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HomeMy WebLinkAboutClast Investigation_191008Purpose 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. 1 410+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 2 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 3 y e � ♦ y v Figure 5. Thick covering of old marsh vegetation spread across upper beach near vegetation line 4