SW1180402_Outlet Calcs_20180605

June 5th, 2018

Mr. Corey Anen
North Carolina Department of Environmental Quality
Division of Energy, Mineral and Land Resources
Stormwater Group
1612 Mail Service Center
Raleigh, NC 27699-1612

RE: Request for Additional Information - SW1180402
Stormwater Management Permit Application
Jo Ellen Ammons Fieldhouse for Mars Hill University
Madison County, North Carolina
Dear Mr. Anen:

Please find below responses to the comments in the Request for Additional Information dated
June 5th, 2018 and in regard to Stormwater Permit No. SW1180402.

1. Please submit calculations that show that the proposed erosion control measures are sufficient
to prevent erosion at the inlets, outlets, and the receiving water body.

A: Please review the calculations included with this submittal for information about the erosion
control measures on the site. The erosion and sedimentation control plan for the project has
been approved by the North Carolina Department of Environmental Quality-Land Quality
Section (NCDEQ-LQS) and construction operations are covered under permit NCG010000.

2. Provide pdfs of all revisions, 2 hardcopies of revised plan sheets, and 1 hardcopy of other
documents. Pdfs must be uploaded using the form at:
https://edocs.deq.nc.gov/Forms/SW_Project_Submittal

A: Noted

One (1) hardcopy of the additional calculations and a digital copy of the additional calculations
have been provided for this Request for Additional Information. Please feel free to contact us if you
have any questions or need any additional information.

Sincerely,
McGILL ASSOCIATES, P.A.

Brad Johnson, P.E.
Project Manager

I:\Projects\2016\16.00164 - MHU Fieldhouse\Misc\Stormwater\June 2018 Response to NCDEQ Comments

Type II 24-hr 10-year Rainfall=3.96"MHU Fieldhouse Stormwater
Printed 6/5/2018Prepared by McGill Associates
Page 1HydroCAD® 10.00-20 s/n 04263 © 2017 HydroCAD Software Solutions LLC
Summary for Pond 3P: Bio. Cell
[82] Warning: Early inflow requires earlier time span
Inflow Area = 56,628 sf, 57.12% Impervious, Inflow Depth > 2.09" for 10-year event
Inflow = 4.218 cfs @ 11.95 hrs, Volume= 9,855 cf
Outflow = 3.967 cfs @ 11.98 hrs, Volume= 9,855 cf, Atten= 6%, Lag= 1.7 min
Primary = 3.967 cfs @ 11.98 hrs, Volume= 9,855 cf
Routing by Stor-Ind method, Time Span= 5.00-36.00 hrs, dt= 0.05 hrs
Peak Elev= 2,229.71' @ 11.98 hrs Surf.Area= 2,631 sf Storage= 2,410 cf
Plug-Flow detention time= 100.6 min calculated for 9,852 cf (100% of inflow)
Center-of-Mass det. time= 100.2 min ( 865.5 - 765.3 )
Volume Invert Avail.Storage Storage Description
#1 2,228.50' 3,197 cf Custom Stage Data (Irregular) Listed below (Recalc)
Elevation Surf.Area Perim. Inc.Store Cum.Store Wet.Area
(feet) (sq-ft) (feet) (cubic-feet) (cubic-feet) (sq-ft)
2,228.50 1,020 168.0 0 0 1,020
2,229.00 2,030 203.0 748 748 2,057
2,230.00 2,893 228.0 2,449 3,197 2,942
Device Routing Invert Outlet Devices
#1 Primary 2,225.00'18.0" Round 18" HDPE Pipe L= 81.0' Ke= 0.900
Inlet / Outlet Invert= 2,225.00' / 2,223.25' S= 0.0216 '/' Cc= 0.900
n= 0.012, Flow Area= 1.77 sf
#2 Device 1 2,229.50'36.0" W x 6.0" H Vert. Orifice/Grate X 4.00 C= 0.600
#3 Device 1 2,228.50'2.000 in/hr Exfiltration over Surface area
Conductivity to Groundwater Elevation = 2,220.00'
Primary OutFlow Max=3.822 cfs @ 11.98 hrs HW=2,229.71' (Free Discharge)
1=18" HDPE Pipe (Passes 3.822 cfs of 13.366 cfs potential flow)
2=Orifice/Grate (Orifice Controls 3.688 cfs @ 1.47 fps)
3=Exfiltration ( Controls 0.134 cfs)
Type II 24-hr 10-year Rainfall=3.96"MHU Fieldhouse Stormwater
Printed 6/5/2018Prepared by McGill Associates
Page 2HydroCAD® 10.00-20 s/n 04263 © 2017 HydroCAD Software Solutions LLC
Pond 3P: Bio. Cell
Inflow
Primary
Hydrograph
Time (hours)
36353433323130292827262524232221201918171615141312111098765Flow (cfs)4
3
2
1
0
Inflow Area=56,628 sf
Peak Elev=2,229.71'
Storage=2,410 cf
4.218 cfs
3.967 cfs
User Input Data
Calculated Value
Reference Data
Designed By:ZPR Date:06.05.2018
Checked By:BKJ Date:06.05.2018
Company:McGill Associates
Project Name:MHU Fieldhouse
Project No.:16.00164
Site Location (City/Town)Mars Hill
Outlet I.D.EX-1.1 Outlet
Total Drainage Area (acres)-
Rational Method for Flow
Outlet pipe diameter, Do (in.)18
Tailwater depth (in.)0
Minimum/Maximum tailwater?Min TW (Fig. 8.06a)
Discharge (cfs)3.85
Velocity (ft./s)2.18
Minimum TW Maximum TW
Figure 8.06a Figure 8.06b
Riprap d50, (ft.)0.3
Minimum apron length, La (ft.)9
Apron width at pipe outlet (ft.)4.5
Apron shape Trapezoid
Apron width at outlet end (ft.)10.5
Minimum TW Maximum TW
Max Stone Diameter, dmax (ft.)0.45
Minimum TW Maximum TW
Apron Thickness(ft.)0.675
Return to Main Worksheet
Figure 8.06a: Design of outlet protection from a round pipe flowing full, minimum
tailwater condition (Tw<0.5 diameter)