Clast Investigation_191008

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 410+00 and 400+00 covering from 22nd Place East to 25th Place East. Before the
investigation began the timing of low tide on October 8, 2019 was identified to allow 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.
In the chosen survey area, no dune was present even though the area had been nourished with
a dune in the Post -Matthew FEMA Emergency Dune Restoration Project in the spring of 2018.
Therefore, the length from the current vegetation line 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 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 22nd Place East and ended west of 25th Place East. The length from
the current vegetation line to MLW was measured to be 100 feet, with the last 25 feet being
actively inundated by the tide. This allowed for the creation of 20, 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.
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410+00
,�,�. _ . • 400 00
Limit of Survey Area
100 fl
Legend
T Survey Baseline 500 it
Oak Island Survey Transects
0 150 300 600
Figure 1. Survey Area Limits
Figure 2. Survey Area Setup — looking from MLW
"iiv::
.F 4' -.0s00
Table 1 shows the detailed results from the investigation; a total of 743 clasts greater than or
equal to 3 inches in diameter were surveyed. Figures 3 and 4 show representative clasts found
in the survey area. A few clasts (-10-15 per row) were observed on the upper beach (from the
vegetation line to mean high water (MHW)), while the majority of clasts were documented below
MHW. The clasts surveyed in this area were mostly rocks, with some shells found. The rocks are
remnants of the 2001 Turtle Habitat Restoration Project conducted by the USACE which was
constructed with material from the upland site, Yellow Banks. As mentioned, this area had been
nourished in 2018 during the Post -Matthew FEMA Emergency Dune Restoration project. It was
noted that the dunes were absent in the area which was attributed to the storm activities
(Florence, Michael, and Dorian) that occurred since the 2018 project was completed. The wrack
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line and upper beach near the vegetation line 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
34
Thick covering of old marsh
vegetation at wrack line and
vegetation line. Dunes were
absent in the area. Clasts mostly
found below MHW, majority rocks
with few shells.
2
22
3
21
4
40
5
23
6
31
7
26
8
43
9
40
10
29
11
27
12
25
13
35
14
57
15
58
16
51
17
53
18
51
19
46
20
31
Total
743
Figure 3. Row 5 documentation
Figure 4. Row 16 documentation
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Figure 5. Thick covering of old marsh vegetation spread across upper beach near vegetation line
4