20080748 Ver 1_More Info Received_20080707

Environmental Consultants, PA
Road • Raleigh, North Carolina 27614 • Phone: [phone] • Fax: [phone]
Division of Water Quality
Attn: Cyndi Karoly/Ian McMillan
2321 Crabtree Boulevard
Suite 250
Raleigh, NC 27604
July 7, 2008
U ?( -il-21115 1 EC Project # 11025.W1
15 DWQ No. # 08-0748
JUL 7 2008
DENR - WATER OUAUTY
WETLANDS AND STORMWATER BRANCH
Re: Reply to the Division of Water Quality's 05/01/08 request for additional information
Creekside Village
Wake County, North Carolina
Dear Ms. Karoly and Mr. McMillan:
This letter is in response to your letter dated May 1, 2008 which requested additional
information for the proposed Creekside Village. The following restates your
questions/comments and is then followed by our response.
Additional Information Requested
1. Diffuse Flow and Buffer Authorization:
a. Please submit bypass channel design information. Channel
capacity and riprap lining material shall be designed for the
10 year storm. Please use the precipitation intensity
corresponding to the time of concentration (5minutes) but not
the 24-hour storm to estimate the peak flow rate.
b. Please use discharge rate from the dry pond to size the level
spreaders, and split the flows to the level spreaders if dB-80
servers that purpose. This will help avoid very long level
spreaders that are difficult to install.
Mr. Patrick Perez (Project Engineer with John A. Edwards & Co.) explained the
following in recent correspondence:
La. Bypass channel design calculations attached. The channel was designed for the 10
yr storm flow leaving pond through Riser #2 through FES #82. By routing the 16yr
storm (Intensity = 7.22 in/hr) flow through pond, flow through fes-82 yields 45.0 cfs.
Charlotte Office: Greensboro Office:
236 LePhillip Court, Suite C 3817-E Lawndale Drive
Concord, NC 28025 Greensboro, NC 27455
Phone: [phone] Phone: [phone]
Fax: [phone] Fax: [phone]
b. The Level spreaders was sized based on the the first flush (1') flow to the dry pond
(18cfs to 9cfs/level spreader). Then Pond riser #1 was sized to convey 18cfs to
FES83184. See attached calcs for level spreader sizing.
Attached please find the appropriate calculations and diagrams.
Please feel free to call me if you require any further explanation.
Sincerely, r...,_.._
Nicole J. Thomson
Regulatory Specialist
Attachments: 5 copies of Response letter
5 copies of level spreader and dry pond calculations and diagrams
cc: Ms. Lauren Witherspoon, DWQ Raleigh Regional Office

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BRIDGE TECHNOLOGIES, LLC.
1.800-344-2102

NORTH AMERICAN GREEN EROSION CONTROL MATERIALS DESIGN SOFTWARE VERSION 4.3
NORTH AMERICAN GREEN CHANNEL PROTECTION - ENGLISH/S.I.
USER SPECIFIED CHANNEL LINING BACK-UP COMPUTATIONS
PROJECT NAME: Creekside Village PROJECT NO.:
COMPUTED BY: Patrick Perez DATE: 6/27/2008
FROM STATION/REACH: TO STATION/REACH:
DRAINAGE AREA: DESIGN FREQUENCY:

INPUT PARAMETERS
Channel Discharge : 45.0 cfs (1.27 MA 3/s)
Peak Flow Period : 0.083 hours
Channel Slope : 0.01 ft/ft (0.01 m/m)
Channel Bottom Width : 10.0 ft (3.05 m)
Left Side Slope :21
Right Side Slope :2:1
Channel Lining : Rock Riprap 13in
Permi. Shear(Tp) :4.33 psf (207.5 Pa)
Phase = 0

CALCULATIONS
Initial Depth Estimate = 0.16 * (45.0 /(0.01010.5))10.375 = 1.58 ft (.48 m)
Final Channel Depth (after 8 iterations) = 1.81 ft (0.55 m)
Flow Area = (10.0 * 1.8)+(0.5 *1.81^2 * (2.0+2.0)) = 24.6 sq.ft (2.3 m^2)
Wet Per. =10.0 +(1.8*(((2.OA2)+1)^.5 +((2.0"2)+1)^.5)) = 18.1 ft (5.5 m)
Hydraulic Radius = (24.6 / 18.1) = 1.4 ft (0.4 m)
Channel Velocity =(1.48610.100)*(1.4^0.667)*(0.010^.5) = 1.8 fps (0.6 m/s)
Channel Effective Manning's Roughness = 0.100
Calculated Shear (Td) = 62.4 * 1.81 * 0.010 = 1.13 psf (53.9 Pa)
Safety Factor = (Tp/Td) _ (4.33 /1.13) = 3.85
odhAmericanGrmn- ECMDS Vasion4.3 7/2008 Q06AMCOMPUTED BY: Patrick Perez
PROJECT NAME: DeekoldeViace OJECT NO.:
ROMSTATIONIREACH: ITOSTATION/REACH ! DRAINAGE AREA: ESIGNFREQUENCY:
HYDRAULIC RESULTS
Discharge Peak Flow Vebciy(fps ArmIsaN) Hydraulic Namal R°d`Rpap(n-0'1001
Period s Red3 k D N
455. .0 0.1 1.83 24.58 1.36 1.81
S-0.0100
1 'width
=1QgON1
Not to Scale

Reach Mat" Type
labily Anays' Vegetatrm Cheacterishos
PemissLb
Calculated
Safely Factor
Remarks
Staple Pattem Phase Gass Type DensRy Shear Stress
(1131) Sheaf Stress
(psf)
Straight RockRpap Umegetated 4.33 1.13 3.85 STABLE
13in
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