20171491 Ver 1_waiver_request_double_20171120

DATE: October 6, 2017
SUBJECT: Project: 45342.1.FR14 (W-5212N) Gaston County
Addition of passing lanes and intersection improvements
along NC 279 from SR 1438 (St. Mark's Church Road) to
Tandem Drive
Request for Waiver
This is a request for a waiver from the culvert burial requirement for the proposed downstream
extension of the existing 2@ 7' x 8' RCBC on Little Long Creek at -L- Sta. 324+60.
If you have any questions, please contact Stephen Morgan at 919-707-6739.
la.) The requirement is question is Corps Regional Conditions for All Nationwide Permits,
section 3.6) Safe Passage Requirements for Culvert Placement. The specific requirement
states:
"Culverts greater than 48 inches in diameter will be buried at least one foot below the bed of the
stream. Culverts 48 inches in diameter or less shall be buried or placed on the stream bed as
practicable and appropriate to maintain aquatic passage, and every effort shall be made to
maintain the existing channel slope. The bottom of the culvert must be placed at a depth below
the natural stream bottom to provide for passage during drought or low flow conditions. "
lb.) A waiver for this requirement is being requested.
1 c.) This project is funded thru the safety program due to the 295 total crashes (five fatals)
occurring along the project limits within a five year period. Total project funding is determined
using a benefit/cost ratio.
The inverts of the existing culvert are not buried below the stream bed. Based on visual
inspection, the existing culvert appears to provide adequate flow for aquatic passage through the
low flow (South) barrel. The site was visited in November 2016 in what would be considered
drought conditions (See Attachment 1) with approximately 0.3' of silt and 0.2' of water
observed in the low flow barrel. The shallowness of the slope on the existing culvert (0.4%) is
the greatest contributor to this silt accumulation and allows adequate flow depth for fish passage
in the culvert. The existing culvert is slightly perched above the stream bed with a small pool
(1.0' deep) forming at the inlet. For both options discussed below, no extension is proposed on
the upstream end of the culvert.
Option 1(preferred option) is to extend the existing culvert at the same size (2 @ 7' x 8') and
slope, (See Attachment 3). This option would be simpler and cheaper to construct when
compared to the 2@ 7' x 9' extension. An in-kind extension would also provide the same
aquatic passage as both the existing condition and the 2@ 7' x 9' extension option. Class `I'
Rip-rap would be installed in the stream bed (backfilled with native material to remove voids) at
in the inlet and outlet of the culvert forming constructed glides. This would remedy the perched
inverts of the culvert and address the pool forming at the inlet.
Option 2 is to extend the existing 2@ 7' x 8' RCBC with proposed 2@ 7' x 9' RCBC
downstream (See Attachment 2). The invert of the extension would be buried 1' below the

stream bed with a 1' sill at the outlet. The structural component of constructing a drop in the
culvert floor will be accomplished by attaching a vertical wall section to the end of the existing
culvert. Additional surcharge load from the fill on the extension may cause slight settlement
near the end of extension and create rotational stresses on the culvert floor at the connection
point. The vertices between the wall and the culvert floors are the weak points in the design and
are subject to failure due to these stresses. Note that the top of the sill would be even with the
invert of the existing culvert, and this would not cause the existing culvert to accumulate any
additional silt as per the intent of the burial requirement. The burial of the extension will not
increase the structure's ability for aquatic passage however an increased cost of $60,000 would
occur due to design costs and labor due an approximate three extra weeks of construction when
compared to extending the existing culvert in-kind. Note that the top of the sill would be even
with the invert of the existing culvert, and would not cause the existing culvert to accumulate
additional silt as per the intent of the burial requirement. The burial of the extension will not
increase the structure's ability for aquatic passage, and would be at increased cost and difficulty
to construct when compared to extending the existing culvert in-kind.
1 d.) Attachments

Attachment ]
2�� 7' x 8' Culvert Photos from November 2016 Site Visit
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2@ 7' x 8' Culvert Photos frotn November 2016 Site Visit
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1 e.) The project letting is tentatively scheduled for Spring 2018. The waiver is requested for
this specific culvert extension only and will not be used for justification on other projects.
1 f.) The in-kind extension (2 @ 7' x 8') provides equal silt accumulation and normal depth flow
for aquatic passage when compared to the buried extension alternate (2 @ 7' x 9') and the
existing culvert. This is supported by silt accumulation and normal flow in the existing culvert
observed at the site, which is due to its shallow slope (0.4%).
1 g.) An Activity Hazard Analysis is not applicable for this waiver request.
lh.) No additional requirements or standards address the requirement in question.