SW6210202_NCDEQ Hydrology Report - Rev. 1_20210512

Hydrology Study
For
Ft. Bragg
Ft. Bragg, North Carolina
Prepared for:
North Carolina Department of Environmental Quality
225 Green Street, Suite 714
Fayetteville, NC 28301
Prepared by:
Steve Waldron
C)WE
ENGINEER S
October 30, 2020
Rev. 1 - May 12, 2021

Contents
I. Executive Summary..............................................................................1
II. Hydrology.............................................................................................1
A. Objectives................................................................................................................... 1
B. Model Development................................................................................................... 1
III. Existing Conditions Hydrologic Analysis (Pre -Development) ................. 3
IV. Proposed Conditions Hydrologic Analysis (Post-Development).............4
V. Conclusion............................................................................................ 7
Tables
Table 1: Rainfall Recurrence Intervals............................................................................2
Table 2: Study Point #1 Pre -Development Peak Flows....................................................3
Table 3: Study Point #2 Pre -Development Peak Flows....................................................4
Table 4: Overall Site Pre -Development Peak Flows........................................................4
Table 5: Study Point #1 Post -Development Peak Flows..................................................5
Table 6: Study Point #2 Post -Development Peak Flows..................................................5
Table 7: Overall Site Post -Development Peak Flows.......................................................6
Table 8: Study Point #1 Pre -Development and Post -Development Comparison ..............6
Table 9: Study Point #2 Pre -Development and Post -Development Comparison ..............6
Table 10: Overall Site Pre -Development and Post -Development Comparison.................7
Exhibits
Exhibit1: Vicinity Map..................................................................................................9
Exhibit2: Soils Map.....................................................................................................10
Appendices
Appendix 1: Pope Army NOAA Precipitation Data........................................................12
Appendix 2: Drainage Basin Maps...............................................................................13
Appendix 3: Calculations & Hydraflow / Storm Sewer Output......................................16

I. Executive Summary
Lowe Engineers (henceforth to be referred to as "Lowe") has prepared this hydrological study on
behalf of the United States Army Corps of Engineers for the proposed development of the SOF
Operations Addition. The facility includes the construction of a 55,000 SF building, pavement, and
approximately 277 parking spaces. The Ft. Bragg SOF Addition project proposes to develop ±5.78
acres of property located at an unnamed roadway near the intersection of Lamont Rd/McKellars
Rd, Ft. Bragg, North Carolina in Cumberland County. The project location is depicted on the Vicinity
Map, Exhibit 1.
II. Hydrology
A. Objectives
This hydrology study evaluates the existing (pre -development) and proposed (post -development)
hydrologic conditions of the site to determine the requirements for stormwater quality, detention,
and extreme flood protection. This study was developed using the federal and state laws which
govern the National Pollutant Discharge Elimination System (NPDES), as part of the CFR 40 Clean
Water Act, and mandates local authorities to review, permit, and enforce all point discharges into
waters of the United States, such as land disturbing activities in excess of 1-acre.
B. Model Development
The NRCS Discrete Method, Hydraflow version 2020, and Storm Sewers version 2020 (which are
extensions of AutoCAD Civil 3D 2020) were used to model the existing and proposed drainage,
bioretention cells, and pipe sizes. The following information is necessary to utilize the SCS method:
1. Determination of curve numbers in the drainage areas which is representative of the land
uses and their hydrologic soil types.
2. Calculations of the times of concentration to their respective bioretention cells.
3. Type II rainfall distribution hydrographs, which are used to determine the total and excess
rainfall amounts.
The model uses a standard time series plot generated for an SCS Type II 24-hour rainfall
distribution event for the 1-, 2-, 10-, 25-, 50-, and 100-year rainfall depths specific to that
geographic location. Rainfall depths for this project were obtained from the NOAA Precipitation
Frequency Data Server (PFDS). The design precipitation depths are listed below in Table 1.
Additionally, Lowe designed proposed additions to the existing storm pipe network by entering
Hydraflow bioretention cell outflow data into Storm Sewers. The graphical analysis and
calculations can be reviewed in Appendix 3.

Table 1: Rainfall Recurrence Intervals
Rainfall Recurrence Depth
Interval (Inches)
Required Retention 1.00
1-year
3.06
2-year
3.70
10-year
5.46
25-year
6.52
50-year
7.38
100-year
8.26
To accurately determine total discharge volumes and rates, Lowe divided pre -developed and
post -developed sites into sub -basins with varying curve numbers. Because each of the sub -basins
are relatively small, times of concentration for each sub -basin are 5 minutes. Per the USGS Soils
Map (Exhibit 2), Lowe entered curve numbers for Hydrologic Soils Groups (HSGs) "A". To show
retention of the first 1" from impervious area, Lowe utilized NRCS Discrete Method calculations.
See Appendix 3 for NRCS-modeled runoff calculations.
Areas of relatively concentrated flows were assigned hydrological study point numbers, of which
there are 2. The study points and descriptions of their discharge areas are listed below...
• Study Point #1 includes...
o Discharge area for "PRE -BASIN #1", which includes building O190U, its associated
roadways/parking, adjacent embankments, and offsite runoff originating from
the South.
o Discharge area for "POST -BASIN #1", which includes the proposed building,
surrounding roadways, and adjacent embankments.
• Study Point #2 includes...
o Discharge area for "PRE -BASIN #2", which is composed primarily of trees and
vehicle -compacted soil.
o Discharge area for "POST -BASIN #2", which includes additional facility parking.
To review the sub -basin maps as well as all study point locations and pre -basin and post -basin
routing, see drainage basin maps in Appendix 2. To ensure adequate bioretention cell volumes
and properly designed outlet control structures, Lowe routed the hydrographs for various "POST -
#X" hydrographs to their corresponding bioretention cells. To confirm 100-Yr conveyance, 25-Yr
2

hydraulic grade lines, and 10-Yr event outlet velocities adherence to NPDES standards, Lowe
entered flow rates from pipe outlets into Storm Sewers. Results from the Storm Sewers analysis
can be reviewed in Appendix 3.
III. Existing Conditions Hydrologic Analysis (Pre -Development)
The project site totals approx. 5.78 acres of pervious and impervious area. The impervious area
consists of two buildings and roadways/parking that total approx. 2.5 acres. The remaining
pervious area is comprised of woods and open space with soils primarily consisting of well -
draining soils (HSG "A"). The topography slopes mildly to moderately across the site from East to
West. The site is located within the Cape Fear River Basin and drains Northwestwardly to Cypress
Creek which connects to Little River approximately 3 miles downstream. Per 2018 NCDEQ "2018
Final 303(d) List" no portion of Cypress Creek is classified as a 303(d) impaired waterbody. Most
upstream runoff, including much of that denoted as "OFFSITE", appears to flow through the
existing on -site stormwater network to the nearby existing detention area.
Lowe calculated pre -development peak flowrates and hydraulic volumes. The results of those
calculations are depicted in the tables below. Approximately 14 acres of upstream runoff appears
to drain to the nearby detention area, but because the upstream runoff is routed through the
existing network (which will be re-routed through a portion of the proposed network to be used
exclusively for conveyance of existing upstream runoff), it was not included in the calculations for
this report. Table 2 shows the flowrates and hydraulic volumes for "Study Pt. #1". Table 3 shows
the flowrates and hydraulic volumes for "Study Pt. #2". Table 4 shows the overall site flowrates
and hydraulic volumes for each pre -developed stormwater event.
Note: Values for the following tables are the results of Hydraflow Analysis (Appendix 4).
Table 2: Study Point #1 Pre -Development Peak Flows (Pre -Basin #1)
Frequency (yr)
Peak Discharge (cfs)
Hydraulic Volume (ft3)
1
14.96
34,925
2
18.16
42,587
5
24.02
56,083
10
29.25
68,059
25
36.94
85,714
50
43.21
100,134
100
49.73
115,2

Table 3: Study Point #2 Pre -Development Peak Flows (Pre -Basin #2)
Frequency (yr)
Peak Discharge (cfs)
Hydraulic Volume (ft3)
1
2.46
4,981
2
3.42
6,877
5
5.17
10,408
10
6.75
13,670
25
9.17
18,612
50
11.14
22,723
100
13.20
27,067
Table 4: Overall Site Pre -Development Peak Flows (Pre -Basin #1 & Pre -Basin #2)
Frequency (yr)
Peak Discharge (cfs)
Hydraulic Volume (ft3)
1
17.33
39,907
2
21.50
49,464
5
29.15
66,491
10
36.00
81,729
25
46.11
104,326
50
54.35
122,856
100
62.94
142,275
IV. Proposed Conditions Hydrologic Analysis (Post -Development)
The proposed site consists of a 55,000 SF -footprint building, pavement, and approximately 263
parking spaces. The addition of these materials will result in the percentage of post -developed
impervious area being greater than that of the impervious area in pre -developed conditions.
Impervious area percentages for individual sub -basins can be reviewed in Appendix 2.
To meet NCDEQ and NPDES requirements, the post -developed site stormwater control measures
were designed to ensure that the first 1" of post -developed runoff generated by impervious area
is retained on -site (required and designed retention volumes for each drainage area can be
reviewed in Appendix 3). Additionally, Lowe designed the retention measures to ensure that the
annual post -developed hydraulic volume is less than a 10% increase over the pre -developed
hydraulic volume.
Differing site conditions between the pre -developed and post -developed site will result in
increased impervious area. As a result of the increased impervious area, soil infiltration rates will
be reduced. Because the re -developed site will be infiltrating at a lower rate, stormwater runoff
will increase. Increased runoff generated by expanded parking areas and roadways will be routed
to seven Bioretention Cells. The existing detention area inflows, where all "POST -BASIN #1"
treated stormwater will be routed, is "Study Pt. #1". Table 5 shows the flowrates and hydraulic
4

volumes for "Study Pt. #1". Table 6 shows the flowrates and hydraulic volumes for "Study Pt. #2".
Table 7 shows the overall site flowrates and hydraulic volumes for each post -developed
stormwater event. The increased post -developed runoff will be controlled by routing the
applicable post -developed area to bioretention cells for stormwater quality and quantity control.
Five of the seven bioretention cell OCSs control and direct flow to "Study Pt. #1" (existing
detention area). Due to topographical constraints, portions of the post -developed site runoff will
not be collected. However, uncollected runoff generated from said proposed impervious area is
approximately equivalent to the existing area and location which currently discharges to the
existing detention area. Uncollected runoff from proposed pervious area is included in the total
runoff calculations for each study point as well as the entire site. The results of these calculations
can be broadly reviewed in Table 10 and in greater detail in Appendix 3.
Lowe also calculated the pre -developed and post -developed comparisons of peak flowrates and
hydraulic volumes for each study point and rain event. Table 5 shows the flowrates and hydraulic
volumes for all events for "Study Pt. #1". Table 6 shows the flowrates and hydraulic volumes for
all events for "Study Pt. #2". Table 7 shows the flowrates and hydraulic volumes for all events
sitewide. Tables 8 and 9 show pre- and post -development comparisons for the site. Table 10
shows that the overall post -developed site peak hydraulic volumes are less than a 10% increase
for all events, and therefore adheres to the limits set by the NCDEQ Stormwater Design Manual.
Note: Values for the following tables are the results of Hydraflow Analysis (Appendix 3).
Table 5: Study Point #1 Post -Development Peak Flows (Post -Basin #1)
Frequency (yr)
Peak Discharge (cfs)
Hydraulic Volume (W)
1
16.89
35,686
2
20.08
43,870
5
25.48
58,000
10
30.27
70,341
25
37.41
88,330
50
43.27
102,905
100
49.19
118,070
Table 6: Study Point #2 Post -Development Peak Flows (Post -Basin #2)
Frequency (yr)
Peak Discharge (cfs)
Hydraulic Volume (W)
1
4.10
7,432
2
4.87
9,471
5
6.32
13,079
10
7.65
16,306
25
9.71
21,102
50
11.39
25,053
100
13.17
29,21

Table 7: Overall Site Post -Development Peak Flows (Post -Basin #1 & Post -Basin #2)
Frequency (yr)
Peak Discharge (cfs)
Hydraulic Volume (ft3)
1
20.99
43,118
2
24.94
53,341
5
31.80
71,079
10
37.92
86,648
25
47.12
109,432
50
54.65
127,958
100
62.36
147,28
Table 8: Study Pt. #1 Pre -Development & Post -Development Comparison
Frequency (yr)
Pre -Development
Peak Discharge
(cfs)
Pre -Development
Hydraulic Volume
(ft3)
post -Development
Peak Discharge (cfs)
Post -Development
3
Hydraulic Volume (ft )
1
14.96
34,925
16.89
35,686
2
18.16
42,587
20.08
431870
5
24.02
56,083
25.48
58,000
10
29.25
68,059
30.27
70,341
25
1 36.94
1 85,714
37.41
881330
50
43.21
100,134
43.27
102,905
100
49.73
115,208
49.19
118,070
Table 9: Study Pt. #2 Pre -Development & Post -Development Comparison
Frequency (yr)
Pre -Development
Peak Discharge
(cfs)
Pre -Development
Hydraulic Volume
(ft3)
post -Development
Peak Discharge (cfs)
Post -Development
3
Hydraulic Volume (ft )
1
2.46
4,981
4.10
7,432
2
3.42
6,877
4.87
9,471
5
5.17
10,408
6.32
13,079
10
6.75
13,670
7.65
16,306
25
9.17
18,612
9.71
21,102
50
11.14
22,723
11.39
251053
100
13.20
27,067
13.17
29,210

Table 10: Overall Site Pre -Development & Post -Development Comparison
Frequency (yr)
Pre -Development
Peak Discharge
(cfs)
Pre -Development
Hydraulic Volume
(ft3)
post -Development
Peak Discharge (cfs)
Post -Development
3
Hydraulic Volume (ft )
1
17.33
39,907
20.99
43,118
2
21.50
49,464
24.94
531341
5
29.15
66,491
31.80
71,079
10
36.00
81,729
37.92
86,648
25
46.11
104,326
47.12
1091432
50
54.35
122,856
54.65
127,958
100
62.94
142,275
62.36
147,280
V. Conclusion
The proposed site plan's increase in impervious area will result in varying pre -developed and
post -developed peak flowrate and hydraulic volume changes. Study Point #1 results show that
pre -developed and post -developed flowrate and hydraulic volume percentage changes increase
steadily as the event frequency decreases, with all runoff from all rain events resulting in slight
volume increases. Study Point #2 shows similar reductions as those calculated for Study Point #1.
Because the site was designed to retain the post -developed runoff generated by the first 1" of
rainfall, the overall site post -developed conditions will increase hydraulic volumes for all rain
events, but by less than 10%. Therefore, runoff from all storms will fall within the NC Stormwater
Rule 15A NCAC 02H .1002 (44) "Runoff Volume Match" definition which states that "annual
runoff volume after development shall not be more than ten percent higher than the annual
runoff volume before development."

Exhibits
Exhibit 1 Vicinity Map
Exhibit 2 Soils Map

Exhibit 1
Vicinity Map
E

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—vww
Exhibit 2
3 Soil Map —Cumberland County, North Carolina
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USDA Natural Resources Web Soil Survey 611512020
21111111111111 Conservation Service National Cooperative Soil Survey Page 1 or 3
Map Unit Legend
Map Unit Symbol
Map link Name
Acres in AOI
percent of AOI
BaB
Blaney loamy sand, 2 to 8
1.3
0.4%
percent slopes
BaD
Blaney loamy sand, 8 to 15
60.1
20.3%
percent slopes
B 6
Bragg sandy loam, 1 to 4
19.7
6.7%
percent slopes
CaB
Candor sand, 1 to 8 percent
129.2
43.7%
slopes
JT
Johnston loam
14.7
5.01A
NoB
Norfolk loamy sand, 2 to 6
29.8
10.1 %
percent slopes
WaB
Wagram loamy sand, 0 to 6
40.5
13.7%
percent slopes
Totals for Area of Interest
295A
100.0%
10

Appendices
Appendix 1: Pope Army Airfield NOAA Precipitation Data
Appendix 2: Drainage Basin Maps
Appendix 3: Calculations & Hydraf low / Storm Sewer Output
11

Appendix 1
Pope Army Airfield NOAA Precipitation Data
NOAA Atlas 14, Volume 2. Version 3 POPE AFB
Station ID: 314=1
Location name: Pope PuTuy Air45eld, North
Carolina, USA*
Latitude: 35.173P, LongikAe:-79.00W ��
Elevation_
Elevation Istation nleladata): 218 ft*'
' wu'ce: E3R- Map:
" spuroa: UEG3
POINT PRECiPrrATION FREQUENCY ESTIMATES
.a _- -- - 1 - 6 _ - ==zrosk, M.'rekta a9d G. Rehr
`JClAA. F4a:.•2 .'.e:e Z-he 3prft Maryor,d
PF tabular r=3r_pr=;i I MLapL & aerials
PF tabular
PDS-based
point precipitabon "uency estimates with 90% confidence intervals [in inche!W
duration
Average
recurrence iniprval wears}
1 2
5
10
25 11
50
100
200
1 5E10
1OW
5-"n
0-AM D.512
0,59E
Dim
0.732
0-783
0.132
0-075
0-921
0.96E
0.391-0-97I 0.461-0a?4
-OM9
0.592-0335
6.55".817
-74X-OA73
'0.738-0.925
.7744k973
,813-1.03
.8431
10-Min
GAM 0.819
11
0,955
1-05
11-17
1.25
1-32
1.39
1AT
1,53
Qs2A-11.776' 0.73M-918
RH5Q1.
_4i5-1. ll
1.11-1.39
1.17-1.4
123-1.51
129-1.63
1.33-1.69
15-min
0-1167
1.03
121
1-33
1-48
1.5E
1-67
1.75
1.25
1,9'I
0.780-0.97
AV-1-15
1.09-i-35
12i"AlI1.32-1.E5
1A1-175
q 1.£2-2.0E
1.67-2.12
30-min
1.19
1AI2
1.72
1-93
L79
2.38
2-56
2.73
2,94
3-10
1.07-1.33
12P 1
1.55-1-42
1-73-2-1
196-2.44
2-13-2�fi5Y
�258-327
70-3.41
1.48
1.79
2.20
251
292
3.22
153
3.22
422
4M
60�r1in
1.33-1.£5
111-2.
1. &2AI
2A1
.E13.25
2-ti8-3S9'i
3.13-3.
3.38-425
3.70,4.68
.94-5.02
1.74
210
2.63
3.03
3-%
3.9E
4-AD
4.22
5,37
5M
2fir
2.34-297
70-3 +12
-15-4.63
3-51AM
3A5-4.9E
4.19-SA3
4.fO-&W
4.%-6.FA
1m
223
2A0
326
&W
4.37
d_RR
5A1
G_14
6.73
1.55-2.09
2,53
2.50- t18
90-3�fi8
.43-4.37
3.&dA94Y
42&6.50
4.b3�s-10
525{k91
5.70-7.Sfi
220
2-"
3 35
3-90
&65
5-26
5-911
G.56
7.45
R22
6 �1r
1.99-2,45
Al-2.%
3.023-32
1-4.3x
-ib5.15
&M,5d2
520-6.51
5.73-722
6AS324
.01�.Q5
2G0
3.1A
3.97
4-65
am
-r.
R.04
92B
10,312a1r
I
{2.35-2-CA1
f2AS�3.48}V
11AA1
4.1&.15
9M.15
-
5.G67AnY
d.30
.98-W11
2
&57-11.2
2A�1r
3.07
ii 3.71
4.68
-1.44
6 %
7.34
821
9.11
1 d-3
113
;2.85-3.315
II E3-d4d.06}
4.3d-5.D4
S-U43.8
e-00-7.00
15--m 90
.54-8.63
8.MAr.;9
9.42-11-1
10-3122
2_day
335E
411
1.38
6.24
TA2
835
-32
10.3
11.7
128
{3.32-363}
fAA1d.d1S
5.0Y-5.7a
5-90-5.59
5.87-7.95
7-72�.96Y
8.58-16.0
.4E-11.1
10.7-12.6
11-5-13.7
3-day
3.78
4.55
5.67
G.55
T-77
8.73
9-73
14.8
12.2
13-3
;3.53-4.045
f426d.87}
5.29£.-05
10-7.00
213.30
E08-9.33'
fa-1D
.89-11.5
11.1-13-0
121-142
4-dap
4.00
4.R8
5-95
6-86
8-11
9.12
10-1
11-2
12-7
13-8
3.74-425
.50-5.12
5.5663d
1-7.30
.55-B.54
6A&900
.37-1Q6
113-11.9
11.E-135
126r147
7-dap
4-63
5M
6.77
7-75
9-10
1QZ
11.3
12.4
14A
152
4.32,&95
15-�5-92
fi.30-7-M
=41-30
A4-9.74
9A2-10.9
10.4-12.1
11.4-13.3
12.8-15.0
13-9-ifk3
10
5.28
629
TM
8-61
9,98
11-1
133
14-8
159
4.97-5-62
2-6
7.12 Ufi
07A-t4
.33-10.5
i�3-11-7
11-3-12.9
123-14-1
13Jfi-15.7
14-7-17J
20-d3y
7-11
8-Ai
9.97
112
12-9
I.12-1
14-2
15-5
16.1
11-6
20-0
I
5.57-7-58
A9-8-97
9.34-10jfi
10,5-11-
=13.7
132-15.1
14.4-ifk5
iS5-17.9
17.1-19.9
fl&4-21A)
am
10-A
122
13-5
15,3
19-6
1&0
19.3
21-1
225
y
6.34-9A2
9.82-11-t
11A-12.9
-7-t4.4 13
IL H,J
15Ap-17.7
16.E-1
1&0-2a�5
19�fi122E
_
f26.&.2&1
11-2
13-1
15.1
16.6
1"
20-6
21-A
22-1
24-6
26-0
y
10.E-11.9
12.4-13.
142-150
11 17.
17A-19�E
183-21.1
0.122
21.3242
23 .2
! j242-27.7
13.4
15 7
1-1-8
19.5
21-G
23-2
24-7
262
21.1
25-6
12.7-142Y
f14.9-16.5
F6}-18-8
18.4-20.
0A-2_8
21.9-2d.4
3.3-26.1
24-5r27-7
26.4-29.E
-731A
1 Pr9NR=cn teeja-: • e: -. � '9Is tatAe are based on frequency anaysle arpartial dmilon series [Pd3�
Nrrnters :n parmtl_a a o-e =_ : _: = a : a-� o:rsr a-d uppeFbornds oT the 90% canlldenoe Wtereal. The prob3tClty that pre*Itatim frequency EVInnates i.ror
r agiven durationand average t:. -_ :e :_- , ' be greater dean ire upper board {or less than me b1«eF board}is 5%. Estimaleaat uppertaurds are
not Mecked agalnsi probable rat .. .-a:. :.:: : FMP} esti hates and may be higher liar orrrerNy valld aMP VAK%.
Please reierta NOAA Atiaa 14 docu^ -,:':r more Irdormatlon.
12

Appendix 2
Drainage Basin Maps
13

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Calculations & Hydraflow / Storm Sewer Output
Underdrain Calculations
where: D = Diameter of single pipe (inches)
n = Roughness factor (recommended to I)e 0.011)
= Internal slope (recommended to be 4.5%)
Q = Underdrain design flow (cfs)
Bioretention Cell #1
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in ft
1 hr = 0.00002315 s
3
Q = 1443 ft2 * 0.00002315 st * 10 = 0.334fs
(0.334 f t3I * 0.011
D = 16 0 So s = 2.23 inches
Table 1: Number of 4" Pipes Required in the Underdrain
)f D is Less Than No- of 4'Pipes
5.13 2
&66 4
7.22 5
7.75 6
8 20 7
16

Bioretention Cell #2
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in ft
1 hr = 0.00002315 s
3
Q = 816 f tZ * 0.00002315 st * 10 = 0.189 f S
(0.189 f t3I * 0.011
D = 16 0 So s = 1.80 inches
Table 1: Number o€4"Pipes Required in the Und4erdrain
WD Is Less Then No, of Pipes
5.13
5.95 a
6.66 4
7.22 5
7.75 6
8.20 ?
Bioretention Cell #3
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in ft
1 hr = 0.00002315 s
3
Q = 1,649 f tZ * 0.00002315 st * 10 = 0.382 fs
(0.382 f t3I * 0.011
D = 16 = 2.34 inches 0 So s
17

Table 1: Number o€4"Pipes Required in the Undi�rdrain
If D is Less Than No, of Pipes
5.1 L
5.95 3
6.66 4
.22 5
7.75 b
8.20 7
Bioretention Cell #4
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in ft
1 hr = 0.00002315 s
3
Q = 610 ft2 * 0.00002315 st*10=0.141fs
3
(0.141ft )*0.011
D = 16 = 1.61 inches 0 So s
Table 1: Number o€4"Pipes Required in the Und4erdraln
If D is I ess Than No. of 4" Pipes
5.93 2
5.95 3
6.66 4
7.22 5
7.75 6
8.20 7
18

Bioretention Cell #5
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in ft
1 hr = 0.00002315 s
3
Q = 1,015 f tZ * 0.00002315 st * 10 = 0.235 f S
(0.235 ft3I * 0.011
D = 16 0 So s = 1.95 inches
Table 1: Number o€4"Pipes Required in the Und4erdrain
WD Is Less Then No, of Pipes
5.13
5.95 a
6.66 4
7.22 5
7.75 6
8.20 ?
Bioretention Cell #6
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in f t
1 hr = 0.00002315 s
3
Q = 1,921 f tZ * 0.00002315 st * 10 = 0.445 fs
3
(0.445 fs I * 0.011
D = 16 = 2.48 inches 0 So s
19

Table 1: Number of 4" Pipes Required in the Undi�rdrain
If D is Less Than No, of Pipes
5.1 L
5.95 3
6.66 4
.22 5
7.75 6
8.20
Bioretention Cell #7
Q = Media Surface Area (ft2) * Media Infiltration Rate (f t1s) * Safety Factor
in f t
1 hr = 0.00002315 s
3
Q = 2,222 f tZ * 0.00002315 st * 10 = 0.514 f S
3
(0.514fs I*0.011
D = 16 0 So s = 2.62 inches
Table 1: Number of 4" Pipes Required in the Underdrain
y No, of Pipes
.} 2
5.95 3
6.66 4
7.22 5
7.75 6
8.20 7
20

Runoff Volume Match Calculations
Discrete NRCS Curare Number Method for Runoff Depth
_ 1000 - 1
CN
Where: S = Maximum retention after rainfall begins (in)
N = Curve number (unitless)
ZZ (P—O.S)2
(P + 0. S)
Where: Q' = Runoff depth (in)
P = Rainfall depth (in)
21

FT. BRAGG SOF ADDITION BIORETENTION CELLS VOLUME (FT3) (NRCS)
ImperviousCN
Retention Depth (in) =
1
BASIN
Area (sf)
Area (ac)
S =
0.20
1" Q (in) =
0.79
1
24829
0.57
98
1" Q Volume Required (ft3) =
1636
Retention Volume Provided (ft3) =
1752
S =
0.20
1" Post Q (in) =
0.79
2
13504
0.31
98
1" Post Q Volume (ft3) =
890
Retention Volume Provided (ft3) =
950
S =
0.20
1" Post Q (in) =
0.79
3
29621
0.68
98
1" Post Q Volume (ft3) =
1952
Retention Volume Provided (ft3) =
1969
S =
0.20
1" Post Q (in) =
0.79
4
20909
0.48
98
1" Post Q Volume (ft3) =
1378
Retention Volume Provided (ft3) =
1616
S =
0.20
1" Post Q (in) =
0.79
5
18295
0.42
98
1" Post Q Volume (ft3) =
1206
Retention Volume Provided (ft3) =
1258
Spost =
0.20
1" Post Q (in) =
0.79
6
30056.4
0.69
98
1" Post Q Volume (ft3) =
1981
Retention Volume Provided (ft3) =
2246
S =
0.20
1" Post Q (in) =
0.79
7
39204
0.90
98
1" Post Q Volume (ft3) =
2584
Retention Volume Provided (ft3) =
2627