RE_ R-5759 (DWR#20240303)

Carpenter, Kristi
From:
Hining, Kevin J
Sent:
Friday, March 1, 2024 4:00 PM
To:
Ridings, Rob
Subject:
RE: R-5759 (DWR#20210303)
Attachments:
Pages from OutfallAnalysis_HYD_R5759.pdf, R-5759 HYD_SMPv3.01.pdf, Outlet of 36
inch HDPE at Stat 139+91 Rt.pdf
Hey Rob,
I was able to talk to Paul Cameron, our consulting hydrologist for R-5759, who works for HNTB. He said to tell you hey —
sounded like the two of you worked together for a few years as part of the U-71 project in Durham.
Paul said he was not with HNTB when the design was made, but he didn't expect there to be any issues with the new
design. Once I talked to him, he pointed out a few things that will hopefully support the current plan.
Regarding the inlet question — I wasn't sure until checking, but the headwall does have wingwalls. It will be an 8' wide
headwall, with 3' wings on each side. Also, the proposed turn from the stream into the proposed new 54" WSP will be
more gradual than the existing situation — currently, the stream has to make a 90 degree turn into the existing 48"
CMP. So, we are hoping that lessening the angle from a 90-degree angle to more of a 135-degree angle, will ease the
transition. Also, since the headwall will have wings, it should help direct the flow and act as bank stabilization. Hoping
that since the structure has a wingwall and will be a more gradual turn into the pipe than the existing situation, that this
will help lessen concerns. But, let me know if you need more information.
Regarding the outlet question - the proposed pipe makes a sharp turn into the stream because it is following the same
angle/footprint of the existing 36" HDPE. This allowed us to stay within the PDE. I'm hoping that is ok, as we might have
to do an additional right of way claim and expand the PDE, if we tried to angle the pipe differently. But, in support of
the existing design, while we are increasing stormwater flow from new impervious surfaces, the proposed larger pipe
and existing low gradient, should handle the flow pretty well. Also, currently the outlet has scoured out a hole, as there
is no armoring (photo attached, and other photos in the Outfall Analysis). The proposed scour hole will result in the
banks being laid back at 2:1, and the banks and stream bed will be lined for 20' with Class I rip rap. As such, the
proposed rip rapped bank should help dampen flow out of the pipe and direct it downstream, and the rock lined stream
bed should help prevent additional scour. Also, we will bury the pipe 1', and get rid of the existing perch. This should
also reduce scour. Paul thought that the new design should function much better than the existing condition, and didn't
expect any issues. I'm hoping by reshaping the outfall and lining with geotextile and class I, that we will be able to
redirect the flow into the channel. But, we can definitely commit to replacing the Class I with Class II if the constructed
scour hole doesn't remain stable.
Finally, there are some hopefully supporting numbers in the attached outfall analysis for this system. I'm not a
hydrologist, so I can't claim to understand all of it, but my main take home is that the drainage area didn't really change,
and while we are increasing the amount of impervious surfaces within our ROW, the proposed velocity and shear stress
are similar to pre-existing conditions. Paul thought that the larger pipe sizes coupled with the increased stability at the
inlet and outlet of the system should result in less erosion issues overall, and that the proposed system should function
well without any issues, even during high flows/heavy storm events.
I'm hoping this helps support our current plans, but let me know if you need any additional informaiton. Also, I attached
the stormwater mangement plan sheet. Let me know if it's ok, and happy to make changes if not.
Thanks and hope you have a great weekend,

Kevi n
Kevin Hining
Division 11 Environmental Supervisor
North Carolina Department of Transportation
828-386-7202 cell
kihining ncdot.gov
801 Statesville Rd.
PO Box 250
North Wilkesboro, NC 28659
Email correspondence to and from this address is subject to the
North Carolina Public Records Law and may be disclosed to third parties.
From: Ridings, Rob <[e-mail]>
Sent: Thursday, February 29, 2024 10:21 AM
To: Hining, Kevin J <[e-mail]>
Subject: R-5759 (DWR#20210303)
Kevin,
Thanks for taking my call on this project. As we discussed, I'll put it on Hold for more info, but here is a summary of the
things we'll be looking for:
1- Mitigation for Stream SG. DWR requires mitigation perennial stream impacts over 300 feet per stream. SG is
combined with sites 4 & 5, which have permanent impacts over this threshold. We'll need DMS letter to cover
this impact to issue 401.
2- Site 4 Inlet: has a sharp turn before going into pipe. Has DOT considered some stabilization at that sharp turn?
3- Site 4 Outlet: This one also has a very sharp angle going out. This one has riprap, but has DOT considered
possibly using a junction box with a stub, to create a less sharp turn for the water coming out of here? This pipe
carries a good deal of drainage, including the water from both streams SG & SH, in addition to stormwater
drainage.
4- The typical SMP (Stormwater Management Plan) for the project was not included in the permit drawings.
Let me know if any questions or details you need to address. Thanks for your time,
Rob Ridings
Environmental Specialist
401/Buffer Transportation Permitting (DOT Divisions 4-5; Temporary Divisions 6, 11, 14)
Division of Water Resources, NC Department of Environmental Quality
rob.ridingsa-deg.nc.gov
919-707-8786

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NORTH CAROLINA 7d� Q
Department of Environmental Quality
Email correspondence to and from this address is subject to the North
Carolina Public Records Law and may be disclosed to third parties.
Email correspondence to and from this address may be subject to the North Carolina Public Records Law and may be disclosed to third parties by an authorized
state official.
Email correspondence to and from this sender is subject to the N.C. Public Records Law and may be disclosed to third parties.