20110661 Ver 1_Staff Comments_20120301

110061 Chastain II —take 2
AML comments
March 1 2012
The scour hole appears to be significantly undersized for the peak flows that will be directed to
it The length of level spreader needed to handle the estimated 10 year peak flow that you have
estimated is over 1200 feet This capacity is not likely to be replaced by a grassed scour hole
equipped with a rock dam that is only 10 feet in length The DWQ strongly recommends
providing a bypass system for larger storms and designing a scour hole that is appropriately
sized and lined for the one inch /hour storm Please redesign and provide calculations for the
maximum velocity that will be discharged from the scour hole
It appears that your calculations have underestimated both the percentage of impervious
surface in the drainage area and the C value Please re visit these numbers before you address
Item (1) above

Lucas, Annette
From
Lucas Annette
Sent
Thursday March 01 2012 12 28 PM
To
Chapman Amy
Cc
Lucas Annette
Subject
FW 11 0061 Chastain II — take 2
Attachments
11 0061 Chastain II —take 2 docx
Sorry — NOW they are attached
From Lucas Annette
Sent Thursday, March 01, 2012 12 27 PM
To Chapman, Amy
Cc Lucas, Annette
Subject 110061 Chastain II — take 2
Amy
My comments are attached Let me know when you have set up the meeting
Thanks
Z
Annette M Lucas PE
Environmental Engineer
NCDENR ( DWQ I Wetlands and Stormwater Branch
1650 Mail Service Center Raleigh NC 27699 1650 (M ill)
512 N Salisbury St Raleigh NC 27604 1 911, Floor (Lo( itior7 & P noels)
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