20160728 Ver 1_RE Br Nos 221 and 132 in Orange Co_20160811

Wrenn, Brian L
From:Wrenn, Brian L
Sent:Thursday, August 11, 2016 2:14 PM
To:Thomson, Nicole J
Cc:[e-mail]
Subject:RE: Br. Nos. 221 and 132 in Orange Co.
Nikki,
Thanks for the information. After reading the Br. 132 explanation multiple times, I think I understand the engineering
lingo. Although my original question was related to the armoring of banks above the floodplain benches not the
benches themselves, I will defer to the technical analysis of the design team on the amount of armoring necessary to
build a stable channel.

As for Br. 221, can you send me a detail of the floodplain benches that detail coir fiber matting will be used. The only
detail I have found in the application is this one, and I don’t see any mention of coir fiber matting.

Thanks,
Brian Wrenn
919-707-8792 (office)
919-710-6516 (mobile)

From: Thomson, Nicole J
Sent: Thursday, August 11, 2016 11:40 AM
To: Wrenn, Brian L <[e-mail]>
Cc: [e-mail]
Subject: RE: Br. Nos. 221 and 132 in Orange Co.

I’m sorry – I ended up staying home yesterday with Odin. We got him into the vet – steroids are pretty much our only
option at this point.

As for these bridges – below is what was sent to me from the Engineering firm(s):

Dan and I have reviewed the approved CSR and hydraulic conditions at each of the referenced projects. Below are our
comments relative to the channel improvements:

BR132:

There are two basic components that drive the proposed outlet channel design – the hydraulic model and the

standard engineering protocol for tying high flow benches.

The hydraulic model for this project is a bit delicate. The outlet channel improvements do have an effect on the

water surface elevation (WSE), in that the improved outlet conditions are necessary to achieve a no-rise
conditions and prevent effects to the WSE.

It is standard NCDOT practice to design for one culvert barrel to handle low/normal flow conditions. For BR 132,

this is the easternmost barrel (see screen shot below). The westernmost barrel is the high flow barrel, which
obviously activates when the surface water rises during rain events. We have sought to minimize the channel
disturbance while meeting the no-rise condition, and provide a stable connection to the adjoining stream.

It is standard NCDOT practice to tie the high flow bench to the natural channel in a manner that both provides

the needed conveyance (to make the model work) and to provide a connection to the natural stream that does
not induce instability. As noted above, the proposed outlet channel improvements are integral to the hydraulic
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model for the stream. The limits or the proposed channel improvements also tie to the natural streambank at a
location that will best insure stability when high flow conditions exist. A close review of the image below shows
that the blue line of the natural stream ties to the center wall of the culvert (and thus the low flow barrel), but
also note the meander (inward jog ) of the blue line as it does so. The proposed high flow bench is designed to
tie to this inward jog, thus creating a high flow conveyance that blends into the natural curvature of the stream.
If we force a bench tie-in closer to the culvert, there will be a section of natural stream bank that will project out
into the stream. Such a projection would ultimately be unstable after a series of high flow events as the high
flow stream would eventually carve it out.

The proposed channel improvement in the NW quadrant extends approximately 45’ from the headwall of the

culvert, but we feel it will produce the most stable outlet condition in the long term.

BR 221:

Normal/low flow barrel is the westernmost barrel, and it aligns with the existing stream.


Easternmost barrel is the high flow barrel and minimal channel improvements are proposed to tie to the

existing floodplain. The proposed improvements only extend 20’ from the headwall of the proposed culvert. It
should also be noted that this included lateral ditches from the roadway that have been designed to tie to
existing discharge points near the stream to the greatest degree possible.

Inlet and outlet channel improvements are minimal (15’ max. off the headwalls of the culvert, not including the

aforementioned ditches) and are design to tie to the surrounding floodplain as quickly as possible and in a
manner that will produce stable conditions for high flow events (see screen shot below) .

All efforts will be made to minimize disturbance during construction.

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Second email from the design team:
I am attaching a screen shot (which didn’t come through, but I believe it’s the same as above)of the inlet and outlet
channel details for BR 132. They both call for the floodplain benches as depicted in the attached photos. Please note the
details call out for rip rap armoring of the outer channel slopes as well as the internal bench slope immediately adjacent
to the stream. In the area of the floodplain bench, the detail call for Coir Fiber Matting so the areas will grass up.

We have no issue with the channel details the agencies want, it is in fact what we have designed. Our position remains
the same relative to the length/limits of the channel improvements. We have designed per NCDOT protocol. If the
agencies want us to deviate from this protocol, then Nikki (as a representative of NCDOT) needs to coordinate with
NCDOT Hydro, gain concurrence on the requested changes in length/limits and the potential effect on the hydraulic
models, and have NCDOT Hydro provide us the directive they want followed.

Please let me know if this addresses your concerns with respect to the floodplain or if more discussion is needed.

Thanks to you both for your patience
Nikki

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From: Wrenn, Brian L
Sent: Thursday, August 11, 2016 8:31 AM
To: Thomson, Nicole J
Cc: [e-mail]
Subject: Br. Nos. 221 and 132 in Orange Co.

Nikki,
Do you have an idea of when we will get responses on the design questions regarding the subject bridge projects? I’m in
no big rush, but just trying to get a feel for whether I need to officially put the projects on hold. Thanks.

Brian Wrenn
NCDWR – Transportation Permitting Branch
1617 MSC
Raleigh, NC 27699-1617
919-707-8792 (office)
919-710-6516 (mobile)
[e-mail]

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