HE-0013_USACE_Comments_20231016_Responses

Comments from Steve Brumagin (USACE) on October 16, 2023

Responses from Matthew Edwards (Sungate) on October 23, 2023

I have a few comments for you on the HE-0013 Hydro drawing you sent me and things you want to
consider for a permit applicatfon (in no partfcular order):

• You do this anyways, but I wanted to encourage you to tell the story about the reasons why
NCDOT is proposing to do this project in a permit applicatfon discussion. This can include
discussion about the limited capacity of the existfng enclosure system, the flooding of the
convenience store access/parking area, NCDOT’s desire to have the conveyance of this tributary
out of the existfng structure due to maintenance concerns, improved traffic safety, etc.

Project Leap Frog is an economic development project in Scotland County. HE-0013 is an NCDOT
project that proposes roadway improvements to US 15-501 (Aberdeen Rd) and to provide an
industrial access road to the Laurinburg/Scotland industrial site and fire statfon. A history of
flooding issues at Nic’s Pic Kwik, an existfng convenience store, was noted by Greg Davis, PE
(Division Project Engineer). Analysis of the existfng drainage (24” pipe) indicates the existfng
roadway would be impaired by storm events much less than the 2-year storm. The proposed
drainage improvements dramatfcally increase the level of service when compared to existfng
drainage.

• I think it would also be important to indicate that this project will be directfng water into the same
area as it currently flows (minus two trips under the railroad) and if correct, that the proposed
relocatfon will reduce some hydrology from the tributary indicated between the railroad and the
Schwarz property but does not result in a loss of waters in this tributary due to observed flow in
the channel, area of estfmated drainage area, aerial photographs, etc.

The existfng drainage system flows under the convenience store driveways, under the railroad,
under US 15-501, and then back under the railroad. The proposed drainage improvements
ultfmately drain to the same stream outiall as the existfng drainage systems but create
significantly less right-of-way impacts to the convenience store and completely avoids impacts to
the railroad. Approximately 66 acres stfll drain to the existfng 48” RCP underneath US 15-501 on
the south side of the railroad. At this locatfon, there will be a reductfon in hydrology but will not
result in a loss of waters.

Image 1 shows the 48” CMP underneath the railroad in poor conditfon. The 48” CMP pipe
segments are disconnected at the inlet.

Images 2 and 3 show the observed water surface between the 48” RCP underneath US 15-501
(Aberdeen Rd), to the south of the railroad and the 30” CMP underneath the railroad.

Image 1: Outlet of 48” RCP and inlet of 48” CMP (poor condition).

Image 2: Ponding observed upstream of 48” RCP under US 15-501 to the south of the railroad.

Image 3: Ponding observed between inlet of 48” RCP under US 15-501 and outlet of 30” CMP under
railroad.

• You will need to provide a discussion that explains why the 15/501 crossing will use two 83” x 57”
pipes while the downstream crossing of Plant Road includes two 48” RCPs. The goal of this
discussion will be that the design/placement of these pipes will assure that the smaller
downstream pipes do not pose an impoundment threat.

2@48” RCP-Vs are the largest pipe size that can fit underneath SR 1301 (Plant Rd) without
requiring a grade raise. The proximity of the proposed 2@48” RCP-Vs and the intersectfon
between SR 1301 (Plant Rd) and US 15-501 (Aberdeen Rd) also prevents a grade raise at this
crossing. Additfonal pipes could be added underneath SR 1301 (Plant Rd) but avoiding the increase
in proposed channel width was preferred.

The effectfve flow area of a 2@48” RCP buried 1.0 foot is 20.2 square feet. The effectfve flow area
of 2@83”x57” CMP Arch is 40.8 square feet. The proposed 2@48” RCPs under SR 1301 (Plant Rd)
overtops at flow rates greater than 107 CFS, which is less than the 2-year storm event. The
2@83”x57” CMP Arch overtops at flow rate greater than 121 CFS. Therefore, impoundment is not
antfcipated to be a threat due to overtopping.

• Please explore optfons at the outlet of the 48” RCPs under Plant Road to avoid/minimize the
excavatfon/earthmoving/placement of rock in the area indicated on the plan sheet as
wetlands. Since there is some area there at the outlet that is not indicated as wetlands, I would
encourage thoughts of placing energy dissipator or other rock design that does not affect
wetlands. In additfon, since there is minor realignment of the existfng stream at the inlet of the
83” x 57” pipes, is there some way to minimize the rock in the channel like what is proposed in
the channel across 15/501 (utflizing rock on the side slopes but a natural channel to handle normal
flow)? I understand the need to have rock to protect the inlet, I am just hoping to minimize the
rock in the channel along the existfng paved drive to the south end of the Leap Frog site.

Avoiding impacts to railroad Right-of-Way was requested by the Division, and the alignment of
outlet channel provides good transitfon to the receiving stream, with the intent to provide long-
term stability. The design flow is 160 CFS at the outlet of the proposed 2@48” RCPs under SR 1301
(Plant Rd). This flow rate is too high for an energy dissipator pad. The concern is the flow would
not properly diffuse, become reconcentrated, and eventually erode a channel. Therefore, it was
decided to propose constructfng a stable outlet channel. In lieu of an energy dissipator pad,
NCDOT Hydraulics is okay with adding 20’ of Class ‘I’ rip rap in the channel at the outlet of the
proposed 2@48” RCP-Vs.

Due to velocity and proximity of US 15-501, there does not appear to be a way to construct a
natural stream and stay within existfng Right-of-Way. There is not enough room between existfng
Right-of-Way and US 15-501 for a floodplain bench to be constructed, and to add sinuosity for
natural stream design. The Division is not seeking mitfgatfon credits. The special 2’ base ditch with
Class ‘I’ rip rap from 15+65 to 17+18 -L- Left is too steep to not have rip rap in the channel to
prevent erosion and scouring. The max shear stress is 2.17 pounds per square foot and the velocity
is 3.61 feet per second.

For the inlet side of the 2@83”x57” RCPs, the proposed slope is 2.36%, with a flow velocity of 5.3
ft/sec and max shear stress of 3.5 pounds per square foot. Site constraints required steepening
the slope at the channel upstream of the 2@83”x57” CMPAs, which increased velocitfes. The
associated inlet channel was aligned to provide a better transitfon to the outlet channel. To
eliminate the rip rap in the channel bed, the limits of the channel work and associated impacts to
the jurisdictfonal stream would need to be extended to flatten the slope of the proposed channel.