SW8060416_COMPLIANCE_20060505

STORMWATER DIVISION CODING SHEET
POST -CONSTRUCTION PERMITS
PERMIT NO.
SW�� l&
DOC TYPE
❑ CURRENT PERMIT
❑ APPROVED PLANS
❑ HISTORICAL FILE
?�DCOM . PLIANCE EVALUATION INSPECTION
DOC DATE
YYYYMMDD

PINER ROAD OFFICE PARK
NEW HANOVER COUNTY, NC
DESIGN AND STORMWATER NARRATIVE
Prepared for:
DANNEN PROPERTIES, LLC
1515 Chestnut Street
WILMINGTON, NC 28401
Prepared by:
AMOUNTIF
N E f F 1 N O. 1 N C. 1 1
127 Racine Drive, Suite 202
Wilmington, NC 28403
Project f06102.PE
ApQ 2006
1INWID
MAY 0 5 2006
SY:

PINER ROAD OFFICE PARK
NEW HANOVER, NORTH CAROLINA
Project #06102.PE
Table of Contents
Design Narrative
Proposed Improvements
Maintenance
NC DENR Calculations - Wet Detention Pond
Storm Drainage Calculations
New Hanover County Soils Map
USGS Map
Page i

PINER ROAD OFFICE PARK
NEW HANOVER, NORTH CAROLINA
Project #06102.PE
DESIGN NARRATIVE
Dannen Properties, LLC is proposing to develop an office park on a 2.80 acre parcel in New
Hanover County. The site is located on Piner Road just east of the intersection of College and
Carolina Beach Road. The project will consist of 1.37 acres of impervious area and 2.80 acres of
the site will be disturbed. The site is currently a fairly covered grassland lot.
The project is within a drainage basin that drains to an unnamed tributary of Motts Creek (C;
SW, 1.0" runoff). All stormwater will be captured by a storm sewer system which will serve to
convey the runoff into a wet detention pond for treatment and storage. The wet detention
pond has been designed to meet the New Hanover County Storm Water Management
Ordinance and NCDEI`gZ!s design requirements.
PROPOSED IMPROVEMENTS/ STORMWATER
All stormwater will be captured through the means of a storm sewer system which will serve
to convey the runoff into a wet detention pond. The pond will outlet through a riser -barrel
structure which has been designed to hold back the 1" storm for the required 2-5 day
drawdown. In addition, the outlet structure has been designed to discharge the 25 year storm
event at a flow rate less than pre -development and the 100 year storm event will overflow
through the means of an emergency spillway. The soil type for the site is Leon (Le). All
supporting drainage calculations and drainage basin information are provided within the
following pages of this report.
For treatment calculations the procedure in the NCDENR Stormwater Best Management
Practices Manual was used to determine the pond surface area, volume, and depth. The pond's
designed length to width ratio is approximately 3:1, and includes a forebay of at least 20% of
the basin's overall volume. Pond side slopes are 3H to 1V, with 6H to 1V planted littoral shelf.
The permanent pool depth is 4 feet. Stormwater in excess of the design storm will overflow the
pond and discharge to the outfall waterway.
The outlet structure for pond 1 is configured to detain the 1.0" runoff volume. The permanent
pool elevation for Pond #1 was determined to be at elevation 21.50 and the 1" runoff volume to
be at elevation 22.30. In respect to these elevations, a 1" orifice will draw -down the 1" runoff
volume in 3.47 days.
Page 1

In addition to the state stormwater requirements, the pond is designed to meet New Hanover's
County 25-year pre/post requirement. A predevelopment curve number of 69 was used which
resulted in a peak flow of 13.54 cfs. This curve number was developed based upon the
property having a type B soil and open space with fair cover. A post development curve
number of 83.2 was used which resulted in a peak flow of 19.95 cfs. based upon the proposed
condition of the development. The elevation for the riser weir structure was placed at 23.10.
This elevation permits the post -development flow to be less than the pre -development
conditions for the 2,10, and 25 year events. An emergency spillway is placed at an elevation of
24.30. The top of the pond is placed at 25.0, with the 100-year elevation of 24.52. All
supporting calculations and results are provided in the following pages of this report.
MAINTENANCE
Contractors shall be responsible for periodic inspection and maintenance of all indicated
erosion control devices. In addition, inspection and any necessary maintenance will be
required immediately following any significant storm event. Any erosion control measure that
fails to function as intended shall be repaired immediately. Upon completion of construction
and the establishment of stabilized ground cover, the property owner shall be responsible for
any on going site maintenance.
Page 2

NC DENR CALCULATIONS - WET DETENTION POND

Basin #1 Wet Detention Pond Calculations
NC DENR Retention Requirements
Objective: design a wet detention basin with the following characteristics: a permanent water pool depth between 3- and
6-feet, a surface area that meets TSS removal requirements (values set by NC DENR and included here), a forebay that
is approximately 20016 of the total pond volume, a temporary water pool sized to detain the initial 1 inch of rainfall runoff an
outlet device that drains the temporary water pool within 2-5 days, and a length -to -width ratio of approximately 3.1.
Step 1: Determine the surface area required for 90% TSS removal
Post -Development Conditions
Total Drainage Area 1.73 ACRE Value from CAD
Impervious Drainage Area ..1.37 ACRE Value from CAD
o Impervious Cover=(Impervious Drainage
Impervious Cover 79.2 /° Area)/(Total Drainage Area)*100•1
Elevation of Permanent Pool Surface
21.5
FT
Value selected by designer
Depth of Permanent Pool
4.0
FT
Value selected by designer
Elevation of Wet Detention Pond Bottom
17.5
FT
(Bottom Elevation)=(Permanent Pool Surface
Elevation) -(Depth of Permanent Pool)
Approximate Pond Length
122
FT
Value from CAD
Approximate Pond Width
44
FT
Value from CAD
Len th:Width Ratio
3:1
Ratio=(Length)/(Mdth):1
Required SA/DA Ratio for 90%TSS Removal 7.000 Value from chart. - Reference: "90% TSSRemoval
Required Permanent Pool Surface Area 5,275 SF Required Surface Area=(Required SAIDA
Ratio)*(Total Drainage Area)
Provided Permanent Pool Surface Area 5,346 SI= Interpolated value from stage -storage
calculations
Sten 2: Determine the 1-inch runoff elevation within the wet detention pond.
Runoff Coefficient, Rv 0,763 MIN Rv=0.05+0.009`(% Impervious)
Required 1" Runoff Volume (Volume of Temporary 4,790 CIF 1"Runoff Volume=l inch*Rv*1 fooV12
Pool) inch *(Total Drainage Area)
Value from stage -storage calculations
Volume Below Permanent Pool 9,476 CIF (cumulative pond volume at permanent pool
elevation)
Tn 1 V„1....,o +., hn r1A 99p rG Total Volume to be Controlled -(Volume Below
Permanent Pool)+(1 " Runoff Volume)
Storage Elevation at Required Volume 22.27 FT Value is interpolation based upon stage -storage
values. See stage -storage calculations
RkProjectslLisle Arch106102.PE finer Road Office0ocuments0esign CalculationslHydrolWel Detention Pond050506

Ste 3: Calculate the required forebay volume 18-22% of total pond volume and com are to the forebay volume provided.
Total Pond Volume 9,476 CIF Value from stage -storage calculations
Required Forebay Volume 1,895 CIF Forebay Volume=(Total Pond Volume)`20%
Provided Forebay Volume:Total Pond Volume 19% 11""""'""" ' -
Volume) - f 00%
Step 4: Verify that time reauired to drawdown the 1-inch runoff volume is within 2 to 5 days.
Diameter of Proposed Low -flow Orifice
1.00
IN
Value chosen by designer
Weir Elevation of Outlet Structure
22.27
FT
Value chosen by designer
(Total Elevation Head Above Orifice)=(Weir
Total Elevation Head Above Orifice
0.80
FT
Elevation) -(Elevation of Permanent Pool
Surface)
(Average Elevation Head Above
Orifice)=[(Storage Elevation at Required
Average Elevation Head Above Orifice
D.40
FT
Volume)+(Elevation of Permanent Pool
Surface)X-(Storage Elevation at Required
Volume)
Cd, Coefficient of Discharge
0,60
Value chosen by designer
Q, Flowrate Through Low -flow Orifice
0.02
CFS
Q=Cd`(Pi) j(Diameter of Orifice) *(I W12
in 2/4' '32.2 Ave a Head 112
Drawdown Time for 1-inch Runoff
3.34
DAYS
(Drays�dawn Trme)=(1"Runoff Volume)/Q'(1
P:IProjectslLisle Arch106102.PE Piner Road OfficelDocumentslDesign CalculationslHydrolWet Detention Pond050506

PINER ROAD OFFICE PARK
Stage -Storage Calculations for Proposed Wet Detention Pond
determine the storage capacity and characteristics of the proposed wet detention pond.
Pond side slope above Permanent Pool 3
Planted shelf 2' below Permanent Pool 6
Side Slope below planted shelf 3
incremental and cumulative storage of the wet detention nond ,nor contour interval.
Stage/Storage Above Permanent Pool (Including Forebay)
Cumulative
Contour
Incremental
Volume, S
Contour
Area (SF)
Volume (CF)
(CF)
21.50
5,346
0
9,476
—Permanent Pool
22.00
6,333
2,920
12,396
22.27
14,266 1" Elevation
23.00
7,345
6,839
19,235
24.0
8,475
7,910
27,145
25.0
9,045
8,760
35,905
P:1Projects\Lisle Arch106102.PE Piner Road Office\DocumentslDesign Calculations\HydrolWet
Detention Pond05O5O6

PINER ROAD OFFICE PARK
Stage -Storage Calculations for Proposed Wet Detention Pond
Stage/Storage Total Pond (Including Forebay)
Cumulative
Contour
Incremental
Volume, S
Contour
Area (SF)
Volume (CF)
(CF)
17.5
620
0
0 ---Pond Bottom
18.0
850
368
368
19.0
1,536
1,193
1,561
20.0
2,581
2,059
3,619
21.0
4,307
3,444
7,063
21.5
5,346
2,413
9,476 ~Permanent Pool
Forebay Only (20% of Total Pond)
Cumulative
Contour
Incremental
Volume, S
Contour
Area (SF)
Volume (CF)
(CF)
18.5
180
0
0 --Bottom Elev.
19.0
310
123
123
20.0
546
428
551
21.0
901
724
1,274
21.5
1,027
482
1,756 ----Forebay Volume
P:1Projects\Lisle Arch106102.PE Piner Road Office\Documents\Design CalculationslHydrolWet
Detention Pond050506

4/18/2006
Project: Piner Road Commerical
Project Number: 06102.PEI
HYDRAULIC GRADE LINE
025
SHEET 1 of 1
DESIGNED BY: TGC
CHECKED BY: JRB
*D.FROM WS ELEV. TO RIM ELEV.
OUTLET
INLET
RIM
W.S.
W.S.
ELEV.
INLET
ELEV
Do
Qo
Lo
HEAD LOSSES (ft)
ELEV
@ eop
REMARKS*
(in.)
(cfs)
(ft.)
Hf
He
He
Hb
Ht
-_
�ri:tS#10'
24.25
'MH#2=
24.25
24
10.390
47
0.099
O.D42
0.017
0.000
0.158
24.41
25.12
0.71
=C#f
24.41
18
3.080
118
D.101
O.D12
O.D03
O.00D
0.116
24.53
25.00
0.47
C1#7 r
24.53
15
0.930
138
0.028
0.002
0.000
0.000
0.030
24.56
25.15
0.59
Ci#8W
24.56
15
0.080
21
0.000
1 0.000
0.000
0.000
0.000
24.56
25.00
0.44
ES#1"
24.25
lMM.i
24.25
24
10.390
47
0.099
0.042
0.031
0.000
0.172
24.42
25.12
0.70
' C}# Li,RM
24.42
24
7.510
16
O.D18
0.022
0.017
0.000
O.D57
24.48
25.05
0.57
n== C!#42
24.48
24
5.610
135
0.083
O.D12
0.016
0.400
0.111
24.59
25.05
0.46
24.59
18
3.050
167
0.140
0.012
0.000
0.000
0.152
24.74
25.00
0.26
PAProjectsUsle Arch106102.PE Piner Road Office0ocuments0esign Calculations\HydrolHGL 030706 4/18/2006

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