SW4210702_Design Calculations_20210729

Stormwater Calculations
Southern Alamance High School
Additions & Renovations
Project Location: Alamance County, North Carolina Project #: 20-040
July 7, 2021
I_riaddesigngroup
ARCHITECTURE ENGINEERING IINTERIOR DESIGN
4807 - C Koger Boulevard, Greensboro, NC 27407 1 T - 336-218-8282 F - 336-218-8010 I www.triad-designgroup.com

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EROSION CONTROL NARRATIVE:
PROJECT DESCRIPTION:
The project is located at 631 Southern High School. Road south of Graham, NC in
Alamance County. The project will consist of the construction of a 30,1 g0 SF +/-
classroom building, an 8,940 SF +I_ classroom building., and a 3,300 SF +I_ cafeteria
addition. Site improvements will include replacing existing sidewalks between thie.
buildings,. additional student and teacher parking, new bus lot, and. student drop-off
drives at the high school and middle school. Grading the site for the proposed facility
will require cuts up to 6 ft. and fills up to 11 ft. The site is expected to balance therefore
eliminating the need to haul off or bring in fill material to reach finished grades. A
series of dra.inage swales and pipe networks capture and direct the storm runoff to
proposed stormwater control measures and existing pipe networks. The proposed
stormwater control measure will be used as primary.emsion control devices for the site
during construction.
EXISTING SITE CONDITIONS:
The subject project is proposed on the current campus of Southern High School and
Southern Middle School. The total site area is 89.50 acres +1-. The site contains
existing buildings associated with the middle.school and high school, drives., parking
lots, and athletic fields. Portions of the site are wooded areas containing a combination
of evergreens. and. hardwoods. Grades range from 626 down to 58.3 msl. Public
utilities (water aid sewer) are available to. the site to serve the proposed facilities.
STREAM AND WETLAND PERMITTING
There are no streams or wetlands impacted by the. proposed construction.
ADJACENT AREAS:
The site is. bordered primarily by wooded areas and farmland to the north, .south, east
and west. Southern High. School Road forms the southern border.
SOILS:
The soils present within the project area are: CnB2, CnC2, CnD2, Ud, and Ur

PROPOSED CONDITIONS:
A 30,100 SF +/- classroom building, an 8,940 SF +1- classroom building, and. a 3,300
SF +/- cafeteria addition are being proposed on the. subject site and will disturb 14.38
acres during construction. Site improvements will include replacing existing sidewalks:
between the buildings, additional student and teacher parking,. new bus lot, and student
drop-off drives at the high school and. middle school. The site has 21.133 acres of
existing built -upon area. Approximately 4.713 acres of existing built=upon. area will be
demolished during construction., Proposed built -upon area is 7.515 acres bringing the
total built upon .area on the site to .23.935 acres. or 26.74% of the total site.
On -.site stormwater runoff will be collected in existing and proposed stormwater. systems
consisting of pipes and swales. Approximately 6.29'acres will be directed to the
proposed stormwater control... measure and treated for bath water quality and quantity.
The stormwater control measure meets NCDEQ requirements.
The .site water is: served by an existing Well..
Sewer will be served by the City of Graham..
CONSTRUCTION SEQUENCE
The site will be developed according to the construction sequence shown Sheets.
EC101, EC103, EC103A, EG103B, and EC1 07 of the plans. Some deviations from this
sequence may be required dine to local site conditions, or unanticipated problems.
These changes will be coordinated through the engineer and NCDEQ inspections staff.
All work must meat NCDEQ standards. A pre -construction meeting between the
grading contractor and the NCDEQ. inspection staff maybe required.. A copy of the
approved. plans must be kept on the job site at. all times. Additional pre -construction
meetings with other interested parties may be required.

CALCULATIONS
.All calculations necessary for the desigri of the proposed wet pond are in accordance
with the NCDEQ BMP manual. All calculations. are attached and summarized. within
this report.
SUMMARY
Pred evelopment. Flow From. Site (Existing Conditions)
1 yr 81.31 cfs
2yr 125.70 cfs
10yr 294.26 cfs.
100yr 551.42 cfs
Post Development to Pond —
1 yr
13.44 cfs
2yr
17.65 cfs
1 Oyr•
31.75 cfs
I00yr
50.71 cfs
Routed Flows Leaving Pond —
1 yr 3.29 cfs
2yr 9.08 cfs.
10yr 20.47 cfs
100yr 22.53 cfs
Post Development Bypass —
1 yr 75:59 cfs
2yr 116.87 cfs
1Oyr 273.58 cfs
100yr 512.66 cfs
Total Post Development From Site
1 yr 75.66 cfs
2yr 119.64 cfs
1 Oyr 293.09 cfs
100yr 534.05. cfs.

Southern Alamance High School Additions and Renovations
Summary of Site arid Built -Upon Area
Total Site Area = 89.50 Acres = 3,898,620 sf
Existing Built -upon Area = 21.133 .Acres = 920,553: sf (23.61%)
Existing Suiit-upon Area Rem oved:= 4.713 Acres = 205,298 sf (5.27%)
Proposed New Built -upon Area = 7.5.15 Acres := 327,353-sf (&40%)
Total Built-upon.Area = 23.935. Acres .= 1,042,609 sf (26.74%)
Net New Built -upon Area for entire Site = 2.802 Acres=122,055 sf (3.13%)
Drainage Area 1 (To Pond)
Site Area W .6.290 Acres = 273,992 sf
Existing 3uilt-upon Area to Pond = 0.871 Acres.= 37,918 sf
Proposed Built -upon Area to. Pond = 3.030 Acres = 18 1,9 6.6 sf
Total Built -upon Area to Pond = 3.901.:Acres = 169,88.4 sf
The tota1.8dilt-upon Area treated is greater than th6 proposed additional Built -upon Area:
3.901 Acres > 2.802 Acres

Pond Check State Rev 2020-01-17-qpw
07/28/21
PERMANENT WET DETENTION POND DESIGN - RISER/BARREL
Revised B-24-99
o. Alamance High School_
CHECKED BY:
LAH
LOCATION:
I Graham, NC DATE:
September 22, 2020
REVISED:
I
SITE DATA
TOTAL SITE AREA =' 89.501 WATERSHED =
NIA
PROPOSED BUILT UPON AREA 2.80' CLASSIFICATION. WS -
NIA
FUTURE BUILT UPON AREA = 0.00 ! GWAIWCA =
NIA
TOTAL ON -SITE BUILT UPON AREA = 23.94
PERCENT BUILT -UPON = 26.74%,
ON -SITE DRAINAGE AREA TO POND = 6.29
ON -SITE BUA TO POND = 3.90
ONSITE % BUA FOR POND SUB -BASIN = 62.00%
OFF -SITE DRAINAGE AREA TO POND =' 0.00 MAX. OFF -SITE B.U.% =
TOTAL WATERSHED AREA TO POND = 6.29
0%
_
❑ISTURBE❑ AREA = i 4.23
TEMP. SED. STOR. USED YIN i
DESIGN DATA
TEMPORARY SEDIMENT STORAGE 0.5
acre -inch per disturbed acre
PERMANENT SEDIMENT STORAGE 0.125
acre -in per total drainage are
PERMANENT WATER QUALITY
COMPOSITE B.U. % = 62.00
percent
DEPTH minimum 3 feet): 3.5
feet Averse Depth)
SA1DA (Figure 5.6-2 2.01
percent
.TEMPORARY WATER QUALITY 1.0
rainfall to be controlled
PRINCIPAL SPILLWAY
TYPE X
corrugated aluminum
DESIGN STORM 10
reinforced concrete
year
RAINFALL INTENSITY 7.04
RUNOFF COEFFICIENT 0.8
inches per hour
EMERGENCY SPILLWAY
1 TYPE X
DESIGN STORM 100
concrete box
rip-raprip-rap lined
year
RAINFALL INTENSITY' 8.3
inches per hour
RUNOFF COEFFICIENT 0.8
POND DESIGN
TEMPORARY SEDIMENT STORAGE VOLUME REQUIRED
7677
PERMANENT SEDIMENT STORAGE VOLUME REQUIRED
2854
PERMANENT WATER QUALITY SURFACE AREA REQ'D
5507
TEMPORARY WATER QUALITY STORAGE REQUIRED
Rv=
0.608
VOLUME REQUIRED =
13883
Page 1

So. Alamance High School
Average ❑epth
Option 1
Permanent pool voulme divided by permanent pool surface area
Pool Volume Pv = 22533 Hydra Flow
Perm Pool Surface Area PSA = 8172 Pond Check Sheet
Avg Depth
Option 2
A bottom shelf
A perm pool
A bottom pond
Depth
davg = 2 757342
Ab5 = 7388
App =
8172
Abp =
3703
D=
4
davg =
3.478452

Water Quality Draw Down Time
Size of Hole 1 1.5 0.012272
Storage Volume 14162
Surface Area of Pond ft"2 (As) 7944
Head of Water above Hole, ft (h) 1.5
Coefficient of Contraction of an orrifrce (0.6) 0.6
Time to ❑ewater Days 2.26496
Q= 0.07237 cfs
Qmin 0.03278 5 days
Qmax 0.08196 2 days

RISER ANCHOR CALCULATIONS
RISER DIAMETER = 30 IN. BARREL DIAMETER = 18 IN.
TOP OF RISEL EL = 605.5
BOTTOM OF PSS EL = 599
RISER HEIGHT = 6.5 FT.
VOLUME OF RISER (V=Trr2h) = 31.91 CF
VOL OF BARREL (V=Trr2 x 10') = 17.67 CF
TOTAL VOLUME = 49.58 CF
50% FOS (150%) = 74.37 CF
WT. OF WATER IN SYSTEM (62.4 x VOL) = 4640.53 LBS.
WT. OF CONCRETE = 150 - 62.4 = 87.6 LBS
VOL OF CONCRETE REQUIRED - CF
LENGTH 4.5 FT VOL PROVIDED = CF
WIDTH 4.5 FT
DEPTH 3 FT

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