20100735 Ver 1_More Info Received_20120726

I ® -ol35
July 23 2012
North Carolina Department of Environment and Natural Resources
Department of Water Quality
Wetlands and Stormwater Branch
521 N Salisbury Street
Raleigh North Carolina. 27604
Attention Annette Lucas, PE
Amicus ingineering
Sustainable Systems Design & Development
Reference Transmittal Letter
Proposed Professional Budding at Lawyers Road
Stallings, North Carolina
Parcel ID 08324002
NCDENR Job Number 10 0735
Ms Lucas,
On behalf of Mr and Mrs Kevin Bngham Arrucus Engineering, PC (Amrcus) is pleased to
submit the revisions associated with the re grading of Bnoretentnon cell BR 1 for the
proposed professional building at Lawyers Road in Stallings, North Carolina (Parcel ID
08324002) Bnoretentnon Cell BR -1 had to be shifted approximately 10 -feet to the east in
order to accommodate a more robust perimeter buffer requirement for the Town of Stallings
This is a very minor change It would be greatly appreciated if the state could expedite a
review and approval of this change due to the fact that the Town of Stallings is requiring
said documentation before the developer can continue construction on BR 1
Per an email sent on July 22 I ve attached a full set of the revised construction drawings
along with the NCDENR Bnoretentnon Supplement Form Should any questions or
comments about this submittal package anse during your review, please feel free to contact
us at [phone]
Sincerely, ©�
N
Nicholas R Parker, P E�
President

Permit Number
(to be p-v ded by DWG?)
f • O�OF W ATE9oG
�, y y
NCDENR
STORMWATER MANAGEMENT PERMIT APPLICATION FORM
401 CERTIFICATION APPLICATION FORM
BIORETENTION CELL SUPPLEMENT
This form must be filled out printed and submitted
The Required Items Checklist (Part 111) must be printed filled out and submitted along with all of the required information
I PROJECT INFORMATION
Project name Proposed Professional Budding at Lawyer's Road (Revised BR 1)
Contact name Nicholas R Parker PE
Phone number 704 573 -1621
Date June 5 2012
Drainage area number
II DESIGN INFORMATION
Site Characteristics
Drainage area 22 328 fe
Impervious area 9 825 ft
Percent impervious
440% %
Design rainfall depth
10 inch
Peak Flow Calculations
Is pre/post control of the 1 yr 24 hr peak flow required?
y (Y or N)
1 yr 24 hr runoff depth
2 9 in
1 yr 24-hr intensity
012 m/hr
Pre - development 1 yr 24-hr peak flow
0100 ft3 /sec
Post - development 1 yr 24-hr peak flow
1 110 ft3 /sec
Pre/Post 1 yr 24-hr peak control
1010 ft3 /sec
Storage Volume Non -SA Waters
Minimum volume required
837 0 ft3
Volume provided
19310 ft3
OK
Storage Volume SA Waters
15 runoff volume
ft3
Pre - development 1 yr 24 hr runoff
ft3
Post - development 1 yr 24-hr runoff
ft3
Minimum volume required
0 ft3
Volume provided
ft3
Cell Dimensions
Ponding depth of water
12 inches
OK
Ponding depth of water
1 00 ft
Surface area of the top of the bioretention cell
1931 Oft2
OK
Length
70 ft
OK
Width
30 ft
OK
-or Radius
ft
Media and Soils Summary
Drawdown time ponded volume
6 hr
OK
Drawdown time to 24 inches below surface
18 hr
OK
Drawdown time total
24 hr
In-situ soil
Sod permeability
n/a in/hr
OK
Planting media soil
Soil permeability
150 m/hr
OK
Sod composition
% Sand (by volume)
87%
OK
% Fines (by volume)
8%
OK
% Organic (by volume)
5%
OK
Total 100%
Phosphorus Index (P Index) of media
20 (undless)
OK
Forth SW401 Bwretentwn -Rev 8
June 25 2010 Parts I and 11 Design Summary Page 1 of 2

Permit Number
(to be provided by DWQ)
Basin Elevations
Temporary pool elevation 682 50 fmsl
Type of bioretention cell (answer Y" to only one of the two
following questions)
Is this a grassed cells y (Y or N) OK
Is this a cell with trees/shrubs? n/a (Y or N)
Planting elevation (top of the mulch or grass sod layer) 6815 fmsl
Depth of mulch 0 Inches Insufficient mulch depth unless Installing grassed cell
Bottom of the planting media sod 679 5 fmsl
Planting media depth 2 ft
Depth of washed sand below planting media soil 0 67 ft
Are underdrains being Installed?
y (Y or N)
How many dean out pipes are being Installed?
3
OK
What factor of safety Is used for stung the underdrams9 (See
2
OK
BMP Manual Section 12 3 6)
Additional distance between the bottom of the planting media and
1 ft
the bottom of the cell to account for underdrains
Bottom of the cell required
677 83 fmsl
SHWT elevation
670 fmsl
Distance from bottom to SHWT
7 83 ft
OK
Internal Water Storage Zone (IWS)
Does the design include IWS
n (Y or N)
Elevation of the top of the upturned elbow
fmsl
Separation of IWS and Surface
681 5 ft
Planting Plan
Number of tree species
0
Number of shrub species
0
Number of herbaceous groundcover species
3
OK
Additional Information
Does volume in excess of the design volume bypass the
bloretentlon cell?
y (Y or N)
OK
Does volume In excess of the design volume flow evenly distributed
through a vegetated fifter?
y (Y or N)
OK
What is the length of the vegetated fifter7
n/a ft
Does the design use a level spreader to evenly distribute flow?
n (Y or N)
Show how flow is evenly distributed
Is the BMP located at least 30 feet from surface waters (50 feet if
SA waters) ?
y (Y or N)
OK
Is the BMP localed at least 100 feet from water supply wells?
y (Y or N)
OK
Are the vegetated side slopes equal to or less than 31?
y (Y or N)
OK
Is the BMP located in a proposed drainage easement with access
to a public Right of Way (ROW)?
y (Y or N)
OK
Inlet velocity (from treatment system)
ft/sec
Is the area surrounding the cell likely to undergo development in
the future9
n (Y or N)
OK
Are the slopes draining to the bloretentlon cell greater than 20 %?
n (Y or N)
OK
Is the drainage area permanently stabilized?
y (Y or N)
OK
Pretreatment Used
(Indicate Type Used with an X in the shaded cell)
Gravel and grass
(81inches gravel followed by 3-5 ft of grass)
x
Grassed Swale
0
OK
Forebay
0
Other
0
Form SW401 B&oretention Rev 8
June 25 2010 Parts 1 and 11 Design Summary Page 2 of 2

Project No 17 10 033 Sheet No of
4KI Date 01 31 11
Calcs Performed By JLM
Calcs Checked By NRP
Amuus inglneering Project Name Proposed Professional Building at Lawyer s Road
Subject Bioretention — Water Quality (Revised 06 05 12)
OBJECTIVE
Determine required water quality volume for the proposed bioretention area
DESIGN CONSIDERATIONS
The Manual of Storm water Best Management Practices requires the design volume
of the bioretention structure be based on treating the first inch of runoff using the
Simple Method Per a previous draft of the manual a general rule of thumb states
that the surface area of the bioretention area should be between 3% and 8% of the
total drainage area This rule of thumb was used as a starting point for the design
REFERENCES
1 Manual of Storm Water Best Management Practices by The Nort1
Department of Environment and Natural Resources 2007
2 Bioretention Drainage Map by Amicus Engineering PC 06/05/12
3 Charlotte Mecklenburg BMP Design Manual October 10 2008
4 Georgia BMP Design Manual Section 3 2 3 6
CALCULATIONS
1 Bioretention Area BR -1
� Carolina
FESS
.a- SEAL
032Q06
� 4'GIN
EF' ����\�
R
(ft) Volume (ft l/,/,/ � �
Elevation (ft)
[Ref 2]
Area (ft)
[Ref 2]
Height
68250
2 216
100
1931
68150
1 645
1 Total volume available in Bioretention Area BR 1 (elev 682 50 ft) = 1931 ft'
Determine surface area required (conservative rule of thumb)
a Total drainage area = 0 51 acres [Ref 2]
b Surface area of BR 1 = 1 650 ft2 = 0 04acres [Ref 2]
c Percent of area = (0 04 ac /0 51 ac ) = 0 08 or 8% therefore ok
2 Determine water quality volume required for area draining to BR -1
The runoff volume calculations in the Simple Method as described
by Schueler (1987) will be used [Ref lJ
a Rv = 0 05 + 0 009(n
Rv = runoff coefficient = storm runoff (inches) / storm rainfall (inches)
I = percent impervious portion of the drainage area = 43%
Rv = 0 05 + 0 009(43)
Rv= 044(in /in)

Project No 17 10 033 Sheet No of
Date 01 31 11
Cates Performed By JLM
Calcs Checked By NRP
Amicus Engineering Project Name Proposed Professional Building at t Lawyer s Road
Subject Bioretention — Water Quality (Revised 06 05 12)
b For the volume that must be controlled
Volume = (design rainfall) (Rv) (drainage area)
Volume = 100 inch rainfall * 0 44 (in / in) * 1/12 (feet / inch) * 22 327 ft
Volume = 819 ft3
Volume available in BR 1 = 1931 ft3
819 ft3 < 1 931 ft3 therefore ok.
3 Determine Minimum Surface Area
a Af = Minimum surface area required for BR 1 [Ref 4]
b Af = (WQv)(Df) /((k)(Hf+Df)(Tf)) [Ref 4]
c Af = (0 019 ac /ft )(2 ft ) /((0 5 ft /day)(6 in + 2 ft )(1 425 days))
d Af = 809 ft3 < 1931 ft3 therefore ok
4 Compute Filter Media Capacity
a Media Capacity = (Af)(k)(hf+df) /df [Ref 3]
b Media Capacity = (1 650 ft)(0 5 ft/day)(0 5 ft + 2 ft) /(2 ft)
c Media Capacity = 0 01 cfs
5 Design Inlets and Underdrain System for BR -1
a Compute minimum drawdown discharge
Water Quality volume = 1931 ft3
Drawdown = 1931 ft3/[(24 hours)(3 600sec/hour)]
= 0 022 cfs
b Compute perforation capacity
i # of Perforations= (185 lf)(1 rows /0 5 ft)(4 holes /row) = 370 holes
(assume 50% clogging)
50 percent of perforations = 185 holes
Capacity of one hole = CA(2gh)0 s
= (0 6)(3 1416)[(3/8m)(l/24)]2[(64 4)(5 Oft)]o s
= 0 0083 cfs
Total capacity = (0 0083 cfs)(185) = 1 54 cfs
ii 154 cfs > 0 022 cfs > 0 01 cfs therefore ok
c Compute underdram pipe capacity
i For 8 inch PVC underdram pipe at 0 005 ft/ft slope
Capacity of pipe = 0 93 cfs [Ref 2]
Fifty percent assuming clogging = 0 47 cfs
11 0 47 cfs > 0 022 cfs > 0 01 cfs therefore ok

Project No 17 10 033 Sheet No of
Date 01 31 11
Calcs Performed By JLM
Calcs Checked By NRP
Project Name Proposed Professional Building g_ t Lawyer s Road
Subject Bioretenhon — Water Qualgy (Revised 06 -05 12)
6 Rinretenfinn Area RR 2
Elevation (ft)
[Ref 2]
Area (ft)
[Ref 2
Height (ft)
Volume (ft)
68100
4 683
100
4 216
68000
3 749
i Total volume available in Bioretention Area BR 2 (elev 681 00 ft) = 4 216 It
Determine surface area required (conservative rule of thumb)
a Total drainage area = 128 acres [Ref 2]
b Surface area of BR 2 = 3 749 ft2 = 0 09 acres [Ref 2]
c Percent of area = (0 09 ac /128 ac ) = 0 07 or 7% therefore ok
7 Determine water quality volume required for area draining to BR -2
The runoff volume calculations in the Simple Method as described
by Schueler (1987) will be used [Ref 1]
a Rv = 0 05 + 0 009(I)
Rv = runoff coefficient = storm runoff (inches) / storm rainfall (inches)
I = percent impervious portion of the drainage area = 46°x:
Rv = 0 05 + 0 009(46)
Rv =046(in /in)
b For the volume that must be controlled
Volume = (design rainfall) (Rv) (drainage area)
Volume = 100 inch rainfall * 0 46 (in / in * 1/12 (feet / inch) * 55 626 ft2
Volume = 2 132 ft3
Volume available in BR 2 = 4 216 ft3
2 132 ft3 < 4 216 ft3 therefore ok.

Project No 17 10 033 Sheet No of
Date 01 31 11
Calcs Performed By JLM
/
Cates Checked By NRP
Qmicus ingmeenng Project Name Proposed Professional Building at t Lawyer s Road
Subject Bioretenhon — Water Quality (Revised 06 05 12)
8 Determine Minimum Surface Area
a Af = Minimum surface area required for BR 2 [Ref 4]
b Af = (WQv)(Df) /((k)(Ht+Df)(Tf)) [Ref 4]
c Af= (0 049 ac /ft )(2 ft ) /((0 5 ft /day)(6 in + 2 ft )(142 5 days))
d Af = 2 156 ft3 < 4 216 ft3 therefore ok
9 Compute Filter Media CapacitS
a Media Capacity = (Af)(k)(ht+df) /df [Ref 3]
b Media Capacity = (3 749 ft2)(0 5 ft/day)(0 5 ft + 2 ft) /(2 ft)
c Media Capacity = 0 03 cfs
10 Design Inlets and Underdrain System for BR -2
b Compute minimum drawdown discharge
ii Water Quality volume = 4 216 ft3
Drawdown = 4 216 ft3 /[(24 hours)(3 600sec/hour)]
= 0 05 cfs
c Compute perforation capacity
ii # of Perforations = (430 lf)(1 rows /0 5 ft)(4 holes /row) = 860 holes
(assume 50% clogging)
50 percent of perforations = 430 holes
Capacity of one hole = CA(2gh)o s
= (0 6)(3 1416)[(3/81n)(1/24)]2 [(64 4)(5 Oft)]o s
= 0 0083 cfs
Total capacity = (0 0083 cfs)(430) = 3 57 cfs
ii 3 57 cfs > 0 03 cfs > 0 05 cfs therefore ok.
d Compute underdram pipe capacity
ii For 8 inch PVC underdram pipe at 0 005 ft/ft slope
Capacity of pipe = 0 93 cfs [Ref 5]
Fifty percent assuming clogging = 0 47 cfs
u 0 47 cfs > 0 03 cfs > 0 05 cfs therefore ok
Note to Reviewer
The values calculated by Amicus Engineering In these calculations exceed the
minimum values calculated using the NCDENR rain garden sizing spreadsheet
and the Town of Stallings rain garden sizing spreadsheet. Both of which are
included with this re- submittal

11 I
NCDPJr Stormy ater BN F Manual
Re-m-sed D9 ZM7
alloin s the user to select from one of NOAH s numerous data stations throughout the
state Then the user can ask ror precipitation intensity and view a table that displays
precipitation intensity estimates for various annual return intervals (ARIs) (1 year
through 1000 years) and various storm durations (5 minutes through 60 days)
The requirements of the applicable stormwater program vall detemlme the appropriate
values for ARI and storm duration If the design is for a level spreader that is receiving
runoff directly from the drainage area then the value for I should simply be one inch
Per hour (more Information on Ievel spreader design In Chapter 8)
a J Runoff Volume
Many stormwater programs have a volume control requirement that is capturing the
-first 1 or uic o s ormv� a er and re iiLng i�foi r 5 dad s There are-two-p=:ar3
methods that can be used to determine the volume of runoff from a given design storm
the Simple Method (Sclhueler 1987) and the discrete SCS Curve Number Method (MRCS
1956) T5om o these methods are mternded ror use at Lhe scale of a single drainage area
Stormwarer BM's shall be designed to treaL L tiol -L-me that is at least as large as the
volume calculared using the Simple Me-chod ± the SCS Method yields a greate_ volume
tne-11 it can also be used
3 31 Simple Me-lhod
he Smile 7v eznoa L-,es a in -m-n-1 amoint o n-o i-a-auon suc`n as v� ate shea lr_- a- -Iage
area ?' L v Dins a_Tea at-1d Cle�ogn atorm depa =n Io E:, mate 'ale L, oluzzie of iruno- 1 ra
Sin nple Meth oa was caev eloped by measun-ig me -runoff -ro-m m;rny -w a=s Liees y-Im
lino v -n =1 t?` v OILS ?ZS `TnC� C1= ve= �-1D a -ems iO-is L7 b� , "'Z -c eR
-'Paz 10 L-�cs aura ae -aC=o-L O- -a Z,-1-1 con e_ cQ ID L.--o- (me -i-no- Coe LZ� )
1 Lis aelaton_sl n p is prae_ -.tC-ci bel ow
R-V= 005 --09 }1
Vv-here Rv = Runof co facienn: [storm runoff (m) / storm rainfall (in)] uiutless
IA = Impervious fraction [u npen -lous poraon of drainage area (ac)/
drainage area (ac)] urutless
Once the runoff eoefficlent is determined the volume of runoff that must be controlled is
gz�en by the equation below
V = 3630 � RD } R 'A
Where V = Volume of runoff that must be controlled for the design storm (ft3)
RD= Design storm rainfall. depth (in) (Typically 10 07 15 )
A = Watershed area (ac)
Stormwater MaztagemEMx and Calculatxorls 3- fly 2007

NCDENTR Stormn� ater BMIP Manual
12 $xorefentxon
Revised 09 28 07
Description ll
Bioretenfion is the use of plants and sods for removal of pollutants from stormwater runoff via
adsorption filtrafion sedimentation volat hzaton ion ex change and biological decomposition.
In addition bioretention provides landscaping and habitat enhancement benefits
Regulatory Credits
Feasibility
Considerations
Po7lutmit Re7wval
85/ Total Suspended Sohds
Fagh
Land Requirement
35 / Total Nitrogen
Med I Tigh
Cost of Construction
4,5/- Total_ Phosphorus
Med Idijh
Maintenance Burden
Meter Quanirty
Small
Treataole Basin Size
Yes Peal, Runoff AttenLaton
Med
Possible Site Cons>za-mts
poss ble Runofr Volunne Reduction
Med High
Co=un-ty Acceptance
Advantages
F-laaern -zLT+ozal Tnedzod ro- sLs?---nded
solids hz�vy merls adsorbed poliLLmrts
r�- _Too n, p nospho -u parlo;E-rns and
��ra'iLT2
! p -o - d---g 502
CO- CLUOZS i C:.^ --I eTZc-L- Zj LC2 'z.
T-i -no= =a fO Telai14`l y Te-GL�`Z So=-::,
-=dlce -iitD1 LolnL -3 and TE-1La
L LCL 1i iL L-Lu axe ;,� ?1 su eG -0 L52 Z
sma l a -eas and 3ialuple d�5m bau- -a 1iTLu
Ca--I Vro'i 3de trea.-Lme=—nlarge d=a -TLge
ZFa C
Natural mtegraaon an7o landscapin- for
- rbanlannscap- enhancement
Disadvantazes
Su .Ce SD -1 laYE Mmy CIO .- O-1 e= tITIe
(tb0Lgh t Can b= Testored)
- � -eiauan �asn =�o�rz m?� b� -eci�ed
angle 1 -- CF--I flZy SZ-i/e sln i
dZ -tee a e-
�� =b ==:. Le_a iL -' i an —MCe D ?l-a
na�t'ne =?ch1. z=
Bzoretentaon 121 July 2007

XTCDEINTR Stormy, ater BAS' Manual Chapter Revised 09 2s 07
hlalorDesian RIP.mPntc
rn�ss�on'�3,�mer a `c��attb�s"��ecess `� � .sfi�e o i3tan''f� ,e�mmox'.�
=1 shall take into account all runoff at ultimate build out including off -site
drainage
ide slopes stabilized -�A ith vegetation shall be no steeper than 31
MP shall be located in a recorded drainage easement with a recorded access
asement to a public right of -A ay (ROl9
olunme in excess of the design volume as determined from the desgn storm shall
y -pass time bioretention cell
olume in excess of the design volume as determined from time design storm shall_ be
verily dastnbuted moss a minimum 30 feet long vegetated ii-Iter stomp (A 50 ft fzlter
requied um some locations ) If this cannot be atta ned alternate des, g•ms -will be
ons,dered on a case by case bass
��Bfee"E aorerenizon ra�ae� shall nor be used where the seasonLll_y l -?glm -sA a er amble less �n below borro?i or BlvT
/ k'�'fedia -;�- meab. >3-3 o` 0 52-6 Pe- hour L Tem-ired 1 2 in p--r hei- s p- eie: —ea
!'Ie Ge5 g I sr al be ±oza ad a 5���+` -m O 3o ee =off su-nce dvaLL, a'1C 50 ce - =oZ
S S A W a-ers
�! he ae gn s aii be ioca-ea a =u:- Lm+.- -n. o 209 ree roam vva -er sao�ly u e1s
0 "rerznt3on fa-11 ties shali nom be used in here slopes greater than 20 /, or m non
ently stab> > - -d drainage areas
1 OTA must be sheet flo1A (1 ft /sec) or ut lime energy dissipaung devices
2 �ondzng depth shall be 12 inches or less Nine inches ms preferred
3 edra deptli shall be specified for the vegetation used For grassed cells use 2 feet
um For shrubs or trees use 3 feet minimum
14 e geometry of the cell shall be such that no dimension is less than 10 feet (width
ength or radius)
15 ledia should be specified as listed in this section
e phosphorus index (P -mdex) for the soil must be low between 10 and 30 This is
6 hough phosphorus to support plant growth without exporting phosphorus from the
ell
7 onded water shall completely drain into the soil within 12 hours It shall dram to a
level of 24 inches below the soil surface in a maximum of 48 hours
Eioretmtion 122 July 2007

NCDENR 5tormwater BAS' Manual
Revised 09 25-07
�.n underdram shall be typically installed if in -situ soil drainage is less than 2 in /lu or
E there is in situ loamy soil (-1251 or more of fines) This is usually the case for soil
cghter than sandy loan-i or if there has been significant soil compaction from
onstructzon
� 9 f Clean-out pipes must be provided if underdraii s are required I
121 General Ci.axactensfics and Purpose
A bioretentlon cell consists of a depression in the ground filled with a soil media
nature that supports various types of water - tolerant vegetation The surface of the BMP
is depressed un bloretention facilities to allow for ponding of runoff that filters through
the BA\C media Water eats the bioretenton area via axhltratuon into the surrounding
soil flow out an underdram and evapo>zanspination The surface of the cell is protected
from weeds mechanical erosion and desiccation by a layer of mulch Bioretention is an
efficient medzod for removm-, a wide i a_nery of poLutans such as suspended solids
l ieavy merals nut aen`s par-hoge_ns and mmperarure (NIC Cooperatry e Exte�1sroTh 2 ©Do)
B,oret.e.nno-i areas p -ovide some nuTient upral e in addition 'o physical filtration L
lo, ated a a sire wide approp-ia. e soil coriLuDris to provide mriaation bzoreten'Lon ca-1
also be erem%7e in Tedumna peal, runoL ram TeducZg ru nor zoltme� aad TechaTo— 0
ou--idIA ar.e_T
fiv z, de e_oprie_n projecs p e:�,enr a chaile_age m t-e des j a o co-hv `-uo -iz!
sz:0=14 a-et BlVes became o pn3 sical s7 z2 co-3snrz -ins B ore-ermon areas cre zn-znayd. -o
adores 12--e spzual con_t=is Ana- can be foilad ?-n ae-nsely developed urbaTi areas
w he72 =ie iLz--jage a -ear a 2 h'ghi3 ?- ip=-jous (see �'lgLe l2 1) ltle} Ca- be used o�
�-1 -a na, sI -es Via- v'.Oul a no- 'oil,) S-L?70— ui e h a-olD 3 o c Z4 e der-1-17-0--L
porid and where the so-Is v oald not a±ovv -or an in:5- Imago -i de ace Metaia-n s=ipz,
ra3hp loops u aLic circles and parleng log islands are good e).ampl, or typical locations
fo- bioretenaon areas See �et� on L 31 or more iliusu aced e��hples of the --e_rsatdity-
of bloretention faali -es
Bzoretention units are ideal for distributing several units throughout a site to provide
treatment of larger areas Developments that incorporate this decentra]ved approach to
stormiA,ater manage men'- can actueve savings by elimnnaung stormv� aver management
ponds reducing pipes. zrdet structures curbs and gutters and having less grad2ng and
clearing Depending on the type of development and site constraints the costs for using
decentralized bioretention stormwater management methods can be reduced by 10 to
25 percent compared to stormy ater and site development using other BM--Ps (Coffman et
al 1998)
Bioretentron facilities are generally most effective if they receive runoff as close as
possible to the source Reasons for thus include mu imizing the concentration of flow to
reduce entry velocity reducing the need for inlets pipes and downstream controls and
allowing for blending of the facilities with the site (e g parking median facilities) For
sites where infzltrahon is being utilized it also avoids excessive groundwater mounding
Where bxoretention takes the place of required green space the landscaping expenses
Bxoretentxon 123 l -1Y 2X7

NCDENTR Stomw ater Bh P Manual Chapter Revised 09 28-07
that Would be required in the absence of bioretention should be subtracted when
determining the actual cost (LoiAT Impact Development Center 2000) Bioretention cells
may also address landscaping / green space requirements of some local governments
(WOSSIrtk and Hunt 2003)
Figure 721
Bioretentron in Parking Lot Island
12 2- Meet -ng Reguiatory RegiurQirents
—'o Obi.'-' a P---Li i0 CO—SL—LC- a b3oTer---inOZ ce—TI in NoT7ch C--o = itne b oTezan?o-i ce-11
L�}n° sp- -=eQ. i-i L e Majo- Des j E1.=e_1u loco ca a -�C
j Zr1J 1 Ol i -ais Y Lo-
POZZvfartP\EMGrOal Cple�etcons
The pollui:anzremoval calculations for biorere ration fa©liaes are as desmbed in Section
3,14- anci use the poLuta-nt rerTnovai rates provided ui Table 4-2 in Sec aon 4 0
Consuuction or a bioretentlon cell also pasmelly lowers nutnexnt loadu-ng since it is
counted as pe - ,Tious surface when calculating nutnent loading
VOZU777.0 Control CaZmZations
A bioretentzon cell can sometimes be designed with enough storage to provide active
storage control (calculations for which are provided in Section 3 4) however some may
not have enough water storage to meet the volume control requirements of the
particular stormwater program (since its storage potential is limited because the
ponding depth is hmrted) so they may need to be used in series with another BMP with
volume control capabilities All bioretentlon facilities provide some passive volume
control capabilities by providing pervious surface and therefore xeducung the total
zwnO: f volume to be controlled
koretentaon 12-4 July 2007

removal rates for each of the design are given in Table 4 8 1
Table 4 8 1 Design Values and Pollution Removal Rates
Threshold
Minimum
Minimum
Maximum
Pollution Removal
Detention Time
Media Depth
Plndex
Rate
Optimal
85% TSS
Efficiency
2 0 days
2 5 feet
50 ppm
70% TP
Standard
70% TSS
Efficiency
I 1 0 days
2 feet
50 ppm
35% TP
TSS -only
85% TSS
Efficiency
2 0 days
2 5 feet
N/A
0% TP
• A sediment chamber is required as a pretreatment device for all sand filters The sedimentation
chamber storage area above the filter media must be sized to hold 20 percent of the water quality
volume
• Sand filters require a sand filter media with an underdrain system using 6 Inch diameter perforated
pipe surrounded with filter fabric The underdraln collection system should be equipped with a 6-inch
perforated PVC pipe (AASHTO M 252) in a 12 inch gravel layer The pipe should have 318 -Inch
perforations spaced at 6-inch centers with a minimum of 4 holes per row
• The underdraln system must be designed so that runoff exits the facility within the design duration
assuming 50 percent of the underdraln capacity if lost due to clogging A minimum grade of 0 5% or
minimum flow velocity of 1 fps must be maintained The pipe is spaced at a maximum of 10 feet on
center Minimum spacing of clean outs for the underdraln system shall be 50 linear feet
• The top of the sand filter media must be protected with a 1-inch thick debris screen
• The maximum contributing drainage area for a surface sand filter is 10 acres The maximum
drainage area for a perimeter sand filter is 2 acres The maximum drainage area for an underground
sand filter is 5 acres
• Sand filter systems are designed for Intermittent flow and must be allowed to drain and reaerate
between rainfall events They must not be used on sites with a continuous flow from groundwater
sump pumps or other sources
• No runoff should enter the filter's sand bed until the upstream drainage area Is completely stabilized
and site construction if completed Any disturbed areas within the sand filter facility drainage area
must be identified and stabilized Filtration controls must only be constructed after the construction
site is stabilized
• The filtration media surface area should be sized using Darcy s equation using an average filtration
rate of 1 75 inches /hour
where
Af= (WQy)(df) / L(k)(hf + df)(tf)]
Af = surface area of ponding area (ft)
WQ„ = water quality control volume (or total volume to be captured)
di, = filter bed depth (2 0 feet minimum)
k = coefficient of permeability of filter media (3 5 ft/day)
hf = average height of water above filter bed (ft)
ti, = design filter bed drain time (days)
Chadofte- Mecklenburg BMP Design Manual Apnl 30 2008
486

CZIZ F U
apply
P-
U
mss[ o
° �# $ ATUR
Step 3 Compute water quality volume (WQ,) using equations 3 2 and 3 3 — WQ„ = 1 OR A/12
Step 4 Compute site hydrologic parameters using the SCS procedures and /or computer models
that use the SCS procedures
Step 5 Compute water quality peak flow (WQp) using equation 3 4 for a modified curve number
and the SCS hydrograph procedures with a 1 -inch 6 hr balanced storm event
Step 6 Compute channel protection volume (CP„) using the SCS method and a 1 -yr 24-hr storm
event Estimate approximate storage volume for channel protection
Step 7 Size flow diversion structure if needed to divert the water quality volume to the sand
filter
Step 8 Compute the release rates for the water quality control and channel protection volume
control
Step 9 Compute pretreatment volume (if Included in the design) The sedimentation chamber
should be sized to contain 20 percent of the WQ,,
Step 10 Size filtration basin chamber
The fitter area Is sized using the following equation (based on Darcy s Law)
°e A r = (WQ,,) (df) I f(k) (hr } df) (tf)]
where
A r = surface area of filter bed (ft)
WQ„ = Water Quality Protection Volume (or total Volume to be infiltrated)
df = filter bed depth (designer selects either 24 inches or 48 inches)
k = coefficient of permeability of filter media (ft/day) (use 3 5 ft/day for sand)
hf = average height of water above filter bed (ft) (1/2 hma., which vanes based on
site but h,r,� is typically < 1 foot)
tf = design filter bed drain time (days) (designer selects either 27 or 51 hours
which is 24 or 48 hours beyond the center of the water quality storm event 3
hours)
Set preliminary dimensions of filtration basin chamber and sedimentation chamber
Step 11 Derive stage - discharge and stage- storage relations for the sand filter Assume that
discharge occurs for headwater depths at the elevation of the top of the filter media and
higher A zero discharge should be assumed at the elevation of the top of filter media
Step 12 Route flows through sand filter facility and adjust design of facility to meet all design
criteria
Step 13 Design inlets pretreatment facilities underdrain system and outlet structures
Step 14 Compute overflow weir sizes
Charlotte- Mecklenburg BMP Design Manual Apnl 30 2008
4810

{ I 797 < 'r �'
P�F�� ' r`FI
? -TER
Y=
*t—ew
The length slope number of pipes spacing etc is configured per design requirements Based upon the
required area for the sand filter BMP (927 ft) the approximate dimensions of the sand filter area is
selected to be 30 feet wide by 30 feet in length (approximately 927 ft)
The design process uses a trial and error process The capacity of the perforations and pipe (assuming
50 percent of the system is clogged) are computed The computed underdrain capacity is checked
relative to the filter media capacity to ensure that the filter media is the controlling outflow condition The
computed underdrain ca aci if compared to the static outflow discharge that ensures the runoff within
the s s es within 51 hours
_ (0 19 ac ft)(43 560ft3 /ac ft) = 8 276 ft3
= 8 276 ft3/[(51 hours)(3 600sec /hour)]
0 045 cfs
Compute perforation capacity
Since the maximum underd pacing is 10 feet on center and the sand filter area is 30 feet wide by 30
feet in length t rallel underdrain pipes (6 -inch diameter PVC) 30 feet in length were selected For
a ions below the length of pipe containing holes was reduced by 1 foot to account for fittings at
either end
Number of perforations = (3 pipes)(2 rows /ft)(30 -1) ft/pipe)(4 holes /row) = 696 holes
50 percent of perforations = 348 holes
Capacity of one hole = CA(2gh)"
(0 6)(3 1416)[(3/8in)(1/24)12[(64 4)(5 Oft)]o s
0 0083 cfs
Total capacity = (0 0083 cfs)(348) = 2 89 cfs
The perforations capacity (2 89 cfs) is greater than the filter media capacity (0 051 cfs computed in step
11b) and the minimum drawdown capacity requirement (0 045 cfs computed in this step) Therefore the
design is acceptable
Note that the headwater depth used to determine the filter media capacity is 1 0 feet the average
headwater depth above the filter media for the water quality storm event The drawdown computation is
also based on the water quality volume The headwater depth for the perforations is also based on the
pflle average headwa ations 1 feet above the fitter media or 5 0 feet above the perforations
Compute underdraln pipe capacity
For 6-inch PVC underdrain pipe at 0 ft/ft slope
Capacity of pipe - (1 49 /n)(A)(A/P)° 67(S)o s
(1 49/0 013)(0 1963 ft2)(0 125 ft)° 67(0 005)o s
0 40 cfs
Fifty percent assuming clogging = 0 20 cfs
The perforations capacity (0 20 cfs) is greater than the filter media capacity (0 051 cfs computed in step
11b) and the minimum drawdown capacity requirement (0 045 cfs computed in this step) Therefore the
design is acceptable
Step 14 Design Overflow Weir
The final step is to route the 50 year 6 -hour storm event through the sand filter facility to ensure that a
minimum of 6- inches of freeboard is provided and that a maximum of 5 feet of depth is over the sand filter
media An eight (8) foot weir at 738 0 is proposed as the emergency overflow The peak stage is 738 25
which is 4 25 feet above the filter media (less than 5 feet) and therefore meets design standards The
following HEC -1 output file illustrates the results
Charlotte - Mecklenburg BMP Design Manual Apnl 30 2008
4826

3 2 3 6 Design Procedures
Step 1 Compute runoff control volumes from the Unified Stormwater Sizing Criteria
alculate the Water Quality V61ume,6NQ„) Channel Protection Volume �Cp,)UOverbank
food Protection` Volume (Qp) and the Extreme Flood Volume {Qf) '
;tails on the Unified Stormwater Sizing Cntena are found in Section 14
Step 2 Determine if the development site and conditions are appropriate for the use of a
bioretention area
'yx+ --F - c'+rr� -� x Z' vxnisrm nz- �vix�v- .— 'S -c
onsider the Application and Site fFeasibilitytCnteni rn subsections 32 4 and 3 2 3 5-A
j_o b8n and Sltll1gl .._��. d <. } my z w
Step 3 Confirm local design criteria and applicability
Consider any special site - specific design; conditions /cntena from subsection 3 2 S b-d
l(Adddional `Sitej;Spec�c "Design Cntena and Issues) � �
r-heckwithxlocal officials and other agenciesio determine if there are any additional
testncfions andLorD u_qaj;Lq)yater or�ivatershed re�,uiremgnts thatanayapply
Step 4 Compute WQ„ peak discharge (Q,Nql
7fie peakate of discha�gelorwater qualify design storm is needed for sizing of off line
tiversion structures {seezubsechon,21 7) M
ka) rUsing V1/Q (or#otal volume to be captured) compute CN
4b) Compute time of concentration using TR 55 method -
c) Determine appropriate unit peak discharge from time of concentration�K
f) Compute . rom iinit _peak dise arge drainage area and WQ„ _
Step 5 Size flow diversion structure, if needed
iA fiow.regujator (or flow splrtter diversion structure) sbdu d be suppliedio divert the �d-to
Ahe bioretention area -
L�
e jto flow onfice weir or other device to ass
Step 6 Determine size of bioretention ponding /filter area
Ttie rec ui�ed pl na ding soil filter bed area s computed using following equati6W based
�n =Darcy's iawy s x
Af i= ,r(WQv) (df)1((k) (hf } df) (tf)1
wiz � t 8x s� t� 3r r �4'k _ 4
Af - n surface area of ponding area (ft) y w
{ WQ„ u water quality m
volue (orlotal volume" be captured)
df = filter bed depth; r
xa �-14 -Jeet minimum)
k 10 1P= j,n „coefficientof permeability of filter media (ft/day)
i 11 ��(use 0 5 ft/dayforsiltlaoamn .
h average height of water above flterbed Al
` � 3 %typically 3 Inches 4,which� is half of the 6 i eti ¢ ndin depth)
�'th�5 �
f i1 F `” r` k J b i '� � 4 p� `C )
7 � _ � design filterbedArainlime i(day�)
g � daySiOr 5,,,bgurq)s recon1mended maXim11171) t y
Volume 2 (Technical Handbook) Georgia Stormwater Management Manual 3 2 51

Step 7 Set design elevations and dimensions of facility
See subsection 3 2 3 5 C (Physical Specifications /Geometry)
Step 8 Design convevances to facility (off line systems)
LSee,the example figures to determine the type bf conveyances needed for the site
Step 9 Design pretreatment
Pretreat with a grass filter strip {on line configuration) orgrass channel (off line) and stone
sdiaphragm -
L
Step 10 Size underdrain system
€Seesubsectton 3 2 3 6C {Physical SpecificattonslGeometry) , k
Step 11 Design emergency overflow
An overflow must be provided to`bypass and %or convey larger flOWsOhe downstream~
4drainage-system or stabilized watercourse Nonerosive velocities need to be ensured at
the ,outlet point
Step 12 Prepare Vegetation and Landscaping Plan
A landscaping plan for the bioretention area should be prepared Z ndicatehowit vinll be h
established with vegetation
See subsection 3 2 3 54 (Landscaping) and Appendix F. -for more details _ µ
See Appendix D 2 for a Bioretention Area Design Example
3 2 52 Georgia Stormwater Management Manual Volume 2 (Technical Handbook)