21_22 Oak Island_TI Coastal Letter_Nearshore Seafloor Investigation

TI Coastal Services
387-B N Green Meadows Drive
Wilmington, NC, 28405
910-821-1358
www.TlCoastal.com
Dawn York
Senior Coastal Scientist
Moffatt & Nichol
RE: Oak Island Nearshore Seafloor Investigations
Dear Mrs. York,
TI =, oastal
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TI Coastal Services was contracted by Moffatt & Nichol to investigate the seafloor along the oceanfront side of
Oak Island in order to identify areas of potential hard bottom. These surveys utilized an Edgetech 4125 sidescan
sonar system to image the seafloor between 2nd Place West and 69t` Place West and confined to the -10 foot and
-30 foot NAVD88 contours. These sidescan surveys were conducted between June 16 and 18, 2021. The dataset
was then processed and a final drawing containing sidescan imaging mosaics and bathymtric soundings was
submitted to Moffatt & Nichol on June 29, 2021.
The sidescan imagery revealed an area of interest extending the full length of the beach, approximately 1,000 feet
seaward of the surfzone and ranging in width from 700 to 2,500 feet. This feature was defined by a distinct and
different seafloor texture in comparison to a normal sand rich environment, however sidescan sonar only provides
imaging of the seafloor and does not actually classify the material type. As such, TI Coastal delineated an area of
interest confined by the textural boundary seen in the sidescan imagery and noted that further methods of
investigation (i.e. Ponar grab samples, divers) were recommended to ground truth the seafloor substrate.
Moffatt & Nichol then contacted TI Coastal in regards to collecting ponar samples from select areas along the
proposed pipeline routes to the Oak Island beachfill area. The purpose of these samples were to determine what
the seafloor sediment actually was—hardbottom/rocky, sandy, or muddy. The ponar sampler is lowered to the
seafloor and a clamshell snaps shut when tension is released and a spring loaded pin is able to pop out of the arms.
As the clamshell snaps shut, it "grabs" a sediment sample from the seafloor, which is hauled back onto the boat
and emptied into a container. TI Coastal, along with representatives from Moffatt & Nichol performed 23 grab
samples along 6 transects on July 13, 2021. All samples were photographed then transferred to a labeled bag for
future reference.
All ponar grab samples returned evidence of silty and/or sandy material. No physical evidence of hard bottom was
discovered during this investigation. While one sample did have a small rock (approximately 1 inch in diameter),
the remainder of the samples contained very fine grained silt or medium grained quartz sand with moderate shell
hash. If the bottom had been exposed hard bottom or rocky, the ponar sampler would not have returned any
sediment at all, or returned with an isolated rock/pebble caught between the edges of the clamshell. Because each
sample location returned a good amount of sediment, it can safely be stated that the area of interest delineated in
the provided drawing contains at least 4 inches of silt and sand. It should be noted that the ponar only retruns
superficial sediments and gives no indication to the thickness of the sediment layer nor what exists below a few
inches of superficial sediments.
Sincerely,
IV14- V�-
Jamie Pratt, PG
Vice President
TI Coastal Services