20110749 Ver 1_Staff Comments_20110915

Lucas, Annette
From: Lucas, Annette
Sent: Thursday, September 15, 2011 12:29 PM
To: Witherspoon, Lauren
Cc: Lucas, Annette
Subject: RE: 11-0749 Dogwood Property at Flowers Plantation
Attachments: 11-0749 Dogwood Property at Flowers Plantation.docx
Lauren,
Slight correction - please use this version instead.
Thanks,
Annette
From: Lucas, Annette
Sent: Thursday, September 15, 2011 12:23 PM
To: Witherspoon, Lauren
Subject: 11-0749 Dogwood Property at Flowers Plantation
Lauren,
Here is some more info for your letter.
Thanks,
A
Annette M. Lucas, PE
Environmental Engineer
NCDENR I DWQ I Wetlands and Stormwater Branch
1650 Mail Service Center, Raleigh, NC 27699-1650 (Mail)
512 N. Salisbury St, Raleigh, NC 27604 1 9th Floor (Location &: Parcels)
Phone: [phone] Fax: [phone] Email: tUnct'..I 's clE°nr. c;?
Website: http://portal.ncdenr.org/web/wq/swp/ws/401
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11-0749 Dogwood Property at Flowers Plantation
AML comments
September 15, 2011
Pertaining to Sheet D-2:
a. Please allocate more space for the blind Swale, level lip and #57 stone. The plan view
shows about six feet in width for these BMP elements; however, your detail shows that
over 11 feet of width is needed.
b. Please provide watering specifications to establish the sod in the vegetated filter strip.
c. Please provide dimensions for the level lip and footer in section view.
d. The construction sequence discusses installing a forebay; however, it appears that the
blind swale is being used as the forebay in this design.
e. If it is your intent to use the blind swale as the forebay, then the depth will need to be
increased to 18 inches.
f. Please provide proposed topographic contours for the blind swale and provide proposed
grading directions that will prevent stormwater from short-circuiting around each end of
the level spreader.
g. Please provide plans for each of the three proposed flow splitting devices.
h. Please provide details for the dissipater pads located downslope of the bypass channel
outlets.
2. Pertaining to Level Spreader 1:
a. The Supplement Form shows that the drainage area for this level spreader is 0.41 acre.
However, the Runoff Map (Sheet C-1) that you provided shows that there are portions of
a number of lots that also drain to this device. Please correct the drainage area size and
adjust the size of the level spreader accordingly.
b. Sheet C-1 shows that the drainage from the road and the lots is initially conveyed in
ditches that flatten out prior to hitting the level spreader. The DWQ is concerned that this
approach may result in erosion through the flattened area and recommends retaining the
ditches until the water is dispersed into the blind swale.
c. Please accurately calculate the surface area of the forebay and report this in the
Supplement Form.
Pertaining to Level Spreaders 2 and 4:
a. Please relocate the level spreader and vegetated filter strip such that is it completely
outside of the protected riparian buffer. See Figure 8-5 of the BMP Manual, available at:
htto://nortal.ncdenr.ors/c/document library/aet file?uuid=5d698fO0-caaa-4f64-aclf-
d 1561 b4fd53d&groupld=38364.
b. Please accurately calculate the surface area of the forebay and report this in the
Supplement Form.
4. It appears that there is a ditch located between the drainage areas of level spreaders 2 and 3 that
discharges directly to the protected riparian buffer. Please either re-grade this area to avoid
concentrated flow or provide an appropriately designed LS-VFS to diffuse the flow from the
ditch.