infiltration practices, Dry Wells require regular and effective maintenance to ensure
or Dry Wells:
• Dispose of sediment, debris/trash, and any other waste material removed from a Dry Well at
es and in compliance with local, state, and federal waste
• Evaluate the drain-down time of the Dry Well to ensure the maximum time of 72 hours is not
-down times are exceeding the maximum, drain the Dry Well via
and clean out perforated piping, if included. If slow drainage persists, the system may
need replacing.
• Regularly clean out gutters and ensure proper connections to facilitate the effectiveness of the
• Replace filter screen that intercepts roof runoff as necessary.
per year.
Cost I
The construction cost of a Dry Well/Seepage Pit can vary greatly depending on design variability,
con
from $4
Annual maintenance costs have been reported to be approximately 5 to 10 percent of the capital costs
(Schueler, 1987). The cost of gutters is typically included in the total structure cost, as opposed
Specif
The following specifications are provided for information purposes only. These specifications include
info
limiting.
The de
accord
1. Stone
Maintenance Issues
A
prolonged functioning. The following represent minimum maintenance requirements f
• Inspect Dry Wells at least four times a year, as well as after every storm exceeding 1 inch.
suitable disposal/recycling sit
regulations.
being exceeded. If drain
pumping
dry well.
• If an intermediate sump box exists, clean it out at least once
ssues
figuration, location, site-specific conditions, etc. Typical construction costs in 2003 dollars range
- $9 per cubic foot of storage volume provided (SWRPC, 1991; Brown and Schueler, 1997).
ications
rmation on acceptable materials for typical applications, but are by no means exclusive or
signer is responsible for developing detailed specifications for individual design projects in
ance with the project conditions.
for infiltration trenches shall be 2-inch to 1-inch uniformly graded coarse aggregate, with a
19th
y ASTM-C29.
wash loss of no more than 0.5%, AASHTO size No. 3 per AASHTO Specifications, Part I,
Ed., 1998, or later and shall have voids 40% as measured b
2. Nonwoven Geotextile
shall consist of needled nonwoven polypropylene fibers and meet the
following properties:
b. Mullen Burst Strength (ASTM-D3786)
³ 225 psi
500 hrs (ASTM-D4355) ³ 70%
endared fabrics are not permitted
Acceptable types include Mirafi 140N, Amoco 4547, and Geotex 451.
a. Grab Tensile Strength (ASTM-D4632)
³ 120 lbs
c. Flow Rate (ASTM-D4491)
³ 95 gal/min/ft2
d. UV Resistance after
e. Heat-set or heat-cal
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3. Topsoil
See Appendix C
4. Pipe
shall be continuously perforated, smooth interior, with a minimum inside diameter of 4-
sity polyethylene (HDPE) pipe shall meet AASHTO M252, Type S or AASHTO
M294, Type S. 12 gauge aluminum or corrugated steel pipe may be used in seepage pits.
inches. High-den
5. Gutters and splashboards
shall follow Manufacturer’s specifications.
Re e
New Je
Practices Manual
. 2004.
New Yo
Des
Fre h
fer nces
rsey Department of Environmental Protection.
New Jersey Stormwater Best Management
rk Department of Environmental Conservation.
New York State Stormwater Management
ign Manual.
2003.
nc Drains. http://www.unexco.com/french.html
. 2004.
SW
C
Watersheds, US Environmental
rown and Schueler,
Stormwater Management Fact Sheet: Infiltration Trench.
1997.
RP , The Use of of Best Management Practices(BMPs) in Urban
Protection Agency,1991.
B
Schueler, T., 1987.
Controlling urban runoff: a practical manual for planning and designing
urban BMPs
, Metropolitan Washington Council of Governments, Washington, DC
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BMP 6.4.7: Constructed Filter
Filters are structures or excavated areas containing a
layer of sand, compost, organic material, peat, or other
filter media that reduce pollutant levels in stormwater
runoff by filtering sediments, metals, hydrocarbons, and
other pollutants.
Water Quality Functions
85%
85%
30%
TSS:
TP:
NO3:
Stormwater Functions
*
Depends on if infiltration is used
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low-High*
Low-High*
Low-High*
High
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Limited
Yes
Yes
Yes
Yes
.
Follow Infiltration Systems Guidelines in Appendix C
Yes
.
Drain down – should empty within the guidelines in Chapter 3
.
Minimum permeability of filtration medium required
.
Minimum depth of filtering medium = 12"
.
Perforated pipes in stone, as required
.
May be designed to collect and convey filtered runoff down-
gradient
.
May be designed to infiltrate
.
Pretreatment for debris and sediment may be needed
.
Should be sized for drainage area
.
Regular inspection and maintenance required for continued
functioning
.
Positive overflow is needed
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed, see Appendix C
Certain applications may warrant spill containment.

Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
Description
A stormwater filter is a structure or excavation filled with material and designed to filter stormwater
runoff to improve water quality. The filter media may be comprised of materials such as sand, peat,
compost, granular activated carbon (GAC), perlite, or other material. Additional filtration media will be
acceptable for use as long as data is available o verify the media is capable of meeting performance
goals. In some applications the stormwater ru
to
allow the large particles and debris to settle ou
option for pretreatment. The runoff then passe
filtered out, and is collected in an under-drain a
or infiltrated into the soil mantle.
t
noff flows through an open air, “pretreatment” chamber
t (sedimentation). Surface vegetation is another good
s through the filter media where additional pollutants are
nd returned to the conveyance system, receiving waters
Variations
There are a wide variety of Filter Applications, including surface and subsurface, vegetated, perimeter,
infiltration, and others. There are also a variety of filter products that may be purchased. Examples of
these variations include:
Surface Non-vegetated Filter
A Surface Non-vegetated Filter is constructed by excavation or by use of a structural container. The
surface may be covered in sand, peat, gravel, river stone, or similar material.
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Vegetated Filter
Filters may be designed to allow some portion of the treated water to infiltrate. Infiltration Design
Criteria apply for all Filters designed with infiltration. In all cases, a positive overflow system is
recommended.
A layer of vegetation is planted on top of the filtering
medium. Composted amended soil may serve as a
filter media. For filters composed of filtering media
such as sand (where topsoil is required for
vegetation) a layer of nonwoven, permeable
geotextile should separate the topsoil
and vegetation from the filter media.
Infiltration Filter
Contained Filter
In contained Filters, infiltration is not
incorporated into the design. Contained Filters
may consist of a physical structure, such as a
precast concrete box. For excavated filters, an
impermeable liner is added to the bottom of the
excavation to convey the filtered runoff
downstream.
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Linear “Perimeter” Filters
Perimeter Filters may consist of enclosed
chambers (such as trench drains) that run along the
perimeter of an impervious surface. Perimeter
Filters may also be constructed by excavation and
vegetated. All perimeter filters should be designed
with the necessary filter medium and sized in
accordance with the drainage area.
Small Subsurface Filter
A Small Subsurface filter is an inlet designed to treat runoff at the collection source by filtration. Small
Subsurface filters are useful for Hotspot Pretreatment and similar in function to Water Quality Inserts.
Small Subsurface filters should be carefully designed and maintained so that runoff is directed through
the filter media.
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Linear “Perimeter” Filters
Perimeter Filters may consist of enclosed
chambers (such as trench drains) that run along the
perimeter of an impervious surface. Perimeter
Filters may also be constructed by excavation and
vegetated. All perimeter filters should be designed
with the necessary filter medium and sized in
accordance with the drainage area.
Small Subsurface Filter
A Small Subsurface filter is an inlet designed to treat runoff at the collection source by filtration. Small
Subsurface filters are useful for Hotspot Pretreatment and similar in function to Water Quality Inserts.
Small Subsurface filters should be carefully designed and maintained so that runoff is directed through
the filter media.
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Large Subsurface Filter
Large Subsurface Filters receive relatively large amounts of flow directed into an underground box that
has separate chambers, one to settle large particles, and one to filter small particles. The water
discharges through an outlet pipe and into the stormwater system.
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Manufactured Filtration Systems
There are a considerable number of manufactured filtration systems available, some of which also
incorporate oil/water separators, vortex systems, etc. The Designer should obtain product specific
information directly from the manufacturer.
Applications
Filters are applicable in urbanized areas having high pollutant loads and are especially applicable
where there is limited area for construction of other BMPs. Filters may be used as a pretreatment
BMP before other BMPs such as Wet Ponds or Infiltration systems. Filters may be used in Hot Spot
areas for water quality treatment, and spill containment capabilities may be incorporated into a filter.
Examples of typical areas that benefit from the use of a Filter BMP include:
• Parking lots
• Roadways and Highways
• Light Industrial sites
• Marina areas
• Transportation facilities
• Fast food and shopping areas
• Waste Transfer Stations
• Urban Streetscapes
esign Considerations
1. Filters should be sized as per the Control Guideline that applies. All filters should be designed
so that
larger storms may safely overflow or bypass the filter
. Flow splitters, multistage
chambers, and other devices may be used. A flow splitter may be necessary to allow only a
portion of the runoff to enter the filter. This would create an “off-line” filter, where the volume
and velocity of runoff entering the filter is controlled. If the filter is “on-line”, excess flow should
be designed to bypass the filter and continue to another quality BMP.
2.
Entering velocity should be controlled
. A level spreader may be used to spread flow evenly
3.
Pretreatment
may be necessary in areas with especially high levels of debris, large sediment,
etc. Pretreatment may include oil/grit separators, vegetated filter strips, or grass swales.
These measures will settle out the large particles and reduce velocity of the runoff before it
enters the filter.
4. The
Filter Media
may be a variety of materials and in most cases should have a minimum depth
of 12 inches and a maximum depth of 30 inches, although variations on these guidelines are
acceptable if justified by the designer. Coarser materials allow for more hydraulic conductivity,
but finer media filter particles of a smaller size. Sand has been found to be a good balance
between these two criteria, but different types of media remove different pollutants. While sand
is a reliable material to remove TSS, (Debusk and Langston, 1997) peat removes slightly more
D
across the filter surface during all storms without eroding the filter material. Parking lots may be
designed to sheet flow to filters. Small riprap or riverstone edges may be used to reduce
velocity and distribute flow.
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TP, Cu, Cd, and Ni than sand. The Filter Media should have a minimum hydraulic conductivity
(k) as follows:
• Sand 3.5 ft/day
• Peat 2.5 ft/day
• Leaf compost 8.7 ft/day
5. A
Gravel Layer
at least 6” deep is recommended beneath the Filter Media.
6.
Under drain piping
should be 4” minimum (diameter) perforated pipes, with a lateral spacing of
no more than 10’. A collector pipe can be used, (running perpendicular to laterals) with a slope
of 1%. All underground pipes should have clean-outs accessible from the surface.
7. A
Drawdown Time
of not more than 72 hours is recommended for Filters.
8. The
Size
of a Filter is determined by the Volume to be treated:
A = V x d / (k x t(h+d))
A =
Surface area of Filter (square feet)
V =
Water volume (cubic feet)
d =
Depth of Filter Media (min 1.5 ft; max 2.5 ft)
t =
Drawdown time (days), not to exceed 72 hours
h =
Head (average in feet)
k =
Hydraulic conductivity (ft/day)
9. When a Filter has accumulated sediment in its pore space, its hydraulic conductivity is reduced,
and so is its ability to removal pollutants.
Maintenance and Inspection
are essential for
continued performance of a Filter. Based upon inspection, some or all portions of the filter
media may require replacement.
10. Filters should be designed with
sufficient maintenance access
(clean-outs, room for surface
cleaning, etc.). Filters that are visible and simple in design are more likely to be maintained
correctly.
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Detailed Stormwater Functions
er
Infiltration Volume = Bottom Area (sf) x Infil. Rate (in/hr) x Drawdown time** (hr)
= Area of filter (sf) x Depth (ft) x 20%***
*Fo ilt
ly
** N t t
urs
***For sand, amended soil, compost, peat; Use 20% unless more specific data is available
Pea R
See Chapter 8 for Peak Rate Mitigation methodology which addresses link between volume reduction
nd peak rate control.
ee Chapter 8 for Water Quality Improvement methodology, which addresses pollutant removal
effe
Constr
. Permanent Filters should not be installed until
ity benefits.
Stabilize all contributing areas before runoff
2.
. Excavated filters that infiltrate or structural filters that infiltrate should be excavated in such a
4.
5.
6.
7.
8. Saturate filter media and allow media to drain to properly settle and distribute.
9. For vegetated filters, a layer of nonwoven geotextile between non-organic filter media and
plan ng
ended.
10. The s
pace (head) between the top of the filtering bed and the overflow of
the ilt
m head designed to be stored before filtration.
Volume Reduction Calculations
If a Filter is designed to include infiltration, the Volume Reduction is a function of the Area of the Filt
and infiltration rate. There is minimal volume reduction for Filters that are not designed to infiltrate.
Volume = Infiltration Volume* + Filter Volume
Filter Volume
r f ers with infiltration on
o o exceed 72 ho
k ate Mitigation Calculations
a
Water Quality Improvement
S
ctiveness of this BMP.
uction Sequence
1
the site is stabilized. Excessive sediment
generated during construction can clog the
Filter and prevent or reduce the anticipated
post-construction water qual
enters filters.
Structures such as inlet boxes, reinforced
concrete boxes, etc. should be installed in
accordance with the manufacturers’ or design engineers guidance.
3
manner as to avoid compaction of the subbase. Structures may be set on a layer of clean,
lightly compacted gravel (such as AASHTO #57).
Infiltration Filters should be underlain by a layer of permeable non-woven-geotextile.
Place underlying gravel/stone in minimum 6 inch lifts and lightly compact. Place underdrain
pipes in gravel during placement.
Wrap and secure nonwoven geotextile to prevent gravel/stone from clogging with sediments.
Lay filtering material. Do not compact.
ti
media is recomm
re hould be sufficient s
F er to allow for the maximu
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Maintenance and Inspection
Filters
filtering
effective
upon installation, but quickly decrease in efficiency as sediment accumulates in the filter. (Urbonas,
Urb
hat are not
maintained. Inspection of the filter is recommended at least
four times a year
.
uring inspection the following conditions should be
conside
• Standing water
– any water left in a surface filter
afte
sig
is not optimally
• Film r
col
– th in
es
filte u ce.
Filter Maintenance
/removal has reduced depth of filtering media
In a
metals)
te
and fed
tions.
inter concerns
Pennsylvania’s winter temperatures go below freezing about four months out of every year, and surface
filtration may not take place as well in the winter. Peat and compost may hold water, freeze, and
become impervious on the surface. Design options that allow directly for subsurface discharge into the
filter media during cold weather may overcome this condition.
require a regular inspection and maintenance program in order to maintain the integrity of the
system and pollutant removal mechanisms. Studies have shown that filters are very
an Drainage and Flood Control District, CO) Odor is also a concern for filters t
D
red:
r the de n drain down time indicates the filter
functioning.
o dis
oration
of any surface filter material
is dicat organics or debris have clogged the
r s rfa
Remove trash and debris as necessary
Scrape silt with rakes
Till and aerate filter area
Replace filtering medium if scraping
reas where the potential exists for the discharge and accumulation of toxic pollutants (such as
, filter media removed from filters must be handled and disposed of in accordance with all sta
eral regula
W
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Cost Issues
Filter costs vary according to the filtering medial (sand, peat, compost), land clearing, excavation,
1.
S
grading, inlet and outlet structures, perforated pipes, encasing structure (if used), and maintenance
cost. Underground structures may contribute significantly to the cost of a Filter.
Specifications
tone/Gravel
shall be uniformly graded coarse aggregate, 1 inch to ¾ inch with a wash loss of
no more than 0.5%, AASHTO size number 57 per AASHTO Specifications, Part I, 19th Ed.,
1998, or later and shall have voids 40% as measured by ASTM-C29.
2.
Peat
shall have ash content <15%, pH range 3.3-5.2, loose bulk density range 0.12-0.14 g/cc.
3.
Sand
shall be ASTM-C-33 (or AASHTO M-6) size (0.02” – 0.04”), concrete sand, clean, medium
4.
Non-Woven Geotextile
to fine sand, no organic material.
shall consist of needled nonwoven polypropylene fibers and meet the
Strength (ASTM-D3786)
³ 225 psi
c. Flow Rate (ASTM-D4491)
³ 95 gal/min/ft
2
d. UV Resistance after 500 hrs (ASTM-D4355) ³ 70%
e. Heat-set or heat-calendared fabrics are not permitted
4547, Geotex 451, or approved others.
5.
P
following properties:
a. Grab Tensile Strength (ASTM-D4632)
³ 120 lbs
b. Mullen Burst
Acceptable types include Mirafi 140N, Amoco
ipe
shall be continuously perforated, smooth
inches. High-density polyethylene (HDPE) p
M294, Type S.
interior, with a minimum inside diameter of 8-
ipe shall meet AASHTO M252, Type S or AASHTO
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References
tlanta Regional Commission.
Georgia Stormwater Management Manual.
August 2001.
ucker
A
University of Minnesota Extension Service, Northeast Regional Correction Center (NERCC)
“Field Evaluation of a Stormwater Sand Filter” Ben R. Urbonas, John T. Doerfer and L. Scott T
www.udfcd.org/fhn96/flood1.html
“An Evaluation of Filter Media For Treating Stormwate
Langston
r Runoff” Thomas A DeBustk and Michael A.
, Benefict Schwegler, Scott Davidson, Fifth Biennial Stormwater Research Conference,
“A Denitrification System For Septic Tank Effluent Using Sphagnum Peat Moss” E. S. Winkler, and P.
“Stormwater Sand Filter Sizing and Design – A Unit Operations Approach” Urbonas
ent of Environmental Conservation.
New York Stormwater Management Manual.
Cal rn
ociation.
California Stormwater BMP Handbook.
January 2003.
November, 1997
L. M. Veneman
New York Departm
2003.
ifo ia Stormwater Quality Ass
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BMP 6.4.8: Vegetated Swale
parabolic channel, densely planted with a variety of trees,
shrubs, and/or grasses. It is designed to attenuate and in
ut
in the process. In steeper slope situations, check dams
A Vegetated Swale is a broad, shallow, trapezoidal or
some cases infiltrate runoff volume from adjacent
impervious surfaces, allowing some pollutants to settle o
may be used to further enhance attenuation and infiltration
opportunities.
Key Design Elements
Potential Applications
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Limited
Yes Yes
Yes
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low/Med.
Low/Med.
Med./High
Med./High
Water Quality Functions
TSS:
TP:
NO3:
50%
50%
20%
.
Plant dense, low-growing native vegetation that is water-resistant,
drought and salt tolerant, providing substantial pollutant removal
capabilities
.
Longitudinal slopes range from 1 to 6%
.
Side slopes range from 3:1 to 5:1
.
Bottom width of 2 to 8 feet
.
Check-dams can provide limited detention storage, as well as
enhanced volume control through infiltration. Care must be taken
to prevent erosion around the dam
.
Convey the 10-year storm event with a minimum of 6 inches of
freeboard
.
Designed for non-erosive velocities up to the 10-year storm event
.
Design to aesthetically fit into the landscape, where possible
.
Significantly slow the rate of runoff conveyance compared to
pipes
Residential:
Commercial:
Yes Yes
Other Considerations
Protocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems
Guidelines
should be followed whenever infiltration of runoff is desired, see Appendix C
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Description
Vegetated swales are broad, shallow channels designed to slow runoff, promote infiltration, and filter
pollutants and sediments in the process of conveying runoff. Vegetated Swales provide an
environmentally superior alternative to conventional curb and gutter conveyance systems, while
providing partially treated (pretreatment) and partially distributed stormwater flows to subsequent
BMPs. Swales are often heavily vegetated with a dense and diverse selection of native, close-growing,
water-resistant plants with high pollutant removal potential. The various pollutant removal mechanisms
of a swale include: sedimentary filtering by the swale vegetation (both on side slopes and on bottom),
filtering through a subsoil matrix, and/or infiltration into the underlying soils with the full array of
infiltration-oriented pollutant removal mechanisms.
A Vegetated Swale typically consists of a band of dense vegetation, underlain by at least 24 inches of
permeable soil. Swales constructed with an underlying 12 to 24 inch aggregate layer provide
significant volume reduction and reduce the stormwater conveyance rate. The permeable soil media
should have a minimum infiltration rate of 0.5 inches per hour and contain a high level of organic
material to enhance pollutant removal. A nonwoven geotextile should completely wrap the aggregate
trench (See BMP 6.4.4 Infiltration Trench for further design guidelines).
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A major concern when designing Vegetated S
lope, and other factors do not combine to pr
wales is to make certain that excessive stormwater flows,
oduce erosive flows, which exceed the Vegetated Swale
apabilities. Use of check dams or turf
reinforcement matting (TRM) can enhance swale
performance in some situations.
A key feature of vegetated swale design is th
swales can be well integrated into the landsc
character of the surrounding area. A vegeta
swale can often enhance the aesthetic value
site through the selection of appropriate nati
vegetation. Swales may also discreetly blen
with landscaping features, especially when
adjacent to roads.
Variations
Vegetated Swale with Infiltration Trench
This option includes a 12 to 24 inch aggregate bed or trench, wrapped in a nonwoven geotextile (See
BMP 6.4.4 Infiltration Trench for further design guidelines). This addition of an aggregate bed or trench
substantially increases volume control and water quality performance although costs also are
increased. Soil Testing and Infiltration Protocols in Appendix C should be followed.
s
c
at
ape
ted
of a
ve
d in
Vegetated Swales with Infiltration Trenches are best fitted for milder sloped swales where the addition
of the aggregate bed system is recommended to make sure that the maximum allowable ponding time
of 72 hours is not exceeded. This aggregate bed system should consist of at least 12 inches of
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uniformly graded aggregate. Ideally, the underdrain system shall be designed like an infiltration trench.
The subsurface trench should be comprised of terraced levels, though sloping trench bottoms may also
be acceptable. The storage capacity of the infiltration trench may be added to the surface storage
volume to achieve the required storage of the 1-inch storm event.
Grass Swale
Grass swales are essentially conventional drainage ditches. They
typically have milder side and longitudinal slopes than their
vegetated counterparts. Grass swales are usually less expensive
than swales with longer and denser vegetation. However, they
provide far less infiltration and pollutant removal opportunities.
Grass swales are to be used only as pretreatment for other
structural BMPs. Design of grass swales is often rate-based.
Grassed swales, where appropriate, are preferred over catch
basins and pipes because of their ability to reduce the rate of
across a site.
Wet Swales
Wet swales are essentially linear wetland cells. Their design
often incorporates shallow, permanent pools or marshy
conditions that can sustain wetland vegetation, which in turn
provides potentially high pollutant removal. A high water
table or poorly drained soils are a prerequisite for wet
swales. The drawback with wet swales, at least in
flow
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residential or commercial settings, is that they may promote mosquito breeding in the shallow standing
water (follow additional guidance under Constructed Wetland for reducing mosquito population).
Infiltration is minimal if water remains for
extended periods.
Applications
Parking
Commercial and light industrial facilities
Roads and highways
Residential developments
Pretreatment for volume-based BMPs
Alternative to curb/gutter and storm sewer
Design Considerations
1. Vegetated Swales are sized to temporarily store and infiltrate the 1-inch storm event, while
providing conveyance for up to the 10-year storm with freeboard; flows for up to the 10-year
storm are to be accommodated without causing erosion. Swales should maintain a maximum
ponding depth of 18 inches at the end point of the channel, with a 12-inch average maintained
throughout. Six inches of freeboard is recommended for the 10-year storm. Residence times
between 5 and 9 minutes are acceptable for swales without check-dams. The maximum
ponding time is 48 hours, though 24 hours is more desirable (minimum of 30 minutes). Studies
have shown that the maximum amount of swale filtering occurs for water depths below 6 inches.
It is critical that swale vegetation not be submerged, as it could cause the vegetation to bend
over with the flow. This would naturally lead to reduced roughness of the swale, higher flow
velocities, and reduced contact filtering opportunities.
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2. Longitudinal slopes between 1% and 3% are generally recommended for swales. If the
topography necessitates steeper slopes, check dams or TRM’s are options to reduce the energy
gradient and erosion potential.
3. Check dams are recommended for vegetated
swales with longitudinal slopes greater than 3%.
They are often employed to enhance infiltration
capacity, decrease runoff volume, rate, and
velocity, and promote additional filtering and
settling of nutrients and other pollutants. In effect,
check-dams create a series of small, temporary
pools along the length of the swale, which shall
drain down within a maximum of 72 hours. Swales
with check-dams are much more effective at
mitigating runoff quantity and quality than those
without. The frequency and design of check-dams
in a swale will depend on the swale length and
slope, as well as the desired amount of
storage/treatment volume. Care must be taken to
avoid erosion around the ends of the check dams.
Check-dams shall be constructed to a height of 6 to
12 in and be regularly spaced. The following
materials have been employed for check-dams:
natural wood, concrete, stone, and earth. Earthen
check-dams however, are typically not
recommended due to their potential to erode. A
weep hole(s) may be added to a check-dam to
allow the retained volume to slowly drain out. Care
should be taken to ensure that the weep hole(s) is
not subject to clogging. In the case of a stone
am, a better approach might be to allow low flows (2-year storm) to drain through the
ugh a weir in the center of the dam.
tone size, flow depth, flow width, and flow
used to estimate the flow through a
L = length of flow (ft)
D = average stone diameter (ft) (more uniform gradations are preferred)
e actually more influential on flow than
eck-dam as a function of flow depth can be
check-d
stone, while allowing higher flows (10-year storm) drain thro
Flows through a stone check-dam are a function of s
path length through the dam. The following equation can be
stone check dam up to 6 feet long:
q = h
1.5
/ (L/D + 2.5 + L
2
)
0.5
where:
q = flow rate exiting check dam (cfs/ft)
h = flow depth (ft)
For low flows, check-dam geometry and swale width ar
stone size. The average flow length through a ch
determined by the following equation:
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L = (ss) x (2d – h)
where:
ss = check dam side slope (maximum 2:1)
d = height of dam (ft)
h = flow depth (ft)
ale flows overwhelm the flow-through capacity of a stone check-dam, the top of the
dam shall act as a standard weir (use standard weir equation). (Though a principal spillway, 6
inches below the height of the dam, may also be required depending on flow conditions.) If the
check-dam is designed to be overtopped, appropriate selection of aggregate will ensure stability
during flooding events. In general, one stone size for a dam is recommended for ease of
construction. However, two or more stone sizes may be used, provided a larger stone (e.g. R-
4) is placed on the downstream side, since flows are concentrated at the exit channel of the
weir. Several feet of smaller stone (e.g. AASHTO #57) can then be placed on the upstream
side. Smaller stone may also be more appropriate at the base of the dam for constructability
purposes.
4. The effectiveness of a vegetated swale is directly related to the contributing land use, the size of
the drainage area, the soil type, slope, drainage area imperviousness, proposed vegetation, and
the swale dimensions. Use of natural low points in the topography may be suited for swale
location, as are natural drainage courses although infiltration capability may also be reduced in
these situations. The topography of a site should allow for the design of a swale with sufficiently
mild slope and flow capacity. Swales are impractical in areas of extreme (very flat or steep)
slopes. Of course, adequate space is needed for vegetated swales. Swales are ideal as an
terna
bs and gutters along parking lots and along small roads in gently sloping
rrain.
iting of vegetated swales should take into account the location and function of other site
atural areas, etc.). Siting should also attempt to aesthetically fit
e swale into the landscape as much as possible. Sharp bends in swales should be avoided.
plementing vegetated swales is challenging when development density exceeds four dwelling
ay culverts often increases to the point where
wales essentially become broken-pipe systems.
here possible, construct swales in areas of uncompacted cut. Avoid constructing side slopes
pes can be prone to erosion and/or structural damage by burrowing
6.
7.
es
8.
t
stabilization or energy dissipation is
When sw
al
tive to cur
te
S
features (buffers, undisturbed n
th
Im
units per acre, in which case the number of drivew
s
W
in fill material. Fill slo
animals.
5. Soil Testing is required when infiltration is planned (see Appendix C).
Guidelines for Infiltration Systems should be met as necessary (see Appendix C).
Swales are typically most effective, when treating an area of 1 to 2 acres although vegetated
swales can be used to treat and convey runoff from an area of 5 to 10 acres in size. Swal
serving greater than 10-acre drainage areas will provide a lesser degree water quality
treatment, unless special provisions are made to manage the increased flows.
Runoff can be directed into Vegetated Swales either as concentrated flows or as lateral shee
flow drainage. Both are acceptable provided sufficient
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included (see #6). If flow is to be directed into a swale via curb cuts, provide a 2 to 3 inch
at the interface of pavement and swale. Curb cuts should be at least 12 inches wide to prevent
clogging and should be spaced
drop
appropriately.
reatment devices for other structural BMPs,
themselves are intended to effectively treat
ent measures are recommended to enhance
tically extend the functional life of any BMP, as
y by settling out some of the heavier sediments.
lling check dams at pipe inlets and/or
e a vegetated filter strip, a sediment forebay (or
vel diaphragm (or alternative) with a 6-inch
wale.
10.
d adequate support for proposed
nsuitable (clayey, rocky, coarse sands, etc.)
oximately 12 inches of loamy or sandy soils is
should be used to further reduce and retain metals.
ration capacity is compromised during
nd replaced with a blend of topsoil and
sand to promote infiltration and biological growth.
11.
abolic or trapezoidal in nature.
o 5:1 and should not be greater than 2:1 for
.
12.
mance of swales, the bottom widths typically
ible only when obstructions such as berms or
d sub-channel formation. The maximum
hould be 12:1.
13.
r-
pecific site and therefore should be chosen carefully (See Appendix B). Use of native plant
species is stro
vasive plant species. Swale vegetation must
lso be salt tolerant, if winter road maintenance activities are expected to contribute
rides.
9. Vegetated swales are sometimes used as pret
especially roadway runoff. However, when swales
runoff from highly impervious surfaces, pretreatm
swale performance. Pretreatment can drama
well as increase its pollutant removal efficienc
This treatment volume is typically obtained by insta
driveway crossings. Pretreatment options includ
plunge pool) for concentrated flows, or a pea gra
drop where parking lot sheet flow is directed into a s
The soil base for a vegetated swale must provide stability an
vegetation. When the existing site soil is deemed u
to support dense vegetation, replacing with appr
recommended. In general, alkaline soils
Swale soils should also be well-drained. If the infilt
construction, the first several feet should be removed a
Swales are most efficient when their cross-sections are par
Swale side slopes are best within a range of 3:1 t
ease of maintenance and side inflow from sheet flow
To ensure the filtration capacity and proper perfor
range from 2 to 8 feet. Wider channels are feas
walls are employed to prohibit braiding or uncontrolle
bottom width to depth ratio for a trapezoidal swale s
Ideal swale vegetation should consist of a dense and diverse selection of close-growing, wate
resistant plants whose growing season preferably corresponds to the wet season. For swales
that are not part of a regularly irrigated landscaped area, drought tolerant vegetation should be
considered as well. Vegetation should be selected at an early stage in the design process, with
well-defined pollution control goals in mind. Selected vegetation must be able to thrive at the
s
ngly advised, as is avoidance of in
a
salt/chlo
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ommo
s
lkai Sa
inellia distans
Cool, good for wet, saline swales
Fowl Bluegrass
Poa palustris
Cool, good for wet swales
sses are sod forming and can withstand frequent inundation, and are idela for the swale or
grass channel environment. A few are also salt tolerant. Cool refers to cool season grasses that grow
only used vegetation in swale (New Jersey BMP Manual, 2004)
Table 6.8.1
Comm
C
n Name
Scientific Name
Note
A
ltgrass
Pucc
Canada Bluejoint
Calamagrostis canadensis Cool, good for wet swales
Creeping Bentgrass
Agrostis palustris
Cool, good for wet swales, salt tolerant
Red Fescue
Festuca rubra
Cool, not for wet swales
Redtop
Agrostis gigantea
Cool, good for wet swales
Rough Bluegrass
Poa trivialis
Cool, good for wet, shady swales
Switchgrass
Panicum virgatum
Warm, good for wet swales, somwe salt tolerance
Wildrye
Elymus virginicus/rigarius Cool, good for wet, shady swales
Notes: These gra
during the colder temperatures of spring and fall. Warm refers to warm season grasses that grow most
vigorously during the hot , mid summer months.
By landscaping with trees along side slopes, swales can be easily and aesthetically integrated
into the overall site design without unnecessary loss of usable space. An important
consideration however, is that tree plantings allow enough light to pass and sustain a dense
ground cover. When the trees have reached maturity, they should provide enough shade to
markedly reduce high temperatures in swale runoff.
4. Check the temporary and permanent stability of the swale using the standards outlined in the
nvey
<0.10 ft/ft; use of the maximum permissible shear stress is acceptable for all bed slopes. Flow
icularly vulnerable to scour and erosion and
therefore its seed bed must be protected with temporary erosion control, such as straw matting,
tress
ng material.
confirmed. Permanent
turf reinforcement may supersede temporary reinforcement on sites where not exceeding the
culvert
capacity may supersede Manning’s equation for determination of design flow depth. In these
ed
xit
issible
e implemented.
The following tables list the maximum permissible shear stresses (for various channel liners)
t
1
Pennsylvania Erosion and Sediment Pollution Control Program Manual. Swales should co
either 2.75 cfs/acre or the calculated peak discharge from a 10-year storm event. The
permissible velocity design method may be used for design of channel linings for bed slopes
capacity, velocity, and design depth in swales are generally calculated by Manning’s equation.
Prior to establishment of vegetation, a swale is part
compost blankets, or curled wood blankets. Most vendors will provide information about the
Manning’s ‘n’ value and will specify the maximum permissible velocity or allowable shear s
for the lini
The post-vegetation establishment capacity of the swale should also be
maximum permissible velocity is problematic. If driveways or roads cross a swale,
cases, the culvert should be checked to establish that the backwater elevation would not exce
the banks of the swale. If the culverts are to discharge to a minimum tailwater condition, the e
velocity for the culvert should be evaluated for design conditions. If the maximum perm
velocity is exceeded at the culvert outlet, energy dissipation measures should b
and velocities (for channels lined with vegetation) from the Pennsylvania Erosion and Sedimen
Pollution Control Program Manual.
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Lining Type
lb/ft2
Unlined - Erodible Soils*
Silts, Fine - Medium Sands
0.03
0.45
1.45
Coir - Double Net
2.25
0.25
0.50
R-3
1.00
5.00
R-8
8.00
Manufacturer's shear stress values based on independent tests may be used.
Lining Category
Maximum Permissible Shear Stresses for Various Channel Liners
Coarse Sands
0.04
Very Coarse Sands
0.05
Fine Gravel
0.10
Erosion Resistant Soils**
Clay loam
0.25
Silty Clay loam
0.18
Sandy Clay Loam
0.10
Loam
0.07
Silt Loam
0.12
Sandy Loam
0.02
Gravely, Stony, Channery Loam
0.05
Stony or Channery Silt Loam
0.07
Temporary Liners
Jute
Straw with Net
Coconut Fiber - Double Net
2.25
Curled Wood Mat
1.55
Curled Wood - Double Net
1.75
Curled Wood - Hi Velocity
2.00
Synthetic Mat
2.00
Vegetative Liners
Class B
2.10
Class C
1.00
Class D
0.60
Riprap***
R-1
R-2
R-4
2.00
R-5
3.00
R-6
4.00
R-7
*** Permissible shear stresses based on rock at 165 lb/cuft. Adjust velocities for other rock
weights used. See Table 12.
* Soils having an erodibility "K" factor greater than 0.37
** Soils having an erodibility "K" factor less than or equal to 0.37
Slope Range
Erosion
Cover
Percent
resistant Soil
1
Easily Eroded Soil
2
Kentucky Bluegrass
<5
7
3
5
Tall Fescue
5-10
6
3
4
>10
5
3
Grass Mi
Maximum Permissible Velocities for Channels Lined with Vegetation
xture
<5
5
4
Reed Canarygrass
5-10
4
3
Serecea Lespedeza
<5
3.5
2.5
Weeping Lovegrass
Redtop
Red Fescue
Annuals
<5
3.5
2.5
Temporary cover only
Sudangrass
2
Soils with K values greater than 0.37.
3
Use velocities exceeding 5 ft/sec only where good cover and proper maintenance can be obtained.
1
Cohesive (clayey) fine grain soils and coarse grain soils with a plasticity index OF 10 TO 40
(CL, CH, SC and GC). Soils with K values less than 0.37.
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15. Manning’s
roughness
coefficient, or ‘n’
value, varies with
type of vegetative
cover and design
flow depth. Two
common methods
are based on
design depth (see
adjacent graph )
and based on
vegetative cover (as
defined in the
Pennsylvania
Erosion and
Sediment Pollution
Control Program
Manual). Either of
16.
section, significant levels of pollutant
.
18.
19.
stormwater infrastructure, or a stable outfall.
Detaile
Infiltra
Volum
The vo
Storage
(Top W
f Check Dam) / 2
these can be used
in design.
If swales are
designed according to the guidelines discussed in this
reduction can be expected through filtration and infiltration. In a particular swale reach, runoff
should be well filtered by the time it flows over a check-dam. Thus, the stabilizing stone apron
on the downhill side of the check-dam may be designed as an extension of an infiltration trench.
In this way, only filtered runoff will enter a subsurface infiltration trench, thereby reducing the
threat of groundwater contamination by metals.
17 Culverts are typically used in a vegetated swale at driveway or road crossings. By oversizing
culverts and their flow capacity, cold weather concerns (e.g. clogging with snow) are lessened.
Where grades limit swale slope and culvert size, trench drains may be used to cross driveways.
Swales should discharge to another structural BMP (bioretention, infiltration basin, constructed
wetlands, etc.), existing
d Stormwater Functions
tion Area (if needed)
e Reduction Calculations
lume retained behind each check-dam can be approximated from the following equation:
Volume = 0.5 x Length of Swale Impoundment Area Per Check Dam x Depth of Check Dam x
idth of Check Dam + Bottom Width o

Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
Peak Rate Mitigation
See Chapter 8 for Peak Rate Mitigation methodology, which addresses link between volume reduction
and peak rate control.
Water Quality Improvement
See Chapter 8 for Water Quality Improvement methodology, which addresses pollutant removal
effectiveness of this BMP.
Construction Sequence
n vegetated swale construction only when the upgradient temporary erosion and sediment
control measures are in place. Vegetated swales should be constructed and stabilized early in
the construction schedule, preferably before mass earthwork and paving increase the rate and
volume of runoff. (Erosion and sediment control methods shall adhere to the Pennsylvania
Department of Environmental Protection’s
Erosion and Sediment Pollution Control Program
Manual,
March 2000 or latest edition.)
2. Rough grade the vegetated swale. Equipment shall avoid excessive compaction and/or land
disturbance. Excavating equipment should operate from the side of the swale and never on the
bottom. If excavation leads to substantial compaction of the subgrade (where an infiltration
trench is not proposed), 18 inches shall be removed and replaced with a blend of topsoil and
sand to promote infiltration and biological growth. At the very least, topsoil shall be thoroughly
deep plowed into the subgrade in order to penetrate the compacted zone and promote aeration
and the formation of macropores. Following this, the area should be disked prior to final grading
of topsoil.
3. Construct check dams, if required.
4. Fine grade the vegetated swale. Accurate grading is crucial for swales. Even the smallest non-
conformities may compromise flow conditions.
1. Begi
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5. Seed, vege
list. Plant th
most likely. Howe
Vegetation should
Once all tributary a
controls. It is very
flow.
Follow maintenance
Note: If a vege
regraded and rese
damaged areas sh
ance Issues
red to other stormw
nt removal e
tate and install protective lining as per approved plans and according to final planting
e swale at a time of the year when successful establishment without irrigation is
ver, temporary irrigation may be needed in periods of little rain or drought.
be established as soon as possible to prevent erosion and scour.
6.
reas are sufficiently stabilized, remove temporary erosion and sediment
important that the swale be stabilized before receiving upland stormwater
7.
guidelines, as discussed below.
tated swale is used for runoff conveyance during construction, it should be
eded immediately after construction and stabilization has occurred. Any
ould be fully restored to ensure future functionality of the swale.
Mainten
Compa
ater management measures, the required upkeep of vegetated swales is
relatively low. In general, maintenance strategies for swales focus on sustaining the hydraulic and
polluta
fficiency of the channel, as well as maintaining a dense vegetative cover.
Experience has proven that proper maintenance activities ensure the functionality of vegetated swales
for many years. The following schedule of inspection and maintenance activities is recommended:
Mainte
(>
1 inch
Inspect vegetation on side slopes for erosion and formation of rills or gullies, correct as needed
to design grade
Mow and trim vegetation to ensure safety, aesthetics, proper swale operation, or to suppress
• Inspect for litter; remove prior to mowing
• Inspect for uniformity in cross-section and longitudinal slope, correct as needed
• Inspect swale inlet (curb cuts, pipes, etc.) and outlet for signs of erosion or blockage, correct as
aintenance activities to be done as needed:
ssful establishment
nance activities to be done annually and within 48 hours after every major storm event
rainfall depth):
Inspect and correct erosion problems, damage to vegetation, and sediment and debris
accumulation (address when > 3 inches at any spot or covering vegetation)
Inspect for pools of standing water; dewater and discharge to an approved location and restore
weeds and invasive vegetation; dispose of cuttings in a local composting facility; mow only
when swale is dry to avoid rutting
needed
M
• Plant alternative grass species in the event of unsucce
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• Reseed bare areas; install appropriate erosion control measures when native soil is exposed or
erosion channels are forming
• Rototill and replant swale if draw down time is more than 48 hours
• Inspect and correct check dams when signs of altered water flow (channelization, obstructions,
erosion, etc.) are identified
• Water during dry periods, fertilize, and apply pesticide
only when absolutely necessary
Most of the above maintenance activities are reasonably within the ability of individual homeowners.
More intensive swales (i.e. more substantial vegetation, check dams, etc.) may warrant more intensive
maintenance duties and should be vested with a responsible agency. A legally binding and enforceable
maintenance agreement between the facility owner and the local review authority might be warranted to
ensure sustained maintenance execution. Winter conditions also necessitate additional maintenance
concerns, which include the following:
mediately after the spring melt, remove residuals (e.g. sand) and replace
damaged vegetation without disturbing remaining vegetation.
rking lot runoff is directed to the swale, mulching and/or soil
aeration/manipulation may be required in the spring to restore soil structure and moisture
he impacts of deicing agents.
ed
pretreated salt.
• Use salt-tolerant vegetation in swales.
Cos
As with
design
ted
Swales
Vegeta
ative to traditional curbs and gutters, including
associated underground storm sewers. The following table compares the cost of a typical vegetated
swa
• Inspect swale im
• If roadside or pa
capacity and to reduce t
• Use nontoxic, organic deicing agents, applied either as blended, magnesium chloride-bas
liquid products or as
t Issues
all other BMPs, the cost of installing and maintaining Vegetated Swales varies widely with
variability, local labor/material rates, real estate value, and contingencies. In general, Vegeta
are considered relatively low cost control measures. Moreover, experience has shown that
ted Swales provide a cost-effective altern
le (15 ft top width) with the cost of traditional conveyance elements.
Structure:
Swale
Underground Pipe Curb & Gutter
Construction Cost (per
linear foot)
$4.50 - $8.50 (from seed)
$15 - $20 (from sod)
$2 per foot per inch
of diameter
$13 - $15
Annual O&M cost (per
linear fo
$0.75
No data
No data
ot)
Total Annual Cost (per $1 (from seed)
$2
No data
No data
Lifetime (years
50
20
linear foot)
(from sod)
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It is important to note that the costs listed above are strictly estimates and shall be used for design
pur
leveling
on
(SEWR
to
$50.00
ruction
activities are considered, it is still likely that the cost of vegetated swale installation is less than that of
trad
howeve
lifespan
Spe
c
The ol
de
informa
g.
The de
ts in
accordance with the project conditions.
poses only. Also, these costs do not include the cost of activities such as clearing, grubbing,
, filling, and sodding (if required). The Southeastern Wisconsin Regional Planning Commissi
PC, 1991) reported that actual costs, which do include these activities, may range from $8.50
per linear foot depending on swale depth and bottom width. When all pertinent const
itional conveyance elements. When annual operation and maintenance costs are considered
r, swales may prove the more expensive option, though they typically have a much longer
.
cifi ations
f lowing specifications are provided for information purposes only. These specifications inclu
tion on acceptable materials for typical applications, but are by no means exclusive or limitin
signer is responsible for developing detailed specifications for individual design projec
1. Swale Soil
shall be USCS class ML (Inorganic silts and very fine sands, rock flour, silty or
clayey fine sands with slight plasticity), SM (Silty sands, poorly graded sand-silt mixtures), SW
d-
.
(Well-graded sands, gravelly sands, little or no fines) or SC (Clayey sands, poorly graded san
clay mixtures). The first three of these designations are preferred for swales in cold climates
In general, soil with a higher percent organic content is preferred.
2. Swale Sand
shall be ASTM C-33 fine aggregate concrete sand (0.02 in to 0.04 in).
3. Check dams
constructed of natural wood shall be 6 in to 12 in diameter and notched as
necessary. The following species are acceptable: Black Locust, Red Mulberry, Cedars,
Catalpa, White Oak, Chestnut Oak, Black Walnut. The following species are not acceptable, as
they can rot over time: Ash, Beech, Birch, Elm, Hackberry, hemlock, Hickories, Maples, Red
and Black Oak, Pines, Poplar, Spruce, Sweetgum, and Willow. An earthen
check dam
shall be
constructed of sand, gravel, and sandy loam to encourage grass cover (Sand: ASTM C-33 fine
check dam
aggregate concrete sand 0.02 in to 0.04 in, Gravel: AASHTO M-43 0.5 in to 1.0 in). A stone
shall be constructed of R-4 rip rap, or equivalent.
4.
planting mix
Develop a native
. (see Appendix B)
References
Alameda Countywide Clean Water Program (ACCWP). “Grassy Swales.”
Catalog of Control Measures
.
5. If infiltration trench is proposed, see BMP 6.4.4 Infiltration Trench for specifications.
<http://www.oaklandpw.com/creeks/pdf/Grassy_Swales.pdf>
AM
Ma
t Manual
. 2001.
California Stormwater Quality Association.
California Stormwater Best Management Practices
ment
. 2003.
Ca
o
.
EC Earth and Environmental Center for Watershed Protection et al.
Georgia Stormwater
nagemen
Handbook: New Development and Redevelop
rac and Claytor.
Stormwater BMP Design Supplement for Cold Climates
. 1997
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2006
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Chapter 6
City f
#2
.
Cen r
ent.
2000 Maryland
tormwater Design Manual
. Baltimore, MD: 2000.
Claytor
ign of Stormwater Filtering Systems. Center for Watershed
rotection
. Silver Spring, MD: 1996.
olwell, S. R. et al.
Characterization of Performance Predictors and Evaluation of Mowing Practices in
Sheets. Bay Area Stormwater Management Agencies
Association (BASMAA). 1997.
Maine
tion BMP Manual for Erosion and
edimentation Control
. 1992.
North C
f
Ma
Texas
. 2002.
Sch l
Re
e Coastal Zone
. 1992.
Un
(USEPA). “Post-Construction Storm Water
Management in New Development & Redevelopment.”
National Pollutant Discharge Elimination
System (NPDES).
<http://cfpub1.epa.gov/npdes/stormwater/menuofbmps/post_8.cfm>
nited States Environmental Protection Agency (USEPA).
Storm Water Technology Fact Sheet:
o Portland Environmental Services.
City of Portland Stormwater Management Manual: Revision
2002.
te for Watershed Protection and Maryland Department of the Environm
S
, R.A. and T.R. Schuler.
Des
P
C
Biofiltration Swales
. 2000.
Fletcher, T., Wong, T., and Breen, P. “Chapter 8 – Buffer Strips, Vegetated Swales and Bioretention
Systems.”
Australian Runoff Quality (Draft)
. University of New Castle – Australia.
Lichten, K. “Grassy Swales.” BMP Fact
Department of Transportation.
Maine Department of Transporta
S
entral Texas Council of Governments.
Stormwater Best Management Practices: A Menu o
nagement Plan Options for Small MS4s in North Central
ue er, T. et al.
A Current Assessment of Urban Best Management Practices: Techniques for
ducing Nonpoint Source Pollution in th
ited States Environmental Protection Agency
U
Vegetated Swales
(EPA 832-F-99-006). 1999.
Vermont Agency of Natural Resources.
The Vermont Stormwater Management Manual
. 2002.
Virginia Stormwater Management Handbook, Volumes 1 and 2, First Edition, 1999
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2006
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
BMP 6.4.9: Vegetated Filter Strip
The EPA defines a Vegetated Filter Strip as a “permanent, maintained strip of planted or indigenou
vegetation located between nonpoint sources of pollution and receiving water bodies for the purpos
removing o
s
e of
r mitigating the effects of nonpoint source pollutants such as nutrients, pesticides,
suspended solids.”
sediments, and
Water Quality Functions
TSS:
TP:
NO3:
30%
20%
10%
s
Key Design Elements
Potential Applications
Industrial:
Retrofit:
Limited
Yes
.
Sh
.
Fi
soil
.
Fi
are
.
Le
she
adja
.
M
%, unless energy dissipation is provided.
ontributing drainage area.
o
xisting vegetation at the site as possible.
pendix B for list of acceptable filter strip vegetation.
Residential:
Commercial:
Ultra Urban:
Yes
Yes
Limited
eet Flow across Vegetated Filter Strip
lter Strip length is a function of the slope, vegetative cover, and
type.
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low/Med.
Low/Med.
Low
High
Stormwater Functions
.
Maximum Filter Strip slope is based on soil type and vegetated
cover.
lter strip slope should never exceed 8%. Slopes less than 5%
generally preferred.
vel spreading devices are recommended to provide uniform
et flow conditions at the interface of the Filter Strip and the
cent land cover.
aximum contributing drainage area slope is generally less than
Highway/Road: Ye
Minimum recommended length of Filter Strip is 25 ft, however
shorter lengths provide some water quality benefits as well.
.
5
.
Minimum filter strip width should equal the width of the
c
.
C nstruction of filter strip should entail as little disturbance to
e
.
See Ap
Other Considerations
Regular maintenance required for continued performance
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Description
Filter strips are gently sloping, densely vegetated areas that filter, slow, and infiltrate sheet flowing
ts,
c
s for other BMPs, such as
SS levels,
nt
rem
ic
gth, a filter
he
f:
ilter strips may be comprised of a variety of trees, shrubs, and native vegetation to add aesthetic value
as well as water quality benefits. The use of turf grasses will increase the required length of the filter
strip, as compared to other vegetation options. The use of indigenous vegetated areas that have
surface features that disperse runoff is encouraged, as the use of these areas will also reduce overall
site disturbance and soil compaction. Runoff must be distributed so that erosive conditions cannot
develop.
The vegetation in Filter Strips must be dense and healthy. Indigenous wooded areas should have a
healthy layer of leaf mulch or duff. Indigenous areas that have surface features that concentrate flow
are not acceptable.
stormwater. Filter strips are best utilized to treat runoff from roads and highways, roof downspou
small parking lots, and pervious surfaces. In highly impervious areas, they are generally not
re ommended as “stand alone” features, but as pretreatment system
Infiltration Trenches or Bioretention Areas. Filter Strips are primarily designed to reduced T
however pollutant levels of hydrocarbons, heavy metals, and nutrients may also be reduced. Polluta
oval mechanisms include sedimentation, filtration, absorption, infiltration, biological uptake, and
m robial activity. Depending on hydrologic soil group, vegetative cover type, slope, and len
strip can allow for a modest reduction in runoff volume through infiltration.
T
vegetation for Filter Strips may be comprised o
Turf Grasses
Meadow grasses, shrubs, and native vegetation, including trees
Indigenous areas of woods and vegetation.
F
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The following example shows three filter strips that vary only
oils and has a slope of 6%. Using the recommende
by cover type. Each strip is on type ‘C’
d sizing approach, the filter strip covered with turf
rass required a length of 100 ft, while the strip with indigenous woods required only 50 ft. The strip
overed with native grasses and some trees required 75 ft. Where the required length is not available,
filter strip can still be used but it will be less effective.
s
g
c
a
Filter Strip Example #1: Turf Grass
Filter Strip Example #3: Indigenous Woods
Filter Strip Example #2: Native Grasses and Planted Woods Grass
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Variations
Filter strip effectiveness may be enhanced through the addition of a pervious berm at the toe of the
slope. A pervious berm allows for greater runoff velocity and volume reduction and thus better pollutant
removal ability, by providing a very shallow, temporarily ponded area. The berm should have a height
of not more than six to twelve inches and be constructed of sand, gravel, and sandy loam to encourage
vegetative cover. An outlet pipe(s) or overflow weir should be provided and sized to ensure that the
area drains within 24 hours, or to convey larger storm events. The berm must be erosion resistant
under the full range of storm events. Likewise, the ponded area should be planted with vegetation that
is resistant to frequent inundation.
Check dams may be implemented on filter strips with slopes exceeding 5%. Check dams shall be
constructed of durable, nontoxic materials such as rock, brick, wood, not more than six inches in height,
and placed at appropriate intervals to encourage ponding and prevent erosion. Care must be taken to
prevent erosion around the ends of the check dams.
Ap c
reas
Roads and highways
esign Considerations
,
, etc.) site soil group, proposed cover type, and filter strip slope. The filter length
can be determined from the appropriate graph shown below the text.
pli ations
Residential development lawn and housing a
Parking lots
Pretreatment for other structural BMPs (Infiltration Trench, Bioretention, etc.)
Commercial and light industrial facilities
As part of a Riparian Buffer (located in Zone 3)
D
1. The design of vegetated filter strips is determined by site conditions (contributing drainage area
length, slope
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2. Level spreading devices or other measures may be required to provide uniform sheet flow
conditions at the interface of the filter strip and the adjacent land cover. Concentrated flows are
explicitly discouraged from entering filter strips, as they can lead to erosion and thus failure of
the system. Examples of level spreader applications include:
a. A gravel-filled trench, installed along the entire upgradient edge of the strip. The gravel
in the trenches ma
size no. 6, 1/8” to 3/8”) for
most cases to shoulder ballast for roadways. Trenches are typically 12” wide, 24-36”
deep, and lined with a nonwoven geotextile. When placed directly adjacent to an
impervious surface, a drop (between the pavement edge and the trench) of 1-2” is
recommended, in order to inhibit the formation of the initial deposition barrier.
y range from pea gravel (ASTM D 448
b. Curb stops
c. Concrete sill (or lip)
d. Slotted or depressed curbs
e. An earthen berm with optional perforated pipe.
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3. Although in some locations more “natural” spreader designs and materials, such as earthen
berms, are desirable, they can be more susceptible to failure due to irregularities in berm
5. The seasonal high water table should be at least 2 to 4 ft lower than any point along the filter
,
ent of vegetation. However, tilling will only have
ayer. Therefore, other measures, such as planting
trees and shrubs, may be needed to provide deeper aeration. Deep root penetration will
promote greater absorptive capacity of the soil.
7. The ratio of contributing drainage area to filter strip area should not exceed 6:1.
8. The filter strip area should be densely vegetated with a mix of salt- and drought- tolerant and
erosion-resistant plant species. Filter strip vegetation, whether planted or indigenous, may
range from turf and native grasses to herbaceous and woody vegetation. The optimal
vegetation strategy consists of plants with dense growth patterns, a fibrous root system for
stability, good regrowth ability (following dormancy and cutting), and adaptability to local soil and
climatic conditions. Native vegetation is always preferred. (See Appendix B for vegetation
recommendations.)
9. Natural areas, such as forests and meadows, should not be unduly disturbed by the creation of
a filter strip. If these areas are not already functional as natural filters, they may be enhanced
by restorative methods or construction of a level spreader.
10. Maximum lateral slope of filter strip is 1%.
it runoff from laterally bypassing a strip, berms and/or curbs can be installed along the
sides of the strip, parallel to the direction of flow.
.
cular traffic on filter strips should be strictly discouraged. Since the
r strips can be easily overlooked or forgotten over time, a highly visible, physical
er, by simple post
eading device itself.
13. Vegetated filter strips may be designed to discharge to a variety of features, including natural
buffer areas, vegetated swales, infiltration basins, or other structural BMPs.
14. In cold climates, the following recommendations should be considered:
elevation and density of vegetation. When it is desired to treat runoff from roofs or curbed
impervious areas, a more structural approach, such as a gravel trench, is required. In this case,
runoff shall be directly conveyed, via pipe from downspout or inlet, into the subsurface gravel
and uniformly distributed by a perforated pipe along the trench bottom.
4. The upstream edge of a filter strip should be level and directly abut the contributing drainage
area.
strip.
6. In areas where the soil infiltration rate has been compromised (e.g. by excessive compaction)
the filter strip shall be tilled prior to establishm
an effect on the top 12-18 inches of the soil l
11. To prohib
12 Pedestrian and/or vehi
function of filte
“barrier” is suggested. This can be accomplished, at the discretion of the own
and chain, signage, or even the level-spr
a. Filter strips often make convenient areas for snow storage. Thus, filter strip vegetation
should be salt-tolerant and the maintenance schedule should involve removal of sand
buildup at the toe of the slope.
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b. The bottom of the gravel trench (if used as the level spreader) should be placed be
the frost line to prohibit water from freezing in the trench. The perforated pipe in the
trench should be at least 8 inches in diameter to further discourage freezing.
c. Other water quality options may be explored to provide
low
backup to filter strips during the
winter, when their pollutant removal ability is reduced.
Required Length as a Function of Slope, Soil Cover
Hydrologic
Maximum Filter Strip Slope (Percent)
Sand
A
Sandy Loam
B
Loam, Silt Loam
B
Sandy Clay Loam
C
Clay Loam, Silty Clay, Clay
D
8
5
7
8
8
8
7
8
8
8
Filter Strip Soil Type
Soil Group
Turf Grass, Native
Grasses and Meadows
Planted and Indigenous
Woods
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Detailed Stormwater Functions
Volume Reduction Calculations
To determine the volume reduction over the length of a filter strip the following equation is
recommended:
Filter Strip Volume Reduction = Filter Strip Area x Infiltration Rate x Storm Duration
When a berm is positioned at the toe of the slope, the total volume reduction shall be defined as the
amount calculated above plus the following:
Berm Storage Volume = (Cross-sectional Area Behind Berm x Length of Berm) + (Surface Area Behind
Berm x Infiltration Rate x 12 hours)
The inundated area behind the berm should be designed to drain within 24 hours. An outlet pipe or
overflow weir may be needed to provide adequate drain down. In that case, the infiltration volume
behind the berm should be adjusted based on the invert of the overflow mechanism.
Peak Rate Mitigation Calculations
See in Section 8 for Peak Rate Mitigation methodology which addresses link between volume reduction
and peak rate control.
Water Quality Improvement
See in Section 8 for Water Quality Improvement methodology which addresses pollutant removal
ffectiveness of this BMP.
Construction Sequence
1. Begin filter strip construction only when the upgradient site has been sufficiently stabilized and
temporary erosion and sediment control measures are in place. (Erosion and sediment control
methods shall adhere to the Pennsylvania Department of Environmental Protection’s Erosion
and Sediment Pollution Control Program Manual, March 2000 or latest edition.) The strip
should be installed at a time of the year when successful establishment without irrigation is most
likely. However, temporary irrigation may be needed in periods of little rain or drought.
2. For planted (not indigenous Filter Strips) clear and grub site as needed. Care should be taken to
disturb as little existing vegetation as possible, whether in the designated filter strip area or in
adjacent areas, and to avoid soil compaction. Grading a level slope may require removal of
existing vegetation.
3. Rough grade the filter strip area, including the berm at the toe of the slope, if proposed. Use the
lightest, least disruptive equipment to avoid excessive compaction and/or land disturbance.
4. Construct level spreader device at the upgradient edge of the strip. For gravel trenches, do not
compact subgrade (Follow construction sequence for Infiltration Trench).
5. Fine grade the filter strip area. Accurate grading is crucial for filter strips. Even the smallest
irregularities may compromise sheet flow conditions.
e
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Inlets or sediment sumps that drain to filter strips should be cleaned periodically or as needed.
6. Seed or sod, as desired. Plant more substantial vegetation, such as trees and shrubs, if
proposed. If sod is proposed, place tiles tightly enough to avoid gaps and stagger the ends to
prevent channelization along the strip. Use a roller on sod to prevent air pockets between the
sod and soil from forming.
7. Concurrent with #6, stabilize seeded filter strips with appropriate permanent soil stabilization
methods, such as erosion control matting or blankets. Erosion control for seeded filter strips
should be maintained for at least the first 75 days following the first storm event of the season.
8. Once the filter strip is sufficiently stabilized, remove temporary erosion and sediment controls. It
is very important that filter strip vegetation be fully established before receiving upland
stormwater flow. One full growing season is the recommended minimum time for
establishment. Some seed mixtures may require a longer time period to become established.
9. Follow maintenance guidelines, as discussed below.
Note: When and if a filter strip is used for temporary sediment control, it might need to be regraded and
reseeded immediately after construction and stabilization has occurred.
Maintenance Issues
As with other vegetated BMPs, filter strips should be properly maintained to ensure their effectiveness.
In particular, it is critical that sheet flow conditions and infiltration are sustained throughout the life of the
filter strip. Field observations of strips in more urban settings show that their effectiveness can
deteriorate due to lack of maintenance, inadequate design/location, and poor vegetative cover.
Compared with other vegetated BMPs, filter strips require only minimal maintenance efforts, many of
which may overlap with standard landscaping demands.
Vegetated filter strip components that receive or trap sediment and debris should be inspected for
clogging, density of vegetation, damage by foot or vehicular traffic, excessive accumulations, and
channelization. Inspections should be made on a quarterly basis for the first two years following
installation, and then on a biannual basis thereafter. Inspections should also be made after every storm
event greater than 1 in during the establishment period. Guidance information, usually in written
manual form, for operating and maintaining filter strips should be provided to all facility owners and
tenants. Facility owners are encouraged to keep an inspection log, where they can record all
inspection dates, observations, and maintenance activities.
Sediment and debris should be routinely removed (but never less than biannually), or upon
observation, when buildup exceeds 2 inches in depth in either the strip itself or the level spreader. If
erosion is observed, measures should be taken to improve the level spreader or other dispersion
method to address the source of erosion. Rills and gullies observed along the strip may be filled with
topsoil, stabilized with erosion control matting, and either seeded or sodded, as desired. For channels
less than 12 inches wide, filling with crushed gravel, which allows grass to creep in over time, is
acceptable. For wider channels, i.e. greater than 12 inches, regrading and reseeding may be
necessary. (Small bare areas may only require overseeding.) Regrading may also be required when
pools of standing water are observed along the slope. (In no case should standing water be tolerated
for longer than 48-72 hours.) If check dams are proposed, they should be inspected for cracks, rot,
structural damage, obstructions, or any other factors that cause altered flow patterns or channelization.

Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
Sediment should be removed when the filter strip is thoroughly dry. Trash and debris removed from the
ite should be deposited only at suitable disposal/recycling sites and must comply with applicable local,
. In the case where a filter strip is used for sediment control, it
aintaining a vigorous vegetative cover on a filter strip is critical for maximizing pollutant removal
ressure
quipment, as needed to maintain a height of 4-6 inches. Mowing should be done only when the soil is
ed to grow as high as possible, but mowed frequently
nough to avoid troublesome insects or noxious weeds. Fall mowing should be controlled to a grass
ng should be used.
ekly
spections are recommended for at least the first growing season, or until the vegetation is
egetation is established, inspections of health, diversity, and
d be sustained at 85% and reestablished if damage greater than 50% is
bserved. Whenever possible, deficiencies in vegetation are to be mollified without the use of fertilizers
options, as well as any other methods used to achieve optimum
the
.
wo other maintenance recommendations involve soil aeration and drain down time. If a filter strip
e and/or vegetative cover, periodic soil aeration may be needed. In
ddition, depending on soil characteristics, the strip may need periodic liming. The design and
ma
time it
rainfall
specifie
ur
maxim
should
be, the
specified drain down time of the system.
Cost I
The real cost of filter strips is the land they require. When unused land is readily available at a site,
filte
premiu
establishing a filter strip itself is relatively minor. Of course, the cost is even less when an existing
gra
The cost of filter strips includes grading, sodding (when applicable), installation of vegetation (trees,
shr
ed.
Depen
hanced vegetation use or design
variations, construction costs may range anywhere from $0 (assuming the area was to be grassed
regardless of use as treatment) to $50,000 per acre. The annual cost of maintaining filter strips
s
state, and federal waste regulations
should be regraded and reseeded immediately after construction has concluded.
M
efficiency and erosion prevention. Grass cover should be mowed, with low ground p
e
dry, in order to prevent tracking damage to vegetation, soil compaction, and flow concentrations.
Generally speaking, grasses should be allow
e
height of 6 inches, to provide adequate wildlife winter habitat. When and where cutting is desired for
aesthetic reasons, a high blade setti
If vegetative cover is not fully established within the designated time, it should be replaced with an
alternative species. It is standard practice to contractually require the contractor to replace dead
vegetation. Unwanted or invasive growth should be removed on an annual basis. Biwe
in
permanently established. Once the v
density should be performed at least twice per year, during both the growing and non-growing season.
Vegetative cover shoul
o
or pesticides. These treatment
vegetative health, should only be used under special circumstances and if they do not compromise
functionality of the filter strip
T
exhibits signs of poor drainag
a
intenance plan of filter strips, especially those with flow obstructions should specify the approximate
would take for the system to “drain down” the maximum design storm runoff volume. Post-
inspections should include evaluations of the filter’s actual drain down time compared to the
d time. If significant differences (either increase or decrease) are observed, or if the 72 ho
um time is exceeded, strip characteristics such as soils, vegetation, and groundwater levels
be reevaluated. Measures should be taken to establish, or reestablish as the case may
ssues
r strips may prove a sensible and cost-effective approach. However, where land costs are at a
m (i.e. not readily available), this practice may prove cost-prohibitive in the end. The cost of
ss or meadow area is identified as a possible filter strip area before development begins.
ubs, etc.), the construction of a level spreader, and the construction of a pervious berm, if propos
ding on whether seed or sod is applied, not to mention en
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(mo
d in
fact, m
as they
Sp
The fol
information on acceptable materials for typical applications, but are by no means exclusive or limiting.
The
accord
1.
wing, weeding, inspection, litter removal, etc.) generally runs from $100 to $1400 per acre an
ay overlap with standard landscape maintenance costs. Maintenance costs are highly variable,
are a function of frequency and local labor rates.
ecifications
lowing specifications are provided for information purposes only. These specifications include
designer is responsible for developing detailed specifications for individual design projects in
ance with the project conditions.
Vegetation
– See Appendix B
2.
Erosion and Sediment
Control components shall conform to the Pennsylvania Department of
Environmental Protection’s Erosion and Sediment Pollution Control Program Manual, March
or a gravel trench level spreader:
2000 or latest edition.
F
3.
Pipe
should be continuously perforated, smooth interior, high-density polyethylene (HDPE) with
a minimum inside diameter of 8-inches. The pipe should meet AASHTO M252, Type S or
AASHTO M294, Type S.
4.
Stone
for infiltration trenches should be 2-inch to 1-inch uniformly graded coarse aggrega
a wash loss of no more than 0.5%, AASHTO size number 3 per AASHTO Specifications,
19th Ed., 1998, or later and should have voids
35% as measured by ASTM-C29.
te, with
Part I,
Pea gravel (clean bank-run gravel) may also be used. Pea gravel should meet ASTM D 448
and be sized as per No.6 or 1/8” to 3/8”.
5.
Non-Woven Geotextile
should consist of needled non-woven polypropylene fibers and me
following properties:
a. Grab Tensile Strength (ASTM-D4632)
120 lbs
b. Mullen Burst Strength (ASTM-D3786)
225 psi
c. Flow Rate (ASTM-D4491)
95 gal/min/ft2
d. UV Resistance after 500 hrs (ASTM-D4355)
et the
70%
e. Heat-set or heat-calendared fabrics are not permitted
Acceptable types include Mirafi 140N, Amoco 4547, and Geotex 451.
6.
Check dams
constructed of natural wood should be 6 in to 12 in inches diameter and notch
as necessary. The following species are acceptable: Black Locust, Red Mulberry, Cedars,
Catalpa, White Oak, Chestnut Oak, Black Walnut. The following species are not acceptable
since they can rot over time: Ash, Beech, Birch, Elm, Hackberry, Hemlock, Hickories, Map
Red and Black Oak, Pines, Poplar, Spruce, Sweetgum, and Willow. An earthen check
should be constructed of sand, gravel, and sandy loam to encourage grass cover. (Sand:
ASTM C-33 fine aggregate concrete sand 0.02 in to 0.04 in, Gravel: AASHTO M-43 0.5 in to 1.0
in). A stone check dam should be constructed of R-4 rip rap, or equivalent.
ed
les,
dam
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7.
Pervious Berms
The berm should have a height of 6-12 in and be constructed of sand, gravel,
and sandy loam to encourage grass cover. (Sand: ASTM C-33 fine aggregate concrete sand
0.02”-0.04”, Gravel: AASHTO M-43 ½” to 1”)
tlanta Regional Commission.
Georgia Stormwater Management Manual.
August 2001.
002.
outh Florida Water Management District, 2002.
Best Management Practices for Southern Florida
d Department of the Environment, 2000.
2000 Maryland
Stormwater Design Manual
, Baltimore, MD
y of the Environment, 2003.
Stormwater Management Planning and Design Manual
2003
, Toronto, Ontario
ersity of
References
New Jersey Department of Environmental Protection.
New Jersey Stormwater BMP Manual
. 2004
Environmental Services, City of Portland.
Stormwater Management Manual
. September 2002.
Virginia BMP Manual
A
Delaware Department of Natural Resources.
DURMM: The Delaware Urban Runoff Management
Model.
(March 2001)
TheVermont Agency of Natural Resources.
The Vermont Stormwater Management Manual.
April 2
California Stormwater Quality Association.
California Stormwater BMP Handbook.
January 2003.
Washington State Department of Ecology, 2002.
Stormwater Management Manual for Western
Washington,
Olympia, WA
S
Urban Stormwater Management Systems
, West Palm Beach, FL
United States Environmental Protection Agency (USEPA), 1999.
Storm Water Technology Fact Sheet:
Sand Filters
(EPA 832-F-99-007)
Auckland Regional Council, 2003.
Stormwater Management Devices: Design Guidelines Manual
,
Auckland, New Zealand
Center for Watershed Protection and Marylan
Ontario Ministr
Barr Engineering Company, 2001.
Minnesota Urban Small Sites BMP Manual: Stormwater Best
Management Practices for Cold Climates
, St. Paul, MN.
CRWR Online Report 97-5: Use of Vegetative Controls For Treatment of Highway Runoff (Univ
Texas at Austin)
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BMP 6.4.10: Infiltration Berm & Retentive Grading
An Infiltration Berm is a mound of compacted earth with sloping sides that is usually located along a
con
s. Berms can also be created through excavation/removal of
ups
Berm with the original grade. Berms may serve various
sto
n
for
promot
tentio
ot interfere with use.
tour on relatively gently sloping site
lope material, effectively creating a
rmwater drainage functions including: creating a barrier to flow, retaining flow and allowing infiltratio
volume control, and directing flows. Grading may be designed in some cases to prevent rather than
e stormwater flows, through creation of "saucers" or "lips" in site yard areas where temporary
re
n of stormwater does n
Water Quality Functions
Recharge:
Pe
ontrol:
uality:
Low
Medium
Med./High
Key Design Elements
Potential Applications
Ultra Urban:
Limited
high
imp
nc
n
the
ui
B
e than 24
If
, the berm side slopes should not
exceed
s.
.
The c
e of th
berm, r
r a more n ural,
asymm
.
Berms
however mo
shrubs and trees are recommended.
Residential:
Commercial:
Yes
Yes
Stormwater Functions
o
r guidelines described in Protocol 2: Infiltration Systems
G delines
erms should be relatively low, preferably no mor
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
.
Maintain a minimum 2-foot separation to bedrock and seasonally
water table, provide distributed infiltration area (5:1
ervious area to infiltration area - maximum), site on natural,
u ompacted soils with acceptable infiltration capacity, a d follow
Volume Reduction: Low/Med.
inches in height.
.
berms are to be mowed
.
ak Rate C
a ratio of 4:1 to avoid "scalping" by mower blade
rest of the berm should be located near one edg
e
Water Q
ather than in the middle, to allow fo
at
etrical shape.
should be vegetated with turf grass at a minimum,
re substantial plantings such as meadow vegetation,
TSS:
TP:
NO3:
60%
50%
40%
Other Considerations
rotocol 1. Site Evaluation and Soil Infiltration Testing
and
Protocol 2. Infiltration Systems Guidelines
should be followed, see Appendix C
P
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De
Infiltrat
d along (i.e. parallel to) existing site contours in a
moderately sloping area. They can be described as built-up earthen embankments with sloping sides,
which function to divert, retain and promote infiltration, slow down, or divert stormwater flows. Berms
for reasons independent of stormwater management, such as to add interest to a flat
te a noise or wind barrier, separate land uses, screen undesirable views or to enhance
uch
s pathways through woodlands. Therefore, when used for stormwater management, berms and other
the
ndscape.
filtration Berms create shallow depressions that collect and temporarily store stormwater runoff,
e constructed
series along a gradually sloping area.
n be constructed on disturbed slopes and revegetated as part of the
construction process. Infiltration berms should not be installed on slopes where soils having low
2. They can be installed along the contours within an existing woodland area to slow and infiltrate
3. May be constructed in combination with a subsurface infiltration trench at the base of the berm.
e runoff rate and volume control, though the level to which they do is limited
by a variety of factors, including design variations (height, length, etc.), soil permeability rates,
lots). Systems of parallel berms have
een used to intercept stormwater from roadways or sloping terrain. Berms can sometimes be
ofte
very large, highly impervious sites. Due to their relatively
limited volume capacity, the length and/or number of berms required to retain large quantities of runoff
appropriately used as pre- or additional-treatment for other more distributed infiltration systems closer
the source of runoff (i.e. porous pavement with subsurface infiltration).
Ret
f sites where infiltration has been deemed to be
possible and where site uses are compatible. Ideally, such retentive grading will serve to create subtle
as a very subtle berm, which can be vertically impervious when vegetated and integrated into the
verall landscape.
scription
ion Berms are linear landscape features locate
are also utilized
landscape, crea
or emphasize landscape designs. Berms are often used in conjunction with recreational features, s
a
retentive grading techniques can serve multifunctional purposes and are easily incorporated into
la
In
allowing it to infiltrate into the ground and recharge groundwater. Infiltration berms may b
in
1. Infiltration berms ca
shear strength (or identified as “slip prone” or “landslide prone”, etc.) have been mapped.
runoff from a development site.
Infiltration Berms can provid
vegetative cover, and slope. Berms are ideal for mitigating runoff from relatively small impervious
areas with limited adjacent open space (e.g. roads, small parking
b
threaded carefully along contour on wooded hillsides, minimally disturbing existing vegetation and yet
still gaining stormwater management credit from the existing woodland used. Conversely, berms are
n incapable of controlling runoff from
make them impractical as the lone BMP in these cases. In these situations, berms are more
to
entive grading may be employed in portions o
“saucers,” which contain and infiltrate stormwater flows. The “lip” of such saucers effectively function
o
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Variations
Diversion Berms
Diversion Berms can be used to protect slopes from erosion and to slow runoff rate. They can also be
used to direct stormwater flow in order to promote longer flow pathways, thus increasing the time of
concentration. Diversion berms often:
1. Consist of compacted earth ridges usually constructed across a slope in series to intercept
runoff.
2. Can be incorporated within other stormwater BMPs to increase travel time of stormwater flow by
creating natural meanders while providing greater opportunity for pollutant removal and
infiltration.
Applications
• Meadow/Woodland Infiltration Berms
Infiltration Berms effectively control both the rate and volume of stormwater runoff. The berms
are constructed along the contours and serve to collect and retain stormwater runoff, allowing it
to infiltrate through the soil mantle and recharge the groundwater. Depressed areas adjacent to
the berms should be level so that concentrated flow paths are not encouraged. Infiltration
berms may have a variety of vegetative covers but meadow and woodland are recommended in
order to reduce maintenance. If turf grass is used, berms in series should be constructed with
enough space between them to allow access for maintenance vehicles. Also, berm side slopes
should not exceed a 4:1 ratio. Woodland infiltration berms can sometimes be installed within
existing wooded areas for additional stormwater management. Berms in wooded areas can
even improve the health of existi
hanced groundwater recharge. Care
should be taken during construction to ensure minimum disturbance to existing vegetation,
ng vegetation, through en
especially tree roots.
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• Slope Protection
Diversion Berms can be used to help protect steeply sloping areas from erosion. Berms may
water travel time is increased. For example, berms can be utilized within existing BMPs
as part of a retrofit strategy to eliminate short-circuited inlet/outlet situations within detention
ined.
pervious flow pathways within developed areas. Berms
should be designed to compliment the landscape while diverting runoff across vegetated areas
depths. See BMP 6.6.1 – Constructed Wetlands.
divert concentrated discharge from a developed area away from the sloped area. Additionally,
berms may be installed in series down the slope to retain flow and spread it out along multiple
level berms to discourage concentrated flow.
• Flow Pathway Creation
Berms may be utilized to create or enhance stormwater flow pathways within existing or
proposed BMPs, or as part of an LID (Low Impact Development) strategy. Berms can be
installed such that vegetated stormwater flow pathways are allowed to “meander” so that
storm
basins provided care is taken to ensure the required storage capacity of the basin is mainta
Flow pathway creation can be utilized as part of an LID strategy to disconnect roof leaders and
attenuate runoff, while increasing
and allowing for longer travel times to encourage pollutant removal and infiltration.
Constructed Wetland Berms
Berms are often utilized within constructed wetland systems in order to create elongated flow
pathways with a variety of water
Design Considerations
1. Sizing criteria are dependent on berm function, location and storage volume requirements.
a. Low
berm height
(less than or equal to 24 inches) is recommended to encourage
maximum infiltration and to prevent excessive ponding behind the berm. Greater
heights may be used where berms are being used to divert flow or to create
“meandering” or lengthened flow pathways. In these cases, stormwater is designed to
flow adjacent to (parallel to), rather than over the crest of the berm. Generally, more
berms of smaller size are preferable to fewer berms of large size.
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b.
Berm length
is dependent on functional need and site size. Berms installed along the
contours should be level and located across the slope. Maximum length will depend on
of the slope. Generally speaking, diversion berm length will vary with the size and
constraints of the site in question.
o consist of high quality Topsoil, with well-drained soil making up the remainder of the
of clay
ired due to its cohesive qualities (especially where the berm height is high or
relatively steeply sloped). However, well-compacted soil usually is sufficient provided that the
angle of repose (see below) is not exceeded for the soil medium used.
A more sustainable alternative to importing berm soil from off-site is to balance berm cut and fill
material as much as possible, provided on-site soil is deemed suitable as per the Specifications
below. Ideally, the concave segment (infiltration area) of the berm is excavated to a maximum
depth of 12 inches and then used to construct the convex segment (crest of berm).
4. The
Angle of Repose of Soil
is the angle at which the soil will rest and not be subject to slope
failure. The angle of repose of any soil will vary with the texture, water content, compaction,
and vegetative cover. Typical angles of repose are given below:
a. Non-compacted clay: 5-20%
b. Dry Sand: 33%
c. Loam: 35-40%
d. Compacted clay: 50-80%
5.
Side Slopes
. The angle of repose for th
in the berm should determine the maximum
slope of the berm with additional consideration to aesthetic, drainage, and maintenance needs.
If a berm is to be mowed, the slope should not exceed a 4:1 ratio (horizontal to vertical) in order
to avoid “scalping” by mower blades. If trees are to be planted on berms, the slope should not
exceed a 5:1 ratio. Other herbaceous plants, which do not require mowing, can tolerate slopes
erm side slopes should not exceed a 2:1 ratio.
r the function and form of the berm when selecting
nts,
7.
d
tion
width
2.
Infiltration Berms
should be constructed along (parallel to) contours at a constant elevation.
3.
Soil
. A berm may consist entirely of high quality topsoil. To reduce cost, only the top foot
needs t
berm. The use of gravel is not recommended in the layers directly underneath the topsoil
because of the tendency of the soil to wash through the gravel. In some cases, the use
may be requ
e soil used
of 3:1. B
6.
Plant Materials.
It is important to conside
plant materials. If using trees, plant them in a pattern that appears natural and accentuates the
berm’s form. Consider tree species appropriate to the proposed habitat. If turf will be
combined with woody and herbaceous plants, the turf should be placed to allow for easy
maneuverability while mowing. Low maintenance plantings, such as trees and meadow pla
rather than turf and formal landscaping, are encouraged.
Infiltration Design.
Infiltration berms located along slopes should be composed of low berms
(less than 12 inches high) and should be vegetated. Subsurface soils should be uncompacted
to encourage infiltration behind the berms. Soil testing is not required where berms are locate
within an existing woodland, but soil maps/data should be consulted when siting the berms.
Where feasible, surface soil testing should be conducted in order to estimate potential infiltra
rates.
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8.
Infiltration Trench Option.
Soil testing is recommended for infiltration berms that will utilize a
9.
esthetics.
To the extent possible, berms should reflect the surrounding landscape. Berms
he top of the berm is smoothly convex and the toes of the berms are
than
her
subsurface infiltration trench. Infiltration trenches are not recommended in existing woodland
areas as excavation and installation of subsurface trenches could damage tree root systems.
See BMP 6.4.4 – Infiltration Trench, for information on infiltration trench design.
A
should be graded so that t
smoothly concave. Natural, asymmetrical berms are usually more effective and attractive
symmetrical berms. The crest of the berm should be located near one end of the berm rat
than in the middle.
Detailed Stormwater Functions
filtration Area
The Infiltration Area is the ponding area behind the berm, defined as:
Len
Volume Redu
In
gth of ponding x Width ponding area = Infiltration Area (Ponding Area)
ction Calculations
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Storage vo
area created b
volume can be
Surface Storage Volume is defined as the volume of water stored on the surface at the ponding depth.
Thi
Cross-sectional area of ponded water x Berm length = Surface Storage Volume
Peak R
See Se
een volume reduction
and peak rate control.
Water
See Se
al
effectiveness of this BMP.
Const
The following is a typical construction sequence for a infiltration berm without a subsurface infiltration
trench, tho
epending on design variations.
1. Ins
t
nt and erosion control BMPs as per the Pennsylvania Erosion and
Sed
m Manual.
3.
fore delivering soil to site.
5.
7.
8.
planted and disturbed areas with compost mulch to prevent erosion while plants become
established.
lume can be calculated for Infiltration Berms. The storage volume is defined as the ponding
ehind the berm, beneath the discharge invert (i.e. the crest of the berm). Storage
calculated differently depending on the variations utilized in the design.
s is equal to:
ate Mitigation:
ction 8 for Peak Rate Mitigation methodology which addresses link betw
Quality Improvement:
ction 8 for Water Quality Improvement methodology which addresses pollutant remov
ruction Sequence
ugh alterations will be necessary d
tall emporary sedime
iment Pollution Control Progra
2. Complete site grading and stabilize within the limit of disturbance except where Infiltration
Berms will be constructed; make every effort to minimize berm footprint and necessary zone of
disturbance (including both removal of exiting vegetation and disturbance of empty soil) in order
to maximize infiltration.
Lightly scarify the soil in the area of the proposed berm be
4. Bring in fill material to make up the major portion of the berm. Soil should be added in 8-inch
lifts and compacted after each addition according to design specifications. The slope and shape
of the berm should graded out as soil is added.
Protect the surface ponding area at the base of the berm from compaction. If compaction of this
area does occur, scarify soil to a depth of at least 8 inches.
6. Complete final grading of the berm after the top layer of soil is added. Tamp soil down lightly
and smooth sides of the berm. The crest and base of the berm should be at level grade.
Plant berm with turf, meadow plants, shrubs or trees, as desired.
Mulch
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Ma
Infiltrat
filtration Berms
Trees and shrubs may require annual mulching, while meadow planting
requires annual mowing and clippings removal.
Avoid running heavy equipment over the infiltration area at the base of the berms. The
crest of the berm may be used as access for heavy equipment when necessary to limit
disturbance.
.
Routinely remove accumulated trash and debris.
Remove invasive plants as needed
Inspect for signs of flow channelization; restore level gradient immediately after
deficiencies are observed
Diversion Berms
Regularly inspect for erosion or other failures.
Regularly inspect structural components to ensure functionality.
Maintain turf grass and other vegetation by mowing and re-mulching.
Remove invasive plants as needed.
Routinely remove accumulated trash and debris.
Cost Issues
Infiltration berms can be less expensive than other BMPs options because extensive clearing and
grubbing is not necessary. Cost will depend on height, length and width of berms as well as desired
vegetation.
pecifications
ed for information purposes only. These specifications include
formation on acceptable materials for typical applications, but are by no means exclusive or limiting.
onsible for developing detailed specifications for individual design projects in
ccordance with the project conditions.
intenance Issues
ion Berms have low to moderate maintenance requirements, depending on the design.
In
Regularly inspect to ensure they are infiltrating; monitor drawdown time after major
storm events
Inspect any structural components, such as inlet structures to ensure proper functionality
If planted in turf grass, maintain by mowing. Other vegetation will require less
maintenance.
S
The following specifications are provid
in
The designer is resp
a
1. Soil Materials
a. Satisfactory soil materials are defined as those complying with ASTM D2487 soil
classification groups GW, GP, GM, SM, SW, and SP.
b. Unsatisfactory soil materials are defined as those complying with ASTM D2487 soil
classification groups GC, SC, ML, MH, CL, CH, OL, OH, and PT.
c. Topsoil: Topsoil stripped and stockpiled on the site should be used for fine grading.
the top layer of earth on the site, which produces heavy growths of
crops, grass or other vegetation.
Topsoil is defined as
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d. Soils excavated from on-site may be used for berm construction provided they are
deemed satisfactory as per the above recommendations or by a soil scientist.
2. Placing and Compacting of Berm Area Soil
structions, and deleterious materials from ground surface prior to placement of fill.
horizontal so that fill
material will bond with existing surface.
b. When existing ground surface has a density less than that specified under g. (below) for
particular area classification, break up ground surface, pulverize, bring the moisture-
condition to optimum moisture content, and compact to required depth and percentage
m density.
c. Place backfill and fill materials in layers not more than 8 inches in loose depth for
depth for material to be compacted by hand-operated tampers.
d. Before compaction, moisten or aerate each layer as necessary to provide optimum
. Compact each layer to required percentage of maximum dry density
or relative dry density for each area classification. Do not place backfill or fill material on
ll and fill materials evenly adjacent to structures, piping, or conduit to
required elevations. Prevent wedging action of backfill against structures or
f piping or same elevation in each lift.
f. Control soil and fill compaction, providing minimum percentage of density specified for
g. Percentage of Maximum Density Requirements: Compact soil to not less than the
mpact top 6 inches of subgrade and each layer
of backfill or fill material at 85 percent maximum density.
3.
a. Ground Surface Preparation: Remove vegetation, debris, unsatisfactory soil materials,
ob
Plow strip, or break up sloped surfaces steeper than I vertical to 4
of maximu
material to be compacted by heavy compaction equipment, and not more than 4 inches
in loose
moisture content
surfaces that are muddy, frozen, or contain frost or ice.
e. Place backfi
displacement o
each area classification indicated below. Correct improperly compacted areas or lifts if
soil density tests indicate inadequate compaction.
following percentages of maximum density, in accordance with ASTM D 1557:
Under lawn or unpaved areas, co
Under infiltration areas no compaction shall be permitted.
Grading
a. General: Uniformly grade areas within limits of grading under this section, including
adjacent transition areas. Smooth finished surface within specified tolerances; compact
r between
such points and existing grades.
ot
ade surfaces to the depth and indicated
percentage of maximum or relative density for each area classification.
4.
with uniform levels or slopes between points where elevations are indicated o
b. Lawn or Unpaved Areas: Finish areas to receive topsoil to within not more than 0.10 fo
above or below required subgrade elevations.
c. Compaction: After grading, compact subgr
Temporary Seeding
a. Temporary seeding and mulching shall be required on all freshly graded areas
ll
Straw bale barriers shall be placed in swale areas until vegetation is established.
itself properly, mulch as
described above, pending fine grading or permanent seeding.
5. Finish Grading
immediately following earth moving procedures. Seed-free straw or salt hay mulch sha
be applied at a rate of 75 lbs. per 1,000 square feet over temporary seeded areas.
b. Should temporary seeding not be possible or not establish
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a. Spreading of topsoil and finish grading shall be coordinated with the work of the
Landscape Contractor.
compaction.
bgrades shall be loosened and made friable by cross-discing or harrowing
shall
d grade stakes and rubbish removed.
e.
frozen, excessively wet, or extremely
er placing topsoil rake soil to a smooth, even-draining surface and compact lightly with
clean and well raked, ready for lawn
Reference
AMEC
ental Center for Watershed Protection et al.
Georgia Stormwater
Manag e
Harris, C. a
on
. New York, NY:
McGraw-Hill, 1998.
University
M
an Landscape Information Series
(SULIS)
. 1998. <http://www.sustland.umn.edu/implement/soil_berms.html>
hester County Conservation District.
Chester County Stormwater BMP Tour Guide-Infiltration
.
Canadian Building Digest - Drainage and Erosion at Construction Sites
. National
Research Council Canada. 2004. <http://irc.nrc-cnrc.gc.ca/cbd/cbd183e.html>
b. Verify that the rough grades meet requirements for tolerances, materials, and
c. Surface of su
to a depth of 2 inches. Stones and debris more than 1-1.5 inches in any dimension
be raked up an
d. Topsoil shall be uniformly spread to minimum depths after settlement of 6 inches on
areas to be seeded and 4 inches on areas to be sodded. Correct any surface
irregularities to prevent formation of low spots and pockets that would retain water.
Topsoil shall not be placed when the subgrade is
dry and no topsoil shall be handled when in a frozen or muddy condition. During all
operations following topsoil spreading, the surface shall be kept free from stones over 1-
1.5 inches in size or any rubbish, debris, or other foreign material.
f. Aft
an empty water roller. Leave finish graded areas
work.
s
Earth and Environm
em nt Manual
. 2001.
nd Dines, N.
Time Saver Standards for Landscape Architecture, 2nd Editi
of innesota. “Building Soil Berms.”
Sustainable Urb
C
Trenches (Infiltration Berms).
2002.
Williams, G.P
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6.5 Vo
e
lume/P ak Rate Reduction BMPs
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BMP 6.5.1: Vegetated Roof
An extensive vegetated roof cover is a veneer of vegetation that is grown on and
completely covers an otherwise conventional flat or pitched roof (<
30
o
slope),
endowing the roof with hydrologic characteristics that more closely match surface
vegetation than the roof. The overall thickness of the veneer may range from 2 to
6 inches and may contain multiple layers, consisting of waterproofing, synthetic
insulation, non-soil engineered growth media, fabrics, and synthetic components.
Vegetated roof covers can be optimized to achieve water quantity and water quality
benefits. Through the appropriate selection of materials, even thin vegetated
covers can provide significant rainfall
retention and detention functions.
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
30%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Med/High
None
Low
Medium
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Yes
Yes
None
.
2-6 inches of engineered media; assemblies that are 4 inches
and deeper may include more than one type of engineered media
.
Engineered media should have a high mineral content.
Engineered media for extensive vegetated roof covers is typically
85% to 97% non-organic (wet combustion or loss on ignition
methods).
.
Vegetated roof covers intended to achieve water quality benefits
should not be fertilized
.
Irrigation is not a desirable component of vegetated covers used
as best management practices
.
Internal building drainage, including provisions to cover and
protect deck drains or scuppers, must anticipate the need to
must incorporate supplemental measures to insure stability against
sliding.
Structural considerations are required.
manage large rainfall events without inundating the cover.
.
Assemblies planned for roofs with pitches steeper than 2:12
Other Considerations
The roof structure must be evaluated for compatibility with the maximum predicted dead and live
loads. Typical dead loads for wet extensive vegetated covers range from 8 to 36 pounds per square
foot. Live load is a function of rainfall retention. For example, 2 inches of rain equals 10.4 lbs. per
square foot of live load. It requires 20 inches of snow to have the same live load per square foot.
The waterproofing must be resistant to biological and root attack. In many instances a
supplemental root-fast layer is installed to protect the primary waterproofing membrane from plant
roots.
Standards and guidelines (in English) for the design of green roofs are available from FLL1, a
European non-profit trade organization. In the United States, guidelines are in development by ASTM
(American Standard Testing Methods).
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Description
Extensive vegetated roof covers are usually 6 inches or less in depth and are typically intended to
achieve a specific environmental benefit, such as rainfall runoff mitigation. For this reason they are
most commonly not irrigated. While some installations are open to public access, most extensive
vegetated roof covers are for public viewing only. In order to make them practical for installation on
conventional roof structures, lightweight materials are used in the preparation of most engineered
media. Developments in the last 40 years that have made these systems viable include: 1) recognition
of the value of vegetated covers in restoring near open-space hydrologic performance on impervious
surfaces, 2) advances in waterproofing materials and methods, and 3) development of a reliable
temperate climate plant list that can thrive under the extreme growing conditions on a roof.
Vegetated roof covers that are 10 inches, or deeper, are referred to as ‘intensive’ vegetated roof
covers. These are more familiar in the United States and include many urban landscaped plazas.
Intensive assemblies can also provide substantial environmental benefits, but are intended primarily to
achieve aesthetic and architectural objectives. These types of systems are considered “roof gardens”
and are not to be confused with the simple “extensive” design. Benefits beyond the stormwater
considerations include temperature moderation and roof longevity.
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Variations
Most extensive vegetated roof covers fall into three categories
• Single media with sy
• Dual media
• Dual media with syn
All vegetated roof covers wi
waterproofing materials sele
ary
waterproofing membrane fro
Insulation, if included in the roof covering system, may be installed either above or below the primary
waterproofing membrane. Most vegetated roof cover system can be adapted to either roofing
configuration. In the descriptions that follow, the assemblies refer to the conventional configuration, in
which the insulation layer is below the primary waterproofing.
All three extensive roof cover variations can be installed without irrigation. Non irrigated assemblies are
strongly recommended. While this may place some limits on the type of plants that can be grown, the
benefits are that the assembly will perform better as a stormwater BMP, and the maintenance
requirements will be substantially reduced.
Some assemblies are installed in tray-like modules to facilitate installation, especially in confined
locations.
Single media assemblies
Single media assemblies are commonly used for pitched roof applications and for thin and lightweight
installations. These systems typically incorporate very drought tolerant plants and utilize coarse
engineered media with high permeability. A typical profile would include the following layers.
• Waterproofing membrane
• Root-barrier
(optional, depending on the root-fastness of the waterproofing)
• Semi-rigid plastic
geocomposite drain
or
mat
(typical mats are made from non-biodegradable
fabric or plastic foam)
• Separation geotextile
• Engineered
growth media
• Foliage layer
Pitched roof applications may require th
igid slope stabilization panels,
cribbing, reinforcing mesh, or similar method minimizing sliding instability.
nthetic under-drain layer
thetic retention/detention layer
ll require a premium waterproofing system. Depending on the
cted, a supplemental root-fast layer may be required to protect the prim
m plant roots.
e addition of slope bars, r
Flat roof applications with mats as foundations typically require a network of perforated internal
drainage conduit to enhance drainage of percolated rainfall to the deck drains or scuppers.
Assemblies with mats can be irrigated from beneath, while assemblies with drainage composites
require direct drainage.
Dual media assemblies
Dual media assemblies
asal layer of coarse lightweight mineral aggregate. They do not
include a geocomposite drain. The objective is to improve drought resistance by replicating a natural
utilize two types of non-soil media. In this case a finer-grained media with
some organic content is placed over a b
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growing environment in which sandy topsoil overlies gravelly subsoil. These assemblies are typically 4
to 6 inches thick and include the following layers:
• Waterproofing membrane
• Protection layer
• Coarse-grained
drainage media
• Root-permeable nonwoven separation geotextile
• Fine-grained engineered growth media layer
• Foliage layer
These assemblies are suitable for roofs with pitches less than, or equal to, 1.5:12. Large vegetated
covers will generally incorporate a network of perforated internal drainage conduit.
Dual media systems are ideal for use in combination with base irrigation methods.
Dual media with synthetic retention/detention layer
These assemblies introduce plastic panels with cup-like receptacles on their upper surface (i.e., a
modified geocomposite drain sheet). The panels are in-filled with coarse lightweight mineral aggregate.
The cups trap and retain water. They also introduce an air layer at the bottom of the assembly. A
typical profile would include:
• Waterproofing membrane
• Felt fabric
• Retention/detention panel
• Coarse-grained drainage media
• Separation geotextile
• Fine grained ‘growth’ media layer
Foliage layer
These assemblies are suitable on roof with pitches less than or equal to 1:12. Due to their complexity,
these system are usually 5 inches or deeper.
If needed, irrigation can be provided via surface spray or mid-level drip.
Stormwater Volume and Rate Control
Vegetated roof covers are an “at source” measure for reducing the rate and volume of runoff
released during rainfall events. The water retention and detention properties of vegetated
roof covers can be enhanced through proper selection of the engineered media and plants.
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Runoff Water Quality Improvements
pollutant discharges. Vegetated roof
covers can significantly reduce this source of pollution. Assemblies intended to produce
eered media with 100% mineral content.
e plant establishment period (usually about 18 months), on-going fertilization of
indicates that it will take five or more years
for a water quality vegetated cover to attain its maximum potential pollutant removal
asures
Vegetated roof covers are frequently combined with ground infiltration measures. Vegetated
• Filtering runoff to produce a clear effluent
Habitat Restoration/Creation
itigate the development of open space. This can be accomplished with assemblies as thin
as 6 inches.
1. Live and
dead load
bearing capacity of the roof need to be established. Dead loads should be
ts determined using a standardized laboratory procedure.1
l root-barrier layer should be installed in conjunction with materials that
ould extend 6 inches higher than the top of the growth media surface and be
protected by counter-flashings.
membrane from physical
damage during and after installation of the vegetated cover assembly.
modate
a two-year return frequency rainfall without generating surface runoff flow.
ers, or gravel stops serving as methods to discharge water from the roof
area should be protected with
access chambers
. These enclosures should include removable
ready access for inspection.
Direct runoff from roofs is often a contributor to NPS
water quality benefits should employ engin
Following th
the cover should not be permitted. Experience
efficiency.
• In Combination with Infiltration Me
roof covers improve the efficiency of infiltration devices by:
• Reducing the peak runoff rate
• Prolonging the runoff
Roofs that are designed to achieve water quality improvements will also reduce pollutant
inputs to infiltration devices.
Vegetated roof covers have been used to create functional meadows and wetlands to
m
Design Considerations
estimated using media weigh
2. Waterproofing
materials must be durable under the conditions associated with vegetated
covers. A supplementa
are not root-fast.
3. Roof flashings sh
4. The design should incorporate measures to protect the waterproofing
5. Vegetated roof covers should incorporate internal drainage capacity sufficient to accom
6. Deck drains, scupp
lids in order to allow
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7. The physical properties of the engineered media should be selected appropriately in order to
achieve the desired hydrologic performance.
tain no clay size particles and should contain no more than 15%
t combustion or loss on ignition methods)
.
should have a maximum moisture capacity2
10. Plants should be selected which will create a vigorous, drought-tolerant ground cover. In
Pennsylvania the most successful and commonly used ground covers for non irrigated projects
are varieties of Sedum and Delosperma. In the Pennsylvania climate Delosperma is deciduous.
Both deciduous and evergreen varieties of
Sedum
are available. Deeper assemblies (i.e., 4 to
6 inches) can also incorporate a wider range of plants including
Dianthus, Phlox, Antennaria,
and
Carex.
11. Roofs with pitches exceeding 2:12 should be provided with supplemental measures to insure
stability against sliding
8. Engineered media should con
organic matter
(we
9 Media used in constructing vegetated roof covers
of between 30% and 40%.
363-0300-002 / December 30,
2006
Page 130 of 257

Pennsylvania Stormwater Best Management Practices Manual
Chapter 6
Detailed Stormwater Functions
The perfor
represente
runoff from
discharged
and quanti
of key phy
• Maximum media water retention
• Non-capillary porosity
The maxim
y of water that can be held against gravity
under drain
ically for measuring this quantity
roof media are available from FLL and ASTM (draft).
Pea R
Vegetated
A general
er the first portion of the rainfall fills the volume reduction capacity (see
below).
ions
All v
these v
ater
Functions
).
The int
and lon
., reductions in total seasonal or annual roof runoff). Continuous simulation
using a representative annual rainfall record from a local weather station is required in order to predict
the
accord
all.
Usin
regions
based on one or two
types of cover assemblies and selected regions in PA for which good weather data is available. For the
tabl
Wat
Once th
uld be suspended. Experience indicates that
the efficiency of vegetated covers in reducing pollutant and nutrient releases from roofs will increase
with time. The vegetated cover should reach its optimum performance after about five years.
mance of vegetated roof covers as stormwater best management practices cannot be
d by a simple algebraic expression. Conventional methods are used to estimate surface
various types of surfaces. In the analysis of vegetated roof covers, the water that is
from the roof is not surface runoff, but rather underflow, (i.e., percolated water). The rate
ty of water released during a particular design storm can be predicted based on knowledge
sical properties, including:
• Field capacity
• Plant cover type
• Saturated hydraulic conductivity
um media water retention is the maximum quantit
ed conditions. Standards that have been developed specif
in
k ate Mitigation
roof covers can exert an influence on runoff peak rates derived from roofs.
rule is to consid
Volume Reduction Calculat