SW6091003_HISTORICAL FILE_20091209

STORMWATER DIVISION CODING SHEET
POST -CONSTRUCTION PERMITS
PERMIT NO.
DOC TYPE
❑ CURRENT PERMIT
❑ APPROVED PLANS
HISTORICAL FILE
❑ COMPLIANCE EVALUATION INSPECTION
DOC DATE
YYYYMMDD

STORMWATER PACKAGE FOR
LEE PROFESSIONAL PARK
DUNN,NC
SEAL
22024

Stormwater Narrative for
LEE PROFESSIONAL PARK
This site is off of Tilghman road in Dunn, North Carolina. This site will
eventually drain into the Cape Fear River, though no steams or wetland exist near this
site. The roadway layout is situated in such a way as to minimize the impervious impact
on the site. Seasonal high water table was found it be 178' from a 24" bore drilled just
offsite.
This site will meet stormwater requirements set forth by the NCDENR. The site
will be graded and paved in a way as to convey the stormwater to a two bio-retention
areas. One is in the front of the site near the right-of-way, the other in the rear. All the
impervious areas from the site will drain to these devices.
There are gravel pretreatment areas at both locations. The area will be grassed
and will have a media depth of two (2) feet. The media will consist of 85% sand with
10% fines and 5% organics the device is to have a drawdown time of 48hours. The
bottom of the drain will be washed sand then washed stone to the under -drain of the
device.
An overflow device will be placed in the device to carry away any volume greater
than the first flush. The overflow and the drained water will weir over the top of the
devices and discharge into existing grassed swales. The grassed lines swales will carry
the water away from the site where it will travel for a great ways before it comes near a
riparian buffer or wetland. To place the devices at the points where they will capture the
most impervious area the 30' buffer strip cannot fit onsite, we fill that the existing
grassed swales and the length to which they flow before they enter a buffer is sufficient to
meet any nutrient removal required.

The volume required was found using the simple method.
V = 3630 X Rd X Rv X A
Device #1
Rv = 0.05 + 0.9 (Ia)
Where our imperious percentage is 45.5%.
Rv = 0.46
Where our depth is set to 12".
A = 3.79ac
V=3630XRdXRvXA
V = 6,327 CU.FT.
V provided = 7,129 CURT.
EMERGENCY SPILLWAY DESIGN
QI00 = 0.55(9.72)3.79 = 20 efs
Q = CLH3/2
SOLVING FOR L LEAVES L= Q/(CH3/2)
L=20/(3)1^1.5 = 6.7'
MAKE L = 10'
EMERGENCY SPILLWAY IS P HIGH WITH AN OPENING 10' WIDE.
Device #2
Rv = 0.05 + 0.9 (Ia)
Where our imperious percentage is 46.7%.
Rv = 0.47
Where our depth is set to 12".
A = 6.22ac
V = 3630 X Rd X Rv X A
V = 10,631 CU.FT.
V provided = 10,872 CU.FT.
EMERGENCY SPILLWAY DESIGN
Q100 = 0.55(9.72)6.22 =33 cfs
Q = CLH3/2
SOLVING FOR L LEAVES L= Q/(CH3/2)
L = 33/(3)JAI .5 = IF
MAKE L = 12'
EMERGENCY SPILLWAY IS I'HIGH WITH AN OPENING IT WIDE.

The bio-retention devices will not be put into place until such time as the site is
stabilized. This will limit the amount of sediment build-up in the devices.
A proposed maintenance agreement and stormwater agreement has been
submitted with this application and will be signed and notarized when the plan is
approved.
The peak runoff of the site before any construction taking place was found using
the rational method.
Q=CIA
Where C = 0.22
Where I = 0.12 in/hr
A = 9.42 ac
Q = 0.25 cfs
The post runoff from out site is as follows.
Runoff from the bioretention devices with a 48 hour infiltration rate
Device #1 drawdown cfs = 0.04 cfs
Device #2 drawdown cfs = 0.04 cfs
A soil mixture will be brought in and mixed onsite to meet the drawdown requirements
specified in this package and in the state bmp manual.
Peak runoff from site after bioretention devices are installed = 0.08cfs
Peak flow into device #1
C = 0.53
I = 0.12 in/hr
A = 3.79 ac
Q=0.24cfs
Peak flow into device #2
C = 0.53
I = 0.12 in/hr
A = 6.22 ac
Q = 0.40 cfs

BIORETENTION DEVICE #1
1 YEAR STORM
TIME
INFLOW
STORAGE
STAGE
OUTFLOW
MIN)
(CFS)
(CU. FT.
FT
0
0
0
0.0
0.00
1
1.10
0
0.0
0.04
2
3.97
63
0.0
0.04
3
7.53
299
0.1
0.04
4
10.40
749
0.1
0.04
5
1 11.50
1370
0.2
0.04
6
10.40
2058
0.3
0.04
7
8.09
2680
0.4
0.04
8
6.24
3163
0.5
0.04
9
4.81
3534
0.5
0.04
10
3.71
3820
0.6
0.04
11
2.86
4040
0.6
0.04
12
2.20
4209
0.6
0.04
13
1.70
4339
0.6
0.04
14
1.31
4439
0.6
0.04
15
1.01
4515
0.7
0.04
16
0.78
4573
0.7
0.04
17
0.60
4618
0.7
0.04
18
0.46
4651
0.7
0.04
19
0.36
4677
0.7
0.04
20
0.28
4696
0.7
0.04
21
1 0.21
4710
0.7
0.04
22
0.16
4720
0.7
0.04
23
0.13
4727
0.7
0.04
24
0.10
4733
0.7
0.04
25
0.08
4736
0.7
0.04
26
0.06
4738
0.7
0.04
27
0.04
4739
0.7
0.04
28
0.03
4740
0.7
0.04
29
1 0.03
4739
0.7
0.04
30
0.02
4738
0.7
0.04
31
0.02
4737
0.7
0.04
32
0.01
4736
0.7
0.04
33
0.01
4734
0.7
0.04
34
0.01
4732
0.7
0.04
35
0.01
4730
0.7
0.04
36
0.00
4728
0.7
0.04
37
0.00
4726
0.7
0.04
38
0.00
4724
0.7
0.04
39
0.00
4722
0.7
0.04
40
0.00
4719
0.7
0.04
41
0.00
4717
0.7
0.04
42
0.00
4715
0.7
0.04
43
0.00
4712
0.7
0.04
44
0.00
4710
0.7
0.04
45
0.00
4708
0.7
0.04
46
0.00
4705
0.7
0.04
47
0.00
4703
0.7
0.04
48
0.00
4701
0.7
0.04
49
0.00
4698
0.7
0.04
50
0.00
4696
0.7
0.04

BIORETENTION DEVICE #2
1 YEAR STORM
TIME
INFLOW
STORAGE
STAGE
OUTFLOW
WEIR
MIN)
(CFS)
(CU. FT.
FT)
(CFS)
(CFS
0
0
0
0.0
0.00
0.00
1
1.88
0
0.0
0.04
0.00
2
6.81
110
0.0
0.04
0.00
3
12.89
516
0.1
0.04
0.00
4
17.82
1288
0.1
0.04
0.00
5
19.70
2354
0.2
0.04
0.00
6
17.82
3534
0.3
0.04
0.00
7
13.85
4601
0.4
0.04
0.00
8
10.68
5430
0.5
0.04
0. -0
9
8.24
6068
0.6
0.04
0.00
10
6.35
6560
0.6
0.04
0.00
11
4.90
6938
0.7
0.04
0.00
12
3.78
7230
0.7
0.04
0.00
13
2.91
7454
0.7
0.04
0.00
14
2.24
7626
0.7
0.04
0.00
15
1.73
7758
0.7
0.04
0.00
16
1.33
7860
0.7
0.04
0.60
17
1.03
7937
0.7
0.04
0.00
18
0.79
7997
0.7
0.04
0.00
19
0.61
8042
0.7
0.04
0.00
20
0.47
8076
0.8
0.04
0.00
21
0.36
8102
0.8
0.04
0.00
22
0.28
8122
0.8
0.04
0.00
23
0.22
8136
0.8
0.04
0.00
24
0.17
8147
0.8
0.04
0.00
25
0.13
8154
0.8
0.04
0.00
26
0.10
8160
0.8
0.04
0.00
27
0.08
8163
0.8
0.04
0.00
28
0.06
8165
0.8
0.04
0.00
29
0.05
8166
0.8
0.04
0.00
30
0.04
8167
0.8
0.04
0.00
31
0.03
8166
0.8
0.04
0.00
32
0.02
8166
0.8
0.04
0.00
33
0.02
8165
0.8
0.04
0.00
34
0.01
8163
0.8
0.04
0.00
35
0.01
8161
0.8
0.04
0.00
36
0.01
8160
0.8
1 0.04
0.00
37
0.01
8158
0.8
0.04
0.00
38
0.00
8156
0.8
0.04
0.00
39
0.00
8153
0.8
0.04
0.00
40
0.00
8151
0.8
0.04
0.00
41
0.00
8149
0.8
0.04
0.00
42
0.00
8147
0.8
0.04
0.00
43
0.00
8144
0.8
0.04
0.60
44
0.00
8142
0.8
0.04
0.00
45
0.00
8140
0.8
0.04
0.00
46 1
0.00
8137
0.8 1
0.04
0.00
47 1
0.00
8135
0.8
0.04
0.00
48 1
0.00
8133
0.8
0.04
0.00

Calculations used in spreadsheets for drawdown and I Oyr routing.
For the column for time the number has entered in one min increments for the peak flow
routing and thirty mins for drawdown.
For the inflow column on the drawdown is set to zero, the drawdown just shows the time
it takes for the device to empty from full.
For the inflow column on the peak flow routing two different equations for a time 1.25
times the time to peak.
For time less than 1.25 times the Tp the equation used is:
(Qp/2) X [1-cos((pi X time)/Tp)]
For time greater than 1.25 times the Tp the equation used is:
4.34 X Qp X exp[-1.3(time/Tp)]
Where;
Qp = Peak Flow
Tp = Time to peak
For the storage column for storage the initial amount is set to full for the drawdown
For the storage column for storage the initial amount is set to empty for the peakflow and
the equation for the rest of the column is:
(Outflow @ time y — Inflow @ time y) X time increment) + previous storage
For the stage column the initial amount is set to zero for the peak flow.
For the rest of the column in peak flow and for all the cells in the drawdown this equation
is used:
Stage = (storage / Ks ) ^ (1/b)
Where;
B =[In(Storage2/Storagel)]/[ln(stage2/stagel)]
Ks = Storage 2 / (stage 2^b)
For the Outflow columns the equation used in both the peak and drawdown calculations
is:
Outflow = 48Hr drawdown rate.

For the culvert flow two equations are used depending on the stage.
For a stage less than the diameter of the pipe the equation used is:
Q = 0.372 X Cd X pipe dia X stage^1.5
For a stage greater than the diameter of the pipe the equation used is:
Q = 0.0437 X ((pipe dia)^2) X (stage — (pipe dia /24))^0.5
Where Cd = Coefficient of discharge = 0.65
For the Weir flow column the equation used is:
Q = Cw X weir length X (Stage over spillway)^1.5
Where Cw = weir coefficient = 3