20110156 Ver 1_Staff Comments_20120117

Lucas, Annette
From:
Lipscomb, Brian S
Sent:
Tuesday, January 17, 2012 1:54 PM
To:
Lucas, Annette
Cc:
Lauffer, Matthew S; Wrenn, Brian
Subject:
Mt. Airy Surry County Airport
Follow Up Flag:
Follow up
Flag Status:
Flagged
Annette,
Hope you are doing well.
Matt Lauffer and I spoke with Brian Wrenn on Thursday of last week regarding the Mt. Airy Surry County Airport
extension and road relocation project. It was requested that we provide velocities (2yr and 10yr) and summary of
review for the outlet at HW -1. Please find the requested information below.
The drainage system that discharges at structure HW -1 collects roadway and off -site drainage. The off -site drainage
consists mostly of relatively flat grassed areas and the roadway portion consists of a 2 -lane, low AADT, state route. The
roadway cross - section is in a full super elevation requiring runoff to be collected in a shoulder berm gutter and pipe
system that discharges to a rip rap channel before entering the 66 inch cross pipe. Due to the amount of watershed
impervious cover, low pollutant export and other treatment occurring along the corridor, no additional treatment was
proposed. In addition to water quality, the Department also reviewed the design for water quantity by comparing pre
and post hydrographs and peak flows. Comparison of the pre and post hydrographs showed acceptable increases in
flows, which did not warrant further detention of stormwater and reduction in peak flows. Outlet velocities were shown
to be non - erosive for the 2yr and 10yr storm events with those velocities being 0.82 ft/s and 1.08 ft/s respectively.
Consideration was also given for future maintenance if a stormwater control were to be installed at this location. The
area has an elevation difference from the roadway to the toe of slope of approximately 95 ft., steep slopes and lack of
permanent access. The increased grading and project costs that a stormwater control in this location would require
were deemed inappropriate and unwarranted for a stormwater control that would provide little to no water quality or
hydrologic benefit.
Please feel free to contact me if you have any further questions or comments.
Thanks,
Brian
Brian S. Lipscomb, PE
NCDOT, Hydraulics Unit
Highway Stormwater Program
1590 Mail Service Center
Raleigh, NC 27699 -1590
email:[e-mail]
Direct Phone: 919 - 707 -6735
Office Phone: 919 - 707 -6700
Fax: [phone]
website: http: / /ncdot.ortz/programs /environment /stormwater/

Email correspondence to and from this sender is subject to the N.0 Public Records Law and may be disclosed to third parties.