20190142 Ver 1_Email_20190220

Carpenter,Kristi
From: Thomson, Nicole J
Sent: Wednesday, February 20, 2019 1:07 PM
To: Norton, Apri) R
Cc: Parker, Jerry A; Cail, William G; Patterson, Robert D;
[e-mail]; Holmes, David K;
Powers, Tim
Subject: Request for Additional Information Bridge 164
_NCDWR #20190142_WBSI7BP.7.R.106
Afternoon April —
This is just a follow up from our phone conversation for the above bridge project. There
are 2 proposed pipe outfalls on the upstream end of the project (one on the left and
one on the right). Each of these outfalls outlets prior to Buffer Zone 2(i.e. not new
man-made conveyance through the buffer). As shown, the velocities from these pipe
outlets are non-erosive. In order to meet the diffuse flow/non-erosive requirement of
the buffer rules, the Department needs to keep the Q10 at 2 or less and the V10 at 4 or
less (which has been achieved). For that reason, PSHs aren't required at these locations.
From a practical standpoint, the Department as a whole has shifted away from PSHs
because they often don't work (and after a few years are a glorified trash rack that is
acting as a standard rip rap pad). Instead, the Department focuses on reducing the
project footprint (so that pre and post construction impervious percentages are the
same) and keeping outlet velocities low/non-erosive wherever possible. Additionally,
the Department also makes use of existing ditches so that there are no new
concentrated flows (as allowed by field/site conditions). When those
conditions/requirements cannot be met, the Department then employs other options
within the tool box (i.e. ditch step downs/checks, "supering" or "crowning" the road so
that storm drainage can be split, etc). PSHs are generally a last resort control when all
other efforts/ideas have failed.
Thank you for your time this morning and for working with the Department — you know
we appreciate the opportunity to discuss these issues and concerns with you!
Ta ke ca re,
Nikki
Nicole J. Thomson, PWS
Division Environmental Supervisor Assistant
Division Environmental Office
919-754-7806 Mobile
Nithomson2(c�ncdot.qov
PO Box 14996
Greensboro, NC 27415-4996

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