20061214 Ver 2_More Info Received_20071127

WITHERS L~~~ RAVEN EL
ENGINEERS I PLANNERS I SURVEYORS
November 26, 2007 ~ ... ------~~~
P,,~V L 7 2007
~EhiR - WATER ~?i1ALI fiY
NC-Division of Water Quality ~v~'tn.NOS:~!o srrnp,~~~~~
Attn: Ms. Annette Lucas
2321 Crabtree Blvd.; Suite 250
Raleigh, North Carolina 27604
Re: Woodcreek - Tract F Phase -Stormwater Management Plan
Request for more information: Stormwater Management Plan - Response to DWQ
Comments
W&R Project #: 02040232.55
DWQ Project #: 06-1214, Ver. 2
Dear Ms. Lucas,
Our office received a letter from you dated November 5, 2007 in which you requested
more information for the Woodcreek - Tract F Stormwater Management Plan. Please find
listed below and enclosed the additional items that are necessary forthe DWQto
complete the review of the plan. Listed below are responses to the additional
information that you requested.
1. The drawdown rates forall three of the proposed BMPs appearto be fasterthan
two days. Please reevaluate the orifice sizes of these structures and provide
orifice sizes that will draw down the water quality volume in two to five days.
Response: The orifice sizes forthe BMPs have been adjusted so thatthey are
between 2 and 5 days (BMP 1= 2-in orF. ~a? 49 hrs; BMP 2 = 2-in orF @ 73.5
hrs; BMP 3 = i.5-in orf. ~a 81 hrs). The drawdown spreadsheets have been
enclosed.
2. Please explain how maintenance access will be provided for BMP 3.
Response: A io ft wide berm has been included around the edge of the
wetland that can be used for maintenance purposes. Maintenance access is
provided by traversing down the 3:1 side slopes onto the so ft wide berm. The
upper edge of the wetland can be accessed by usingthe drainage easements
that encompass the contributing pipe system.
3. Please provide details forthe swale that drains BMP 3.
Response: Details for the swale have been included on the attached BMP 3
detail sheet (Sht No.15). Estimated Swale velocities for designated storm
events are also included on the detail.
4. Please adjust the grading for BMP 3 so that side slopes do not exceed 3:1.
ui MacKenan Drive i Cary, NC 275u i tel: [phone] i fax: [phone] i www.withersraveneLcom
7040 Wrightsville Avenue i Suite lol i Wilmington, NC z84o3 i tel: 9io.z56.9z77 i fax: 91o.z56.z584
loz7 Sabbath Home Rd, SW i Supply, NC z846z i tel: 91o.84z•939z i fax: 91o.84z.8o19

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Response: The side slopes for BMP 3 have been adjusted so that they do not
exceed 3:1.
Please feel free to call if you have questions or require additional information to complete
your review (Direct Line: 23$-0323)•
Sincerely,
WITHERS & RAVENEL, INC.
C. Heath Wadsworth, PE
Attachments:
1) Orifice drawdowns (BMP #1, #2, and #3)
2) BMP design details (BMP #1, #2, and #3)

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Project Name: Woodcreek - Tract F Phase
City/State: Holly Springs, NC
Project #: 2040232.6
Date: 26-Nov-07
STORMWATER BMP F-1 -CONSTRUCTED WETLAND (85% TSS Removal 8~ 40% TN Removal)
Orifice Calculator

Variables
WQ Volume:
Head:
Draw down time:
0.213625 Acre-ft
0.50 ft
49 hrs
9305.505 cf
0.25 ft
176400 s
Constants
g=
Cd =
32.2 ft/sz
0.6
Orifice Area =
Orifice Diameter =
0.021912 sq. ft
2.00 in
3.155293 sq. in

Y
Project Name: Woodcreek - Tract F Phase
City/State: Holly Springs, NC
Project #: 2040232.6
Date: 26-Nov-07
STORMWATER BMP F-2 -CONSTRUCTED WETLAND (85% TSS Removal & 40% TN Removal)
Orifice Calculator

Variables
0.391125 Acre-ft
0.75 ft
73.5 h rs
17037.41 cf
0.375 ft
264600 s
Constants
g=
Cd =
32.2 ft/s2
0.6
WQ Volume:
Head:
Draw down time:
Orifice Area =
Orifice Diameter =
0.021838 sq. ft
2.00 in
3.144606 sq. in

Project Name: Woodcreek - Tract F Phase
City/State: Holly Springs, NC
Project #: 2040232.6
Date: 26-Nov-07
STORMWATER BMP F-3 -CONSTRUCTED WETLAND (85% TSS Removal & 40% TN Removal)
Orifice Calculator

Variables
WQ Volume:
Head:
Draw down time:
0.197 Acre-ft
0.50 ft
81 h rs
8581.32 cf
0.25 ft
291600 s
Constants
g=
Cd =
32.2 ft/s2
0.6
Orifice Area =
0.012224 sq. ft
1.760212 sq. in
Orifice Diameter= 1.50 in