SW3230603_Design Calculations_20230821

DOLLAR GENERAL — STORE #25122
ROCKWELL, NC
HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT
BREC
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Prepared By: Prepared For: Project Site:
BREC, P.A. Teramore Development, LLC TBD Crescent Road
1520 Meadowview Drive Joe Strickland, Dir. of Ops., NC Rockwell, NC
Wilkesboro, NC 28697 214 Klumac Rd., Ste. 101
NCBELS Firm #C-3448 Salisbury, NC 28144
[phone] [phone]
[e-mail] [e-mail]
21 August 2023
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%,, C.CN° .\`‘ Justin Church, PE
`� Principal Engineer
BREC, P.A. Dollar General— Store #25122 1

Table of Contents
1.0 General Information 4
2.0 Methodology 4
2.1 Rainfall Data 4
2.2 Time of Concentration Calculations 5
2.3 Runoff Coefficient Calculations 5
2.4 Pipe Design 7
2.4.1 Outlet Design 7
2.5 Hydrology Calculations 8
2.5.1 Modified Curve Number Calculations 9
2.6 Sand Filter Design 9
2.6.1 Chamber and Filter Sizing 10
2.6.2 Underdrain Sizing 10
2.6.3 Emergency Spillway Calculations 11
2.6.4 Anti-floatation Calculations 12
3.0 Results 13
Appendix 14
Post-Construction Design Storm (1 inch)
Post-construction 1 Year, 24 Hour
Post-construction 10 Year, 24 Hour
Post-construction 100 Year, 24 Hour
Soils Report
BREC, P.A. Dollar General— Store #25122 2

List of Figures
Figure 1 - System Storage of Design Storm (1 inch) 13
List of Tables
Table 1 - NOAA Rainfall Depths 4
Table 2 - NOAA Rainfall Intensity 4
Table 3 - Post-Construction ToC 5
Table 4 - Curve Numbers 5
Table 5 - Post-Construction Drainage Areas 6
Table 6 - Composite Rational Coefficients 6
Table 7 - Pipe Design Summary 7
Table 8 - Riprap Outlet Stone Sizing 8
Table 9 - Riprap Apron Dimensions 8
Table 10 - Modified SCS Curve Numbers 9
Table 11 - Sand Filter State-Storage 10
Table 12 - Pipe Design Table 14
BREC, P.A. Dollar General— Store #25122 3

1.0 General Information
This document contains hydrologic and hydraulic calculations demonstrating
compliance with post-construction state and local stormwater regulations for the
construction of a 12,794 ft2 Dollar General retail store at the intersection of US
Highway 52 and Crescent Road in Rockwell, NC. Specifically, regulations for this
area require water quality treatment of the one inch design storm'.
The subject tract is composed of a vacant lot with unmaintained grass and gravel
cover. No jurisdictional features are present on the site. The site is primarily
composed of CfB soils which are classified as hydrologic soil group B. Post
construction stormwater requirements will be satisfied with an above ground sand
filter which is approved by NCDEQ as a primary stormwater control measure
(SCM).
2.0 Methodology
Six post-construction drainage areas covering the subject tract improvements
were identified. See plansheets for delineation.
2.1 Rainfall Data
Rainfall data was taken from NOAA Atlas 14, Volume 2, Version 3 for Rockwell,
NC. Rainfall depths for each design storm are summarized below.
Table 1 - NOAA Rainfall Depths
Storm Rainfall Depth (in)
Design Storm 1.00
1 Year, 24 Hour 2.80
10 Year, 24 Hour 4.91
100 Year, 24 Hour 7.30
Table 2 - NOAA Rainfall Intensity
Storm Rainfall Intensity (in/hr)
10 Year, 5 Min 7.09
1 15A NCAC 02H.1001 -Stormwater Management
BREC, P.A. Dollar General— Store #25122 4

2.2 Time of Concentration Calculations
Time of concentration values were calculated using the Kerby and Kirpich
equations for overland and channel flow time.
Tc = Tov + Tch (min)
To„ = 0.828(Lo„N)o.467S671 o.235 (min) (Kerby)
Lo„ = Overland f low length (f t)
N = Retardance coefficient (dimensionless)
So„ = Average surf ace slope (f t/f t)
Tch = 0.0078L47 7S5y0.385 (min) (Kirpich)
Lch = Channel flow length (ft)
Sch = Average channel slope (f t/f t)
All areas less than or equal to 0.5 acres were assigned a minimum time of
concentration of 5 minutes.
Table 3 - Post-Construction ToC
Name Tov Tch Tc—calc Tc—assumed
(min) (min) (min) (min)
POST-CON:1 - - - 5
POST-CON:2 - - 5
POST-CON:3 - - - 5
POST-CON:4 - - - 5
POST-CON:5 - _ _ 5
POST-CON:6 5
2.3 Runoff Coefficient Calculations
Composite SCS curve numbers and rational coefficients were calculated as
weighted averages using values summarized below.
Table 4 -Curve Numbers
GROUND COVER HSG B
CN C
IMPERVIOUS 98 0.95
GRASS 61 0.20
WOODED 55 0.15
BREC, P.A. Dollar General— Store #25122 5

For post-construction drainage areas, the composite curve number is calculated
as: (Aimppost X CNimp + Agrasspost X CNgrass + Awoodedpost x CNwooded)
CNcomPpost
Atotalpost
Table 5 - Post-Construction Drainage Areas
Name HSG Out Atotal Aim
Agrass Awooded CNcom
f all (acres) (acres) (acres) (acres) l ppost
POST-CON:1+ B 1 0.30 0.30 0.00 0.00 98.0
POST-CON:2+ B 1 0.21 0.20 0.01 0.00 96.7
POST-CON:3+ B 1 0.51 0.50 0.01 0.00 97.0
POST-CON:4+ B 1 0.15 0.00 0.15 0.00 61.0
POST-CON:5+A B 1 0.21 0.02 0.19 0.00 64.7
POST-CON:6 B 1 0.75 0.19 0.56 0.00 70.5
TOTAL 2.12 1.20 0.92 0.00 61.6
SUBTOTAL+ (TO SCM) 1.37 1.10 0.36 0.00 72.5
+ Routed through sand filter prior to discharge.
A Includes areas of assumed off-site 100% BUA.
The site also contains 1,690 ft2 of BUA in the driveway stem that cannot be routed
to the SCM due to topographic constraints. The area does not meet low density
thresholds; therefore, a variance is requested for this area per 15A NCAC 02H
.1003(6).
A composite rational runoff coefficient and associated discharge was also
calculated for each post-construction drainage area for pipe and swale
conveyance sizing.
Aimppost X Cimp + Agrasspost X Cimp + Awoodedpost X Cwooded
Ccomp =
Atotalpost
Q = CcomplA
Table 6 - Composite Rational Coefficients
Name Atotal `gimp Agrass Awooded Ccomp Q f t3
(acres) (acres) (acres) (acres) s
POST-CON:1* 0.30 0.30 0.00 0.00 0.95 11.99
POST-CON:3* 0.51 0.50 0.01 0.00 0.93 3.37
POST-CON:4* 0.15 0.00 0.15 0.00 0.20 0.21
BREC, P.A. Dollar General- Store #25122 6

Name Atotal Aimp Agrass Awooded C ft3
(acres) (acres) (acres) (acres) comp n s
POST-CON:5* 0.21 0.02 0.19 0.00 0.27 0.41
POST-CON:6* 0.75 0.19 0.56 0.00 0.39 2.09
* Qio
** Q25
2.4 Pipe Design
Rational discharges from the delineated drainage areas were used to size pipes
by solving Manning's equation iteratively for the normal depth.
Table 7 - Pipe Design Summary
ft3 rs
NAME DIA (in) Qmax Qdesign
PIPE-1 15 7.0 2.4
PIPE-2 15 5.1 3.8
PIPE-3 18 12.9 7.3
PIPE-4 15 7.6 2.1
PIPE-5 18 I 8.7 I 7.1
See Appendix for complete pipe design table.
2.4.1 Outlet Design
Outlet design for pipes discharging to ground surface was taken from Chapter 10
of HEC-14. Riprap Dso was calculated as:
4
Qdesign 3 ( D
D50 = 0.2D / D2.5 \TWI
Where,
D50 = Riprap median diameter (ft)
Qdesign = Design discharge (ft3/s)
D = Pipe diameter (ft)
TW = Tailwater depth (ft)
g = gravitational acceleration (32.2 ft/s2)
Substituting the corresponding pipe diameters and design discharges into the
equation above yields:
BREC, P.A. Dollar General- Store #25122 7

Table 8 - Riprap Outlet Stone Sizing
NAME DIA (in) Qdesign (fs ) D50calc (in) D50design (in)
PIPE-3 18 7.3 3.3 5 (CLASS A)
PIPE-4 15 2.1 0.9 5 (CLASS A)
PIPE-5 18 7.1 3.2 5 (CLASS A)
A minimum Dso of 5 inches was chosen for each pipe which corresponds to
NCDOT Class A stone. Given the design D50, the dimensions of the apron were
taken from Table 10.1 equations from HEC 142.
La = Apron length (ft) = 4D
Da = Apron depth (ft) = 3.5D50
W1 = Upper width (ft) = 3D
W2 = Lower width (ft) = 3D + (2/3) La
Table 9 - Riprap Apron Dimensions
NAME DIA (in) La (ft) Da (ft) W1 (ft) [ W2 (ft)
PIPE-3 18 6 1.5 5 9
PIPE-4 15 5 1.5 4 7
PIPE-5 18 6 1.5 5 9
2.5 Hydrology Calculations
The water quality volume (WQV) required to be treated from the design storm
(1 inch) was calculated using the Schueler method outlined in the NCDEQ
Stormwater Design Manual3. The Schueler method utilizes the following
equations:
R„ = 0.05 + 0.9IA
WQV = 363ORDR„A
Where IA is the impervious fraction of the drainage area, R„ is a runoff coefficient,
RD is the rainfall depth, A is the drainage area, and WQ„ is the runoff volume. The
following parameters are used for the drainage routed to the sand filter.
Ia = 0.74
R„ = 0.05 + 0.9(0.74) = 0.71
RD = 1.0 inch
2 http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hecl4ch 10.cfm
3 https://deq.nc.gov/sw-bmp-manual
BREC, P.A. Dollar General— Store #25122 8

A = 1.37 acres
WQV = 3,630(1.0)(0.71)(1.37) = 3,531 ft3
WQVadj = 0.75WQ„ = 0.75(3,531) = 2,648 ft3
2.5.1 Modified Curve Number Calculations
To route the WQV through the sand filter, it is necessary to calculate a modified
SCS curve number to avoid underrepresenting the design storm. The modified
curve number is calculated as follows:
CNm = 1000/ [10 + 5RD + 10R„ — 10 JR; + 1.25R„RD]
The modified curve numbers were used to route the first flush using an SCS 6 hour
balanced storm distribution. The remaining design storms were routed with
standard curve numbers and an SCS type II 24 hour storm distribution.
Table 10 - Modified SCS Curve Numbers
Name R„ CNmod
POST-CON:1 0.95 99.57
POST-CON:2 0.92 99.29
POST-CON:3 0.92 99.34
POST-CON:4 0.05 77.22
POST-CON:5 0.14 83.40
POST-CON:6 0.28 88.85
2.6 Sand Filter Design
The combined area of the sediment chamber and sand chamber is determined
from the relationship of the max water depth observed during the water quality
event, Hmax, and the adjusted water quality volume.
Hmax = 1.10ft
A _ WQVadj _ 2,648 — 2,407 f t2
min — Hmax 1.10
Aprovided = 2,539 > Amin
Vmin = WQVadj = 2,648 f t3
Vprovided = 3,263 ft3 > Vmin
BREC, P.A. Dollar General—Store#25122 9

_ Table 11 -Sand Filter State-Storage
Stage Elevation Area Sediment Vol Filter Vol Total Vol
(ft) (ft) (ft2) (ft3) (ft3) (ft3)
0.0 805.0 2,539+ 0 0 0
1.0 806.0 3,316 1,464 1,464 2,928
1.1 806.1 3,396 1,632 1,632 3,263"
2.0 807.0 4,139 3,327 3,327 6,654
3.0 808.0 5,018 5,616 5,616 11,232
+ Aprovided
n
Vprovided
2.6.1 Chamber and Filter Sizing
The sand filter sediment chamber and sand chamber are similarly sized to satisfy
MDC 2. The sediment chamber has an additional 1.0 ft depression to assist with
maintenance and primary sediment deposition; however, this added volume is not
considered in calculations to meet minimum design criteria requirements.
Given the sand chamber area, the sand media area, Asand, was selected as
615 f t2. This dimension creates an easily installable geometric shape for the sand
media while preventing the sand media from being installed adjacent to the toe of
slope grading.
Discharge through the sand media is governed by Darcy's Law.
Asandk(hf + df)
Qsand = df
Where,
Aland = Sand media surface area (f t2) = 615 f t2
k = Sand media permeability (ft/day) = 4.0 ft/day
df = Sand media depth (ft) = 1.5 ft
hf = Height of water above sand media
2.6.2 Underdrain Sizing
Drawdown time is calculated as:
WQVdf 3,531(1.5)
t = _ = 25.8 hours
k(ha + df)Asand 4.0(0.55 + 1.5)615
BREC, P.A. Dollar General—Store#25122 10

Discharge rate is calculated as:
WQV 3,531 f t3
Qunderdrain = t = 25.8 = 0.04
After applying a safety factor of 10,
ft3
Qsand = 10Qunderdrain = 0.40 S
The diameter of a single pipe is calculated as:
3
D = 16 rnQsandl8
I
Where,
n = Manning's roughness for pipe
S = pipe slope
3
0.011(0.4) s
D = 16 = 5.5 inches
V0.005
Per Table 1 of Section A-5 of the NCDEQ Stormwater Design Manual, use (3) 4
inch pipes.
2.6.3 Emergency Spillway Calculations
Modeling shows the emergency spillway being activated only during the 100 year,
24 hour storm. The emergency spillway elevation is designed to provide 1 ft of
freeboard when activated. With those constraints, the maximum head for flow over
the emergency spillway is 0.25 ft.
3 3 ft3
Qspillway = CLHz = 3.33 x 20 x 0.25z = 8.3 —
s
The outlet channel is armored with an HDPE scour mat installed over sod with the
following performance metrics:
ft
Vpermissible = 197
Qspillway ft
Vcalculated = = 1.5 — < Vpermissible
'spillway@0.25'
BREC, P.A. Dollar General—Store#25122 11

2.6.4 Anti-floatation Calculations
The outlet structure is a 3 ft x 3 ft x 4 ft precast concrete box with 6 in side and
bottom wall thicknesses. Including concrete and reinforcing steel, the structure
has an empty mass of Moutietempty = 4,078 lb.
Assuming the total height of the box is exposed to buoyant forces, the volume of
water displaced by the box is calculated from the outside dimensions.
Vboxwater = 4.0 ft x 4.0 ft x 4.5 ft = 72 ft3
The buoyant force on the box is defined from the hydraulic forces acting
Fb = Vboxwaterpw = 72 f t3 x 62.4 lb
= 4,493 lb
Additional mass was added to the outlet structure by pouring a 6 in concrete slab
over the bottom.
Moutlettotat = 4,078 lb + (3 ft)(3 ft)(0.5 ft) (iso ) = 4,753 lb > Fb
BREC, P.A. Dollar General—Store#25122 12

3.0 Results
The design storm (1 inch) is routed through the sand filter and reaches a max
storage depth of 0.98 ft which does not exceed the design Hmax = 1.10 ft. The
water quality volume is discharged over a period of 27.4 hours.
(—Storage:System(POST-CON.ROIRINGBALANCED 2023-04-1812:59:18)D
3100
3000
2900
2800
2700
2800
2500
2400
2300
2200
2100
2000
1900
A 1800
�1100
551600
s 1500
N 1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
0 5 10 16 20 25 30 35 40 45
Time(I,rs)
Figure 1 -System Storage of Design Storm (1 inch)
BREC, P.A. Dollar General— Store #25122 13

Appendix
Table 12 - Pipe Design Table
DIA Q MAX Q DESIGN
NAME (IN) INV UP INV DOWN LEN (FT) S (FT/FT) n (CFS) (CFS)
PIPE-1 15 806.25 804.90 133.3 0.010 0.012 7.0 2.40
PIPE-2 15 804.80 804.44 66.8 0.005 0.012 5.1 3.75
PIPE-3 18 802.75 802.25 39.2 0.013 0.012 12.9 7.33
PIPE-4 15 803.25 802.25 84.8 0.012 0.012 7.6 2.09
PIPE-5 18 804.19 804.10 15.5 0.006 0.012 8.7 7.12
BREC, P.A. Dollar General- Store #25122 14

Post-Construction Design Storm (1 inch)
BREC, P.A. Dollar General— Store #25122 15