20140309 Ver 1_Stormwater Info_20140409

Strickland, Bev
From: Devane, Boyd
Sent: Wednesday, April 09, 2014 8:55 AM
To: Strickland, Bev
Subject: please file this document
Attachments: 2014 -4 -8- Revised Storm Impact Narrative.pdf
Would you please file this email and the attachment with permit # 14 -0309
From: Renee Pfeifer [ mailto :rpfeifer(Dclhdesianpa.com]
Sent: Tuesday, April 08, 2014 5:56 PM
To: 'Deborah Shirley'
Cc: Devane, Boyd
Subject: RE: question on South Garner High School.
Debbie,
The 85% TSS is for the entire site, the wet pond provides 90% TSS. I have revised our narrative accordingly.
I hope this helps!
Thanks!
Renee C. Pfeifer, PLA, ASLA
Principal
CLH Design, P.A.
400 Regency Forest Drive, Ste. 120
Cary, NC 27518
P: (919)319 -6716 ext. 4016
From: Deborah Shirley [mailto:dshirlev @sandec.com]
Sent: Tuesday, April 8, 2014 12:13 PM
To: 'rpfeifer @clhdesignpa.com'
Cc: Devane, Boyd (boyd.devane @ncdenr.gov)
Subject: FW: question on South Garner High School.
Renee,
Can you provide some clarification to Boyd with the DWR on the question below?
Thanks,
Deborah Edwards Shirley
Soil & Environmental Consultants, PA
919 - 846 -5900 (office)
919 - 673 -8793 (mobile)
www.SandEC.com
ATHINK BEFORE YOU PRINT
From: Devane, Boyd [ mailto:boyd.devaneCancdenr.Qov]
Sent: Tuesday, April 08, 2014 12:05 PM

To: Deborah Shirley
Subject: question on South Garner High School.
I noticed that the second page of the " Stormwater Impact analysis" says that "The site peak stormwater discharges shall
meet predevelopment rates for the 1 -year, 10 -year and 25 -year Design Storm events as well as providing 85% removal
of Total suspended solids and stormwater treatment of the first 1 -in run -off to provide nitrogen reduction." However, in
their "Nitrogen Calculation" they showed a 90% pond and the Wet Pond sizing seems to calculate a 90% pond sizing
although I have not gone over the calculations yet. Was the 85% statement for the pond just a typo?
pil

Stormwater Impact Analysis
Pro_lect Description
The proposed Garner High School is located along the western margin of Hebron Church Road between
Clifford Road and New Bethel Church Road. The 57.74 acre vacant site is currently within Wake
County will be annexed into Garner City Limits. The High School project will impact approximately 73
acres both with on -site and off -site development.
The majority of the stormwater runoff from the site, identified as Area A, discharges to the east through a
24 -in culvert under Hebron Church Road with four smaller drainage areas discharging to the south, west
and north identified as Areas B, C, D and E.
Stormwater run -off within Area A will be treated and detained by a 90% TSS wet detention pond, a
detention wetland and a dry detention pond. Area D will be treated and detained by a detention wetland.
Areas B, C and E will be surface areas will be reduced.
The site is within the Neuse River Watershed and the site is designed to meet the Town of Garner current
stormwater requirements. The site peak stormwater discharges shall meet pre - development rates for the
1 -year, 10 -year and 25 -year 24 hour Design Storm events as well as providing 85% removal of total
suspended solids and stormwater treatment of the first 1 -in run -off to provide nitrogen reduction. Site
area calculations are provided to the margin of the public right -of -ways.
No Stream buffers are present and identified on the site, but several off -site buffers are impacted by off -
site utility and roadway improvements. These impacts will be permitted through NC Division of Water
Quality and the US Army Corps of Engineers.
FEMA FIRM # 3720162900J dated May 2, 2006 indicates that the site is outside the limits the 500 year
flood plain.
Soils on the site consist of predominately of Appling which is identified as type B soils.
Conclusions
Stormwater Quality
Pre- Development:
Site Area: 57.74 -ac
Base N Loading Rate: 1.77 - lbs /ac /yr
TN Export: 102.32 - lbs /yr
Post - Development (without treatment):
Site Area: 57.74 -ac
Base N Loading Rate: 9.25 - lbs /ac /yr
TN Export: 534.06 - lbs /yr

Post - Development (with treatment):
Site Area:
Base N Loading Rate:
TN Export:
Target N Loading Rate:
N to be Off -set by Payment:
Payment Maint. Duration:
Total N to be Offset by Payment:
Stormwater Quantity
57.74
-ac
6.63
- lbs /ac /yr
382.65
- lbs /yr
3.60
- lbs /ac /yr
3.03
- lbs /ac /yr
174.95
- lbs /ac
30.00 -yrs
5243.47 -lbs x $21.02= $110,217.97
The calculations are performed with Hydraflow Hydrographs Extension for AutoCAD Civil 3D software
which utilizes TR -55 method by the United States Soil Conservation Service (SCS).
Summary tables of the design calculations are provided below:
Pre- Development / Post - Development Peak Runoff Comparison Summary:
Analysis
Point
Design Event
Pre- Development
Post - Development
Post - Development
With Detention
Detention
Qp- (d')
Qpost (cfr)
Q(cfs)
Provided
1 -yr
0.88
43.22
0.83
A*
10 -yr
29.27
136.69
23.49
Yes
25 -yr
52.00
182.19
48.99
1 -yr
0.15
0.19 **
B
10 -yr
1.92
1.30
No
25 -yr
3.01
1.90
1 -yr
0.04
0.03
C
10 -yr
1.50
0.91
No
25 -yr
2.65
1.55
1 -yr
0.34
5.65
0.24
D
10 -yr
6.36
16.44
3.12
Yes
25 -yr
10.14
21.50
6.96
1 -yr
0.10
0.02
E
10 -yr
1.73
0.11
No
25 -yr
2.74
0.16
* Includes upstream off -site run -off.
** The small increase for the 1 -yr Design Storm is due to the decreased Tc caused by the
smaller post development drainage area. No run -off from impervious areas discharge to
Analysis Point B. Since increase is negligible, no detention provided.