SW3200201_Calcs_20200212

DOLLAR GENERAL — STORE #21867
NEWTON, NC
HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT
Prepared By:
Prepared For:
Project Site:
BREC, P.A.
Teramore Development, LLC
TBD Providence Mill Road
1520 Meadowview Drive
Joe Strickland, Dir. of Ops., NC
Newton NC
Wilkesboro, NC 28697
214 Klumac Rd., Ste. 101
NCBELS Firm #C-3448
Salisbury, NC 28144
[phone]
[phone]
[e-mail]
[e-mail]
4 February 2020
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Principal Engineer
BREC, P.A. Dollar General - Store #21867

Table of Contents
1.0 General Information........................................................................................3
2.0 Methodology...................................................................................................3
2.1 Curve Number Calculations
2.2 Time of Concentration Calculations
2.3 Rainfall Data
2.4 Hydrology Calculations
2.5 Modified Curve Number Calculations
2.6 Sand Filter Design
2.6.1 Chamber and Filter Sizing
2.6.2 Underdrain Sizing
2.6.3 Anti -floatation Calculations
3
4
5
6
6
7
7
8
8
3.0 Results............................................................................................................9
Appendix.............................................................................................................11
List of Figures
Figure 1 - System Storage of First Flush..............................................................9
List of Tables
Table 1 -
Pre -Construction Drainage Area............................................................3
Table 2 -
Post -Construction Drainage Areas........................................................4
Table 3 -
Pre -Construction ToC............................................................................5
Table 4 -
Post -Construction ToC...........................................................................5
Table 5 -
NOAA Rainfall Depths...........................................................................5
Table 6 -
NOAA Rainfall Intensity.........................................................................5
Table 7 -
Modified SCS Curve Numbers...............................................................6
Table 8 -
Sand Filter State-Storage......................................................................7
Table 9 -
10 Year Pipe Discharges.....................................................................10
BREC, P.A. Dollar General — Store #21867
2

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 9,100 ft2 Dollar General retail store at the intersection of
Providence Mill Road and NC Highway 16 in Newton, NC. Specifically, regulations
for this area require water quality treatment of the first flush (1") storm.
The subject tract is composed of a partially wooded vacant lot. No jurisdictional
features are present on the site. The site is almost entirely composed of CaB soils
which are hydrologic soil group A. 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
One pre -construction drainage area and six post -construction drainage areas
covering the subject tract and adjacent off -site areas were identified. See
plansheets for delineation. The pre -construction drainage area outlets at
OUTFALL-1 located at an existing NCDOT drainage ditch in the northeast corner
of the site. All proposed impervious areas are routed to the sand filter for treatment
prior to discharge. Two drainage areas, POST-CON:4 and POST-CON:6, are not
routed to the sand filter due to topographic limitations.
2.1 Curve Number Calculations
Composite SCS curve numbers were calculated as weighted averages using
values of 49 for fair grass coverage (pre -construction) and 39 for good grass
coverage (post -construction, maintained), 30 for wooded areas, and 98 for
impervious surfaces in HSG A soils.
For pre -construction drainage areas, the composite curve number is calculated as:
(Almp x 98 + Agrass x 49 + Awooded x 30)
CNComhpre
Atotal
Table 1 - Pre -Construction Drainane Area
Name
Atotal
(acres)
Aimp
acres
Agrass
acres
Awooded
(acres)
CNcom ppre
PRE-CONA
2.621
0.19
1.42
1.01
45.3
BREC, P.A. Dollar General — Store #21867 3

For post -construction drainage areas, the composite curve number is calculated
as:
(Almp x 98 + Agrass x 39 + Awooded x 30)
CNcompositepost
Atotal
Table 2 - Post -Construction Drainage Areas
Name
Atotal
Aimp
Agrass
Awooded
CN

n = Manning's coefficient
PZ = 2 yr, 24 hr rain f all (in)
ft
S = slope
( )
t
f
l
Tscf _3,600V
V = shallow concentrated flow velocity ( f )
Table 3 - Pre -Construction ToC
Table 4 - Post -Construction ToC
2.3 Rainfall Data
Rainfall data was taken from NOAA Atlas 14, Volume 2, Version 3 for Concord,
NC. Rainfall depths for each design storm are summarized below.
Table 5 - NOAA Rainfall Depths
Table 6 - NOAA Rainfall Intensit
Name
Length
S10�7e
Tc—talc
Tc—assumed
t
t
min
min
PRE-CON:1
190
0.02
14-18
16
Name
Length
t
Slope
t
Tc—talc
min
Tc—assumed
min
POST-CON:1
-
-
-
5
POST-CON:2
-
-
-
5
POST-CON:3
-
-
-
5
POST-CON:4
-
-
-
5
POST-CON:5
135
0.02
9-15
11
POST-CON:6
-
-
-
5
Storm
Rain all Depth in
First Flush
1.00
1 Year, 24 Hour
2.92
Storm
Rainfall
Intensit
in hr
10 Year, 5 Min
7.01

2.4 Hydrology Calculations
The water quality volume (WQV) required to be treated from the first flush was
calculated using the Schueler method outlined in the NCDEQ Stormwater Design
Manual'. The Schueler method utilizes the following equations:
R„ = 0.05 + 0.9IA
WQV = 3630RDRvA
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.
la = 0.69
R„ = 0.05 + 0.9(0.43) = 0.67
RD = 1.0 inch
A = 0.99 acres
WQV = 3,630(1.0)(0.67)(0.99) = 2,392 f 0
WQVadj = 0.75WQ„ = 0.75(2,392) = 1,794 ft3
2.5 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 first flush. The modified curve
number is calculated as follows:
CN,,, = 1000/ 110 + 5RD + 10R„ — 10 RV2 + 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 7 - Modified SCS Curve Numbers
https:Hdeq.nc.gov/sw-bmp-manual
BREC, P.A. Dollar General — Store #21867 6
Name R CNmod
POST-CON:1
0.95
99.57
POST-CON:2
0.67
96.54
POST-CON:3
0.88
98.94
POST-CON:4
0.05
77.22
POST-CON:5
0.18
85.08
POST-CON:6
0.09
80.47

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, Hmax7 and the adjusted water quality volume. The design value for Hmax
was chosen as 1.0 ft. When coupled with the adjusted water quality volume,
WQVadj = 0.75WQV = 1,794 ft37 the resulting stage storage for the sand filter is
summarized below.
Table 8 -Sand Filter State -Storage
Stage
At Elevation = 1033 f t, Hmax = 1.0 f t, WQVprovided = 2,100 f t3 > WQVadj.
2.6.1 Chamber and Filter Sizing
The sediment chamber and filter chamber are equally sized. 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 filter
chamber area, the sand media area, Asand, was selected as 375 ft2. This value
maximizes the available filter chamber area and preserves an easily installable
geometric shape for the sand media while providing space for installation of the
outlet structure outside of the sand media and 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)
Asand = d
f
Where,
Asand = Sand media surface area (f t2) = 375 f t2 ft _ da
k = Sand media permeability (day) 4.0 ft
Y
df = Sand media depth (f t) = 1.5 f t
hf = Height of water above sand media
BREC, P.A. Dollar General - Store #21867 7
/Elevation
Area Sediment Vol Filter Vol Total Vol
fation
(ft) ll t� f t2 ft3 ft3 ft3
0.0 1032 1,776 ! 0 ! 0 0
!
1.0 1033 2,423 1,049 1,051 2,100
2.0 1034 3,151 2,440 2,446 4,887
3.0 1035 3, 960 4, 216 4, 226 8, 442
4.0 1036 4,850 6,416 6,43� 12,847

2.6.2 Underdrain Sizing
Drawdown time is calculated as:
WQVdf
t k(ha + df)Asand
Discharge rate is calculated as:
2,392(1.5) _
28.7 hours
4.0(0.5 + 1.5)375
WQV 2,392 f t3
Qunderdrain — t = 28 7 = 0.023 S
After applying a safety factor of 10,
ft3
Qsand = 10Qunderdrain = 0•23 t
The diameter of a single pipe is calculated as:
Where,
3
D = 16 nQsand 8
C -3
n = Manning's roughness f or pipe
S = pipe slope
3
(0.011(0.23) $
D = 16 = 4.6 inches
0.005
Per Table 1 of Section A-5 of the NCDEQ Stormwater Design Manual, use (2) 4
inch pipes.
2.6.3 Anti -floatation Calculations
The outlet structure is a 5.0 ft tall 3 ft x 3 ft precast concrete box with 6 in side
and bottom wall thicknesses. Including concrete and reinforcing steel, the
structure has an empty mass of Moutletempty = 5,184 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.
BREC, P.A.
Dollar General — Store #21867
N.

Vboxwater = 4.0 f t x 4.0 f t x 5.50 f t= 88.0 f t3
The buoyant force on the box is defined from the hydraulic forces acting
lb
Fb = VboxwaterPW = 88.0 f t3 x 62.4 ft3 = 5,491 lb
Additional mass was added to the outlet structure by pouring a 6 in concrete slab
over the bottom.
Moutlettotal = 5,184lb + (3 ft)(3 ft)(0.5 ft) C150- = 5,895 lb > Fb
3.0 Results
The first flush (1") storm is routed through the sand filter and reaches a max
storage depth of 0.93 ft which does not exceed the design Hmax = 1.0 ft. The
water quality volume is discharged over a period of 30.0 hours.
— Storage: System (POST -CON- ROUTH& BALANCED 2020-01-2816:27:58)
2100
2000
1900
1800
1700
1600
1500
1400
1300
w 1200
0 1100
1000
U)
900
800
700
600
500
400
300
200
100
0
0 5 10 15 20 25 30 35 40 45
Time (hrs)
Figure 1 - System Storage of First Flush
BREC, P.A. Dollar General — Store #21867 9

Routing the 10 year, 24 hour storm yields the following pipe outlet discharges:
Table 9 -10 Year Pipe Discharges
See plansheets for outlet protection calculations.
BREC, P.A. Dollar General — Store #21867
10
Pipe
ft3
Q10 -
s
PIPE — 1
0.58
PIPE — 3
3.33
PIPE-4
4.55

Appendix
BREC, P.A. Dollar General — Store #21867 11