20090430 Ver 1_Staff Comments_20100215

Lucas, Annette
From: Lucas, Annette
Sent: Monday, February 15, 2010 2:35 PM
To: David Kiker
Cc: Lucas, Annette; Mcmillan, Ian
Subject: 09-0430 Johnston Co Airport Runway Extension
David -
I have been reviewing the stormwater management plan for this project and have noted the following issues:
- There are numerous discrepencies in freeboard, flow depth, depth of swales and side slopes between the plan sheets,
supplement forms and "Hydraulic Performance of Existing Grass Swales" table in Appendix F. It is difficult to verify the
calculations until these discrepencies are addressed.
- The velocities of flow in Swale 3 appears to exceed 1 fps even with the proposed retrofit (Swale 5 was not checked yet
at this point).
- Plan sheet C-29A/C-30 does not show an adequate spacing between the two parallel swales proposed as a retrofit for
Swales 3 and 5.
- Every Swale must have a freeboard of 0.5 ft above the 10-year storm depth.
- The C values given in the Supplement Form are consistently lower than expected if C for asphalt = 0.95 and C for grass
= 0.20.
- I need to see scaled cross-sections of every proposed swale.
- Swales 3 and 5 need to be fitted with an engineered flow splitter box (a level spreader will not suffice).
- Much more information needs to be provided on Filter Strip #1 (i.e., information listed in the Supplement Form). This
device appears to be another swale rather than a filter strip.
Please let me know how you would like to proceed - a hold letter for the SMP or a conditional 401 approval.
Thanks,
Annette
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]
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