Memo_DCM_Vol_Caswell_DRAFT_Final. Mar21-Oct21

4700 Falls of Neuse Rd, Suite 300
WhARaleigh, NC 27609
moffatt & nichol [phone]
www. moffattnichol. com
MEMORANDUM
To: Heather Coats (DCM) / Greg Currey (USACE)
From: Brandon Grant (M&N)
Date: 11 /15/2021
Subject: 202012021 Oak Island Nourishment Project: Post -Construction Shoreline Survey
Summary
M&N Job No.: 10128-04
Heather/Greg,
Per Permit No. 31-20 (Condition #8) and SAW-2018-02230 (Special Condition #43), the Town of Oak Island
conducted a post -construction shoreline survey of Caswell Beach as defined in the Shoreline Monitoring
Plan dated January 30, 2020 and the associated amendment letter dated February 24, 2020.
TI Coastal conducted a shoreline survey in March 2021 and October 2021 on Caswell Beach between
stations 20+00 and 200+00 (see Figure 1) as summarized below:
• March 2021 Survey data of Caswell beach stations 20+00 — 200+00 (xyz, bmap)
• October 2021 Survey data of Caswell beach stations 20+00 — 200+00 (xyz, bmap)
Figure 1. Caswell Beach Monitoring Transects
The annual shoreline monitoring survey taken in March 2021 (March survey) occurred prior to the dredging
and beach fill placement event and therefore the October 2021 (October survey) data was collected at the
request of Caswell Beach following phone conversation with Moffatt & Nichol (M&N) on October 4, 2021
noting their concerns of escarpments forming as noted by the Town of Caswell Beach in the vicinity of the

M&N # 10128-04
November 15, 2021 Memorandum
Oak Island Lighthouse within the 300 block of Caswell Beach Rd (approximately Station 80+00). The
pre -construction March 2021 xyz data was sent to Caswell Beach on October 11, 2021.The March 2021
bmap data and the complete October data set was sent to Caswell Beach on October 27, 2021.
M&N had a discussion with the Town of Caswell Beach's Engineer, Coastal Protection Engineering
(CPE), on October 28, 2021, regarding the monitoring data that was sent to confirm the methodology
in which volume change would be calculated to ensure consistency between methods of analysis used.
The shoreline change and volume change calculations were performed in accordance with the
monitoring protocol defined in Appendix G in the major modification to permit 31-20. Shoreline change
at mean high water (MHW) and volume change above -2 ft NGVD and depth of closure (DOC) were
calculated between the March and October surveys. MHW for Caswell Beach is +2.9 ft NGVD29 (+1.8
ft NAVD88) and the DOC elevation chosen is -13.9 ft NGVD29 (-15 ft NAVD88). The Caswell Beach
shoreline was divided into three reaches: Fort Caswell — Inlet (20+00 — 50+00), Caswell — East (60+00
— 150+00), and Caswell — West (160+00 — 200+00). The overall Caswell — Oceanfront includes Caswell
— East and Caswell — West. The divide between East and West was made based on shoal influence
within the East as well as the break between areas where infrastructure is present. For this analysis, the
Caswell — Oceanfront includes stations 60+00 through 200+00. The overall summary of shoreline and
volume change is shown in Table 1. The Caswell — Oceanfront experienced overall shoreline accretion
and volumetric gains above all elevations. These trends are consistent throughout the Oceanfront
reaches with the exception of the shoreline change for Caswell — East, which experienced mild shoreline
erosion.
Table 1. Shoreline Change and Volume Change Summary
avg
avg
cumulative
avg volume
cumulative
March 2021 vs. October 2021
Reach
shoreline
volume
volume
change
volume
(Total Change)
Length
change
change
change
above
change
@ MHW
above -2 ft
above -2 ft
DOC
above
NGVD
NGVD
DOC
Reach
ft
ft
c /ft
cy
c /ft
c
Fort Caswell - Inlet
3,502
-10.1
-0.1
-469
-7.2
-25,383
20+00 - 50+00
Caswell - East
10,031
0.0
4.0
40,590
18.7
187,246
60+00 - 150+00
Caswell — West
4,589
15.1
3.2
14,786
5.5
25,029
160+00 - 200+00
Reach
weighted
weighted
total
weighted
total
Length
avg
avg
avg
Caswell
18,122
1.9
3.0
54,907
10.3
186,892
20+00 - 200+00
Caswell - Oceanfront
14,620
4.8
3.8
55,376
14.5
212,276
60+00 - 200+00
The shoreline change and volume change calculated at each profile are shown in Figure 2 and Figure 3,
respectively. The shoreline change oscillates between erosion and accretion across Caswell Beach (note
there was shoreline erosion at Station 80+00 surrounded by areas of shoreline accretion). However, the
volume change consistently shows a gain in material across every profile along Caswell — Oceanfront at
each elevation except for Station 170+00 above depth of closure. The profile plots for this analysis are
included in Appendix A for reference. Overall, since the first dredging event of Jay Bird Shoal that occurred
between March 2021 and May 2021, Caswell Beach experienced a gain in both shoreline and volume along
the surveyed area. These results indicate there has been no negative impact to the Caswell Beach
shoreline between March and October 2021. While erosion was measured along the inlet shoreline, these
areas are more susceptible to erosion on a routine basis. Shoreline surveys and monitoring will continue
in the future as required by the permits.

November 15, 2021
M&N # 10128-04
Memorandum
Caswell Beach: Shoreline Change March 2021 - October 2021
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Figure 2. Caswell Beach Shoreline Change
Caswell Beach: Volume Change March 2021 - October 2021
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Figure 3. Caswell Beach Volume Change