1
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themselves to positive
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Carolina State University,
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University, U.S. Department
of Agriculture, and local
governments cooperating.
Bioretention Performance, Design, Construction,
and Maintenance
Bioretention has become a common stormwater treatment practice in communi-ties
across North Carolina. Recent state and federal rules, including those for
the Neuse and Tar-Pamlico river basins and EPA Stormwater Phase II, require
that innovative devices, such as bioretention, be used in treating stormwater. In
Cary and Greensboro, for example, bioretention is one of the two most frequently
installed practices.
Basic design guidance for bioretention (also termed rain gardens) was provided
in 2001 in Designing Rain Gardens (Bio-Retention Areas), AG-588-3 part of this
Urban Waterways Series by W. F. Hunt and N. M. White. Since the publication of
that fact sheet, much research has been conducted on the effectiveness of bioreten-tion
in North Carolina and surrounding states. Findings from this research and
anecdotal observation of bioretention function have led to more specific design,
construction, and maintenance recommendations. These recommendations now
address designing bioretention cells specifically to remove target pollutants, as
well as preserve the fragile nature of bioretention cells.
OVERVIEW OF RESEARCH
Research in North Carolina by NC
State University has examined the per-formance
of bioretention cells installed
in Greensboro, Chapel Hill, Louisburg,
and Charlotte. Findings from this re-search
reveal that bioretention cells will
efficiently remove nutrients and other
pollutants from stormwater. The four
studies are summarized in Table 1.
GREENSBORO. Two cells located off
Battleground Avenue were studied from
2002 through 2004. One cell was filled
with a high P-Index soil media1 and
had a standard drainage configuration.
The second cell contained a medium
P-Index media and utilized an alterna-tive
drainage configuration, an internal
water storage (IWS) zone (Figure 1).
Both cells were four feet deep.
1 P-Index, or Phosphorus Index, is the measure of phosphorus already present in soil. The value is determined by testing at the North Carolina Department
of Agriculture and Consumer Services soil analysis laboratory in Raleigh. Values greater than 100 are considered very high. Values ranging between 50 and
100 are considered high. Values between 25 and 50 are medium; values less than 25 are low. A soil with a very high or high P-Index is less able to retain
phosphorus because it is already “full.”
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The first cell, with a high P-Index (86-100), increased
phosphorus loads by 240 percent during the first year
of study. During the second and third years, total
phosphorus (TP) loads also increased, but by less
than 40 percent. It is possible that initial phosphorus
loads were being washed out during the study. The
second cell, with a lower P-Index (35-50), marginally
decreased TP load (9 percent) during the second and
third years of testing.
The cells also reduced the amount of total nitrogen
(TN) entering the storm sewer by 33 percent, 40 per-cent,
and 43 percent, depending on the cell and year
examined. The variation in load reduction reflected
the amount of nitrogen entering the cell: cleaner influ-ent
equaled lower pollutant load removal. Similar
findings held for copper and zinc, as load reduction
ranged from 56 percent to 99 percent.
CHAPEL HILL. One cell was studied from 2002 through
2003 at University Mall. The fill soil, which had a low
P-Index (4-12), was four feet deep. During the year-long
study, TP was reduced by 65 percent and TN was
reduced by 40 percent.
LOUISBURG. Two cells were examined from 2004
through 2005 at Joyner Park. The fill soil in both cells
had a very low P-Index (1-2), and the soil was nomi-nally
2.5 feet deep. TN removal from both cells was
between 60 and 70 percent. TP removal ranged from
22 to 66 percent, depending on the cleanliness of the
influent runoff. In Louisburg, outflow TP concentra-tions
were the lowest among those measured at all
four locations. This indicated that the lowest P-Index
fill soil released the lowest amount of phosphorus.
CHARLOTTE. One cell was studied at the Hal Marshall
county government complex from 2004 through 2005.
The cell was nominally 4 feet deep and was con-structed
with low P-Index (7-14) soil. Both nitrogen
and phosphorus load removals exceeded 60 percent.
This cell was also tested for pathogen removal and
was found to remove well over 90 percent of fecal
coliform bacteria.
TABLE 1. SUMMARY OF RESEARCH FINDINGS ON BIORETENTION EFFICIENCY.
Cell (Study Period) Soil P-Index TN Removal TP Removal Other Findings
Greensboro–cell 1
(2002-2004)
86 – 100
40% - year 1
33% - year 2-3
240% increase – yr 1
39% increase – yr 2-3
Cu and Zn
reduced 65 to 99%
Greensboro–cell 2
(2003-2004)
35 – 50 43% - year 2-3 9% - year 2-3
Cu and Zn
reduced 56 to 86%
Chapel Hill
(2002-2003)
4 – 12 40% 65%
Louisburg – cell 1
(2004-2005)
1 – 2 64% 66%
Higher inflow [TP]=
higher TP removal
Louisburg – cell 2
(2004-2005)
1 – 2 68% 22%
Low inflow [TP] =
lower TP removal
Charlotte
(2004-2005)
7 - 14 65% 68%
Fecal coliform
removal > 90%
FIGURE 1. An Internal Water Storage Zone (IWS) is incorpo-rated
into a bioretention cell by forcing water to move “uphill”
through the underdrain system.
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When examined together, these four studies yielded
the following information:
(1) Nitrogen load removal from bioretention is high,
typically meeting or exceeding 40 percent.
(2) Phosphorus removal can be enhanced with proper
fill-soil selection. Using low P-Index fill soils
reduced phosphorus loads, while high P-Index
fill soils increased phosphorus loads in the
effluent drainage.
(3) Cleaner runoff coming into the bioretention cells
decreases load removal. Low concentrations of
pollutants in inflow tend to decrease load removal
efficiency.
(4) When designed to remove particular pollutants,
bioretention cells appear to be very effective.
(5) Bioretention cells can partially recharge ground
water supplies, even in the clayey soils of
piedmont North Carolina. This feature of bio-retention
grows more important as concerns about
water supplies increase.
The main reason for pollutant load reduction is that
runoff entering the bioretention cell partitions into
outflow drainage, exfiltration, evapotranspiration, and
high-flow bypass (during large storms). During most
storm events, only the outflow drainage directly enters
the storm sewer network. All six cells studied exhib-ited
substantial reductions in outflow volume, ranging
from 33 percent to well over 50 percent. Even if the
inflow and outflow pollutant concentrations were the
same, load removal would still occur. Outflow vol-ume
reduction is a very important part of bioretention
function. Without outflow reduction, most bioreten-tion
systems would actually increase some pollutant
loads.
POLLUTANT-SPECIFIC BIORETENTION DESIGN
In current bioretention design standards, one general
guideline is used to locate, size, and design bioreten-tion
cells. This design guideline gives no regard to
target pollutants. However, the research discussed
earlier, in addition to studies conducted at the Uni-versity
of Maryland, Pennsylvania State University,
and NC State University, allows more refined design
guidelines to be developed. These pollutant-specific
guidelines are summarized in Table 2.
TOTAL SUSPENDED SOLIDS (TSS). The trapping mecha-nism
for most TSS is sedimentation. This occurs in
the bioretention cell’s depression storage volume,
which temporarily stores runoff. Some fine suspended
particles are removed by filtration through the very
top portion of the media and mulch layer. No specific
fill-soil depth is required because nearly all TSS re-moval
occurs before water infiltrates the cell. Higher
infiltration rates (exceeding 2 inches per hour) for the
fill media work best. When located in drainage areas
with high TSS loads, however, a maintenance issue
will arise, as is discussed later.
METALS. A study conducted by researchers at the Uni-versity
of Maryland showed that more than 95 percent
of metal removal occurred in the top 8 inches (20 cm)
of bioretention fill soil. Metal accumulation rates in
Maryland and North Carolina are not high enough
to retard plant growth or pose a disposal problem in
most applications. Fill-soil depth in bioretention cells
does not need to exceed 18 inches to effectively re-move
metals from stormwater runoff. The infiltration
rate of the media can vary. It is best that the cell’s top
layer remain unsaturated, so infiltration rates exceed-ing
2 inches per hour may be most appropriate.
PATHOGENS/BACTERIA. While limited data exist for bac-teria
removal by bioretention systems, most scientists
and engineers agree that bacteria die-off occurs at the
surface where stormwater is exposed to sunlight and
the soil can dry out. While no minimum soil depth is
required to remove pathogens, it is best for these bio-retention
cells to not be densely vegetated. Minimal
plant coverage allows for greater exposure to sunlight
and consequent die-off of bacteria.
TEMPERATURE. Increased temperature is a form of pollu-tion
important to western North Carolina’s trout fish-eries,
but very little information has been collected on
bioretention’s ability to reduce outflow temperature.
Some data were collected in the Greensboro study in
2003 showing that the two bioretention cells reduced
temperature by 5 to 10oF. It is recognized that deeper
soil media and ample shade can reduce the tempera-ture
of effluent. Whether this means bioretention cells
should contain fill-soil depths of 2 feet or 4 feet, for
example, has yet to be determined. An IWS volume
at the bottom of the fill media, where it is cooler, may
reduce temperature as well.
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TOTAL NITROGEN (TN). Research conducted at Penn
State University found that nitrogen removal can be
improved by retaining water in the bioretention cell
for a longer period. Soil media infiltration rates of 1
inch per hour are preferable to higher rates. Tests ex-amining
the effectiveness of introducing an IWS zone
(Figure 1) have not yielded any statistically significant
results; however, it does appear that the introduction
of the IWS layer may reduce the outflow concentra-tion
of NO3-N and, consequently, TN. A minimum
fill-media depth of 30 inches is recommended for TN
removal; 36 inches are preferred.
TOTAL PHOSPHORUS (TP). Lower P-Index soils reduce
phosphorus loads leaving the bioretention cell. If
phosphorus is a target pollutant, it is imperative that
the fill soil be tested to verify it has a relatively low
P-Index, ranging between 10 and 30. P-Indices lower
than 10 either retard or do not support plant growth.
Infiltration rates greater than 1 inch per hour are likely
the best for effective TP removal. As with metals, it
is important that the zone where phosphorus is col-lected,
the surface layer, does not become saturated,
which would cause some of the trapped phosphorus to
go into solution and leave the bioretention cell. If an
IWS layer is used for TN removal, it is important to
keep the “top” of this zone at least 18 inches from the
surface of the bioretention cell. A minimum fill-soil
depth of 24 inches is recommended.
TABLE 2. BIORETENTION DESIGN GUIDELINES FOR SPECIFIC POLLUTANTS.
Target Pollutant
Minimum Fill
Media Depth
Target Infiltration Rate Other Design Guidance
TSS
No minimum fill
depth required
Any rate is sufficient. 2 to 6
inches per hour recommended
If high TSS influent, fre-quent
maintenance required.
Pathogens
No minimum fill
depth required
Any rate is sufficient. 2 to 6
inches per hour recommended
Limiting plant coverage
allows more direct sunlight
to kill pathogens.
Metals 18 inches
Any rate is sufficient. 2 to 6
inches per hour recommended
Must keep top layer of cell
from being saturated for
extended periods.
Temperature
To be determined.
Conservatively, at
least 36 inches
To be determined. Slower
rates may be preferable
(less than 2 inches per hour)
Introduction of IWS volume
at the bottom of the cell may
reduce effluent temperature.
Total Nitrogen (TN)
At least 30 inches
(36 inches preferred)
1-2 inches per hour.
Slower rates are better.
Introduction of IWS
volume may reduce
TN concentrations.
Total Phosphorus (TP) 24 inches 2 inches per hour
A low P-Index is essential.
Recommended range is
from 10 to 30.
SPECIFYING FILL-SOIL MEDIA
Fill-soil selection is a crucial component of bioreten-tion
design, particularly in the tighter clay soil regions
of North Carolina’s piedmont, because fill media:
• Provide adequate drainage.
• Reduce pollutant levels.
• Support plant growth.
The following “recipe” for a bioretention soil
media, or fill-soil mix, works best:
• 85 to 88 percent sand. A washed, medium sand is
sufficient. A USGA greens mix is not necessary
and can be costly.
• 8 to 12 percent fines. Fines include both clay and silt.
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• 3 to 5 percent organic matter. Studies in Maryland
have shown newspaper mulch to be an ideal source
of organics. In North Carolina, peat moss has been
successfully used.
When mixing soil components to create the engi-neered
media, it is essential that the components be
well mixed and consistent.
If the fill mix is designed to capture a specific pollut-ant,
the percentage of fines may change, depending on
what the target pollutant is. When nitrogen removal
is the goal, for instance, an infiltration rate of 1 inch
per hour is needed. Incorporating a higher percentage
of fine soil particles will reduce the infiltration rate.
Roughly 12 percent of the fill soil should be made up
of fines to achieve the 1-inch-per-hour rate that is best
for removing nitrogen. To remove phosphorus, met-als,
and other pollutants, a 2-inch-per-hour infiltration
rate is recommended, and the fines mixture should be
approximately 8 percent. Organic matter, the lesser
ingredient in the fill-mix “recipe,” will not change
volume based on target pollutant, and its portion
should remain the same—3 to 5 percent.
Organics are included to “kick-start” nitrogen removal
and plant growth while the bioretention cell matures.
If the original organic matter is depleted by microbial
activity, the bioretention system is expected to provide
some organic content to the fill through mulch decom-position,
grass clippings, and root infiltration.
To support plant growth while removing phosphorus
from runoff, the fill soil must have a P-Index between
10 and 30. If the bioretention area is not designed
to reduce phosphorus in runoff, a P-Index for the
fill soil of 25 to 40 is recommended. In addition to
having a low P-Index, it is best for fill media to have
a relatively high cation exchange capacity (CEC).
Higher CECs describe soils that have a greater ability
to capture and retain phosphorus. Some “designer”
soils with low P-Indices and higher infiltration rates
have been tested and found to have CECs exceeding
20. While a minimum CEC has yet to be established,
CECs exceeding 10 are expected to work relatively
well at removing target pollutants in bioretention
systems.
The types of vegetation expected to grow in the bio-retention
cell also affect the depth of media selected.
Grassed covers do not need more than 15 to 18 inches
of media to survive, while certain small trees speci-fied
to grow in bioretention require a minimum of 36
inches. Most bioretention shrubs can survive and even
flourish with a minimum of 24 inches of fill media.
SELECTING VEGETATION
A detailed list of plant species is provided in Design-ing
Rain Gardens (Bio-Retention Areas) and in other
documents. (Refer to www.bae.ncsu.edu/topic/rain-garden
for more information on plant choices.)
Most vegetation does not survive when planted in the
middle, or deepest part, of the bioretention cell. Most
shrubs and trees suggested in bioretention planting
guides grow much better if they are located along the
edge of the cell. Certain tree species, such as River
Birch (Betula nigra), do well in the “bottoms” of
cells, but they must be staked during the first year of
growth, so that the roots can establish themselves. Be-ing
toppled by wind is always an issue in hurricane-prone
North Carolina (Figure 2).
FIGURE 2. While River Birch (Betula nigra) typically grows
well in bioretention cells, saplings need to be staked during
the first year to keep them from being blown over in hurri-cane-
prone North Carolina.
A horticulturalist should be consulted for plant se-lection.
Bioretention vegetation can be classified as
either “dry,” “average,” or “wet.” Most species cannot
tolerate all three conditions. A few species observed
to tolerate a wide degree of wetness include Virginia
Sweetspire (Itea virginica), Inkberry (Ilex glabra),
River Birch (Betula nigra), and Red Maple (Acer
rubrum).
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The vegetation of choice for some bioretention cells
is grass. Varieties of centipede, Bermuda, and zoysia
can survive in cells that are well drained. Bioreten-tion
cells with higher percentages of fines tend to be
wetter, making it difficult for these grasses to grow.
Because of this, grass-only bioretention cells are not
currently recommended for TN removal.
PROTECTING YOUR INVESTMENT
Bioretention is a fragile stormwater practice. Across
the state, thousands of dollars have been wasted be-cause
measures were not taken to prevent bioretention
cells from becoming clogged with sediment dur-ing
construction. Much of this cost could have been
avoided if simple precautions had been taken.
BIORETENTION PLACEMENT. Avoid locating bioretention
cells near disturbed areas. Excessive sedimentation
ruins bioretention. During construction of bioreten-tion
cells, take protective measures, such as lining the
perimeter of the cell with either straw bales or a silt
fence.
Construction phasing of a bioretention cell is critical
and must be well planned and executed. Sometimes
timing the construction of the bioretention cell may
be complicated. The principal excavation of the cell
may occur any time during the construction process.
Often, sediment traps or basins are transformed into
bioretention cells (Figure 3). This is an excellent use,
provided the sediment trap is excavated prior to its
conversion to a bioretention cell. If the bioretention
facility is constructed as a median in a parking lot,
it is best to wait until the parking lot’s base gravel
course is placed before installing the underdrains,
gravel layer, or fill media of the bioretention cell. Ide-ally,
the initial asphalt layer is placed before bioreten-tion
construction (postexcavation) starts. Once the fill
soils are brought on site, paving of the parking lot can
be completed. When the parking lot and surrounding
landscape are stable, vegetation can be planted and
mulch can be spread.
Be wary of out-parcel development (future develop-ment
occurring upslope). Even if the bioretention cell
immediately treats a stable parking lot, subsequently
developed out-parcels, such as a bank or fast-food
establishment constructed after the main portion of
the shopping center is built, can add sediment to the
bioretention cell, causing it to clog (Figure 4).
WHEN TO USE PERMEABLE GEOFABRIC. Recently, designers
have debated whether to use a permeable filter fabric
between the gravel layer and the overlying fill soil. If
the designer has any concern regarding the stability of
the site during construction or if out-parcels may be
developed at a later time, filter fabric should be avoid-ed.
In lieu of the permeable fabric, a thin layer (nomi-nally
two inches) of choking stone (such as #8 stone)
can be incorporated between the gravel drainage layer
(typically a washed 57 stone) and a thin, 2- to 4-inch
FIGURE 3. This sedimentation basin was converted to a bio-retention
cell at the end of construction. The concrete outlet
was later utilized as the high-flow bypass. The slow draw-down
skimmer was attached to the concrete structure in the
same location as the bioretention cell’s underdrains.
FIGURE 4. An out-parcel was not stabilized (foreground),
leading to the clogging of the bioretention cell pictured next
to the pickup truck. The trail of sediment from the out-parcel
to the bioretention cell is clearly visible, as is the sediment
eyebrow around the bioretention cell.
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layer of pure sand. The fill media is placed on top of
the pure sand layer (Figure 5). If the drainage area
where the bioretention cell is to be installed is stable,
which is often the case when bioretention is retrofit-ted,
using filter fabric to separate the gravel drainage
layer from the fill soil is acceptable. Filter fabric does
prevent the migration of finer soil particles through
underlying gravel. A choking stone can also prevent
this migration if designed correctly. To verify whether
a choking material will keep overlying soil in place,
the following equations (from the Federal Highway
Administration) can be utilized:
D15 open-graded base ÷ D50 choke stone < 5
and
D50 open-graded base ÷ D50 choke stone > 2
DX is the particle size at which X percent of particles
are finer. For example, D15 is the diameter at which
85 percent of the gravel particles are coarser and 15
percent of the materials are finer. This information is
provided by the quarry supplying the material. If both
equations are satisfied, the choking material will not
migrate.
tion helps disperse flow entering the bioretention area.
The gravel strip should be the width of a garden rake
(approximately 8 inches). Installing sod downslope
of the verge provides an immediate layer of pretreat-ment
before runoff enters the bioretention cell proper.
The sod serves as a grassed filter strip (Figure 6).
The minimum width required for the sod filter strip
is three feet, with four to five feet recommended. In
addition to trapping pollutants before they reach the
bioretention cell, the sod immediately stabilizes the
perimeter of the bioretention cell, preventing “inter-nal”
erosion from occurring. Centipedegrass has been
successfully used as a sod verge in central and eastern
North Carolina. Fescue and bluegrass are best suited
for western North Carolina.
FIGURE 6. Installing a gravel verge and sod stabilizes the
perimeter of bioretention cells and is highly recommended.
The resulting filter strip shown here is 5 feet wide.
Fill Soil Media:
85 – 88% Washed Sand
8 – 12% Fines (Silt + Clay)
3 – 5% Organic Matter
Washed Sand 2 to 4 inches
Choking Stone (typically #8
or #89 washed)
2 inches
Washed #57 stone or similar,
and underdrain pipe.
6 to 8 inches
In-situ soil
FIGURE 5. Schematic of bioretention bottom layers construct-ed
without filter fabric.
PRETREATMENT. To prevent premature clogging of bio-retention
cells, designers are strongly recommended
to specify pretreatment devices. The most commonly
used in North Carolina, in descending order, are: (1)
gravel verge (thin strip) with sod surrounding the
perimeter, (2) grass swale, and (3) forebays. A level
gravel verge between the pavement edge and vegeta-
A simple grassed swale is another pretreatment op-tion.
A minimum length is not specified, but most sus-pended
sediment has been observed to fall out in the
first 10 to 15 feet of the swale (Figure 7). The exact
minimum length depends on drainage area size and
composition and the swale’s slope, width, and cover.
Occasionally, large bioretention areas incorporate a
forebay for pretreatment. The forebay should be sized
so that it stills runoff water entering the bioreten-tion
cell, allowing some sediment to settle. Forebay
depth ranges between 18 and 30 inches. Bioretention
applications utilizing forebays are limited to loca-tions
where standing water is not considered a hazard
and there is not enough room to incorporate either a
sod/gravel verge or a grassed swale. Forebays must
be hydraulically isolated from the underdrains so that
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runoff does not short-circuit the bioretention media.
Forebays can be lined to prevent direct flow into the
underdrains.
BIORETENTION MAINTENANCE. To preserve bioretention
performance, the cells must be maintained. Like any
landscape feature, bioretention areas must be pruned,
mulched, and even initially watered and limed
(Figure 8.) Grassed bioretention cells are usually
mowed.
Because plants are an important monetary investment
and essential to the aesthetic appeal of bioretention
systems, they need to be established as quickly as
possible. The need for rapid establishment requires
bioretention cells to be limed, if indicated by a soil
test. Additionally, plants may need to be spot-fertil-ized
to ensure growth and survival in low P soils.
Watering the plants every 2 to 3 days for a month or
two helps ensure vegetation survival. The frequency
of these tasks varies seasonally, with more frequent
maintenance required in summer than in winter.
Maintenance tasks unique to bioretention include
occasional removal of mulch and the top layer of fill
soil. Because clogging occurs most frequently at the
top of the soil column, the bioretention basin rarely
needs to be completely excavated. However, this has
been necessary when the bioretention cell was located
in an unstable drainage area. Table 3 lists some bio-retention
maintenance tasks and the frequencies with
which they should be conducted.
FIGURE 7. Two
other methods of
pretreatment: (a)
a grassed swale
leading from the
roadway to the
bioretention cell;
and (b) a biore-tention
forebay
constructed
adjacent to a
parking lot.
a
b
TABLE 3. BIORETENTION MAINTENANCE TASKS.
Task Frequency Maintenance Notes
Pruning 1 – 2 times / year
Nutrients in runoff often cause
bioretention vegetation to flourish.
Mowing 2 – 12 times / year
Frequency depends upon location
and desired aesthetic appeal.
Mulching 1 – 2 times / year
Mulch removal 1 time / 2 – 3 years
Mulch accumulation reduces available water
storage volume. Removal of mulch also increases
surface infiltration rate of fill soil.
Watering
1 time / 2 – 3 days for first
1 – 2 months. Sporadically
after establishment
If droughty, watering after the
initial year may be required.
Fertilization 1 time initially One time spot fertilization for “first year” vegetation.
Remove and replace
dead plants
1 time / year
Within the first year, 10 percent of plants may die.
Survival rates increase with time.
Miscellaneous
upkeep
12 times / year
Tasks include trash collection, spot weeding,
and removing mulch from overflow device.
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OTHER RESOURCES
Designing Rain Gardens (Bio-Retention Areas). 2001. W. F. Hunt and N. M. White. AG-588-3.
North Carolina Cooperative Extension Service. Raleigh, N.C.
For information on: general design guidelines for bioretention areas, how bioretention works,
plant selection, construction cost estimates.
Available at: http://www.bae.ncsu.edu/stormwater/PublicationFiles/DesigningRainGardens2001.pdf
Urban Stormwater Structural Best Management Practices (BMPs). 1999. W. F. Hunt. AG-588-1.
North Carolina Cooperative Extension Service. Raleigh, N.C.
For information on: overview on grass swale and vegetated filter strip design.
Available at: http://www.bae.ncsu.edu/stormwater/PublicationFiles/UrbanBMPs1999.pdf
NCSU Backyard Rain Garden Web Page
http://www.bae.ncsu.edu/topic/raingarden
For information on: bioretention vegetation selection for North Carolina, images of bioretention
cells/rain gardens from across N.C. and surrounding states.
NCSU BAE Stormwater Web Page
http://www.bae.ncsu.edu/stormwater
For information on: clearinghouse of bioretention and other BMP information including fact sheets,
reports, images for download, upcoming design workshops, and design specifications.
State of North Carolina Stormwater BMP Manual
http://h2o.enr.state.nc.us/su/stormwater.htm
For information on: detailed stormwater practice design guidelines, including bioretention, grass
swales, and vegetated filter strips.
State of North Carolina Stormwater Page
http://www.ncstormwater.org
For information on: stormwater issues, technical and nontechnical, from across North Carolina,
including related news, upcoming workshops, and educational public service announcements.
FIGURE 8. Various
bioretention maintenance
activities: (a) annual to
semi-annual pruning, (b)
an initial lime application
to ensure plant survival,
(c) and (d) removal of
biological films, which
cause the bioretention cell
to clog and may be needed
every 2 to 3 years.
a b
a
c d
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Prepared by
William F. Hunt, Extension Specialist and Assistant Professor of Biological Engineering,
North Carolina State University
and
William G. Lord, Area Environmental Agent, Franklin County,
North Carolina Cooperative Extension
Published by
NORTH CAROLINA COOPERATIVE EXTENSION SERVICE
1/06—JL/DB AGW-588-05
E06-44609