SW5240101_BUA Allocations_20240325

6di
mcgill & w
March 25, 2024
Siler City WWTP Improvements
SW Permit No. SW5240101
Calculations to Demonstrate the Proposed Stormwater Pipes Offer Improved Control and
Protection:
The proposed project, specifically the replacement stormwater piping provides equal or better
Stormwater control.
EXISTING CONDITIONS:
• The existing flow in the old piped system which currently discharges directly to the creek
is 14.40 cfs.
• The current amount of impervious area accounted for within the flow calculation which
discharges from the existing piped system put in place prior to the current stormwater
rules is approximately 9,034 sf, 0.21 ac.
PROPOSED CONDITIONS:
• Due to the addition of the proposed piped system the proposed flow directly into the
creek is now reduced to 3.25 cfs.
• Due to the necessary location of the improvements to the existing WVVfP the location of
the new headworks increases the proposed impervious area to the existing pipe system
by 3254 sf, 0.07 ac.
• The proposed piped system is designed to handle the majority of the flow, mitigating
existing flow discharged directly into the creek. The flow in the new piped system is
11.06 cfs. This flow includes the upland undisturbed area. The new pipe system is
able to discharge prior to the 30' buffer and a blind swale has been designed to allow the
discharge to disperse flow prior to entering the buffer.
• The amount of impervious to the proposed pipe system is 890 sf, 0.02 ac. (includes
726 sf existing plus 164 sf proposed imp.).
Summary: Total outflow predevelopment 25-year 5 min. (existing system) = 14.40 cfs
Total outflow postdevelopment 25-year 5 min. (ex. and prop. system) = 14.31 cfs
Total impervious predevelopment (existing system)= 0.21 acres
Total impervious postdevelopment (ex. and prop. system)= 0.28 acres
The proposed project will provide better stormwater control by greatly reducing the discharge
directly into the creek (approximately 77% reduction). The majority of the discharge will now be
dispersed before entering the buffer, adding better protection to the waters of the State.
* The provided quantities do not include all discharge to the creek via the existing pipe system.
Only the areas that changed were analyzed. '
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MCGILL ASSOCIATES, P.A. a (SEAL
DAVID L. HONEYCUTT, P.E. IT 034999 I
37.c/ 3 i
MCGILL ASSOCIATES 5 REGIONAL CIRCLE, SUITE A, PINEHURS<N' Q "tG,\295.3159/MCGILLASSOCIATES.COM

mcgill
Project Name: SILER CITY
Project#: 18.04015
Date: 3/12/2024
RATIONAL METHOD RUNOFF CALCULATION,Q=CiA
Ex.1-15"Pipe-Pre-dev Flow
A(ac)= 8.28 (offsite)
125(in/hr)= 8.17 (NOAA)
C= 0.16 (wooded)
Q25(cfs)= 10.82
+ Weighted"C"Calculation
A(ac)= 0.44 (onsite) C
125(in/hr)= 8.17 (NOAA) imp.(sf)= 1336 0.9
C= 0.39 maintained lawn(sf)= 17675 0.35
Q25(cfs)= 1.40 total(sf)= 19011 0.39
Q25tot-Ex.1(Cfs)= 12.22
Ex.2-24"Pipe-Pre-dev Flow
A(ac)= 0.49 Weighted"C"Calculation
125(in/hr)= 8.17 (NOAA) C
C= 0.54 imp.(sf)= 7698 0.9
Q25tot-Ex.2(cfs)= 2.18 maintained lawn(sf)= 14127 0.35
total(sf)= 21825 0.54
Q25 tot EX Flow(cfs)= 14.40
Ex.1-15"Pipe-Post-dev Flow
Weighted"C"Calculation
A(ac)= 0.29 C
125(in/hr)= 8.17 (NOAA) imp.(sf)= 4590 0.9
C= 0.55 maintained lawn(sf)= 7897 0.35
Q25tot-Ex.l(cfs)= 1.3 total(sf)= 12487 0.55
Ex.2-24"Pipe-Post-dev Flow
A(ac)= 0.44 Weighted"C"Calculation
125(in/hr)= 8.17 (NOAA) C
C= 0.54 imp.(sf)= 6972 0.9
Q25tot-Ex.2(cfs)= 1.94 maintained lawn(sf)= 12247 0.35
total(sf)= 19219 0.55
Q25 tot EX Post Flow(Cf5)= 3.25

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SuiteA / �9�°°� VP �A(� C GRAPHIC SCALE DIVISION VALUE=60 FEET IMPERVIOUS AREA
oPinehurst,NC 28374 0So o3ases ° ` V m OFFICE MANAGER DESIGNER
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mcgillassociates.com CHATHAM COUNTY, NORTH CAROLINA
'���3/1/2024 NO. DATE BY DESCRIPTION D.HONEYCUTT N.SAWYER November 2023 18.04015 N/A