WQ0037686_Variance Request_20190826

EXECUTIVE DIRECTOR CHAIRMAN
Nicole Johnston 6van Varie J.L.Lowe,Jr.
* VICE-CHAIRMAN
TREASURER * Wayne Moore
Wayne Moore * -
lit 9„e BOARD OF DIRECTORS
Anita Darnell
SECRETARY
•P An't Woody Faulk
Woody Faulk Victor Varela
August 6, 2019 RECEIVED/NGDEQ/DWR
NCDEQ—Division of Water Resources
Attn: Deborah Gore, PERCS Unit Supervisor AUG 2 6 itiig
512 N. Salisbury Street, Suite 925
Raleigh,NC 27604 Water Quality
Permitting Section
RE: Collection System Extension to Serve Weyerhaeuser (WQ0037686 MOD)
Request for Variance
Dear Ms. Gore:
The Yadkin Valley Sewer Authority is submitting this request for a variance from the Minimum
Design Criteria for the Permitting of Gravity Sewers,part III.D.1 "Minimum Slope". This
request is for a 12" gravity sewer segment where our contractor installed the pipe slightly flatter
than the minimum slope of 0.22%. The actual slope,based on an as-built survey, is 0.19%. Since
the pipe has a smooth surface (PVC) and will provide at least two feet per second scour velocity,
and since the pipe is close to a force main that has broken twice this year, the Sewer Authority
prefers to accept this segment of pipe as constructed. Furthermore, this segment cannot be
corrected without replacing at least one other segment of pipe and adjusting a manhole, thus it
would be very disruptive to the private property and possibly to the adjacent force main. The
requested variance will meet the requirements of 15A NCAC 02T .0105(n) as follows:
(n) The Director shall approve alternative Design Criteria and Application Submittal
requirements if the applicant can demonstrate that the alternative will provide:
(1) equal or better treatment of the waste;
Not applicable for collection lines.
(2) equal or better protection of the waters of the state;
Capacity will be sufficient for expected flows and therefore not subject to
SSO's.
(3) no increased potential for nuisance conditions from noise, odor or vermin.
Velocity is estimated to be greater than 2.0 feet per second despite the
•
0.19% slope and, therefore, potential solids accumulation is not
increased.
500 NC Hwy 268 W—Elkin,NC 28621
Phone: [phone]—www.yvsa.org—Fax: [phone]

The following items are enclosed in support of this request:
• Copy of permit WQ0037686 MOD
• As-built drawings
• Specifications
• Engineering calculations
Thank you for considering this request. If you need additional information, please contact me at
[phone] or Benjie Thomas at [phone] or at [e-mail].
Sincerely, 'd
Nicole Johnston
Executive Director
Enclosures
500 NC Hwy 268 W—Elkin,NC 28621
Phone: [phone]—www.yvsa.org—Fax: [phone]

YADKIN VALLEY SEWER AUTHORITY
COLLECTION SYSTEM EXTENSION
TO SERVE WEYERHAEUSER
ELKIN, NORTH CAROLINA
12-inch Gravity Sewer
Engineering Calculations
August 6, 2019
Benjamin B. Thomas, PE
West Consultants, PLLC
405 South Sterling Street
Morganton, North Carolina 2865513
Phone: [phone] —/-\\HI\V W E,2
Fax: [phone] CONSULTANTS PLIC
[e-mail]
North Carolina License No. P-0210

12" Gravity Sewer to Allow Elimination of Dutchman Creek Pump Station and to
Accept Flow from Weyerhaeuser Elkin OSB Mill
Existing Dutchman Creek PS:
Duplex Pumps: 75 gpm each
Current Average Daily Flow: 69,458 GPD (based on average run times August, 2018, prior
to industrial connection)
Weyerhaeuser will be discharging to this pump station with a permitted average flow of 60,000
GPD and a peak flow of 300 gpm.
Normal (using PF=2.5) Peak Flow = (69,458 GPD + 60,000 GPD) x 2.5 = 323,645 GPD = 225 gpm
However, since the industrial discharge peak flow is 300 gpm, the proposed gravity sewer will
need to carry 300 gpm + (69,458 x 2.5)/1440 = 421 gpm
The available slope between the Dutchman Creek PS (DCPS) and the Regional PS is 0.24%. The
minimum slope per State Minimum Design Criteria is 0.22%. However, one segment was
installed at 0.19% slope. Using Manning's equation with n=0.013 and actual diameter of 11.54"
(PVC SDR 26), the full-pipe velocity would be 1.93 feet per second and the capacity would be
630 gpm which is sufficient for the 421 gpm peak flow. Actual n-value for PVC is 0.010, in which
case the full-pipe velocity is 2.51 feet per second and the capacity 819 gpm.
Therefore, the 12" PVC pipe with actual slope of 0.19% will have sufficient velocity for scouring
solids and sufficient capacity for the expected peak flows.
Downstream Capacity - Regional Pump Station:
Triplex Pumps with 1670 gpm per pump, 2770 gpm capacity with largest pump out of service
Current peak flow = 1,871,309 GPD = 1,300 gpm
New peak flow from eliminated Dutchman Creek Pump Station = 323,645 GPD = 225 gpm
1300 gpm + 225 gpm = 1525 gpm which is less than 2,770 gpm - OK