Clast Investigation_191029Final

Purpose
As part of the State Sediment Criteria (15A NCAC 07H.0312(1)(h)) to ensure the compatibility of
sand characteristics, the number of clasts (rocks and shells) greater than or equal to 3-inches in
diameter present on the native beach is to be quantified. This value shall be calculated from
observable clasts on the beach surface found between the frontal dune toe and mean low water
(MLW) for a 50,000 square foot area. This representative sample will quantify the "background"
value of the entire project area. As a result, the criteria stipulate that the borrow area material is
considered incompatible if more than twice the background level that existed on the native beach
is placed in the project area.
Method
The survey area that was chosen to conduct the investigation was located in Oak Island between
Stations 460+00 and 470+00 covering from 3rd Place East to 6th Place East. Before the
investigation began the timing of low tide on October 28, 2019 was identified to allow for easier
survey of the area between the frontal dune toe and MLW. Once onsite, a walkthrough was
completed at the survey area in order to determine a representative section to sample.
The chosen survey area had been nourished with a dune in the Post -Matthew FEMA Emergency
Dune Restoration project in the spring of 2018. Escarped dunes from this project were present in
the area. The length from the dune toe to MLW was measured and marked with a 100-ft rope
that was staked at both ends with survey flags. Then another rope was laid out parallel to the
first, offset by 25 feet. This created an easily identifiable 'row' to search within. Following along
the frontal dune toe survey flags were staked at 25-foot spacing to allow for the continual
creation of rows once the previous row was fully surveyed. General notes were made about the
location, number, and type of clasts found (above mean high water (MHW), rock, etc.). A picture
of a representative clast(s) was taken from each row to document the findings.
Results
The survey began just east of 3rd Place East and ended west of 6th Place East. The length from the
dune toe to MLW was measured to be 200 feet, with the last 25 feet being actively inundated by
the tide. This allowed for the creation of 10, 25-foot wide rows to calculate a 50,000 square foot
area. The 50,000 square foot area is shown in Figure 1. Photos of the row setup are documented
in Figure 2.
1

Figure 1. Survey Area Limits
AIN-
Figune2.Survey Area Setup — looking from dune toe
Table 1 shows the detailed results from the investigation; a total of 58 clasts greaterthan or equal
to 3 inches in diameter were surveyed. Figures 3 and 4 show representative dastsfound in the
survey area. Afevv c|asts /~10 in total) were observed below MHVV, while the majority ofdasts
were documented in the upper beach (from dune toe to K4HVV). The dominant c|ast type
surveyed in this area were shells. About 3/3 of the dasts found were shells and 1/3 were rocks.
2

The wrack line and upper beach near the dune toe were covered by old marsh vegetation, an
effort was made to search for clasts that might have been covered by this debris (Figure 5).
Table 1. Survey Area Results
Row #
# Clasts
Comments
1
2
All found near wrack line, 1 rock
2
10
All found above MHW, 4 rocks
3
5
One found below MHW, 1 rock
4
6
4 rocks
5
5
2 rocks
6
3
All found above MHW
7
1
All found above MHW
8
11
4 rocks
9
9
2 rocks
10
6
3 rocks
Total
58
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Figure 3. Row 1 documentation
Figure 4. Row 5 documentation

Figure 5. Thick covering of old marsh vegetation spread across upper beach near dune toe