NC0024911_Hourly Peak Flow Determination_20160710

PAT MCCRORY
y!� Governor
DONALD R. VAN DER VAART
Secretary
Environmental
Quality
June 10, 2016
Mr. Thomas E. Hartye, PE
General Manager
Metropolitan Sewerage District of Buncombe County
2028 Riverside Drive
Ashville MC 28804
Subject: Hourly Peak Flow Determination and Application
Authorization to Construct# 024911A03
French Broad River WRF
Buncombe County
Dear Mr. Hartye:
As requested,the following is a formal summary in regards to our May 9, 2016 phone discussion.
(1) The grit removal system is considered a non-critical component and there is no bypass
proposed,therefore,
- with both proposed units in service they must be able to pass through the hourly peak
flow sustaining in the new upstream structures a free board no less than 12 inches.
(2) The new fine screen system is considered a critical headworks component, therefore,
- with two of the proposed three fine screens in service they must be able to pass through
the hourly peak flow sustaining in the new upstream structures a free board no less than
12 inches.
(3) With the assistance of Ken Pohlig, PhD, PE from our Division of Water Infrastructure a re-
evaluation to determine your facility hourly peak flow was conducted. The last 10 years of
reported effluent flow data was evaluated to determine the daily/monthly average peaking
factor (DPF) history for this facility. It was determined 2.2 was the appropriate DPF. Your
consultant as requested by the Division had previously provided a hourly/daily average
peaking factor(HDF) of 1.2 for non-rain event conditions. To calculate the hourly peak flow:
DPF x HDF x Permitted Flow(Design Flow) =Hourly Peak Flow
2.2x 1.2x40MGD= 105.6 MGD
In this case, 105.6 MGD is the hourly peak flow that is to be applied. There is a possibility the current
design may meet the free board requirements. To confirm the current design will meet free board
requirements at 105.6 MGD,the hydraulic profile will need to be updated and submitted to the Division.
1617 Mail Service Center,Raleigh,North Carolina 27699-1617
Location:512 N.Salisbury St.Raleigh,North Carolina 27604
Phone:[phone] Fax:[phone]
Internet:www.ncdenr.gov
An Equal OpportunitylAffirmative Action Employer

Mr. Thomas E. Hartye, PE
General Manager
Metropolitan Sewerage District of Buncombe County
June 10, 2016
Page2of2 •
There is no issue with the proposed design to meet reliability requirements at the NPDES permitted flow,
40 MGD.
We discussed the default design peaking factor of 2.5. The default factor only applies when no data is
available for assessment of a treatment facility anticipated flow history.
I hope this information is helpful and if you have any further questions let me know. Please advise if as
stated in your May 31, 2016 letter you still desire a meeting.
Sincerely,
/21/1
Ron Berry, Engineer
Division of Water Resources, Complex Permitting
Cc: Asheville Regional Office/Tim Heim
DWI/Ken Pohlig, PE
Central Files
NPDES File