Monitoring Report 2017

Transcription

1 CAPTAIN SAMS INLET RELOCATION 2015 SEABROOK ISLAND SOUTH CAROLINA Monitoring Report 2017 Prepared for: Seabrook Island Property Owners Association Johns Island South Carolina

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3 SEABROOK ISLAND, SOUTH CAROLINA CAPTAIN SAMS INLET RELOCATION 2015 Monitoring Report Year 2 Prepared for: 1202 Landfall Way Johns Island SC Prepared by: PO Box 8056 Columbia SC [2460YR2 MR] MARCH 2017 COVER PHOTO: Aerial view of Seabrook Island at low tide on 12 October 2016, four days after Hurricane Matthew. The storm helped straighten the North Beach shoreline around old Captain Sams Inlet, rounded the headland at Renken Point (right edge of photo), and produced a scour hole at Deveaux Villas (bottom of photo). (TW Kana)

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5 TABLE OF CONTENTS 1.0 INTRODUCTION & KEY FINDINGS BACKGROUND AND METHODOLOGY Background and Key Events Survey Methodology BEACH CHANGES AND RECENT EVENTS Unit Volumes and Net Volume Changes South Beach Scour Hole Reach Hurricane Matthew 8 October CHANGES AROUND CAPTAIN SAMS INLET & HABITAT MAPPING Post-Relocation Inlet Movement Habitat Mapping Results SUMMARY & RECOMMENDATIONS Scour Hole at Deveaux Villas Recommendations REFERENCES & BIBLIOGRAPHY ACKNOWLEDGMENTS Appendix A) Stationing and CSE Profiles Appendix B) Profile Volume Changes by Line and Reach (January 2006 to January 2017) Monitoring Report Year 17 i Seabrook Island, South Carolina

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7 1.0 INTRODUCTION & KEY FINDINGS This is the second report following the third relocation of Captain Sams Inlet in It also follows over 30 years of beach surveys and reports covering Seabrook Island (SC) and the impacts of prior beach restoration efforts. The focus of the present report is the condition of the beach associated with the third inlet relocation. The report includes comparative profiles, tables, and graphs presenting sand volume changes reach-by-reach along the shoreline. Also included are sections describing changes around Captain Sams Inlet, extensive erosion and dune breaching along North Beach, and events occurring before and after the June 2015 relocation of Captain Sams Inlet. The overall history of Seabrook s beach has been widely reported in various publications since the 1970s (see References & Bibliography at the back of this report). Key milestone events are repeated in Section 2.1, including the initial development of the community, construction of seawalls, implementation of soft-engineering solutions (ie sand management), and sustained restoration of the beach. Since 1990, Seabrook Island has gained over 1.5 million cubic yards via inlet relocation (1983, 1996, and 2015) and nourishment (1990), which is equivalent to a gain of over 50 acres of beachfront lands seaward of the seawall. Restored dunes and beach extend from Beach Court to Captain Sams Inlet and between Beach Club Villas and Camp St Christopher. Only the area around the Beach Club now lacks a dry-sand beach. Prior monitoring reports tracked changes by means of beach-profile surveys with ongoing estimates of how much sand existed along the oceanfront from section to section. Reaches were defined to isolate the erosion and accretion trends around Captain Sams Inlet, along North Beach (Oystercatcher to Renken Point), along South Beach (Renken Point to the Beach Club), and along North Edisto River Inlet (Beach Club Villas to Camp St Christopher). The present report retains the same reaches but rather than comparing back to surveys of the 1980s and 1990s, CSE has elected to focus on changes in the past decade or so. The primary reason for this change is to provide a more realistic estimate of the sand volumes that constitute Seabrook s active beach. This will be explained in more detail in Section 2. But simply stated, CSE s surveys in recent years encompass more profiles and extend further offshore than the early surveys. The newer data sets allow apples-to-apples comparisons to the depth at which sands are moving in the littoral zone (ie the zone directly connected with Seabrook s visible beach. For the present and next series of annual monitoring reports, CSE will use November 2010 as a baseline year. This date marked a time when the beach was relatively healthy but was beginning to exhibit erosion along North Beach due to downcoast migration of Captain Sams Inlet. While efforts were underway to obtain permits for the third inlet relocation (2015), it would take nearly five years Monitoring Report Year 17 1 Seabrook Island, South Carolina

8 of planning and appeals before the inlet was relocated. During the permitting period, CSE surveyed Seabrook s beach in November 2010, January 2012, January 2014, and January These are the principal pre-project survey dates referenced herein. Following the third inlet relocation, CSE completed a Year 1 survey in April 2016 and the present Year 2 survey in January Figure 1.1 shows the network of 50 oceanfront profiles along with the applicable reaches which have been monitored by CSE since November Profiles 1 40 are situated between Camp St Christopher and new Captain Sams Inlet. The remaining profiles are along Kiawah spit. CSE tracks changes along the spit because they relate to future trends at Captain Sams Inlet. Following each inlet relocation, Kiawah spit regrows to the southwest, while the Inlet Conservation Zone (ie the area between profiles 28 and 40) becomes shorter. The background image of Figure 1.1, for example, shows conditions in 2014 when Captain Sams Inlet was centered between profiles 34 and 35. In June 2015, the inlet was relocated to the area around profile 39. Not surprisingly, the greatest changes along Seabrook occur where Captain Sams Inlet is migrating through a particular profile. Previous reports have referenced Reaches 2 8 as the primary Seabrook development reaches. Reach 1 is Camp St Christopher, and Reaches 9 10 are the Inlet Conservation Zone (ICZ) for Captain Sams Inlet. Reach 11 is the new inlet, and Reach 12 is Kiawah spit. The most recent survey confirmed that Reaches 2 8 lost ~105,000 cubic yards (cy) between April 2016 and January This nearly equals the net losses during the 15-month period between January 2015 and April Despite this overall trend, the area between Amberjack Court and Pelican Watch Villas (Reaches 2 5) gained ~30,000 cy between April 2016 and January Renken Point (Reach 6) lost ~14,650 cy, and North Beach (Reaches 7 8) lost nearly 121,000 cy. The abandoned shoals of old Captain Sams Inlet (Reaches 9 10) moved landward, adding ~128,000 cy to the oceanfront. Camp St Christopher (Reach 1) has continued an erosion trend since January 2012 and lost ~5,300 cy between April 2016 and January New Captain Sams Inlet (Reach 11) lost ~15,000 cy, and Kiawah spit (Reach 12) lost ~65,000 cy. The present report discusses the effect of Hurricane Matthew (8 October 2016) on these results. Other sections of the report draw on three-dimensional (3D) topographic and bathymetric maps from recent years to evaluate changes. CSE s older methods of analysis are less accurate but offer the perspective of time, such that long-term trends can be identified and quantified. The 3D topographic maps, by comparison, provide a more detailed accounting of sand trapped in shoals or moving alongshore. The latter maps illustrate the subtleties of habitat evolution after a major change, like inlet relocation, and allow CSE to provide more accurate estimates of areas and volumes gained or lost due to the project. Monitoring Report Year 17 2 Seabrook Island, South Carolina

9 FIGURE 1.1. Network of beach profile stations monitored in Numbers in boxes are shoreline segments (reaches) discussed in the text. See Table 2.2 for old and new station equivalents. Monitoring Report Year 17 3 Seabrook Island, South Carolina

10 The present report summarizes the condition of the beach along Seabrook Island between November 2010 and January 2017, providing detailed survey results. Captain Sams Inlet was relocated on 11 June It deepened and widened to an equilibrium condition with most of the change occurring on the Seabrook side of the new channel. The edge of the new inlet was ~500 feet (ft) closer* to Seabrook and maintained a depth of 14 ft NAVD in January 2017, 19 months after relocation. This rate of migration is almost double the rate of change during the first years after the 1996 inlet relocation. *[Measured along the closure dike alignment.] While erosion has dominated since 2010, much more sand remains between Camp St Christopher and Oystercatcher compared with conditions right after nourishment in February This is apparent along North Beach and Renken Point where seawalls remain buried and safely distant from the normal high-tide mark. Similarly, the dunes along North Edisto River Inlet are much wider than they were before nourishment. In July 2016, a large scour hole formed along the wet-sand beach near Deveaux Villas (ie between the Beach Club and Beach Club Villas). This is an area marking the confluence of the northern channel and the main channel of North Edisto River Inlet. Scour is common in such areas, but generally occurs underwater and well moved from the beach. The first scour hole extended about 150 ft along the shore and nearly reached the seawall. After a couple of weeks, sand naturally infilled the area. A second event was observed after Hurricane Matthew (8 October 2016). That hole was longer and reached the toe of the seawall, possibly contributing to settlement of armor stone around Deveaux Villas. Like the first event, the October scour hole healed naturally by sand moving alongshore. A third scour hole formed at Deveaux Villas after a rainstorm between 19 and 25 January CSE was able to collect profiles and map the area before and after the event. These measurements confirmed the scour hole was an underwater landslide between the beach and the 25-ft depth contour. Nearly 35,000 cy eroded and slid into deep water (35 55 ft depths) along the North Edisto River Inlet channel. Three possible causes of the erosion are discussed in a later section of this report. Despite uncertainty regarding the cause of the scour holes, CSE recommends that the SIPOA initiate planning and permitting for sand transfers along the beach. Areas with a sand surplus like the Inlet Conservation Zone (ICZ) should be used as a borrow source to build up vulnerable areas along South Beach, including the profile at Deveaux Villas. This type of sand management is consistent with the Town s Beach Management Plan (Town of Seabrook 2014). Monitoring Report Year 17 4 Seabrook Island, South Carolina

11 2.0 BACKGROUND AND METHODOLOGY CSE (and its predecessor company) have collected beach profiles at Seabrook Island dating back to the 1970s. Surveys have evolved from a series of eight wading-depth profiles (CSE-0 through CSE-8) using rod-and-level techniques to the present network of 50 profiles and additional coverage for surface-elevation modeling (LiDAR and digital terrain models DTMs) collected using state-of-the-art GPS and drone systems. Building on the initial series of profiles, additional transects were established by CSE or SCDHEC s Office of Coastal & Ocean Resource Management (OCRM) (formerly the South Carolina Coastal Council) in the late 1980s (2500 series and the SBK series) and were included in monitoring efforts through The new transects increased the spatial resolution of beach profiles and extended the coverage east to include the area near Captain Sams Inlet. Additional lines were established in 2008 to expand survey coverage to the area near Beachwalker Park on Kiawah Island. These lines are more evenly spaced and numbered sequentially from 1 to 50. Line 16, for example, corresponds to CSE 5 or OCRM Each transect has been surveyed from backshore monuments (or survey baselines) to some distance and depth offshore, depending on the specific requirements of each yearly monitoring. While technology used in the surveys has also varied over time, the raw profile data provide direct comparisons among any survey dates. As noted in CSE s most recent monitoring reports (CSE 2014, 2016), some profile azimuths and applicable shore lengths between profiles have been revised to better reflect the shape of the shoreline around Renken Point. This change results in minor differences in volumes (relative to the overall sand supply along Seabrook Island) compared with earlier reports. For the present and future reports, CSE has elected to focus on the data sets collected over the past decade. These profiles generally extend further seaward than earlier data and allow calculations to deeper depths. For example, prior reports cut off calculations along Camp St Christopher at low-tide wading depth because that was the limit of earliest surveys. Similarly, many of the North Beach profiles terminated in depths around 7 ft NAVD; thus this depth became the reference for some reaches. The new data sets provide reliable data into deeper water and better indicate the approximate depths at which there is no observable change in bottom elevation over a period of several years. When sequential profiles converge at a particular depth and show negligible change beyond that depth for some distance, CSE refers to this as the depth of closure (DOC). DOC indicates the approximate outer limit of sand exchange between the visible beach and the underwater beach. Nearly all littoral sand transport occurs landward of DOC and consists of movement downcoast as well as cross-shore. During storms, waves will shift sand from the visible beach to shallow water, Monitoring Report Year 17 5 Seabrook Island, South Carolina

12 which is nature s way of absorbing and dissipating wave energy. After storms, sand will move shoreward and gradually rebuild the visible beach. Longshore transport is the zigzag motion of sand associated with waves breaking at an angle to the shoreline. This transport shifts sand downcoast, builds sand spits such as Kiawah spit, and redistributes sand along the beach. Seabrook s beach restoration efforts during the past 35 years have been directly related to managing sand transport and redistribution alongshore. The following section repeats key milestones and events related to the beach for the benefit of the new reader of Seabrook s history. 2.1 Background and Key Events The beach along Seabrook Island is in a constant state of flux in response to variations in wave energy and sediment supply due to effects of Captain Sams Inlet and North Edisto Inlet. Extensive descriptions of the morphological changes, causes of erosion, and restoration history of Seabrook Island are provided in CSE s (2011) Captain Sams Inlet Relocation Project: Design Report. Generally, the beach sediment supply has been controlled by natural and artificial relocations of Captain Sams Inlet, while localized erosion between Renken Point and the Beach Club is influenced by the position of the marginal flood channel of North Edisto River Inlet. To date, there have been four large-scale projects designed to restore or maintain sediment supply to the beach: three inlet relocations (1983, 1996, 2015) and a nourishment project to realign the northern channel (1990). In addition, there have been at least ten small-scale, sand transfer events since the 1980s whereby some of the excess sand north of Oystercatcher is excavated and hauled by trucks to eroding downcoast areas. Table 2.1 provides an updated event log (building from CSE s 2007 Survey Report 11) of significant shoreline activities along Seabrook Island dating to initial development in the early 1970s. Major events all bear some relation to erosion and sand transport processes around Captain Sams Inlet and North Edisto River Inlet. What is particularly notable about the events is their cyclic nature. Monitoring Report Year 17 6 Seabrook Island, South Carolina

13 TABLE 2.1. (shown on 9 pages) Seabrook Island major shoreline events Captain Sams Inlet breaches Kiawah spit near present-day Beachwalker Park, creating multiple channels. A single channel becomes dominant by early 1950s (Fig T-2) Mouth of Captain Sams Inlet is aligned with the mouth of Captain Sams Creek about 1.3 miles north of the present-day Oystercatcher beach access. This shoreline and inlet configuration becomes the model for the 1983 and 1996 inlet relocations (Fig T 3). 1960s Seabrook s beach is healthy and generally growing seaward. In some places like Renken Point, the rate of growth is over 30 feet per year (ft/yr). FIGURE T-1. Aerial view of Seabrook Island in November Circa 1970 Seabrook Island becomes a planned-unit development. Roads, golf course, and lots are platted using the existing dune/ vegetation line as a basis for the plan. (Development allowed behind the normal limit of tides and waves without regard to historical shoreline trends.) FIGURE T-2. Vertical photograph (1949) of Seabrook Island before development. Sometime in 1948, Captain Sams Inlet breached Kiawah spit near present-day Beachwalker Park (right side of image). The northeastern channel became dominant in the 1950s. Monitoring Report Year 17 7 Seabrook Island, South Carolina

14 FIGURE T-3. Seabrook Island and Captain Sams Inlet in 1963 (upper) and 1983 (lower). The 1963 condition served as a model for the plan to relocate Captain Sams Inlet. Lower photo shows the new channel (A) open before the old channel (B) was closed on 4 March s Seabrook Island is in a rapid erosion cycle with some areas like Renken Point eroding at over 20 ft/yr Beach Club under construction Erosion impacts the Beach Club before construction is complete. First shore-protection measures consist of large sand bags, sandbag groins, and sheet-pile bulkheads (Fig T-4). FIGURE T-4. Shore-protection structures at the Beach Club in September 1974 prior to the club s opening. Monitoring Report Year 17 8 Seabrook Island, South Carolina

15 Succession of sandbag revetments, timber and concrete bulkheads/seawalls, and quarry-stone revetments are installed along Seabrook Island between Pelican Watch Villas and the 13 th fairway of the golf course (~2 miles). Individual property owners are generally responsible for the cost of shore-protection structures which, by the late 1980s, totals over $5 million for the island (Fig T-5) RPI (c/o Prof Miles Hayes) completes the first shoreline assessment of the island, identifies three principal erosion-causing processes, and recommends soft solutions involving inlet relocation and nourishment. SEP 1979 Hurricane David causes extensive damage to the seawall (Fig T-6). Mouth of Captain Sams Inlet is near the Oystercatcher beach access. Seabrook s only dry beach areas are a 2000-ft reach around Oystercatcher and the North Edisto Inlet shoreline along Pelican Watch Villas. FIGURE T-5. During the early 1980s, much of Seabrook lacked any beach even at low tide. [UPPER] View north from Renken Point at mid tide. [LOWER] Oblique aerial (1982) looking north at low tide showing no beach around Renken Point. FIGURE T-6. Collapse of the concrete seawall at Renken Point in September 1979 during Hurricane David. Monitoring Report Year 17 9 Seabrook Island, South Carolina

16 MAR 1983 First relocation of Captain Sams Inlet ~1.3 miles north to its 1963 position. Old inlet closed by trucks hauling sand from the new channel basin. Cost of project is (~)$300,000 (Fig T-7). LATE 1980s North Beach is restored by natural processes as sand from the delta of abandoned Captain Sams Inlet migrates onshore, adding over 1 million cubic yards to Seabrook s beach. North Beach is upward of 1,000 ft wide in places, a dry beach is restored, and the rock revetment north of Renken Point begins to be buried by windblown sand. FIGURE T-7. February-March [UPPER] Excavation of the basin for the new channel by landbased equipment. [MIDDLE] The new channel across Kiawah spit and closure dike under construction in the distance on 18 February two weeks before project completion. [LOWER] Closure of the old channel on a falling tide on 4 March Monitoring Report Year Seabrook Island, South Carolina

17 1980s Several sections of the seawall (south of Renken Point) breach during minor storm events (Fig T- 8). No new sand reaches Beach Club Villas or Pelican Watch Villas for nearly a decade, causing loss of the dry beach The northern channel of North Edisto Inlet is forced shoreward by the shoal off Renken Point, causing dangerous encroachment along the seawall (Fig T-8). At Amberjack Court, the channel 50 ft from the wall is 22 ft deep. Property owners continue to add rock in this area to shore up the seawall. FIGURE T-8. Encroachment (upper) of the northern channel (deep blue area) of North Edisto Inlet and lack of maintenance leads to collapse (lower) of a section of seawall near Beach Court in Monitoring Report Year Seabrook Island, South Carolina

18 FEB 1990 The northern channel is realigned by an ocean-going dredge (Great Lakes Dredge & Dock Company dredge Illinois) which builds a parallel channel 600 ft seaward while filling the existing channel along the seawall (Fig T-9). The project adds 685,000 cubic yards to the beach between Renken Point and Pelican Watch Villas. A narrow dry beach exists south of Renken Point for less than one year before the project adjusts. A narrow wet-sand beach persists through the 1990s, giving the seawall protection. Cost of nourishment project is $1.6 million. FIGURE T-9. [UPPER] 1989 plan for realignment of the northern channel and nourishment south of Renken Point. [LOWER] Start of dredging operations in February 1990 at Renken Point. Monitoring Report Year Seabrook Island, South Carolina

19 CIRCA 1995 Nourishment losses south of Renken Point begin to reverse as the area stabilizes and begins a long period of accretion by natural and artificial means. Captain Sams Inlet has migrated about 3,000 ft since the 1983 relocation. APR 1996 Captain Sams Inlet relocated again to its 1963/1983 position (Fig T-10). Cost of construction is (~)$400,000, which is comparable to the cost of one oceanfront lot at this time Winter sand scraping around the abandoned inlet is implemented to accelerate adjustment of the shoreline. An outer dike is constructed 500 ft seaward of the closure dike, leaving a small lagoon between the two dikes. This creates a straighter, longer North Beach and leads to more efficient sand transport to the south Winter sand scraping from North Beach is performed to transfer ~350,000 cubic yards to South Beach. This adds to the natural sand transport from north to south and accelerates recovery of South Beach. By 2005, only about 1,200 ft of shoreline (vicinity of the Beach Club and Beach Court) lack a dry beach during normal high tides. FIGURE T-10. The second relocation of Captain Sams Inlet in April [UPPER] First tide into the channel basin on 4 April during a rising tide. [LOWER] The new channel (left side) before completion of the closure dike across the old channel. Monitoring Report Year Seabrook Island, South Carolina

20 Migration of Captain Sams Inlet leads to focused erosion along North Beach. After review of outside opinions and alternatives, the POA Environmental Committee decided to initiate engineering and permitting for the third inlet relocation project Permit application submitted for third relocation of Captain Sams Inlet Additional reviews, studies, and revisions to permit application. Permit application resubmitted in 2010 and issued by SC DHEC OCRM in January 2012 and by USACE in October The SC permit is appealed by one Seabrook Island property owner Captain Sams Inlet continues to migrate to the west, reaching the approximate location of the 1996 channel. Erosion intensifies along portions of North Beach. Without sand scraping, sediment supply to the rest of Seabrook is reduced, resulting in erosion of the area near the Beach Club. FIGURE T-11. Composite image of the Captain Sams Inlet area from the Seabrook side in January The lagoon formed in the abandoned 1996 channel is on the left side of the image Portions of Kiawah spit which have been stable for a least 40 years become developable under periodic revisions to state jurisdictional setback lines. The new lines leave a wide buffer of foredunes for protection and terminate near the Town of Kiawah Island/Town of Seabrook Island easement boundaries positioned immediately north of the 1983 and 1996 positions of Captain Sams Inlet Kiawah Development Partners (owners of Kiawah spit) sell the land to Kiawah Partners, who announce plans to build 50 homes on the spit north of Captain Sams Inlet Kiawah Partners request a modification of the proposed alignment of Captain Sams Inlet relocation to place the cut ~400 ft south of its planned location near the Town easement line In December, the Administrative Law Court dismisses the lawsuit against SIPOA (which was brought by a property owner in 2012), clearing the way for the third inlet relocation to occur Between 18 May and 18 June, Captain Sams Inlet is relocated for the third time (Fig T-12). The contractor, RE Goodson Construction Inc (Darlington SC) opened the new channel on 2 June, although significant flow did not occur until 12 June because of a plug of marsh at the landward end. The first closure attempt on 4 June failed. The old channel was successfully closed during the second attempt on the evening of 11 June. Final grading and equipment removal occurred on 18 June. Total construction cost was $930,500. The volumes required for channel and dike construction were ~165,000 cy. (CSE 2015a) Monitoring Report Year Seabrook Island, South Carolina

21 FIGURE T-12. Captain Sams Inlet after inlet relocation in June Kiawah spit is to the right and Captain Sams Creek is at the upper right corner of the image. The orthorectified aerial photo was prepared by Independent Mapping Consultants Inc (Charlotte NC) First monitoring survey after the third inlet relocation project is completed March April Seabrook Island is selected for an ASBPA* Best Restored Beaches Award. *[American Shore and Beach Preservation Association Hurricane Matthew, a Category 1 hurricane, tracks along the South Carolina coast, impacting Seabrook Island with a storm surge ~5 ft above normal tides on 8 October Second annual monitoring survey (after the 2015 inlet relocation) is completed in January. Monitoring Report Year Seabrook Island, South Carolina

22 2.2 Survey Methodology The present study builds on the results of previous surveys and updates conditions along Seabrook s shoreline. CSE mobilized field personnel and equipment to Seabrook Island and completed a resurvey of the beach and inshore zone between 16 and 20 January Surveys were performed using a Trimble Model R10 GNSS with VRS RTK-GPS* for backshore, intertidal, and surf-zone work (Fig 2.1). Bathymetry seaward of the surf zone was obtained using an Applanix POS MV Surfmaster positioning system linked to a precision fathometer (Odom Echotrac CV 100 and SMSW200-4a transducer) mounted on CSE s research vessel, the RV Southern Echo. *[RTK GPS Real-time kinematic geographic positioning system which utilizes signals from satellites to triangulate true position and elevation to a high degree of accuracy.] FIGURE 2.1. CSE s monitoring methods include land-based data collection via RTK- GPS (upper left) and hydrographic data collection via RTK-GPS linked to a precision echo-sounder. CSE s shallow-draft vessel the R/V Southern Echo is shown in the image to the right. Monitoring Report Year Seabrook Island, South Carolina

23 Raw data were collected at 50 Hz or 50 points per second. The seaward limit of the survey was generally greater than 0.5 mile offshore. Data were collected in x y z format and converted to x z format (distance elevation pairs) referencing survey monuments for direct comparison with historical data. Raw data were filtered and averaged using HYPACK software and were reduced to manageable size for each profile. With improved positioning, CSE s standard for tracklines over water is ±20 ft in the horizontal and ~5 centimeters (~2 inches) in the vertical. Table 2.2 provides an updated list of profile names and reaches with cross-references to line names used in earlier reports. Figure 1.1 previously showed the location and orientation of monitoring profiles. Appendix A provides profile plots for the principal survey dates referenced herein. Table 2.2 also lists the start and end points for each profile line followed by an offset distance which indicates the starting point for unit-volume calculations. The cutoff distance generally indicates where DOC occurs at each line and marks the outer boundary for volume calculations. Some profiles are cut off close to shore because they are situated along the channels of North Edisto River Inlet. CSE considers the active beach to terminate near the center of the channel or in shallower water where there is an underwater terrace (eg along Camp St Christopher). Sand bars on the seaward side of channels are not considered part of the beach system until they migrate into shallow water. Thus, the bars off South Beach are not included in volume calculations because the northern channel prevents onshore accretion. However, bars at the mouth of Captain Sams Inlet are generally separated by shallow runnels and freely exchange with the beach. CSE includes ridge-and-runnel features when computing littoral volumes. The profiles in Appendix A show the limits for volume calculations and illustrate CSE s interpretation of the dimensions of the active beach zone. Table 2.2 lists the applicable calculation depths ( Lens Limit ), which range from 10 ft NAVD to 24 ft NAVD. Normal DOC away from inlets is (~) 12 ft NAVD in this setting. Deeper DOC is assumed for profiles along the northern channel of North Edisto River Inlet (ie between Renken Point and Beach Club Villas). Monitoring Report Year Seabrook Island, South Carolina

24 TABLE 2.2. Stationing information and volume calculation limits. The limits are generally deeper for the present report because recent survey data sets extend further offshore with greater accuracy. Monitoring Report Year Seabrook Island, South Carolina

25 After preparing comparative data files and checking for quality and completeness, CSE generated graphic profiles for selected dates between November 2006/November 2010 and the present survey. This period reflects changes before the 2015 inlet relocation and nearly two years after the recut. It corresponds with the time that downcoast migration of Captain Sams Inlet triggered planning for the third relocation project. Quantity estimates are derived by applying profile changes over representative shoreline reaches and cross-shore boundaries, using the average-end-area method. Normally, along straight beaches, some uniform depth limit for volume calculations can be established and used over time for consistency of comparisons. Seabrook s shoreline, by contrast, is fronted by two major channels of varying depth as well as by migrating Captain Sams Inlet, giving it an irregular and varying orientation. Beach lengths around the curving shoreline also vary over time under the effects of accretion. Monitoring Report Year Seabrook Island, South Carolina

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27 3.0 BEACH CHANGES AND RECENT EVENTS Between 1983 (first inlet relocation) and 2010, Seabrook Island gained upward of 1.5 million cubic yards along the beachfront (Fig 3.1). This accretion is reflected in burial of much of the seawall and the extensive dune area along North Beach. Therefore, comparisons in the present report should be evaluated in the context of these previous improvements. Since 2010, some areas of Seabrook have continued to gain sand while other sections have eroded. Figure 3.2 illustrates the changes at two localities: profile 13 along South Beach at the old emergency boat ramp near the Beach Club, and profile 21 along North Beach near Loggerhead Court. The general trend along most of South Beach has been accretion since November At profile 13, the visible beach is incrementally higher and wider in 2017, but the biggest change is underwater. FIGURE 3.1. Quantity of 1990 nourishment remaining within the project area along Seabrook Island compared to the pre-nourishment condition. The decrease in volume between 2007 and 2016 reflects the diminished sand supply from North Beach due to migration of Captain Sams Inlet closer to Seabrook Island before it was relocated in June Monitoring Report Year Seabrook Island, South Carolina

28 FIGURE 3.2. Example profiles from South Beach (#13) and North Beach (#21) showing changes between November 2010 and January Also indicated are the boundaries used in calculating unit volumes. Profile 13 shows a general trend of accretion since 2010, whereas profile 21 shows extensive dune recession. Note: MHW is the approximate elevation of mean high water relative to the survey datum, NAVD (North American Vertical Datum), which is roughly 0.5 ft above mean sea level in the Charleston area. Monitoring Report Year Seabrook Island, South Carolina

29 Figure 3.2 (upper) shows a nearly 5-ft buildup of the profile 300 ft from the seawall. This underwater buildup has kept the northern channel from encroaching on the beach during the past seven years. The beach volume in that section is computed between the crest of the seawall and the approximate centerline of the channel 500 ft offshore. The cutoff depth is 15 ft NAVD, which is slightly deeper than the northern channel at that locality. Unit volumes within reference bound - aries are computed by taking the applicable cross-section of the profile and assuming it is 1 ft (unit) thick, which provides a quantity of sand contained in one linear foot of beach. Note the volume calculation at profile 13 does not include sand in the offshore shoal because that quantity is not directly attached to the beach. The lower profile of Figure 3.2 shows an example from North Beach. Profile 21 is much wider and extends about 1,750 ft offshore to the center of the channel separating offshore shoals. The reference cross-section for unit volumes necessarily extends into deeper water and will contain more sand than profile 13. Note that profile 21 sustained nearly 400 ft of dune recession between 2010 and Some of this erosion has produced a buildup underwater in the area 1,200 2,000 ft offshore. The absolute unit volumes given on Figure 3.2 show that profile 21 has much more sand in the active beach zone than profile 13. It also shows the differences between 2010 and 2017 range from ~26 cubic yards per foot (cy/ft) to nearly 130 cy/ft. In the following tables, keep in mind that unit volumes depend on the depth limits applied for a particular profile as well as other factors, such as proximity to channels. In general, largest unit volumes occur around the abandoned shoals of Captain Sams Inlet which have become an integral part of the active beach as they migrate into shallow water. Smallest volumes occur where the northern channel is closest to the seawall. 3.1 Unit Volumes and Net Volume Changes Tables 3.1 and 3.2 provide unit volumes for the primary survey dates discussed herein. Some of the data sets (January 2006 to December 2008) do not include all 50 profile lines, so comparisons with data from 2010 forward are incomplete. Table 3.1 includes average unit volumes by reach, unweighted according to the applicable distances between profiles. Table 3.2 gives the unit volume change and net volume change between profiles (via the average-end-area method). The bottom of the table tallies results by reach and gives the weighted unit volume change by date relative to November Table 3.2 also gives the net changes between April 2016 and January 2017, and the bottom of the table indicates the following: Monitoring Report Year Seabrook Island, South Carolina

30 TABLE 3.1. Seabrook updated unit volumes. (See Table 2.2 and Appendix A for calculation boundaries.) Monitoring Report Year Seabrook Island, South Carolina

31 TABLE 3.2. Seabrook unit volume change and net volume change between profiles. Monitoring Report Year Seabrook Island, South Carolina

32 Camp St. Christopher has eroded over the past year and since November South Beach to Pelican Watch Villas has generally accreted over the past year and since November The Inlet Conservation Zone (ICZ) around old Captain Sams Inlet has built up by over 900,000 cy since November Kiawah spit has eroded significantly since January 2014, losing about 60,000 cy/yr over the past two years. Figure 3.3 illustrates the net change north and south of Renken Point and the change for Reaches 2 10 relative to November Renken Point (Reach 6) accounts for most of the change along the southern half of the island. Reaches 9 and 10 (ICZ) more than offset net losses in Reaches 7 and 8. Island wide (Reaches 1 10) Seabrook Island in January 2017 has about 414,000 cy more sand than November However, the net gain between April 2016 and January 2017 has only been about 18,000 cy. As previously mentioned, if only the development reaches (2 8) are considered, Seabrook Island has lost about 105,000 cy in each of the past two years. FIGURE 3.3. Net change in volume along Seabrook Island since November Reaches 2 6 are Pelican Watch Villas to Renken Point. Reaches 7 10 are North Beach and the Inlet Conservation Zone (excluding new Captain Sams Inlet). Monitoring Report Year Seabrook Island, South Carolina

33 Figures 3.4 and 3.5 show the weighted unit volume change by reach relative to the November 2010 condition. Figure 3.4 (upper) shows results for Reaches 1 5. Reaches 4 and 5 were most variable whereas Reaches 1, 2 and 3 showed general stability and low rates of change. Figure 3.4 (lower) shows Reaches 6 8 (Renken Point and North Beach). The south half of this area shows a rapid and fairly steady decrease in sand volume since November Reach 8 around Oystercatcher eroded at similar rates through January Since then, Reach 8 has remained fairly stable. Figure 3.5 (upper) shows trends for the ICZ (Reaches 9 10) and new Captain Sams Inlet (Reach 11). Reach 9 is the primary area of old Captain Sams Inlet. It has accreted by over 300 cy/ft due to sand accumulation in the shoals as the old inlet and its delta migrated into the reach. While its volume change has stabilized in the past year, the beach along Reach 10 is beginning to rapidly accrete. This reflects landward movement of the abandoned shoals of the old inlet as well as accumulations near the mouth of the new inlet. Reach 11 is the area of the new inlet. Its volumes have decreased with the excavations and evolution of the new inlet. Figure 3.5 (lower) shows the average change along Kiawah spit. Volumes were diminishing prior to inlet relocation as the old channel and delta shifted further south. After inlet relocation, erosion accelerated along the oceanfront as the new channel adjusted and drew off sand to build up a new ebb-tidal delta. Hurricane Matthew in October 2016 exacerbated erosion along all of Kiawah Island causing upward of 850,000 cy losses island wide (CSE 2017). Appendix B provides graphs of unit volumes for each station for the period January 2006 to January The average by reach and a trend line are given on each graph. The results indicate the following. Reach 1 Camp St. Christopher has been relatively stable over the past decade with a low net erosion rate. This trend reverses over 15 years of steady buildup following the 1990 nourishment project. Reach 2 Pelican Watch Villas has gained sand at a rate of about 3 cy/ft/yr, continuing the long-term trend of accretion. Reach 3 Beach Club Villas has eroded by nearly 4.5 cy/ft/yr since A scour hole has periodically formed at the eastern end of the reach (discussed in a later section of this report). Figure 3.6 shows the area of Reach 1, Reach 2 and part of Reach 3 on 12 October 2016, four days after Hurricane Matthew. Reach 4 Beach Club has changed negligibly in the past decade; although there is great variation in profile volumes because of various positions of the northern channel (see Fig 3.7). Monitoring Report Year Seabrook Island, South Carolina

34 Reach 5 South Beach shows a trend of accretion at nearly 7 cy/ft/yr over the past decade. Results vary considerably from station to station, but present conditions are generally much healthier than conditions in 2006 (see Fig 3.2, upper). Reach 6 Renken Point experienced a dramatic buildup after the 1990 nourishment project. However, in 2009, the reach began to rapidly erode and continues to lose sand at a rate of ~21 cy/ft/yr (Fig 3.8). Reach 7 North Beach (south) has experienced a steady loss of sand in the past decade at an average rate of ~22 cy/ft/yr, which equates to several hundred feet of dune recession [see Fig 3.2 (lower) and Fig 3.9]. Reach 8 North Beach (north) (around Oyster Catcher) has also eroded, but at a more variable rate. Sand losses have lessened in the past two years. However, the decadal loss rate has averaged about 7.5 cy/ft/yr, which is equivalent to ~11 ft/yr of dune recession. Reach 9 Inlet Conservation Zone (ICZ) (south) has an average trend of accretion at ~40 cy/ft/yr since This rapid buildup is associated with old Captain Sams Inlet and its shoals building up as the delta migrated into the reach. With abandonment of the shoals after inlet relocation, CSE expects this reach to continue accumulating sand, but for more of the volume to move into higher elevations along the visible beach. Reach 10 ICZ (north) has high unit volumes associated with the ebb-tidal delta of Captain Sams Inlet. The changes at each station are highly variable, but the average trend for six profile lines has been accretion at ~10 cy/ft/yr. Most stations show an uptick since the 2015 inlet relocation as new shoals accumulate and abandoned shoals move toward the visible beach (Fig 3.10). Reach 11 New Captain Sams Inlet encompasses ~1,000 ft in the vicinity of the new inlet. Its unit volumes have dropped by over 100 cy/ft since inlet relocation as a result of the channel cutting through the previous beach on Kiawah spit. CSE expects this reach to begin rebuilding as the new inlet builds up a delta and shifts downcoast into Reach 10 (Fig 3.11). Reach 12 Kiawah spit has eroded along its southern half, while remaining relatively stable over the northern half to Beachwalker Park. Profiles close to the new inlet show the most erosion, which is similar to trends following the first and second inlet relocations (1983 and 1996). When the new inlet is cut, sand is drawn off from Kiawah spit to form a new ebb-tidal delta. Longshore transport shifts sand toward the new inlet and immediately starts rebuilding the spit, forcing the inlet to the south. Inlet migration and evolution are discussed in a later section of this report (Fig 3.12.). Monitoring Report Year Seabrook Island, South Carolina

35 FIGURE 3.4. Weighted average unit volume change by reach since November 2010: Reaches 1 5 (upper), Reaches 6 8 (lower). Monitoring Report Year Seabrook Island, South Carolina

36 FIGURE 3.5. Weighted average unit volume change by reach since November 2010: Reaches 9 11 (upper), Reach 12 (lower). Monitoring Report Year Seabrook Island, South Carolina

37 FIGURE 3.6. [ABOVE] Oblique aerial photo of the North Edisto River Inlet shoreline of Seabrook Island at low tide on 12 October 2016, four days after Hurricane Matthew impacted the area. While there was minor dune scarping and accumulation of wrack (dead marsh grass), there was negligible washover into the broad dune field fronting development. FIGURE 3.7. [RIGHT] Oblique aerial photo on 12 October 2016 looking west along the northern channel of North Edisto River Inlet (Reach 4 and Reach 5). Hurricane Matthew cut back the dunes around Renken Point (lower right corner) and exposed more of the seawall between Beach Court and the Beach Club. Note wave-breaking over the shoals flanking the northern channel. The shoals reduce the size of waves along the beach, but also lessen the rate of sand movement downcoast from Renken Point to the Beach Club. Monitoring Report Year Seabrook Island, South Carolina

38 FIGURE 3.8. Renken Point on 12 October 2016 showing dune washout, washovers into the backshore, and rounding of the point. Portions of the quarry-stone seawall are visible at the walkovers. While erosion has exceeded 21 cy/ft/yr over the past 7 years, a 50-ft buffer of dunes continues to front the seawall. FIGURE 3.9. North Beach (Reach 7 and Reach 8) on 12 October Extensive erosion over the past 7 years has cut back the foredune by hundreds of feet. Hurricane Matthew exacerbated conditions, leaving dead myrtle bushes at the edge of the beach and washover deposits at low breaks in the dune. Monitoring Report Year Seabrook Island, South Carolina

39 FIGURE The Inlet Conservation Zone (Reach 9 and Reach 10) on 12 October This area is generally accumulating sand that was formerly part of the delta of old Captain Sams Inlet. Hurricane Matthew shifted shoals landward while washing over the broad dune field that had formed in recent years. Oystercatcher beach access is just out of the photo along the left edge. FIGURE View of the mouth of Captain Sams Inlet, looking seaward at low tide on 12 October Note the buildup of shoals flanking the channel ( ebb-tidal delta ) and the accumulation of sand on the Kiawah spit side of the channel. Breaking waves such as the ones depicted move sand landward and downcoast, ultimately for the benefit of Seabrook Island. Monitoring Report Year Seabrook Island, South Carolina

40 FIGURE Kiawah spit on 12 October 2016 after Hurricane Matthew. The foredune and first row of waxed myrtle were washed out near new Captain Sams Inlet during the storm. The northern end of the spit around Beachwalker Park remained fairly stable. Note the broad, sparsely vegetated dune lines along the seaward margin of the spit. This area leaves a 300+ ft protective buffer to the heavily vegetated beach ridges (dark green vegetation). 3.2 South Beach Scour Hole Reach 3 In July 2016, a localized scour hole developed in the vicinity of profile 10 at one of the community walkovers adjacent to Deveaux Villas. To the best of CSE s knowledge, this was the first time such a localized erosion feature occurred along North Edisto River Inlet. The scour hole extended about 150 ft alongshore and cut out the wet sand beach to within about 30 ft of the walkover (Fig 3.13, upper). During subsequent months, the hole filled in naturally with sand moving alongshore. When Hurricane Matthew impacted Seabrook Island (8 October 2016), a similar scour hole reformed at one of the Deveaux Villas walkovers, encroaching even closer to the seawall (Fig 3.13, lower). The hurricane damaged the steps and adjacent seawall, causing settlement of rock and overtopping of the crest. Some backshore sand was washed out behind the seawall leaving an erosional escarpment between the two Deveaux Villas. Similar to the July 2016 event, the scour hole infilled within weeks with longshore sand moving downcoast. Monitoring Report Year Seabrook Island, South Carolina

41 FIGURE [UPPER] The first localized scour event near Deveaux Villas on 15 July 2016 (photo by S Hirsh, SIPOA). The scour hole refilled naturally within weeks! [LOWER] A larger scour hole reformed during Hurricane Matthew in October 2016, reaching the toe of the seawall at Deveaux Villas on 12 October (TW Kana). That scour hole similarly refilled by natural processes in the ensuing weeks. Monitoring Report Year Seabrook Island, South Carolina

42 In late January 2017 around the time of CSE s present beach survey, the scour hole reformed around the community beach access and Deveaux Villas (Fig 3.14, upper). The hole was not present on 19 January when CSE was conducting the annual survey, but a few days later appeared and rapidly enlarged. CSE surveyed this event in detail on 27 January via drone and profile surveys. Figure 3.14 (lower) is a rectified drone mosaic showing the scour hole extending ~250 ft alongshore and encroaching on the toe of the seawall. The community boardwalk is the right-hand walkover. CSE prepared digital bathymetry around the scour hole and confirmed that it extended to a depth of (~) 25 ft NAVD. The bathymetric map also indicated the localized erosion produced a corresponding underwater buildup beyond the 25-ft depth contour which is illustrated with profiles in Figure Between 19 and 27 January 2017, a large wedge of sand in the visible beach at profile 9 slumped into deep water at the edge of the main channel of North Edisto River Inlet. Losses above the 25- ft contour were offset by gains between the 35-ft to 55-ft depth contours (Fig 3.15, upper). Adjacent sections of beach 200 ft to the west and 150 ft to the east did not show comparable scour at the time (Fig 3.15, lower). Monitoring Report Year Seabrook Island, South Carolina

43 FIGURE [UPPER] Ground image of the scour-hole event on 25 January 2017 viewed from the community boardwalk next to Deveaux Villas (D Giles). [LOWER] Orthorectified mosaic of drone images of the third scour hole obtained at low tide on 27 January 2017 (D Giles). Monitoring Report Year Seabrook Island, South Carolina

44 FIGURE Representative profiles obtained on 19 and 27 January 2017 before and after formation of the third scour hole. Profile 9 shows extensive sand loss above the 25-ft depth contour and buildup along the margin of the inlet channel between 35 ft and 55 ft depths. The zone between 25 ft and 35 ft did not change, presumably because this area consists of denser consolidated sediments that hold the inlet in place (Moslow 1980, Imperato et al 1988). Monitoring Report Year Seabrook Island, South Carolina

45 CSE is uncertain about the underlying cause(s) of the intermittent scour holes around Deveaux Villas, but believes there are three possible factors. 1) Where two tidal channels intersect, there is often a scour hole induced by circulation eddies. The northern channel of North Edisto River Inlet is pinched between a shoal and the beach immediately downcoast of the Beach Club. As flood flows meet outgoing tides in the main channel, circular vortices form in shallow water (depth controlled by the depth of the northern channel). This may produce focused scour where the two flows meet, cutting back the wet sand beach and undermining the profile. While this may be a plausible explanation, it leaves the question as to why this has not been observed before. The marginal flood channel and main ebb channel of the inlet have interacted in this manner for decades without such focused scour of the beach. The two channels have generally been linked to the shark hole, a deep section of the main channel off Beach Club Villas. 2) Historical charts indicate that a small inlet may have emptied near the vicinity of Deveaux Villas a century ago (Fig 3.16). Such channels infill over time with unconsolidated sand sediments as the channel is closed off by longshore sand transport. If the surrounding sediments are more resistant to erosion, it would be easier for currents to scour the channel fill, producing the observed pattern of erosion at Deveaux Villas. The weakness in this argument is that nearly all of Seabrook s sands above the 20-ft contour are of recent origin. The lands from Beach Club Villas seaward have formed within the past century or so, because of a healthy sand supply from Kiawah Island. 3) Each of the scour events described in this section appear to have formed around the time of a major rainfall event. If water ponds behind the seawall and percolates under the wall at particular localities, it may liquefy underlying sediments as groundwater flows toward the ocean. If underground flows are sufficient, they could potentially initiate a collapse of the slope and move beach sand into deeper water. The weight of unsaturated sediments and the seawall above a saturated zone could lead to this instability. Monitoring Report Year Seabrook Island, South Carolina

46 FIGURE Historical maps of Seabrook Island for various dates between 1696 and Note the presence of a small channel at the western end of Seabrook Island in This may have been situated in the vicinity of present-day Deveaux Villas. [After Hayes et al 1979] Given the increasing size of the scour holes with each event so far, this issue bears close observation because of the threat to the seawall. If the seawall is undermined, it will eventually collapse and open that part of the island to wave action and flooding. Historically, responsibility for seawall construction and maintenance has been with the property owner. A portion of the critical scour area fronts private villas and another portion fronts a SIPOA access easement and Beach Club property. If the problem continues to recur, the most practicable alternatives are to pre-position armor stone for emergency repairs to the wall and build up the beach along the Beach Club to maintain an ample flow of sand to the critical area. 3.3 Hurricane Matthew 8 October 2016 During the present survey year, Seabrook was impacted by the worst hurricane to strike the South Carolina coast since Hugo (1989). Hurricane Matthew formed in the lower Caribbean around 28 September 2016, briefly reached Category 5 status, then tracked along the East Coast between Cape Canaveral and Cape Hatteras. It entered South Carolina the morning of 8 October 2016 and passed along Seabrook Island by late morning (Fig 3.17). Monitoring Report Year Seabrook Island, South Carolina

47 FIGURE The storm track for Hurricane Matthew (28 September to 12 October 2016). [Source: NOAA] The peak surge at Charleston was 9.29 ft mean lower low water (MLLW) or about 5 ft above the predicted tide that day (Fig 3.18). MLLW at Seabrook is 3.48 ft below NAVD'88 datum referenced in CSE surveys. Therefore, assuming tides at Seabrook were similar to those at Charleston, the highest tide was ~5.8 ft NAVD. This elevation is close to the normal dry-sand beach level, but below the crest of the seawall, which varies from ~10 ft to 15 ft NAVD. With wave action, hurricane tides were more than adequate to overtop the dry beach and cut into the foredune. This is apparent along North Beach where rafts of dead myrtle accumulated at the back beach and washed into low areas or over boardwalks (Fig 3.19). Since the storm, there has been some natural recovery of the dry-sand beach as illustrated in the profiles of Appendix A. FIGURE Tide record for Charleston Harbor during Hurricane Matthew, which peaked at 9.29 ft MLLW. The peak tidal surge that day was ~5 ft above normal tide levels. Monitoring Report Year Seabrook Island, South Carolina

48 FIGURE Oblique aerial image around Loggerhead Court on 12 October 2016 showing rafted myrtle bushes and marsh detritus along the foredune and end of the walkway. Hurricane Matthew washed over the beach in the ICZ, but did not breach the closure dike (Fig 3.20). Sand in the abandoned shoals of old Captain Sams Inlet was moved landward and built up the platform fronting the dike. This also had the effect of straightening the shoreline somewhat between the new inlet and Renken Point. As Figure 3.20 shows, the Seabrook side of Captain Sams Inlet was washed over, leaving exposed marsh along the channel and covering some marsh on the landward side of the dike. The net result was to effect more rotation of the channel to the south. To the best of CSE s knowledge, Hurricane Matthew caused no significant structural damage beyond loss of steps at some walkovers and minor displacement of armor stone along exposed sections of the seawall. As previously noted, one section of seawall at Deveaux Villas was overtopped and some sand washed out behind the wall. Monitoring Report Year Seabrook Island, South Carolina

49 FIGURE Seabrook s Inlet Conservation Zone looking south from Captain Sams Inlet (foreground) to North Edisto River Inlet (top of image). Hurricane Matthew moved shoal sand landward and helped straighten the shoreline, but did not breach the closure dike. Old Captain Sams Inlet (along the right side of the image) was nearly filled in seaward of the dike. Monitoring Report Year Seabrook Island, South Carolina

50 THIS PAGE INTENTIONALLY LEFT BLANK Monitoring Report Year Seabrook Island, South Carolina

51 4.0 CHANGES AROUND CAPTAIN SAMS INLET & HABITAT MAPPING Captain Sams Inlet was relocated in April 1996 and June Between these dates, it migrated ~3,000 ft at an average rate of ~160 ft/yr. As CSE (2014) reported, the rate of migration from 2000 to 2006 averaged ~135 ft/yr, whereas the rate increased between 2006 and 2014 to ~180 ft/yr (Fig 4.1). The rate of migration depends on where the measurement is made along the channel because the inlet tends to rotate south as it gets closer to Seabrook s development. The 1996 channel cut was oriented nearly perpendicular to the shoreline of Kiawah spit. By 2015, the channel had rotated over 35 from perpendicular and its rate of migration was accelerating. FIGURE 4.1. Approximate distances from the downcoast edge of the 1996 Captain Sams Inlet to the low-water shoreline on the indicated date. The average migration rate is 160 ft/yr. A similar channel rotation was noted for the period prior to the first inlet relocation in 1983 (Kana et al 1981). Using a shore-parallel alignment for purposes of tracking the spit growth/inlet migration rates, Sexton and Hayes (1983) reported a migration rate averaging 225 ft/yr from ~1948 to This period encompassed spit growth starting from the vicinity of Beachwalker Park. By 1963, the inlet was immediately downcoast of the present-day Kiawah Monitoring Report Year Seabrook Island, South Carolina

52 Seabrook town easement lines near the mouth of Captain Sams Creek. CSE (1995) reported migration rates of 172 ft/yr before the 1983 inlet relocation (measured along the 1983 dike alignment) and 222 ft/yr after the cut. Using an alignment matching the 1992 shoreline, CSE (1995) determined the pre- and post-inlet relocation migration rates to be 280 ft/yr and 270 ft/yr (respectively). Significantly, then, the migration rate after the 1996 inlet relocation was markedly lower by ~40 percent (160 ft/yr vs 270 ft/yr) than the 1983 to 1996 rate. This slower rate helped improve the longevity of the project by increasing the time before the new inlet relocation was required. It is not clear why inlet migration rates were lower between 1996 and One factor was probably the extensive marsh that had formed in the Inlet Conservation Zone (ICZ) after the 1983 project. Cohesive marsh muds can offer more scour resistance than unconsolidated sandy sediment, although the incipient marsh in the old channel floodway was underlain by sandy sediments. The other factors that affect inlet migration rates are the frequency and height of waves from the northeast and the sediment supply. CSE (2007) noted a temporary reduction in sand supply along Kiawah s beach during the 1990s and early 2000s associated with a shoal-bypassing event at the east end of Kiawah Island. For a number of years, nearly all the sand released to the beach from Stono Inlet was accumulating at the east end and not moving downcoast. This may have reduced the net sand transport rates along Kiawah s beach which, in turn, reduced rates of spit growth, the controlling mechanism for Captain Sams Inlet migration. Regardless of the cause of lower migration rates, the result was of benefit to Seabrook Island by prolonging the time before relocation was required. 4.1 Post-Relocation Inlet Movement CSE acquired orthorectified imagery in March 2016 (c/o Independent Mapping Consultants, Charlotte, NC) and in January 2017 by means of drone. The latter images were a mosaic with detailed ground control, which was processed via special software to produce digital topographic data. Coverage of the latter image is not as extensive as the March 2016 data, but it encompasses the abandoned inlet as well as the new inlet. The two images are shown in Figure 4.2. Note the southerly rotation of the new inlet between 2016 and 2017 as well as recession of the shoreline immediately downcoast of the channel along the eastern end of the dike. CSE believes that Hurricane Matthew accounts for much of the change. Extensive erosion along Kiawah during the storm shifted sand to the west (south), lengthening Kiawah spit and overextending the updrift shoals of the ebb-tidal delta, which forced the channel to rotate-migrate south. Monitoring Report Year Seabrook Island, South Carolina

53 FIGURE 4.2. Orthorectified aerial photos of the Captain Sams Inlet showing conditions in January 2015 (upper) and July 2015 (middle). LIDAR data in 2016 and drone elevation data in 2017 were used to develop DTMs with color-coded elevation data (see Fig 4.3). Monitoring Report Year Seabrook Island, South Carolina

54 The orthorectified images and digital elevations were used to create a color-coded DTM of the inlet area. Figure 4.3 shows the DTM for January 2017 along with an oblique aerial photo of the area taken after Hurricane Matthew. The model utilizes drone data over the exposed shoals, beach, and dunes, combined with bathymetry in the channels and subtidal areas. CSE used the Kiawah-Seabrook town easement line for reference and prepared cross-sections along two alignments (8+00 and 14+00) as shown in Figure 4.3 (upper). Section 8+00 is ~600 ft landward of the 2015 closure dike cutting through one of the narrowest parts of the channel. Section generally follows the alignment of the closure dike. These sections are depicted in Figure 4.4 (viewed seaward) and are overlain with earlier data to illustrate changes in channel dimensions and positions. Because section 8+00 runs landward of the dike, both channels appear open. The new channel widened by ~60 ft at the mean tide elevation, but maintained about the same depth during the past year. Along transect 8+00, the new channel moved 100 ft closer to Seabrook. At Section 14+00, the new channel scoured to almost 14 ft deep (NAVD datum) while migrating ~225 ft toward Seabrook between March 2016 and January Section also widened due to rotation of the channel (Fig 4.4, lower). Because the cross-section is more oblique to the channel, it appears wider than conditions in 2015 and That section also indicates the dike did not change significantly during Hurricane Matthew. As Figure 4.4 shows, the centers of each channel were about 2,000 ft apart at transect in January Along the centerline of the dike, the new channel has moved about 500 ft closer to Seabrook since June This change is significantly greater than the average inlet migration rate after the 1996 project. CSE also developed comparative cross-sections more perpendicular to the new channel for purposes of computing the mean flow cross-section. [The sections in Figure 4.4 run obliquely across the channel and yield a higher section value.] Figure 4.5 shows the as built cross-section of June 2015 and the recent sections for April 2016 and January Mean flow cross-section (A C) is a commonly referenced parameter for studies of inlet stability and hydraulics (O Brien 1969). It has been shown that natural inlets equilibrate at certain dimensions which are related to the volumes of water entering or exiting the inlet during the flood or ebb tide. Kana and Mason (1988) reported the 1983 channel had an initial A C of 1,206 square feet (sf). By June 1985, 2.3 years after the first relocation, A C had expanded to 2,283 sf. Figure 4.5 shows that for the third relocation, A C was initially 1,965 sf. By April 2016, it had expanded to 3,382 sf. In January 2017, the effective flow cross-section was ~3,884 sf, which suggests the present inlet has scoured rapidly and is likely approaching equilibrium. Some of the differences between results in the 1980s and those of the third inlet relocation stem from different datums. Monitoring Report Year Seabrook Island, South Carolina

55 FIGURE 4.3. [UPPER] Digital terrain model (DTM) of new Captain Sams Inlet and the ICZ based on the January 2017 survey. The reference line extending across the end of Kiawah spit (right side of image) is the Kiawah Seabrook town easement line. Two shore-parallel cross-sections (8+00 and 14+00) were used to illustrate cutaways of the old and new channels. The closure dike approximately parallels transect Reference flow cross-section is oriented approximately perpendicular to the channel azimuth. [LOWER] Oblique aerial image of the new inlet and closed channel, looking west at low tide on 12 October Note infilling of the old channel seaward of the closure dike and extension of Kiawah spit and the updrift shoal platform of the new inlet. [T Kana] Monitoring Report Year Seabrook Island, South Carolina

56 FIGURE 4.4. Transects looking seaward across Captain Sams Inlet and along the closure dike. See Figure 4.3 for transect locations. The greater movement of new Captain Sams Inlet at transect reflects southerly rotation of the channel during the past year. Monitoring Report Year Seabrook Island, South Carolina

57 FIGURE 4.5. Comparative channel sections of new Captain Sams Inlet near the deepest part of the inlet. See Figure 4.3 (upper) for location of the AC section. Earlier data were collected in NGVD (National Geodetic Vertical Datum of 1929) which was several inches lower than mean sea level in the Charleston area in the 1980s. A new datum, NAVD (North American Vertical Datum of 1988), was established roughly 1 ft higher than NGVD. This newer datum is presently about ft higher than the mean tide level at Seabrook. Therefore, using NAVD as a proxy for MSL tends to overestimate A C by ~ sf. The main reason to present these details here is to demonstrate that the new inlet has adjusted rapidly and is large enough to handle the volumes of water entering and exiting during each tidal cycle. Kana and Mason (1988) found the 1983 inlet cross-section, depth, and width achieved stable dimensions within ~8 months. Monitoring Report Year Seabrook Island, South Carolina

58 4.2 Habitat Mapping CSE utilized a drone and Pix4Dmapper Pro software to generate a rectified orthophotograph and digital point cloud around Captain Sams Inlet in January Hundreds of overlapping images were merged into a single file and were rectified in the State Plane Coordinate System using surveyed control points on the ground. This methodology provides highly accurate topography data and resolutions of 3-inch pixels or better. Prior habitat mapping in March 2016 was performed using LIDAR imagery (c/o IMC). The 2015 habitat mapping (January and July) was accomplished using traditional topographic surveys plus orthorectified aerial photos. Additional upland detail, where vegetation is stable, was added to the 2015 data sets using the March 2016 LIDAR data. These aerial images provide four discrete dates to evaluate the evolution of habitats around the inlet: Pre-project January 2015 One month post-project 5 23 July 2015 Nine months post-project 16 March months post-project 25 January 2017 Digital terrain models (DTMs) were prepared for each survey date encompassing a common reference area totaling acres. This constant control area encompasses the inlet, portions of Kiawah River, Seabrook's ICZ, beaches, bars, and the ebb-tidal deltas of old and new Captain Sams Inlet. Figures 4.6 and 4.7 show the color-coded DTMs and control area. The DTMs allow isolation of contours to delineate various elevation bands such as areas between high water and low water. Habitats around barrier spits and inlets are directly related to elevation with respect to tide and wave levels. Principal habitats and their relation to tide stage are shown in Figure 4.8. Starting from the ocean, land below mean lower low water (MLLW) ( 3.48 ft NAVD) is subtidal. Land between MLLW and mean higher high water (MHHW) (+2.81 ft NAVD at Seabrook Island) is exposed intertidal. This zone will often include a broad terrace incorporating bars separated from the beach by a shallow trough ("runnel"). Bars move onshore across the intertidal terrace under the force of breaking waves then merge with the beach. They reform offshore after storms cut away the visible beach or inlets release offshore bars to become part of the beach system. Therefore, these distinct features are referred to as longshore bars or bypassing bars (shoals). Monitoring Report Year Seabrook Island, South Carolina

59 FIGURE 4.6. [UPPER] Digital terrain model (DTM) of the Captain Sams Inlet in January 2015 prior to the third inlet relocation. The ~575-acre habitat reference area is boxed, and the DTM is superimposed on a January 2015 orthophotograph. [LOWER] DTM of new Captain Sams Inlet in July 2015, one month after inlet relocation. High ground details (red) are based on the March 2016 LIDAR image. Monitoring Report Year Seabrook Island, South Carolina

60 FIGURE 4.7. [UPPER] DTM of the Captain Sams Inlet in March 2016, nine months after inlet relocation based on LIDAR imagery. Note enlarged channel compared with July 2015 (Fig 4.6, lower). [LOWER] DTM of Captain Sams Inlet in January 2017 based on a drone survey by CSE. Note southerly deflection of the channel relative to the March 2016 condition. Monitoring Report Year Seabrook Island, South Carolina

61 FIGURE 4.8. Relationship of Kiawah spit and Captain Sams Inlet conservation zone habitats delineated for the present study. See Table 4.1 for elevation criteria used to map habitats on the DTMs. The beach is a sloping accumulation of sand that is shaped by wave action. Its seaward slope is the wet sand, intertidal area, plus a swash zone produced by wave up-rush at high tide extending well above MHHW. The normal wave run-up limit is typically 2 4 ft above the "stillwater" high-tide mark, a height closely matching the local height of normal waves. The dry sand beach is generally a flat section of the profile which is above normal tides and runup elevation. At Seabrook Island, the dry-sand beach tends to be much narrower than the wet-sand beach because of the ~6-ft tidal range. Where the dry beach is backed by a lagoon or lower topography, the occasional storm will drive waves up and onto the dry beach where it will drain toward the lagoon. This produces "washovers" which flatten any developing low dunes and provide a fresh surface of unvegetated sediments. Where the dry beach builds sufficient elevation to prevent washovers, incipient dunes form and create natural barriers to overwash. At Captain Sams Inlet, the closure dike incorporates a broad, low berm (dry-sand beach) and a narrow, low dune several feet above the beach. On the landward side of the dike/dune, upland areas (above MHHW) are generally sheltered from waves and allow freshwater vegetation to grow closer to MHHW (ie to lower elevations than along the exposed oceanfront). There tends to be a demarcation between freshwater and halophytic (salt-tolerant) vegetation about 1 2 ft above MHHW in the Seabrook setting. Several salt-tolerant species including the shrub Distichlis spicata thrive in that zone. More extensive saltmarsh species propagate in the narrow elevation band around mean high water (MHW) with zonation occurring around small changes in flooding frequency and duration. Monitoring Report Year Seabrook Island, South Carolina

62 TABLE 4.1. Seabrook Habitat Descriptions Elevation data are examined alongside infrared aerial imaging to identify eight habitats: subtidal, exposed intertidal, sheltered intertidal, dry beach/washover, vegetated dry beach, upland marsh, and dike. The defining characteristics are summarized in the table below and expanded upon in the notes at the bottom. Habitat Feet (NAVD) Defining Characteristics Subtidal (<) 3.48 Elevation less than 3.48 ft Exposed Intertidal 3.48 to +4.5 Elevation between 3.48 ft and ft, seaward of the dike Sheltered Intertidal 3.48 to +2.8 Elevation between 3.48 ft and ft, landward of the dike Dry Beach/Washover > 4.5 up to (~)+7.0 Vegetated Dry Beach +4.5 to (~)+7.0 Upland (>)+3.5 to (>)+6.0 Marsh 0 to +3.5 Dike/Dune (>)+5.0 to (>)+7.0 Elevation greater than ft, dry sandy surfaces with little to no vegetation Elevation greater than ft, dry sandy surfaces with lowlying vegetation Dry forested areas on raised land, typical elevation is greater than ft on the exposed face and greater than ft on the sheltered face Wet vegetated regions behind the beach, typical elevations range from 0.00 ft to ft Constructed mound of non-vegetated dry sand at an elevation greater than +5.0 ft for natural dunes, typically (>)+7.0 ft (applicable in future maps) NOTES: Subtidal areas are based on regions in which elevation is below MLLW, 3.48 ft. They are modified to include visible tidal creeks. Intertidal areas are identified as region between MLLW elevation and ft, the typical high water mark. When the high watermark deviates from ft, the visible high watermark (as observed in the imagery) is the dominant defining factor. Exposed intertidal areas are those which are located seaward of the dike, while sheltered intertidal areas lie inland of the dike. Dry beach/washover consists of areas above the high watermark with dry surfaces and little to no vegetation. Vegetated dry beach areas are above the high watermark with visible low-lying vegetation. Marsh areas are identified as wet vegetated regions behind the beach. Elevation data are not strictly used to define these regions; however, typical elevation values range from 0.00 ft to ft bordering the marsh and above ft bordering the beach. The dike/dune region is a constructed mound of non-vegetated sand, defined at an elevation above ft. Monitoring Report Year Seabrook Island, South Carolina

63 "Low marsh" species, such as Spartina alterniflora, thrive at elevations close to MHW, which is ~0.4 ft below MHHW at Seabrook Island. Salicornia sp and Juncus roemerianus are adapted to less frequent flooding at elevations slightly above MHHW (Kana et al 1986a). There are exceptions to these general elevation ranges. Nevertheless, they offer guidance for predicting likely habitat evolution. The profile in Figure 4.8 terminates at the tidal creek and includes sheltered intertidal habitat at the edge of the marsh, commonly a mud embankment transitioning to a gently sloping mud flat. Crassostrea virginica (oysters) thrive in the middle intertidal zone along the mud banks in South Carolina. CSE used these general habitat zones (see Fig 4.8) for purposes of selecting elevation ranges to delineate areas on the DTMs. The boundaries were color-coded and grouped to calculate acreage of each habitat type. CSE used the orthophotos to check delineations, particularly where vegetation offers confirmation of marsh or upland species. Table 4.1 lists the habitat, elevation ranges, and other defining characteristics applied herein. These elevation bands may be revised in the future if there is evidence that an alternative range would be more realistic. As the shoreline evolves around old and new Captain Sams Inlet, CSE expects some areas that are presently highenergy intertidal sand flat to become sheltered by a new outer beach seaward of the closure dike. This will leave a swale in between, where mud can accumulate, modifying the sand bottom and transforming the habitat to an estuarine character or an isolated brackish pond. 4.3 Results Figures show the mapped habitats for pre- and post-inlet relocation within the control area. Table 4.2 lists the results in acres and the changes in areas between each survey. The initial pre-project condition showed the dominant habitat types were exposed intertidal, subtidal, and marsh areas comprising acres (79.4 percent) of the acre control area. Since relocation, the corresponding areas are about the same (totaling ~453 acres), but there has been a major shift with 57.6 fewer acres of exposed intertidal areas and 74.5 more acres of subtidal areas. Some of this change is associated with the excavation of the new channel (increased subtidal habitat) and construction of the closure dike over previous intertidal habitat. However, it also reflects onshore movement and super-elevation of intertidal sand bars consolidating intertidal areas as the old delta is pushed shoreward and upward by waves. Monitoring Report Year Seabrook Island, South Carolina

64 TABLE 4.2. Habitat mapping results in acres and the changes in areas between pre-and post-inlet relocation surveys. The areas of sheltered intertidal habitat along with salt marsh declined by about 22 acres due to channel and dike construction and channel scour. Meanwhile, areas of non-vegetated dry beach plus the dike (initially similar to a high elevation washover devoid of vegetation) increased by ~21 acres. Natural, non-vegetated dry beach increased by 16.2 acres between July 2015 and January This is significant because it is the type of habitat favored by the piping plover, a threatened or endangered species. Vegetated dry-beach area declined mainly around the entrance to new Captain Sams Inlet between January 2015 and March Since then the amount of dry beach within the control area has remained stable at ~14 acres. Natural erosion of the adjacent shorelines after the cut encroached on beach areas with sparser plant cover (eg dune grasses), reducing the amount of that habitat. As Figures 4.11 and 4.12 show, non-vegetated dry-beach/washover habitat formed along the margins of the new inlet and enlarged at the mouth of the abandoned inlet. A remnant of the non-vegetated spit before inlet relocation on the landward side of the closure dike became vegetated dry beach by March Rapid marsh expansion is expected over some of the sheltered intertidal areas behind the dike. Meanwhile, the ocean side of the dike should see diminished wave action as emergent bars form over the abandoned ebb-tidal delta. Once the bars are higher than MHW, their migration rate will slow and sand will accumulate to form an incipient beach ridge (outer barrier beach). Ponded water behind the new outer beach will drain through a small flushing channel across the new beach. Figure 4.13 is a bar graph showing the sizes of each habitat at the times of the s urveys. Continued expansion of unvegetated dry sand beach is expected along the Kiawah side of the inlet as new spit growth forces the channel toward Seabrook Island. CSE also expects an increase in subtidal area as the old ebb-tidal delta migrates landward and merges with the Seabrook shoreline. This increase will be partially offset by seaward growth of the new ebb-tidal delta. A key aspect of future changes in the ICZ will be the continued transformation of habitats. Sand spits will form and other features like the dike will erode as new Captain Sams Inlet migrates through the system. Monitoring Report Year Seabrook Island, South Carolina

65 FIGURE 4.9. Delineation of habitats around Captain Sams Inlet in January 2015 before the third inlet relocation. Habitats are delineated based on the elevation criteria outlined in Table 4.1. See Table 4.2 for computed areas. The control area totals acres. Monitoring Report Year Seabrook Island, South Carolina

66 FIGURE Delineation of habitats around Captain Sams Inlet in July 2015, one month after inlet relocation. Habitats are delineated based on the elevation criteria outlined in Table 4.1. See Table 4.2 for r computed areas. The control area totals acres. Monitoring Report Year Seabrook Island, South Carolina

67 FIGURE Delineation of habitats around Captain Sams Inlet in March 2016, nine months after inlet relocation. Habitats are delineated based on the elevation criteria outlined in Table 4.1. See Table 4.2 for computed areas. The control area totals acres. Monitoring Report Year Seabrook Island, South Carolina

68 FIGURE Delineation of habitat around Captain Sams Inlet in January 2017, 19 months after inlet relocation. Habitats are delineated based on the elevation criteria outlined in Table 4.1. See Table 4.2 for computed areas. The control area totals acres. A remnant of the non-vegetated spit before inlet relocation on the landward side of the closure dike became vegetated dry beach by March Rapid marsh expansion is expected over some of the sheltered intertidal areas behind the dike. Meanwhile, the ocean side of the dike should see diminished wave action as emergent bars form over the abandoned ebb-tidal delta. Once the bars are higher than MHW, their migration rate will slow and sand will accumulate to form an incipient beach ridge (outer barrier beach). Ponded water behind the new outer beach will drain through a small flushing channel across the new beach. Figure 4.13 is a bar graph showing the sizes of each habitat at the times of the surveys. Continued expansion of unvegetated dry sand beach is expected along the Kiawah side of the inlet as new spit growth forces the channel toward Seabrook Island. CSE also expects an increase in subtidal area as the old ebb-tidal delta migrates landward and merges with the Seabrook shoreline. This increase will be partially offset by seaward growth of the new ebb-tidal delta. Monitoring Report Year Seabrook Island, South Carolina

69 A key aspect of future changes in the inlet conservation zone will be the continued transformation of habitats. Sand spits will form and other features like the dike will erode as new Captain Sams Inlet migrates through the system. FIGURE Habitat sizes within a acre control area at the times of the surveys. Monitoring Report Year Seabrook Island, South Carolina