NCDMF Comment Memo - NC24 Causeway

TO: Cathy Brittingham, NCDCM Transportation Project Coordinator
FROM: James Harrison, NCDMF Fisheries Resource Specialist
SUBJECT: NC 24 Causeway, Onslow County
DATE: 31 May 2022
A North Carolina Division of Marine Fisheries (DMF) Fisheries Resource Specialist has
reviewed the permit application for proposed actions that may impact fish and/or fish habitats.
The North Carolina Department of Transportation (NCDOT) is proposing to repair the roadway
embankment and adjacent sidewalk along NC 24 within the project area. NCDOT is also
proposing to construct a living shoreline/offshore sill to protect the area in the future. The living
shoreline is proposed as a means of utilizing a more nature-based method comprised of an outer
sill, closure sill, oyster structure, and created/enhanced wetlands.
The outer sill would be constructed using Class II granite riprap and is proposed to be 845’ in
length, 20’ in width, and 5.7’ tall. The sill would originate at the west end of the causeway,
extend waterward approximately 45’ from the mean high water (MHW) line, and run southeast
parallel to the causeway for approximately 750’ to rejoin the eastern end of the causeway. The
outer sill would be trapezoid shaped with a 3’ wide flat top and 1.5:1 side slopes. There would be
8’ long breaks in the sill at least every 100’ along the sill to allow for fish passage and for
sufficient water movement in and out of the sill.
NCDOT is also proposing granite closure sills at each of the sill breaks (eight in total). The
closure sills would be constructed 1-2’ landward of the outer sills and would be approximately
50’ long and 15’ wide. The closure sills would also be trapezoid shaped featuring 1.5:1 side
slopes but would have 2’ wide tops. The final elevations of the sills are designed to be 1’ above
MHW, while initially constructed 6” higher to provide for the structures to settle into the
sediment. To minimize settlement and provide additional stability for the sills, DOT is proposing
a 12” deep footer constructed of Class A riprap to be placed under the sills and geotextile fabric
placed between the footer and the sills.
NCDOT is also proposing to construct an 8’ wide by 82’ long oyster structure immediately
waterward of the outer sill. This is to provide protection of the sills and to provide additional
ecological benefits. The structures would be constructed using biodegradable jute twine
impregnated with a cement-based binder/hardener that contains oyster spat. Once fully matured,
it is expected that the oysters would form a living hard structure to provide an additional layer of
protection. After completion of the in-water portions of the proposed project, the applicant

proposes to grade the filled areas and plant coastal wetland species landward of the proposed
sills. Planting will occur in all areas with water depths 18” or less at normal high water (NHW).
During site visits by staff from various agencies, submerged aquatic vegetation (SAV) was
observed within the project area. NCDOT has developed and submitted a monitoring plan in
order to address potential impacts to SAV. Based on preliminary design, the proposed living
shoreline will result in 0.56 acres of impacts to SAV. The proposed monitoring will document
the presence and coverage of SAV habitat through pre-construction, during construction, and
post-construction monitoring. The post-construction monitoring is proposed for a minimum of
two years. NCDOT has agreed to mitigate for impacts to SAV at a 1:1 ratio if there is not
sufficient ecosystem uplift as a result of the project.
SAV surveys will be completed twice annually (March/April and August/September) during the
growing seasons and after any significant storm event. These surveys will consist primarily of
low-level drone-based imagery and evaluated for accuracy by ground-truthing points and
margins as directed by initial interpretation of each imagery collection. During construction,
there will be analysis of aerial photography to delineate changes to SAV in comparison to the
pre-construction baseline.
DMF acknowledges and appreciates the avoidance and minimization measures that have been
included through the planning of this project. DMF would recommend, though, that the
mitigation for impacts to SAV be mitigated at a ratio greater than 1:1. This ratio does not provide
any buffer for additional impacts, failure of mitigation, time of lost function before mitigation
reaches full potential, etc. Therefore, DMF would recommend that mitigation be proposed for a
higher mitigation-to-impact ratio. In order to determine an effective and applicable ratio, DMF
recommends reviewing the ratio development detailed in King and Price 2004. This paper
outlines how to develop a mitigation ratio with consideration of numerous factors. A ratio greater
than 1:1 would account for the above issues while also helping to offset the ecosystem service
tradeoffs associated with the proposed habitat conversion. Based on King and Price’s 2004
calculator, DMF would recommend a minimum 1.77:1 ratio, as this accounts for the mitigation
not reaching full function immediately (i.e., a lapse in habitat function between the time of
impact and the time that the mitigation reaches full function).
The proposed surveys would occur closer to the start and end of each growing season, missing
the expected peak period of SAV presence. DMF would recommend that an additional survey be
conducted during the July/August timeframe, as that would be during the period of peak
biological productivity and greatest SAV presence. The August/September monitoring could be
shifted to later in the year (September/October), as this is closer to the expected end of the
growing period. This would be beneficial to provide a better understanding of the shifting areal
coverage of SAV in this area during each growing period and would be aligned with current
DMF efforts to determine the ideal sampling period for SAV in different regions. DMF would
also recommend that the post-construction monitoring occur for 5 years after construction is
completed, similar to what has been included in monitoring plans for other DOT projects with
impacts to SAV (i.e., Harkers Island Bridge).
Thank you for consideration of our comments and concerns. Please contact Jimmy Harrison at
[phone] or at [e-mail] with any further questions or concerns.

REFERENCES
King, Dennis M. and E.W. Price. 2004. Developing defensible wetland mitigation ratios; a
companion to “The Five-Step Wetland Mitigation Ratio Calculator.” Prepared for NOAA,
Office of Habitat Conservation, Habitat Protection Division. September 30, 2004.