20100063 Ver 1_Staff Comments_20100517

Mcmillan, Ian
From: Lucas, Annette
Sent: Monday, May 17, 2010 1:28 PM
To: Mcmillan, Ian
Subject: 10-0063 Franklin Co HUB revised comments
Attachments: 10-0063 Franklin County HUB revised.docx
U l,
Here they are.
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Annette Lucas, PE
NC Division of Water Quality - 401 Oversight/Express Unit
2321 Crabtree Boulevard, Suite 250
Raleigh, North Carolina 27604
Phone: [phone]
Fax: [phone]
Email correspondence to and from this address may be subject to the North Carolina Public Records Law and may be
disclosed to third parties.
1
10-0063 Franklin County HUB
AML Comments
May 17, 2010
1. As was discussed in the meetings held on February 23, 2010 and May 17, 2010, the DWQ will
allow this project to be permitted as a low density development provided that the following
requirements are met:
a. The impervious surface must be limited to less than 24 percent on each lot.
b. The stormwater from the lots may not discharge to the roadway conveyance system.
c. The stormwater from the lots must be conveyed via vegetated conveyances only (with
no stormwater collection systems).
d. As the lots are developed, the site plans for the lots, a Low Density Supplement Form,
and a Diffuse Flow Plan will need to be provided to the DWQ for review and approval.
2. Please provide an updated letter from Scott Hammerbacher that reflects the agreement stated
above. Please provide a date on that letter.
3. Please refer to the revised Chapter 8 (Level Spreader-Vegetated Filter Strip) chapter of the BMP
Manual for additional diffuse flow designs that maybe appropriate on this project. The chapter
is available at: http://Portal.ncdenr.org/web/wq/ws/su/bmp-manual.
4. All diffuse flow plan sheets must show one foot topographic lines throughout the entire sheet to
assist the DWQ with its review.
5. At all outlets, drainage from the both sides of the road must be conveyed to a BMP or diffuse
flow device.
6. At outlet #3, please provide a level spreader to diffuse the stormwater runoff from both sides of
the road from stations 21+30 to 24+00.
7. At outlet #4, please move the 24" RCP further downslope to convey a greater portion of the
roadway drainage from the east side of the road to the bioretention area (this will remove most
of the flow from outlet #5). Please adjust the design of the bioretention area accordingly.
8. At outlet #7, the flow bypass structure will pass the drainage through the vegetated filter strip,
which is not allowed per the BMP Manual.
9. Outlets #8 and #9 may be allowed to discharge to the existing draws provided that calculations
are provided to show that this will not result in erosive velocities within the channels during the
10-year storm.
10. If bioretention cells are used in the redesign of the diffuse flow plan, then please address the
following design issues:
a. Appropriate pretreatment devices must be included.
b. A detail with elevations for the outlet weirs must be included.
c. A planting plan for the sod that specifies the species that will be used, the source, and
the fertilizer and watering requirements must be provided.
d. A soils test that includes borings within the footprints of the proposed bioretention
areas to document the level of the Seasonal High Water Table must be provided.
e. A calculation of the drawdown time for each cell must be provided.
f. Clean-out pipe locations must be shown.
g. The P-value for the media must be specified.
h. All operation and maintenance plans must be signed and notarized.