2. BAE 575 STORMWATER WETLAND DESIGN
PAIGE ERICKSON & TAYLOR CARTER
Objective
The objective of this assignment was to design a stormwater wetland that treated runoff
from a small watershed located off West Woodcroft Parkway in Durham, NC.
Site/Drainage Area
This stormwater wetland is designed for a site located at Hope Valley Road and West
Woodcroft Parkway in Durham, NC. The wetland will be located at the end of a concrete
channel. The existing soils consist of Chewacia and Wehadkee soils with 0 to 2 percent slopes
and are categorized as hydrologic soil group of B/D (Web Soil survey).
The drainage area of the inlet which is the end of the concrete channel was determined to
be 3.45 acres. The watershed was delineated using 2 ft. contours and the flow path feature in
AutoCAD Civil 3D. The Rational Method was used to determine the peak flow of a 1.1 year
design storm. The land uses and the corresponding rational coefficients are shown in Table 1.
Table 1: Land uses and rational coefficients
The time of concentration was estimated with the Kirpich Method as described in equation 1.
The flow path length was determined using the water drop feature in AutoCAD. The change in
elevation was determined by examining 2 ft contours. The time of concentration was estimated
as 4.86 minutes. Therefore, the 5 minute precipitation intensity for 1 year storm with 5 minute
duration was used in the Rational Method which had a value of 4.86 in/hr. (NOAA Precipitation
Frequency Data Server).
Land%Use Area%(ft2
) Area%(acres) C C*Area%(ac)
Roofs 5030 0.12 0.9 0.10
Concrete 16479 0.38 0.95 0.36
Drives9and9Walk 7588 0.17 0.95 0.17
Asphalt 61142 1.40 0.95 1.33
Lawn 60029 1.38 0.15 0.21
Total 150268 3.45 0.63
**average9C9values9obtained9from9Alessa's9lecture9slides
3. T! = 0.0078 L!
H!! !.!"#
[1]
The peak flow determined from the Rational Method as seen in equation 2 was 10.54 cfs.
Q = C!A [2]
Determining the volume of water treated by the Wetland
The volume of water the wetland should treat was determined using the Simple Method
as specified by the NC Stormwater BMP Manual. According to the NC Stormwater BMP
Manual, a stormwater wetland should treat the first flush of a storm which is 1 in. to 1.5 in for
the state of NC. A value of 1 in is used for inland areas while the coast of NC should use a value
of 1.5 in. Therefore, a value of 1 in was used for the design storm depth (RD) in the simple
method. The fraction of impervious area (IA) in the drainage area was 0.590. Equations 3 and 4
were used in determine that the stormwater wetland should treat a volume of 7394 cubic feet.
This is greater than the minimum required volume of 3630 cubic feet specified in the NC
Stormwater BMP Manual.
R! = 0.05 + 0.9×I! [3]
where,
I! = fraction!of!impervious!area, unitless
R! = runoff!coefficient, unitless
V = 3630×R!×R!×A [4]
where,
V = volume!of!stormwater!runoff!treated!(cf)
A = drainage!area, acres
R! = design!storm!depth, in
4. Determining the surface area of the stormwater wetland and its components
The surface area of the stormwater wetland was determined by dividing the volume of
runoff calculated by the Simple Method and the “depth of plants” which is the depth of the
shallow land. The depth chosen for the shallow land was 1 ft. which is within the range of 6
inches to 1 ft. specified by the NC Stormwater BMP Manual. The required surface area of the
stormwater wetland was 7394 cubic feet. The surface area of the stormwater wetland designed
was 7459 cubic feet; therefore, all of the water quality volume from a 1 inch storm will be
treated by the stormwater wetland. The length to width ratio of this stormwater wetland is 2.72:1
which is above the minimum length to width ratio of 1.5:1 specified by the NC Stormwater BMP
Manual.
The stormwater wetland designed in this project was made up of a forebay, other deep
pools, shallow land, shallow water, and an upland area for easier maintenance access. According
to the NC Stormwater BMP Manual, the forebay should account for 10% of the surface area.
However, a study at North Carolina State University determined that the forebay could account
for up to 15% of the stormwater wetland’s surface area (Johnson, 2007). The other deep pools
should account for 5% to 10% of the stormwater wetland’s surface area. The shallow water/low
marsh area should account for about 40% of the stormwater wetland’s surface area and the
shallow land/high marsh area should account for 30-40% of the wetland’s land area. (NC
Stormwater BMP Manual, NCDENR). The values chosen for the surface area of this stormwater
wetland are shown in Table 2.
Table 2: Stormwater wetland surface area
Component Min SA (ft2
) Max SA (ft2
) SA Chosen
(ft2
)
Acceptable
SA?
Forebay 745.9 753.4 750.5 YES
Non-Forebay
Deep Pools
373.0 745.9 616 YES
Shallow Water
(low marsh)
2983.6 3058.2 3001 YES
Shallow Land
(high marsh)
2237.7 2983.6 2773.5 YES
Upland
(maintenance
access)
0.0 1053.9 318 YES
5. Determining the depth of each stormwater wetland component
Then the depth of each component in the stormwater wetland was determined. First, the
soil water balance described in equation 5 was used to determine the minimum depth of the deep
pools. (BAE 575 Stormwater Wetland class notes).
DP = (RF!×EF×WS/WS) − ET − INF − RES [5]
where,
DP = depth!of!deep!pools!(in)
RF! = Monthly!rainfall!during!a!drought, in
EF = Fraction!of!rainfall!entering!wetland!from!watershed
WS/WL = ratio!of!watershed!area!to!wetland!surface!area
ET = monthly!evapotranspiration!(in)
INF = monthly!infiltration!loss, (in)
RES = Reservoir!of!water! safety!factor , in
A value of 1 in was chosen for the monthly rainfall during a drought per the class notes. The
fraction of rainfall entering the wetland from the watershed was 0.20 which is in the acceptable
range of 0.20 to 0.25 specified by the NC Stormwater BMP manual. The infiltration loss for a
drought month was determined to be 7.4 inches by assuming an infiltration rate of 0.01 in/hr
(infiltration rate of a liner). The monthly ET was determined to be 7.2 inches by summing daily
reference evapotranspiration estimated with the Penman-Monteith for July 2008 (a drought
month) and multiplying by a crop coefficient of 1.05 (Allen et al., 2006). Daily reference
evapotranspiration was acquired from the North Carolina State Climate Office and the crop
coefficient was found on the FAO website (Table 3). A reservoir value of 6 inches was used as a
safety factor per the class notes.
6. Table 3: Crop coefficients from the FAO website (Allen et al.2006)
The soil water balance yielded a value of -16.60 inches meaning that the deep pools should have
a depth of at least 18 inches. This is within the range of acceptable depths (1.5 ft to 3 ft)
specified by the NC Stormwater BMP Manual. There are 2 small deep pools in addition to the
forebay and outlet deep pool that are scattered around the wetland to provide refuge for mosquito
predators.
According to the Web Soil Survey, the water table range from 6 inches to 24 inches for
this soil. An impermeable liner with an infiltration rate less than 0.01 in/hr should be installed in
the deep pools to prevent excess seepage. (NC Stormwater BMP Manual, NCDENR). A liner is
not needed in other parts of the wetland because the seasonal high water table (SHWT) is
probably within 6 inches of the PPE. (NC Stormwater BMP Manual, NCDENR). However, if
this design was to be implemented, the water table would need to be confirmed. A 4 inch layer
of top soil should be added to lined and unlined portions of the wetland. A soil analysis would
also be acquired to assess the need for top soil amended with organic material.
7. A transition zone with a slope of no more than 1.5:1 was placed between the deep pool
and shallow water portion of the wetland. The transition zone had a depth of 6 inches which is in
the acceptable range of 6 in to 18 inches specified by the NC Stormwater BMP Manual.
The shallow land (high marsh) was assigned a depth of 1 ft which is in the acceptable
range of 6 inches to 12 inches specified by the NC Stormwater BMP Manual. The shallow water
(low marsh) was assigned a depth of 6 inches which is in the acceptable range of 3 inches to 12
inches specified by the NC Stormwater BMP Manual. The upland area was less than 4 ft above
the shallow land zone as specified by the NC Stormwater BMP Manual.
Outlet
The outlet consisted of a plugged emergency drawdown orifice, an orifice that
maintained the normal pooling elevation (NPE) and detained water from a 1-inch storm 48 to 72
hours, and a weir.
The diameter of the orifice was determined using the orifice equation as described in
equation 6. The driving head was 12 inches, which is the distance from the orifice to the crest of
the weir. The coefficient of discharge was assumed to be 0.6.
Q = C!×A× 2×!×H [6]
where,
Q = discharge, cfs
C! = coefficient!of!discharge, 0.6
A = area!of!orifice, ft!
! = gravitational!acceleration, 32.2!ft!s!!
H = driving!head, ft
The orifice was designed in a way that the stormwater wetland could detain water for 2 to
3 days. A stormwater wetland should not detain water for more than the median length between
storms which is 3 days in Raleigh-Durham, NC (Hunt and Doll, 2000). The orifice was turned
down to prevent clogging.
8. The drawdown was determined by first computing orifice discharge with a driving head
that was 1/3 of the distance between the orifice and the crest of the weir (4 in) as specified in the
class notes. The wetland volume was then divided by the discharge. The drawdown time for the
1-inch storm in this design was computed as 60.7 hours, which is within the recommended range
of 48 to 72 hours.
A weir was designed for a 1.1 year design storm, which was computed to have an outflow
of 10.5 cfs. The weir equation as seen in equation 7 was used to ensure that dimensions were
determined that resulted in a weir that could discharge at least 10.5 cfs. A value of 3.0 was used
for the weir coefficient. The dimensions calculated to handle a 1.1 year storm included a weir
length of 3.5 ft and a 1 ft height above the crest of the weir.
Q = C!×L×H!.!
[7]
where,
C! = crest!coefficient
L = lenght!of!weir, ft
H = height!of!water!above!crest!of!weir, ft
Plant Selection
Table 4: Plant plan with species name, quantity, location and image.
Species Name Quantity/Percent Usage Location Image
Spatterdock
(Nuphar lutea)
40%
Deep
Pool
Fragrant Water
Lily (Nymphaea
odorata)
60%
Deep
Pool
10. Sedge (Carex spp) 25%
Shallow
Land
Common rush
(Juncas spp)
25%
Shallow
Land
Joe-pye weed
(Eupatorium
purpureum)
100% Upland
Plant choice was based on selection variety provided in the NCDENR Handbook and
regional variety outlined in the Urban Waterways article Stormwater Wetland Design Update:
Zones, Vegetation, Soil, and Outlet Guidance. Twelve plant species were chosen for the design
to create biodiversity. None of the plant species chosen included invasive species such as cattails
or willows. Cattails (Typha spp.) tend to overtake the aquatic community and create shelter for
mosquitoes. Spatterdock (Nuphar lutea) and Fragrant Water Lily (Nymphaea odorata) should
only be planted in the transition zone of the deep pools for best survival. The deep pool and
forebay component estimates presented in table 4 are calculated with a 36 inch (nine square feet)
planting density to account for not planting within the deep pool centers.
11. Table 4: Number of each plant broken up by wetland component and species.
Component
SA Chosen
(ft2
)
Planting Units
(4 or 9 ft2
)
Types of
Plants
Number of
Each Plant
Type
Forebay 750.5 83 2 41
Non-Forebay
Deep Pools
616 68 2 34
Shallow
Water (low
marsh)
3001 750 5 150
Shallow
Land (high
marsh)
2773.5 693 4 173
Upland
(maintenance
access)
318 79 1 79
The recommended planting density is one plant per four square feet or one plant per nine
square feet for herbaceous vegetation (Hunt, et al. 2007). Proper site preparation and planting
will help with constructing a successful wetland. Preventing seepage can be prevented during
construction with the addition of clay in the laying on top of a tamped down surface layer (Hunt,
et al. 2007). This type of soil preparation is hard on plant establishment, so it is recommended
that three to six inches of topsoil is added to refresh the organic matter content and promote plant
growth. Therefore, 4 inches of top soil will be spread on the wetland after construction. Plant
location within zones is approximated in attached appendix. Planting pattern should vary, but
only within the permitted zones outlined in this report.
Routing
Inflow Hydrograph
Malcom’s Method was used to construct an inflow hydrograph for a 1.1 year storm. This
method was used because it is preferred by NCDENR and works well for small watersheds. The
peak runoff for the 1.1 year storm was calculated using the Rational Method as described above.
The volume of runoff was computed with the Simple Method which is described above. The
time to peak was computed as 8.48 minutes using equation 8.
12. T! = V× 1.39×Q!
!!
[8]
where,
T! = time!to!peak, sec
V = runoff!volume!from!the!Simple!Method, cf
Q! = Peak!discharge, cfs
A Microsoft Excel spreadsheet was then used to graph discharge against time with equation 9.
The inflow hydrograph is shown along with the outflow hydrograph in Figure 2.
Q =
0.5Q! 1 − cos !"/T!! 0 ≤ ! ≤ 1.25T!
4.34Q!exp!(−1.3!/T!) ! > 1.25T!
[9]
Stage-Storage
A stage-storage relationship was then established to by relating the stage to the
cumulative storage. This was done by using contour areas and elevations. The incremental area
was determined with contour areas and elevations with equation 10.
V! = 0.5 A!!! + A! ∆E [10]
where,
V! = incremental!volume, cf
A = area, ft!
∆E = change!in!elevation, ft
The cumulative storage was determined by adding the incremental volume to the storage
of the previous elevation. Stage was determined by subtracting the contour elevation from the
base elevation which was the bottom of the wetland. The stage-storage relationship constructed
from this analysis is shown below in Figure 1.
13. Figure 1: Stage-Storage relationship
Outflow Hydrograph
A hydrograph was then constructed using the power curve from the stage-storage
relationship shown in Figure 1. The outflow discharge was calculated using the unsubmerged
orifice equation when the stage was below the normal pool elevation (NPE), the submerged
orifice equation when the stage was above the orifice, and the sum of orifice and weir discharge
when the stage was above the temporary pool elevation.
The hydrograph displaying the inflow and outflow discharge as a function of time is
shown below in Figure 2. The peak outflow is lower than the peak inflow; therefore, it can be
concluded that this stormwater wetland provides peak flow mitigation for a 1.1 year storm event.
14. Figure 2: Stormwater wetland hydrograph
Maintenance and Safety
Mosquito prevention was a consideration in this design. There are no cattails or black
willow trees specified in this stormwater wetland design. Maintenance should include regular
control of unwanted trees with an herbicide applicator. Cattails will be controlled by stroking
cattail leaves with hand-dipped aquatic glyphosphate (Rodeo). The herbicide should be applied
by someone who is licensed in North Carolina.
Maintenance activities for the stormwater will be in accordance with Stormwater Wetland
and Wet Pond Maintenance (AGW-588-7)). The drawdown orifice will be kept clean and free-
flowing by checking the orifice for clogging on a monthly basis. Debris and trash should be
removed from the stormwater wetland. In addition, sediment levels in the forebay should be
measured on an annual basis. Sediment and gross solids should be removed from the forebay
when it is half full of sediment. This can be done with a back hoe or track hoe. A study
conducted at North Carolina State University indicates that sediment removal of the forebay
should occur every 5 to 10 years. The emergency drawdown orifice can be unplugged to lower
the water level of the stormwater wetland to aid with maintenance. The deep pool of the outlet
should also have sediment and gross solids removed when these materials are within one vertical
foot of the drawdown hole to ensure that the outlet continues to function properly. (Hunt and
Lord, 2006).
A fence should also be put around the perimeter of the wetland to prevent children from
entering it. This measure will minimize the risk of drowning.
15. Sand Removal
The sediment removal was computed by using Stoke’s Law (equation 11) to determine
the settling velocity of fine sand and course sand then using the average depth of the wetland to
calculate the residence time necessary to capture the particles. The residence time necessary to
capture the particles was compared to the residence time of the wetland, which was determined
to be 60.7 hours. The calculation for determining the residence time of the wetland is described
in an earlier section of this report.
Stoke’s Law was computed with the assumption that water was 20ᵒC and the sand
particles are quartz. Therefore, a particle density of 2650 kg m-3
was used as that is the density
of quartz. The particle size of coarse sand was assumed to be 0.42 mm and the particle size of
fine sand was assumed to be 0.149 mm. These assumptions were made based on values
presented in the class notes.
!! = !!!
ρ! − ρ! 18! !!
[11]
where,
! = acceleration!of!gravity!(9.81!m!s!!
)
! = particle!diameter!(m)
ρ! = particle!density!(2650!kg!m!!
)
ρ! = density!of!water!(1000!kg!m!!
)
! = dynamic!viscosity!of!water!at!20℃, (0.0010!Pa − s)
The settling velocity of coarse sand as calculated with Stoke’s Law was 0.16 m/s and the settling
velocity of the fine sand as computed with Stoke’s Law was determined to be 0.02 m/s. The
hydraulic residence time was determined by dividing the average depth of the wetland by the
settling velocity. The average depth of the wetland was assumed to be 6 inches (0.1524 m).
Therefore, the hydraulic residence time necessary to remove coarse sand is 1.0 seconds and the
hydraulic residence time necessary to remove fine sand is 7.6 seconds. The hydraulic residence
time of the wetland (60.7 hours) exceeds the hydraulic residence time necessary to remove
coarse and fine sands; therefore, it can be concluded that this stormwater wetland will remove
fine sands and coarse sands.
16. References:
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (2006). FAO Irrigation and Drainage Paper
No. 56, Crop Evapotranspiration. Retrieved February 24, 2015, from:
http://www.kimberly.uidaho.edu/water/fao56/fao56.pdf
Hunt, W., Burchell, M., Wright, J., & Bass, K. (2007, December). Stormwater Wetland Design
Update: Zones, Vegetation, Soil, and Outlet Guidance. NC Cooperative Extension.
Hunt, W., C. Apperson, and W. Lord. Mosquito Control for Stormwater Facilities. AG-588-04.
Online: http://www.bae.ncsu.edu/stormwater/ PublicationFiles/ Mosquitoes2005.pdf
Hunt, W., & Doll, B. (2000). Urban Waterways: Designing Stormwater Wetlands for Small
Watersheds. NC Cooperative Extension Service.
North Carolina Department of Environmental and Natural Resources. (2007, July). Chapter: 9.
Stormwater Wetlands.
Soil Survey Staff, Natural Resources Conservation Service, United States Department of
Agriculture. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov/.
Accessed [02/23/2015].
17.
18. BORDER OF WETLAND,
Area: 7459 sq. ft.,
Elevation: 98'
SHALLOW WATER ZONE,
Area of shallow water:3001 sq. ft.,
Elevation: 97'.
FOREBAY
Area: 750.5 sq. ft,
Elevation: 97'
Area: 442 sq. ft,
Elevation: 96'
Area: 187 sq. ft,
Elevation: 94'
UPLAND,
Area: 318 sq. ft,
Elevation: 98.25'
DEEP POOL BORDER
Area: 224 sq. ft.,
Elevation: 96.5'
SHALLOW LAND
Total Area: 2773.5 sq. ft.
TRANSITION ZONE
Slope: 1.5:1
Elevation: 96'
BOTTOM
DEEP POOL:
Area: 124 sq. ft.
Elevation: 95'
DEEP POOL BORDER
Area:184 sq. ft.
Elevation: 96.5'
TRANSITION ZONE:
Slope: 1.5:1
Elevation: 96'
BOTTOM OF
DEEP POOL:
Area: 96.5 sq. ft.
Elevation: 95'
DEEP POOL BORDER:
Area: 184 sq. ft.
Elevation: 96.5'
TRANSITION ZONE:
Slope: 1.5:1
Elevation:96'
BOTTOM
DEEP POOL:
Elevation: 95'
Area: 140 sq. ft.
OUTLET
19. NOTE: BOTTOM OF DEEP POOLS
SHALL BE LINED WITH CLAY HAVING
AN INFILTRATION RATE LESS THAN
0.01 IN/HR
I - DEEP POOLS WITH A DEPTH OF 18 IN (BOTTOM ELEVATION OF 95')
II - TRANSITION ZONE WITH A DEPTH OF 6 INCHES AND SIDE SLOPE OF 1.5:1 (ELEVATION OF 96')
III - SHALLOW WATER ZONE WITH A DEPTH OF 6 INCHES (ELEVATION OF 96.5')
IV- SHALLOW LAND ZONE WITH A DEPTH OF 1 FT, (ELEVATION OF 97')
V-UPLAND AREA (ELEVATION OF 98.25')
IVV III II I
FOREBAY
4 INCHES OF TOP SOIL
EXISTING
CONCRETE
CHANNEL
NPE = 2.5'
STAGE
TPE= 3.5'
STAGE
28. BIORETENTION IN UMSTEAD STATE PARK
PAIGE ERICKSON & TAYLOR CARTER
OBJECTIVE:
The objective of this assignment was to design a bioretention cell in the parking lot of
Umstead State Park’s Reedy Creek entrance. The bioretention cell was designed for the median
shown in Figure 1 and treated a drainage area of 0.75 acres with underlying soils classified as
HSG C. Water was conveyed to the bioretention cell by sheet flow from the parking lot and
from a grassed swale located in the first half of the parking median.
!
Figure 1: Existing parking lot median
29. DESIGN
Rational Method
The swale was designed for peak flow of a 1.1 year storm. The Rational Method was
used to determine the design flow the swale needed to convey. Table 1 shows the percentage of
pervious and impervious area in the drainage area with the corresponding rational coefficients.
The rational coefficients were obtained from Alessa’s lecture slides.
Table 1: Rational method
RATIONAL(METHOD((
Land(Use( Area((ft2
)( Area((acres)( C( C*Area((ac)(
Impervious! 23421.5! 0.54! 0.9! 0.48!
Pervious! 9157.6! 0.21! 0.15! 0.03!
Total( 32579.0( 0.75( !! 0.69!
The time of concentration was determined using the Kirpich Method as seen in equation
1 where L is the longest flow path and H is the change in elevation. The longest flow path was
determined to be 137 ft. using the Water Drop tool in AutoCAD Civil 3D. The change in
elevation was determined to be 1 ft. using the survey given. These values yielded a time of
concentration of 2.29 minutes; therefore, the intensity for a 1.1 year, 5 minute storm was used in
computing the design discharge. The intensity used was 4.73 in/hr. (NOAA Precipitation
Frequency Data Server).
T! = 0.0078 L!
H!! !.!"#
[1]
The discharge determined using equation 2 was determined to be 2.44 cfs for the 1.1 year
storm. The rational method was also used to determine the design discharge for the overflow
structure which used a 10-year, 24 hour design storm. The intensity of the 10 year, 24 hour
storm was 0.206 in/hr and the discharge was 0.1062 cfs.
Q = C!A [2]
30. Swale Design
The swale was designed in the first half of the parking median with a length of 151.33 ft
as seen in the plan view. A pipe then conveyed this water to a forebay that dissipated energy.
Water then entered the bioretention cell.
The swale was designed with a trapezoidal cross section covered with Bermuda grass
sod. Dimensions were determined by ensuring that Manning’s equation as shown in equation 3
yielded a discharge equal to 2.44 cfs. The slope of the swale shall be 1%. A freeboard of 6” was
added to the swale per the state of NC BMP Manual.
Q = 1.486!!!!!
!S!
!/!
!R!/!
!A [3]
where,
! = 13.6 + 6ln!(1.0127VR) !!
(Schwab et al. 1983)
S! = friction!slope!which!is!assumed!to!be!the!slope!of!the!swale!(0.01!ft!ft!!
)
R = !" + !!!
! + 2! !! + 1
!!
, ft
A = !" + !!!
, ft!
!
Figure 2: Trapezoidal dimensions for swale calculations
!
Shear stress was calculated with equation 4 and determined to be 0.6 lb ft-2
. This is less
than the maximum shear stress for Bermuda grass as seen in Table 2. The velocity calculated
with Manning’s equation was 0.7 ft s-1
which is less than the maximum velocity of 4 ft s-1
as seen
in Table 2.
!
τ = FS×γ×S!×! [4]
31. where,
! = shear!stress, lb!ft!!
FS = factor!of!safety!of!1.2
! = specific!weight, 62.4!lb!ft!!
S! = friction!slope!which!is!assumed!to!be!the!slope!of!the!swale!(0.01!ft!ft!!
)
! = depth!of!the!swale (10 in)
Table 2: Maximum shear stress and velocity
Forebay Design
The forebay will serve as a pretreatment for the concentrated flow being received from
the aforementioned swale, reducing the risk of the soil media becoming clogged with sediment.
Forebays need the ability to remove total suspended solids and other debris coming off the
parking lot. In order to dissipate energy from the concentrated flow from the upper swale, the
entry zone needs to be approximately 2.8 feet or 3 feet for easier construction ability. This
additional space will also compensate for sediment accumulation storage. The remainder of the
forebay leading up to the bioretention cell will be the calculated 1.4 feet of depth to assist with
TSS removal.!First, the soil water balance described in equation 7 was used to determine the
minimum depth of the deep pool (forebay) (BAE 575 Stormwater Wetland class notes).
32. DP = (RF!×EF×WS/WS) − ET − INF − RES [5]
where,
DP = depth!of!deep!pools!(in)
RF! = Monthly!rainfall!during!a!drought, in
EF = Fraction!of!rainfall!entering!wetland!from!watershed
WS/WL = ratio!of!watershed!area!to!wetland!surface!area
ET = monthly!evapotranspiration!(in)
INF = monthly!infiltration!loss, (in)
RES = Reservoir!of!water! safety!factor , in
A value of 1 in was chosen for the monthly rainfall during a drought per the class notes. The
fraction of rainfall entering the wetland from the watershed was 0.20, which is in the acceptable
range of 0.20 to 0.25 specified by the NC Stormwater BMP manual. The infiltration loss for a
drought month was determined to be 7.4 inches by assuming an infiltration rate of 0.01 in/hr
(infiltration rate of a liner). The monthly ET was determined to be 7.2 inches by summing daily
reference evapotranspiration estimated with the Penman-Monteith for July 2008 (a drought
month, 6.84 in) and multiplying by a crop coefficient of 1.05 (Allen et al., 2006).
Table 3: Crop coefficients from the FAO website (Allen et al.2006)
33. Daily reference evapotranspiration was acquired from the North Carolina State Climate Office
and the crop coefficient was found on the FAO website (Table 3). A reservoir value of 6 inches
was used as a safety factor per the class notes. The soil water balance yielded a value of -16.75
inches meaning that the deep pools should have a depth of at least 18 inches. This value is
within the range of acceptable depths (1.5 ft to 3 ft) specified by the NC Stormwater BMP
Manual.
Determining the volume of water the bioretention cell will treat
According to the NC BMP Manual, the bioretention cell shall treat the first flush of a
storm. According to the class notes, this first flush has a depth of 1 inch storm and accounts for
80% of the rainfall and ~90% of pollutants. Therefore, 1 in will be used for the first flush value
R! = 0.05 + 0.9×I! [6]
where,
R! = runoff!coefficient![unitless]
I! = impervious!fraction, 0.281![unitless]
V = 3630×R!×R!×A [7]
where,
V = runoff!volume, cfs
R! = design!storm!depth, 1!in
A = watershed!area, 0.75!acres
Equations 6 and 7 were used to compute the volume of water the bioretention cell needs
to treat which was 823 ft3
.
Determining the surface area required
The required surface area of the bioretention cell was calculated by dividing the volume
of water the bioretention cell will treat and the ponding depth. The ponding depth shall be 9
inches for capturing the water quality storm as specified by the NC Stormwater BMP Manual.
An additional 6 inches of ponding depth was added for peak flow mitigation. Therefore, the
34. required surface area of the bioretention cell was determined to be 1097 ft2
. The design surface
area was 1679 ft2
which is greater than the surface area required.
Soil media composition
The soil media shall be uniform and free of any large stones, root clumps, etc. The soil
media will contain 12% fines (silt and clay) to maximize nitrogen removal as Umstead State
Park would be concerned with nitrogen pollution due to its proximity to Jordan Lake and the
Neuse Basin. The soil media will contain 85% sands, and 3% organic material. A soil test
should be performed by the North Carolina Department of Agriculture (NCDA) to ensure the P
index is between 10 and 30 (Hardy et. al., 2003 and Hunt et. al., 2006). This will ensure that the
media does not contribute phosphorus as a pollutant rather than capturing it. After construction,
the media should have a permeability of about 1 in/hr as specified by the NC Stormwater BMP
manual for soil media with 12% fines (silt and clay).
The Soil Water Characteristics Program (SPAW) model from Washington State
University was used to determine the wilting point, field capacity, and saturation. The SPAW
model indicated that a compaction of 1.15 is necessary for a infiltration rate of 1 in/hr. The
results are shown in Figure 3 (Saxton and Rawls, 2006).
!
Figure 3: Results of the SPAW model
!
35. Soil media depth
The depth of the soil media does not affect the removal of TSS as TSS is removed in pre-
treatment and on the surface of the bioretention cell. Metals are removed from about the top 8 inches of
soil as they are often bound to sediment. Particulate bound phosphorus, which accounts for about 2/3 of
TP, is removed in the top surface. The remaining phosphorus that is dissolved in the soil is removed in
the top 1-2 ft of the soil media. Temperature is reduced at about 2 ft below grade. According to the NC
Stormwater BMP Manual, TN is reduced at about 30 inches below the surface. Pathogens are mostly
removed on the surface through desiccation, but there may be some migration of pathogens through the
soil profile if there is enough water to push them through the soil. In considering the depth necessary to
remove pollutants, the depth of soil media in this bioretention cell will be 2.5 ft to ensure the removal of
TN which is a pollutant of concern because Umstead State Park is in the Neuse watershed. Below the soil
media, there filter fabric, and 8 inches of #57 crushed stone surrounding the underdrain per the
specifications of the NC Stormwater BMP Manual. Filter fabric is used in place of choking stone as the
State Park could be considered a non-developed area. There will be 2 inches of mulch placed on top of
the soil media as specified the NC Stormwater BMP Manual.
Underdrain
Underdrains were used per the specifications of the NC Stormwater BMP Manual because the in-
situ soil had a permeability of 1.49 in/hr which is less than 2 in/hr. The underdrain will connect to the
existing stormwater system. An upturned elbow will be used to create internal water storage (IWS).
Research at NC State University indicates that using an up-turned underdrain pipe may increase nitrogen
removal by creating an anaerobic zone that facilitates nitrogen removal. Two capped clean-out pipes
were included in the design with a height of 8" above the surface to prevent damage from maintenance
equipment per the NC Stormwater BMP Manual.
Darcy’s Law as seen in equation 8 was used to determine the design discharge of the underdrains.
!Q = FS!×K!"#!×A!× ∆H!L!!
[8]
Where,
Q = discharge, cfs
K!"# = saturated!hydraulic!conductivity, (1!in!hr!!
)!
A = surface!area!of!the!bioretention!cell, (1679.3!ft!
)
∆H = height!of!water!above!the!gravel!layer!(3.25!ft)
L = depth!of!soil!media, (2.5!ft)
FS = factor!of!safety, 10
36. The discharge calculated from Darcy’s Law in equation 5 was 0.6 cfs. This value was
used to determine the required diameter of the underdrain using Manning’s equation as seen in
equation 9.
!D = 16 Q×!×S!!.! !.!"#
[9]
where,
D = required!diameter!of!the!underdrain, in
Q = discharge!determined!from!Darcy!
s!Law, cfs
! = Manning!
s!roughness!coefficient!(0.014!for!corrugated!platic, Hunt, 2001 )
S = slope!of!the!underdrain!pipe, (0.005!ft!ft!!
)
The diameter calculated with equation 9 was 7.12 in. Table 4 was used to determine the
nominal sizes of the underdrain pipes. According to Table 4, two 6” diameter pipes should be
used if the diameter calculated with equation 9 is less than 7.84 in. Therefore, this design shall
use two 6” corrugated plastic underdrain pipes with a slope of 0.5%, length of 158.6 ft, and a
spacing of 5.25 ft.
Table 4: Underdrain diameters
if#D#is#less#than,#in# ###of#4"#pipes# ## if#D#is#less#than,#in# ###of#6"#pipes#
5.13! 2! !! 7.84! 2!
5.95! 3! !! 9.11! 3!
6.66! 4! !! 10.13! 4!
7.22! 5! !! !! !!
7.75! 6! !! !! !!
8.2! 7! !! !! !!
The underdrain pipes shall be configured in a similar fashion to the underdrains shown in
Figure 4, which depicts construction of a bioretention cell in Alabama (Alabama A&M and
Auburn University Extension).
37. !
Figure 4: Underdrain configuration for bioretention cell in AL (Alabama A&M and Auburn University Extension)
!
! The underdrains will form an upturned elbow in the outlet box to create an 18” IWS layer
in accordance with the NC Stormwater BMP Manual. The upturned elbow was The IWS layer is
within 12” of the bioretention cell’s surface in accordance with the NC Stormwater BMP
Manual. The upturned elbow was placed in the outlet box for easy access in maintenance
activities.
Overflow Structure
In commercial and industrial settings, an overflow pipe often is installed in the middle of
the rain garden. The top of the pipe or overflow box is set at the desired maximum water depth
(ranging from 6 to 12 inches generally, with 9 inches considered a standard). (NC Stormwater
BMP Manual). The overflow structure’s design discharge was calculated based on a 10-year,
24-hour storm. The design discharge was computed using the rational method as described
above. The intensity was obtained from the NOAA Precipitation Frequency Server and was
determined to be 0.206 in/hr. The design discharge was determined to be 0.106 cfs.
The overflow pipe shall be PVC pipe. The orifice equation was used to determine the
diameter needed for the overflow pipe as seen in equation 10 by using Solver in Microsoft Excel
to determine the diameter needed to make the discharge equal 0.106 cfs. The results indicated
38. that the nominal diameter of the overflow pipe should be 6”. A factor of safety of 1.2 was used.
A 6 inch diameter is within the range specified by the NC Stormwater BMP Manual.
!Q = C!A 2!ℎ [10]
Where,
Q = design!discharge, cfs
C! = coefficient!of!discharge, 0.60
A = area!of!the!pipe! 0.25π!!
, ft!
! = acceleration!of!gravity, ft!s!!
ℎ = driving!head, ponding!depth + depht!of!media , ft
The overflow pipe was placed in the outlet box described in the previous section. The
outlet box is located 63.3 ft away from the existing stormwater infrastructure to allow enough
elevation change to drive water flow.
Bioretention Abstraction Volume (BAV)
The bioretention abstraction volume (BAV) was determined to quantify the storage in the
bioretention cell. Calculations were made using the methodology of Davis et al. (2012). This
bioretention cell was designed for a site with in situ impermeable soils (HSG C), so the BAV
was calculated as if it was a bioretention cell with an underdrain. This is because the soils will
not drain readily during a storm event, so the full volume of the IWS is not available for storage.
If the in situ soils were permeable, computations would be made as if the cell was a
bioinfiltration cell with no underdrain. (Davis et al. 2012).
The lowest BAV values will occur when only the root zone is available for water storage.
This calculation is shown below in equation 11. The values for the wilting point, field capacity,
and saturation were determined using the SPAW model as described above. The lowest
bioretention abstraction volume for this bioretention cell was estimated as 440 ft3
.
39. BAV = RZMS × θ!"# − θ!! [11]
where,
RZMS = root!zone!media!storage, (0.98!ft×SA)!(Gregory, 2006; Davis!et!al. 2012)
θ!"# = volumetric!water!content!at!saturation, 0.357!ft!
ft!!
θ!" = volumetric!water!content!at!the!wilting!point, 0.091!ft!
ft!!
The average BAV will occur when the root zone and the lower media storage is available
for storage. The calculation is shown in equation 12. The average BAV for this bioretention cell
was estimated as 1015 ft3
.
BAV = RZMS × θ!"# − θ!" + LMS× θ!"# − θ!" [12]
where,
θ!" = volumetric!water!content!at!field!capacity, 0.131!ft!
ft!!
LMS = lower!media!storage! soil!media!depth − RZMS ×SA , ft!
The highest BAV will occur when the root zone, lower media storage, and bowl volume
are available for storage. The bowl volume was determined by multiplying the average ponding
depth (9 in) and the surface area. The calculation is as follows in equation 13. The highest BAV
was estimated as 1190 ft3
.
BAV = V!"#$ + RZMS × θ!"# − θ!" + LMS× θ!"# − θ!" [13]
Plant Selection
Plant choice was based on selection variety provided in the NCDENR Handbook. Seven
plant species were chosen for the design to create biodiversity. None of the plant species chosen
included invasive species.
“A minimum of one tree, three shrubs, and three herbaceous species should be
incorporated in the bioretention planting plan unless it is a grassed cell. A diverse
plant community is necessary to avoid susceptibility of insects and disease. A
recommended minimum planting density is 400 stems/acre. Bacteria die off
occurs at the surface where stomwater is exposed to sunlight and the soil can dry
40. out. Therefore it is best for bioretention cells to not be too densely vegetated in
order to allow greater exposure to sunlight and consequently die off bacteria”
(NC Cooperative Extension, 2006). (NCDENR, 2007).
Table 5: Plant plan with species name, quantity, location and image.
Species Name Quantity (Stem Count) Type
Loblolly Pine One Tree
Silky Dogwood One Shrub
Possumhaw One Shrub
Arrowwood
Virburnum
One Shrub
Sedge Four Herbaceous
Eastern Blue Star Four Herbaceous
Soft Rush Three Herbaceous
This bioretention cell is approximately 0.039 acres, which corresponds to a total stem count of
15 shown in the above table. Plant location within zones is approximated in attached appendix.
Planting pattern should vary, but only within the permitted zones outlined in this report.
Hydrograph/Routing
A hydrograph was constructed to determine how much the bioretention cell mitigates
peak flow from a 1.1 year storm. Malcom’s method was used to develop the inflow portion of
the hydrograph. The peak discharge was the discharge associated with the 1.1 year storm and
was estimated using the Rational Method. The peak discharge was then used to determine the
time to peak as seen in equation 14.
T! = V× 1.39Q!
!!
[14]
T! = time!to!peak, s
!V = volume, ft!
Q! = peak!discharge, cfs
41. The inflow portion of the hydrograph which is depicted in blue in Figure 5 was computed
using the equation 15.
Q =
0.5Q! 1 − cos!(π!T!
!!
) 0 ≤ ! ≤ 1.25T!
4.34Q!exp!(−1.3!T!
!!
) ! ≥ 1.25T!
[15]
The outflow portion of the hydrograph was computed using methodology from the
bioretention design spreadsheet on the NCSU stormwater website. The grate at the top of the
outlet box is treated like a weir in and exfiltration is taken into account in this methodology. The
hydrograph constructed shows that there was no outflow through the weir with the 1.1 year
storm. The maximum depth was 0.54 feet away from the grate indicating that the extra 6 inches
of ponding depth did not provide additional peak flow mitigation.
!
Figure 5: Hydrograph
!
42. REFERENCES
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (2006). FAO Irrigation and Drainage Paper
No. 56, Crop Evapotranspiration. Retrieved February 24, 2015, from:
http://www.kimberly.uidaho.edu/water/fao56/fao56.pdf
Brown, R., Hunt, W., & Kennedy, S. (2009, November). Designing Bioretention with an Internal
Water Storage (IWS) Layer. Raleigh, NC: North Carolina Cooperative Extension.
Davis, A., Traver, R., Hunt, W., Lee, R., Brown, R., & Olszewski, J. (2012). Hydrologic
Performance of Bioretention Storm-Water Control Measures. J. of Hydrol. Engr, 604-
614. http://dx.doi.org/10.1061/(ASCE)HE.1943-5584.0000467
Hunt, W., & White, N. (2001). Urban Waterways: Designing Rain Gardens (Bio-Retention
Areas). (Technical Report No. AG-588-3 E01-38929). Raleigh, NC: North Carolina
Cooperative Extension.
North Carolina Department of Environmental and Natural Resources. (2007, July). Chapter: 12
Bioretention.
Saxton,(K.(E.,(and(Rawls,(W.(J.((2006).(“Soil(water(characteristic(estimates(by(texture(and(
organic(matter(for(hydrologic(solutions.”(Soil(Sci.(Soc.(Am.(J.,(70(5),(1569–1578.(
Soil Survey Staff, Natural Resources Conservation Service, United States Department of
Agriculture. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov/.
Accessed [03/09/2015].
(
Water Resources: Our Progress. (n.d.). Retrieved March 16, 2015, from Alabama A&M, Auburn
University Extension website: http://www.aces.edu/natural-resources/water-
resources/watershed-planning/watershed-projects/millcreek/mc-progress.php
52. PERMEABLE PAVEMENT BAE 575 HOMEWORK
PAIGE ERICKSON & TAYLOR CARTER
Site Description
This project is located Piney Wood Park in Durham, NC. The address is 400 E.
Woodcroft Parkway, Durham, NC, 27713. The objective of this project is to convert a 0.21 acre
portion of the parking lot from conventional pavement to permeable pavement for pollutant
removal and hydrologic mitigation. The soils are classified as HSG D and have a saturated
hydraulic conductivity of 0.01 in/hr. The depth to the seasonally high water table (SHWT) is 6
feet.
Overview of Design
This design involves installing permeable pavement in the two rows of parking stalls.
Porous asphalt is the permeable pavement material for this project. There shall be an 8”
underdrain installed under each set of parking stalls. This is also a detention design that involves
an upturned pipe in the existing catch basin that sets the water quality depth at 4” and allows for
bypass of larger storms, an orifice sized at 0.25 inches for detention of water quality volume for
3.5 days. The seasonal high water table (SHWT) is also more than 2 feet below the base of the
aggregate storage layer as specified by the State of NC Stormwater BMP Manual.
Pavement Type Selection
Porous asphalt was selected as the pavement material for this project. It is similar to
conventional asphalt except it has a coarser appearance. Porous asphalt is good for parking lots
and streets. It would also blend in well aesthetically with the surrounding landscape in this
project.
The asphalt mixture will be an open graded friction course mixture in accordance with
NAPA IS 131 where the laboratory air voids will be at least 16% to ensure the permeability of
the mixture (CAPA). The liquid asphalt binder will be PG 76-22 as specified by the Carolina
Asphalt Pavement Association. The asphalt content shall be at least 6% to ensure the necessary
coating of aggregates for long term durability (CAPA).
53. The contractor for this project requires contractors that are certified and qualified to lay
porous asphalt in accordance with CAPA (NC Stormwater BMP Manual).
Infiltration vs. Detention Design
Next, it was determined whether this was an infiltration or a detention system. This was
done by computing the ponding time for the water quality event which is 1 inch in the piedmont
of North Carolina. The ponding time as calculated with equation 1 seen below should be
between 2 and 5 days for an infiltration design. An infiltration design is desirable because there
is credit for BUA reduction as well as higher pollutant removal credit than a detention design.
T = P 1 + R (24×SF×!)!!
[1]
Where,
T = ponding!time!(days)
R = ratio!of!additional!BUA!to!permeable!pavement!area!(0.92!for!this!design)
SF = factor!of!safey!(2!per!the!State!of!NC!Stormwater!BMP!Manual)
! = infiltration!rate!(0.01!in!hr!!
)
The ponding time was determined to be 3.4 days for the water quality storm which is an
appropriate ponding/detention time according to the State of NC Stormwater BMP Manual. The
ponding time was 17.4 days for the 10-year, 24-hour storm. Therefore, a detention design is
necessary as the ponding time must be less than 10 days for an infiltration design to be
appropriate.
Subgrade Slope
The slope of the subgrade in this design is 0.5% which is within the recommended slope
range of less than or equal to 0.5% specified by the State of NC Stormwater BMP Manual. The
slope was chosen because the slope should be as flat as possible to maximize the storage capacity
of the system.
54. Aggregate Base
The aggregate base should be washed #57 stone and have 2% or less pass through an
ASTM No. 200 sieve. This is because unwashed aggregates contain fine particles that can clog
the soil subgrade and minimize the ability of the porous pavement to store and infiltrate water
(State of NC Stormwater BMP Manual). According to the class notes and the CAPA, the
porosity of the aggregate base can be assumed to be 0.40. The depth of aggregate needed to
store the water quality volume of water is calculated using equation 2.
D!" = P(1 + R)!!!
[2]
Where
D!" = depth!of!aggregate!needed!to!treat!the!water!quality!storm!(in)
! = porosity!of!the!aggregate!(0.40)
The depth of aggregate needed to treat the water quality volume according to equation 2
is 4 inches. Next, the depth of aggregate needed to treat the 10-year, 24-hour storm was
determined using equation 3. The depth of aggregate needed to store the 10-year storm was
determined to be 21 inches.
D!" = P!" 1 + R − (!×!×SF !!!
[3]
Where,
P!" = depth!of!precipitation!for!the!10!year, 24!hour!storm!(5.09!in)
SF = 0.2!as!specified!by!the!NC!Stormwater!BMP!Manual
! = duration!of!the!storm!(24!hours)
Underdrains
One corrugated plastic underdrain will be placed under each set of parking stalls
containing permeable pavement. Each underdrain shall contain its own cleanout pipe. The
underdrains were sized by first determining the drainage area of the underdrain which was the
length of the pipe multiplied by the area of the parking stall. The following equation was then
used to determine the flow into the underdrain.
55. Q = DA×Inf×FS [4]
Where,
DA = drainage!area, ft!
Inf = surface!infiltration!rate, (assumed!to!be!4!in!hr!!!
as!specified!in!the!example!spreadsheet)
FS = factor!of!safety, (2!per!the!BMP!manual)
The diameter of the underdrain was then determined with equation 5. The nominal
diameter of the underdrains was determined to be 8 inches.
D = 16(Q×!×S!!.!
)!/!
[5]
Where,
D = diameter!of!the!pipe, in
Q = dishcarge!calculated!from!equation!3, cfs
S = slope!of!the!underdrain, 0.2%!in!this!design
! = Manning!
scoefficient!, 0.014! Hunt, 2001 !for!corrugated!plastic
A 0.25 inch orifice shall be used to the water quality storm for 2 to 5 days. This was
determined using the orifice equation with 1/3 of the maximum head for the water quality storm
(wetland class notes). The orifice is used because water will “prefer” to flow out the pipe rather
than infiltrate. Although the orifice size seems small, Oregon State University Extension states
that the orifices are often less than 1” in diameter (OSU Extension).
Structural Support
The required structural support necessary was determined by the cumulative ESAL
Traffic for a 10 year span using the diagram below (AASHTO). A cumulative ESAL of 3.2
million was determined from this chart when a 10 year lifespan was assumed.
56. !
Figure 1: Cumulative ESAL (AASHTO)
Next, a reliability of 50% was assumed as this is an appropriate number for local roadways
according to the table below from AASHTO.
Table 1: Suggested Reliability Values (AASHTO)
57. Next, these values were put into the equation seen below and the structural number was
determined using a Microsoft Excel spreadsheet. The required structural number was
determined to be 3.45.
log!" W!" = !!S! + 9.36 log!" SN + 1 − 0.20 +
!"#!"
∆!"#
!.!
!.!!!"#$ !"!! !.!" + 2.32 log!" M! − 8.07 [6]
Where,
W!" = estimated!total!18 − kip!equivalent!(3.2×10!
)
!! = standard!normal!deviate!of!the!reliability!(!!" = 0)
S! = combined!standard!error!of!the!traffic!prediction!and!performance!prediction!(0.45)
∆PSI = serviciable!life!difference!between!construction!and!end!of!life!(3.5)
M! = subgrade!resilient!modulus! 5000!psi
The structural support number of the pavement was calculated using equation 7.
SN = !!D! + !!D! + !!D!M [7]
Where,
!! = strength!of!porous!asphalt, 0.41!from!Table!2!(AASHTO)
D! = depth!of!porous!asphalt, 4!in.
!! = strength!sand!layer, 0.07, (value!obtained!from!notes)
D! = depth!of!sand!layer, 2!in!
!! = strength!of!aggregate!material, 0.14, (value!obtained!from!notes)
M = environmental!factor, (0.8!for!Fair!Soil!as!the!site!contains!HSG!D!soils)
Table 2: AASHTO strength coefficients
58. The structural support number of the pavement was determined to be 3.77. This is larger
than the required structural support number calculated with equation 6; therefore, structural
support provided by this pavement is adequate.
Observation Wells
An observation well will be placed at the lowest point of the permeable pavement which
is located on the west set of parking stalls at an elevation of 97 ft. The observation well will
consist of a 4 inch perforated pipe that is placed 4 inches into the soil subgrade. A lockable cap
shall be placed flush with the pavement. Quarterly inspections and maintenance will include
observing the water depth in the observation well throughout the ponding time.
Peak Flow Mitigation
Hydrographs were constructed to illustrate peak flow mitigation of the 1.1 year storm and
the water quality storm. The hydrographs were constructed using the permeable pavement
spreadsheet developed by the NCSU Stormwater Team. The total depth of runoff and time to
peak were determined using the Simple Method. A depth of 1 inch was used for the water
quality rainfall depth (class notes). A depth of 2.93 inches was used for the 1.1 year storm
(NOAA Precipitation Frequency Server). The peak discharge from the 1.1 year storm and the
water quality storm were determined using the Rational Method. A time of concentration of 5
minutes was assumed for both storms in obtaining values from the NOAA Precipitation
Frequency Server because the site is an impervious parking lot. An intensity of 4.86 in/hr was
used for the 1.1 year storm and an intensity of 1 in/hr was used for the water quality storm
(NOAA Precipitation Frequency Server; Class Notes). The hydrograph shows that there was no
runoff from the water quality storm and very little runoff from the 1.1 year storm.
59. !
Figure 2: WQ Storm Hydrograph
!
Figure 3: 1.1 Year Storm Hydrograph
Edge Restraints
Edge restraints shall be flush with the pavement. A concrete curb will extend to the
bottom of the permeable base. The separation will be helpful when the conventional asphalt is
resurfaced (State of NC BMP Manual).
60. Signage
Signage should be installed on permeable pavement sites because it is maintained
differently than traditional pavement. An example of the signage is shown below.
!
Figure 4: Sign available as a 24 in by 18 in image from NCDENR
Construction Sequencing
Construction steps are listed and detailed in chapter 18 section 4 of NCDENR Manual.
Below are the main points of construction:
1. Have a preconstruction meeting with contractors to review the special needs of permeable
pavement i.e. preventing subgrade compaction and clogging of pavement surface.
2. Ensure acceptable conditions for construction.
3. Excavate the pavement area and prepare subgrade surface.
a. Dig the final 9 to 12 inch by using the teeth of the excavator to loosen the soil.
b. Rip soils to improve or maintain the soil’s predisturbance infiltration rate.
4. Test the subgrade soil infiltration rate. (Only needed for infiltration systems. Since this
design is detention based, this step may not be needed.)
5. Place observation wells and underdrain system.
6. Place and compact aggregate base. (Be sure to rinse all aggregate of fines.)
7. Install curb restraints/barriers then bedding.
8. As-Built inspection.
61. Maintenance Plan
Thorough directions for maintenance staff can be found in chapter 18 section 6 of
NCDENR Manual. Schedule parking lot sweeping during and after construction to prevent
sediment from accumulating on the pavement (NCDENR). Surfaces should be cleaned with
portable blowers frequently, especially during the fall and spring to remove leaves and pollen.
Do not stockpile soil, sand, mulch or other materials on the permeable pavement. Do not wash
vehicles parked on the permeable pavement. If you must stockpile, then place tarps to collect any
spillage from soil, mulch, sand or other materials transported over the pavement. Also, cover
stockpiles that are near the pavement. Do not apply sand or deicers during winter storms.
Schedule routine pavement sweeping to increase the life of the permeable pavement.
References
American Association of State Highway and Transportation Officials (AASHTO). (1993).
Obtained from: < http://www.pavementinteractive.org/article/1993-aashto-flexible-
pavement-structural-design/ >
Carolina Asphalt Pavement Association (CAPA). Porous Pavement Parking Lots. Obtained from
the CAPA website: < http://www.carolinaasphalt.org>
Oregon State Extension. Porous Pavement Hydrologic Calculator. Obtained from OSU
Extension website: <extension.oregonstate.edu/stormwater/porous-pavement-calculator>
