monitoring report — 2017 - sipoa€¦ · shows conditions in 2014 when captain sams inlet was...

CAPTAIN SAMS INLET RELOCATION — 2015

SEABROOK ISLAND SOUTH CAROLINA

Monitoring Report — 2017

Prepared for:

Seabrook Island Property Owners Association Johns Island South Carolina

SEABROOK ISLAND, SOUTH CAROLINA

CAPTAIN SAMS INLET RELOCATION — 2015

Monitoring Report — Year 2

Prepared for:

1202 Landfall Way Johns Island SC 29455

Prepared by:

PO Box 8056 Columbia SC 29202-8056

[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)

— THIS PAGE INTENTIONALLY LEFT BLANK —

CSE [2460–YR2] Annual Beach & Inshore Survey

Monitoring Report—Year 17 Seabrook Island, South Carolina i

TABLE OF CONTENTS

1.0 INTRODUCTION & KEY FINDINGS ..................................................................................................................................................... 1

2.0 BACKGROUND AND METHODOLOGY ............................................................................................................................................... 5 2.1 Background and Key Events ................................................................................................................................................... 6 2.2 Survey Methodology .............................................................................................................................................................. 16

3.0 BEACH CHANGES AND RECENT EVENTS ...................................................................................................................................... 21 3.1 Unit Volumes and Net Volume Changes............................................................................................................................. 23 3.2 South Beach Scour Hole — Reach 3.................................................................................................................................... 34 3.3 Hurricane Matthew — 8 October 2016 ................................................................................................................................ 40

4.0 CHANGES AROUND CAPTAIN SAMS INLET & HABITAT MAPPING ............................................................................................. 45 4.1 Post-Relocation Inlet Movement ......................................................................................................................................... 46 4.2 Habitat Mapping .................................................................................................................................................................... 52 4.3 Results — 2015–2017 ............................................................................................................................................................. 57

5.0 SUMMARY & RECOMMENDATIONS ................................................................................................................................................ 65 5.1 Scour Hole at Deveaux Villas ................................................................................................................................................ 66 5.2 Recommendations ................................................................................................................................................................ 67

6.0 REFERENCES & BIBLIOGRAPHY ..................................................................................................................................................... 69

7.0 ACKNOWLEDGMENTS ..................................................................................................................................................................... 73

Appendix A) Stationing and CSE Profiles

Appendix B) Profile Volume Changes by Line and Reach (January 2006 to January 2017)

CSE [2460–YR2] Annual Beach & Inshore Survey

Monitoring Report—Year 17 Seabrook Island, South Carolina ii


CSE [2460–YR2] Annual Beach & Inshore Survey Monitoring Report—Year 17 1 Seabrook Island, South Carolina

1.0 INTRODUCTION & KEY FINDINGS

This is the second report following the third relocation of Captain Sams Inlet in 2015. 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


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 2015. 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 2017.

Figure 1.1 shows the network of 50 oceanfront profiles along with the applicable reaches which have

been monitored by CSE since November 2010. 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 2017. This nearly equals the net losses during the 15-month period between

January 2015 and April 2016. 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 2017. 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 ocean-

front. Camp St Christopher (Reach 1) has continued an erosion trend since January 2012 and lost

~5,300 cy between April 2016 and January 2017. 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.


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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 2015. 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 1990. 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 2017. 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).


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 2008. 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 2530. 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 calcula-

tions 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 refer -

ence 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,


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.


FIGURE T-1. Aerial view of Seabrook

Island in November 2013.

TABLE 2.1. (shown on 9 pages) Seabrook Island — major shoreline events.

1948 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).

1963 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).

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.


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 1983.

1970s Seabrook Island is in a rapid erosion cycle

with some areas like Renken Point eroding at over 20

ft/yr.

1973 Beach Club under construction.

1974 Erosion impacts the Beach Club before con-

struction 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.


1975–1981 Succession of sandbag revetments,

timber and concrete bulkheads/seawalls, and

quarry-stone revetments are installed along Sea-

brook Island between Pelican Watch Villas and the

13th 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).

1979 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-6. Collapse of the concrete seawall at Renken Point in September 1979 during

Hurricane David.

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.


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 1983.

[UPPER] Excavation of the basin for the new channel by land-

based 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

1983.


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 1983.

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.

1989 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-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.

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-10. The second relocation of Captain Sams Inlet in April 1996. [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.

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.

1998–2001 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.

2002–2007 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-11. Composite image of the Captain Sams Inlet area from the Seabrook side in January 2014. The lagoon

formed in the abandoned 1996 channel is on the left side of the image.

2007–2008 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.

2008 Permit application submitted for third relocation of Captain Sams Inlet.

2009–2012 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 2012. The SC permit is

appealed by one Seabrook Island property owner.

2008–2015 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.

2009 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.

2013 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.

2014 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.

2014 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.

2015 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 signifi-

cant 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)


2016 First monitoring survey after the third inlet relocation project is completed March–April.

2016 Seabrook Island is selected for an ASBPA* Best Restored Beaches Award. *[American Shore and

Beach Preservation Association—www.asbpa.org]

2016 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.

2017 Second annual monitoring survey (after the 2015 inlet relocation) is completed in January.

FIGURE T-12. Captain Sams Inlet after inlet relocation in June 2015. 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).

FIGURE T-12. Captain Sams Inlet after inlet relocation in June 2015. 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).


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.

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 2017. 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.]


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 Cap-

tain 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 interpreta-

tion 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).


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.


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 chan-

nels 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.


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 2015.

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 emer-

gency 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 2010. At profile

13, the visible beach is incrementally higher and wider in 2017, but the biggest change is under-

water.


FIGURE 3.2. Example profiles from South Beach (#13) and North Beach (#21) showing changes between November

2010 and January 2017. 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.


Figure 3.2 (upper) shows a nearly 5-ft buildup of the profile 300 ft from the seawall. This under-

water 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 2017. 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 2010. Table 3.2 also gives the net changes between April 2016 and January

2017, and the bottom of the table indicates the following:


TABLE 3.1. Seabrook updated unit volumes. (See Table 2.2 and Appendix A for calculation boundaries.)


TABLE 3.2. Seabrook unit volume change and net volume change between profiles.


FIGURE 3.3. Net change in volume along Seabrook Island since November 2010. 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).

Camp St. Christopher has eroded over the past year and since November 2010.

South Beach to Pelican Watch Villas has generally accreted over the past year and since

November 2010.

The Inlet Conservation Zone (ICZ) around old Captain Sams Inlet has built up by over

900,000 cy since November 2010.

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 2010. 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 2010. 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.


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 2010. Reach 8 around Oystercatcher eroded

at similar rates through January 2014. 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 accumula-

tions 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 2017. 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 2006. 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).


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 2006. 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.).


FIGURE 3.4. Weighted average unit volume change by reach since November 2010: Reaches 1–5 (upper),

Reaches 6–8 (lower).


FIGURE 3.5. Weighted average unit volume change by reach since November 2010: Reaches 9–11 (upper),

Reach 12 (lower).


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 move-

ment downcoast from Renken Point to the

Beach Club.


FIGURE 3.9. North Beach (Reach 7 and Reach 8) on 12 October 2016. 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.

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 3.11. View of the mouth of Captain Sams Inlet, looking seaward at low tide on 12 October

2016. 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.

FIGURE 3.10. The Inlet Conservation Zone (Reach 9 and Reach 10) on 12 October 2016. 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 3.12. 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 re-

formed 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.


FIGURE 3.13. [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.


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 3.15.

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).


FIGURE 3.14. [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).


FIGURE 3.15. 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).


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 out-

going tides in the main channel, circular vortices form in shallow water (depth con-

trolled 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 unconsoli-

dated 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.


FIGURE 3.16. Historical maps of Seabrook Island for various dates between 1696 and 1979. Note the presence

of a small channel at the western end of Seabrook Island in 1922. 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).


FIGURE 3.17. The storm track for Hurricane

Matthew (28 September to 12 October 2016).

[Source: NOAA]

FIGURE 3.18. 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.

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 refer-

enced in CSE surveys. Therefore,

assuming tides at Seabrook were simi-

lar 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 sea-

wall, 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 3.19. 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.


FIGURE 3.20. 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.


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.

4.0 CHANGES AROUND CAPTAIN SAMS INLET & HABITAT MAPPING

Captain Sams Inlet was relocated in April 1996 and June 2015. 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.

A similar channel rotation was noted for the period 1963–1981 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 1979. This period encompassed spit growth starting from the vicinity of

Beachwalker Park. By 1963, the inlet was immediately downcoast of the present-day Kiawah–


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 2014. 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.


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).


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 14+00 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 2017. Section 14+00 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 2016. 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 14+00 in January 2017. Along the centerline of the

dike, the new channel has moved about 500 ft closer to Seabrook since June 2015. 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 2017. Mean flow cross-section (AC)

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 AC of 1,206 square feet (sf). By

June 1985, 2.3 years after the first relocation, AC had expanded to 2,283 sf. Figure 4.5 shows that

for the third relocation, AC 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.


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 14+00. 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 2016. 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]


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 14+00 reflects southerly rotation of

the channel during the past year.


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 0.5–0.7 ft higher than the mean tide level at Seabrook. Therefore, using

NAVD as a proxy for MSL tends to overestimate AC by ~135–200 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.


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 2017. 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 10–17 January 2015

One month post-project 5–23 July 2015

Nine months post-project 16 March 2016

19 months post-project 25 January 2017

Digital terrain models (DTMs) were prepared for each survey date encompassing a common

reference area totaling 573.3 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).


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.


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.


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.


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.

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 +4.50 ft, the typical high water mark. When

the high watermark deviates from +4.50 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 +3.50 ft bordering the marsh and above +6.00

ft bordering the beach.

• The dike/dune region is a constructed mound of non-vegetated sand, defined at an elevation above +5.00 ft.

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 +4.50 ft, seaward of the dike

Sheltered Intertidal −3.48 to +2.8 Elevation between −3.48 ft and +2.80 ft, landward of the dike

Dry Beach/Washover > 4.5 up to (~)+7.0 Elevation greater than +4.50 ft, dry sandy surfaces with little to

no vegetation

Vegetated Dry Beach +4.5 to (~)+7.0 Elevation greater than +4.50 ft, dry sandy surfaces with low-

lying vegetation

Upland (>)+3.5 to (>)+6.0

Dry forested areas on raised land, typical elevation is greater

than +6.00 ft on the exposed face and greater than +3.50 ft on

the sheltered face

Marsh 0 to +3.5 Wet vegetated regions behind the beach, typical elevations

range from 0.00 ft to +3.50 ft

Dike/Dune (>)+5.0 to (>)+7.0

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)


"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 high-

energy 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 — 2015–2017

Figures 4.9–4.12 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 455.3 acres (79.4 percent) of the 573.3 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.


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 2017. 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 2016. 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 2015. 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.


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FIGURE 4.12. 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

573.3 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 2015. 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.


FIGURE 4.13. Habitat sizes within a 573.3-acre control area at the times of the surveys.

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.


5.0 SUMMARY & RECOMMENDATIONS

This second annual monitoring report following the third inlet relocation of Captain Sams Inlet

(June 2015) focuses on changes since November 2010. As discussed in the report, recent surveys

have extended further offshore with greater detail and accuracy. Therefore, it is possible to

evaluate changes to the beach and inshore zone out to the depth of measurable bottom elevation

change, which yields a more realistic accounting of littoral sand volumes connected directly with

Seabrook’s beach. Since 2010, CSE has surveyed 50 profile lines between Camp St Christopher and

Beachwalker Park on Kiawah spit. These lines match historical lines and augment earlier data sets

to provide more detailed coverage.

Around Captain Sams Inlet, CSE has obtained “blanket” coverage point clouds by means of LIDAR

(March 2016) and drone (January 2017). These data were supplemented by detailed bathymetry

over channels and subtidal areas to develop digital terrain models (DTMs) around the inlet. The

color-coded DTMs allow delineation of habitats on the basis of defined elevation bands.

During the past year, new Captain Sams Inlet achieved equilibrium and migrated 100–225 ft closer

to Seabrook. The migration distance depends on what point at the inlet is used. Nevertheless,

these results confirmed that the inlet rotated to the south (hence increasing migration distances

proceeding out of the inlet). Inlet migration was greater than the 1996–2009 average rate, but was

comparable to historical rates prior to any inlet relocation projects.

Hurricane Matthew (a Category 1 storm that tracked along the South Carolina coast) impacted

Seabrook Island on 8 October 2016. It produced tides about 5 ft above normal and caused

extensive erosion and dune scarping/washovers at various localities. The southern end of North

Beach and Renken Point lost perhaps the most beach area, continuing a trend that had begun

around 2010. The storm had the effect of rounding off Renken Point and causing sand losses

greater than 20 cy/ft in the area, which is equivalent to about 30 ft of dune recession.

Exacerbating recent erosion in the area, Matthew washed out waxed myrtle shrubs and left rafts of

vegetation at the edge of the dune line (see Fig 3.19). While some of the largest woody plants and

logs should be removed and disposed inland, the majority of the shrub material can be left in place

(or pushed landward in swales) where it will trap sand, decay, and provide a seed bed for future

dune growth. [A traditional practice on many beaches is to place Christmas trees along the dune

line to build up sand by the next summer season.]

The hurricane also pushed shoal sand into low areas seaward of the closure dike and old Captain

Sams Inlet, which resulted in buildup of the dry beach (ie – washovers), a habitat favored by certain

threatened species like the piping plover. The new beach and closure dike have merged and given


the Inlet Conservation Zone (ICZ) a more natural appearance although vegetation remains sparse.

CSE expects areas of dune vegetation to expand rapidly in the next few years.

Overall, Seabrook Island gained 18,000 cy between Camp St Christopher and new Captain Sams

Inlet from March 2016 to January 2017 (Reaches 1–10). If only the developed reaches of the com-

munity are considered (Reaches 2–8: Pelican Watch Villas to Oystercatcher), Seabrook has lost

~105,000 cy since March 2016, which matches the volume lost between January 2015 and April

2016.

5.1 Scour Hole at Deveaux Villas

Since July 2016, there have been at least three episodes of severe scour of the beach around

Deveaux Villas. The first was reported by Steve Hirsh (SIPOA Director of Engineering). The first

scour hole extended about 150 ft along the shoreline and carved out a basin nearly reaching the

seawall. This scour hole healed naturally by sand moving alongshore to fill up the hole. A second,

larger event occurred during Hurricane Matthew and encroached on the toe of the seawall. The

storm caused some settlement of rock and overtopped the wall at Deveaux Villas, leaving a

localized escarpment ~20 ft landward of the seawall. The second scour hole refilled within weeks,

and the area remained stable until January 2017.

CSE’s regularly scheduled survey of the area on 19 January 2017 did not detect any localized scour.

However, SIPOA officials reported the formation of a third scour hole a few days later. CSE

remobilized to the site and obtained more profiles and orthorectified drone imagery for compari-

son. This report describes the third event and provides profiles from 19 January and 27 January

2017. The latter survey shows that a massive part of the upper profile (high water to a depth of ~25

ft) slumped into the main channel of North Edisto River Inlet with the scour area extending roughly

250 ft alongshore. CSE estimates 30,000–35,000 cy shifted downslope into the channel. Infilling

by natural processes restored the area via sand drawn off from the adjacent beach.

Because of the likelihood of a recurrence of the scour, CSE recommends close observation of the

Deveaux Villas area and offers three theories for the cause of each scour hole:

1) Current eddies where flows in the northern channel meet currents in the North Edisto

River Inlet.

2) More erodible sediments at a section of beach where an old inlet may have existed 100

years ago.

3) High rainfall events which accumulate water around Deveaux Villas and saturate the

soils as the water percolates into the ground and under the seawall.


Each theory has weaknesses as discussed in the report. Regardless of cause, the problem is poten-

tially quite serious because such events can lead to catastrophic failure of the seawall. With a

diminished supply of sand along the Beach Club, each scour event draws off more sand from the

immediate upcoast area and increases the vulnerability of other sections of the seawall.

5.2 Recommendations

Because of the potential for recurrence of scour along the seawall, CSE recommends initiation of

planning and permitting to transfer sand from the accreting zone around Captain Sams Inlet to the

Beach Club and Deveaux Villas area. As long as there is a healthy supply of sand in that area, scour

will be less likely to reach the toe of the seawall. Other measures that should be considered are to

stockpile armor stone on the island (or recycle stone from other areas) for emergency seawall

repairs.

CSE also recommends that the SIPOA apply for a sand-scraping permit to shift sand from accreting

areas to South Beach. This will redistribute sand to areas where it is needed for continued protec-

tion of the seawall. Between 2002 and 2007, the SIPOA moved about 350,000 cy (under permit) in

several events during winter months with little impact to beachgoers. The cost of those efforts

was generally less than $3/cy. The primary permitting issue with such projects today is the desig-

nated critical habitat around Captain Sams Inlet. However, such sand transfers have recently been

performed at Isle of Palms and the eastern end of Kiawah Island with appropriate environmental

protection measures.

Finally, CSE recommends the majority of rafted myrtle shrubs be left in place along North Beach.

The dry beach will likely rebuild by summer, begin to bury the material, and initiate growth of a

new dune ridge. The dead myrtle are analogous to Christmas trees, which are commonly placed

along the dunes to help trap sand before the summer buildup. CSE expects to see stabilization of

North Beach and eventual recovery as Captain Sams Inlet shoals move downcoast at an accelerat-

ing rate.

CSE’s next regularly scheduled beach survey is January 2017.


6.0 REFERENCES & BIBLIOGRAPHY

Basco, DR. 1993. Review of beach management plans: Seabrook Island, SC. Review Rept., Seabrook Island Property

Owners Association; Coastal Engineering Center, Norfolk, VA, 25 pp.

Basco, DR, and GF Oertel. 2007. North Beach shoreline changes and management options. Final Report for Seabrook

Island POA. Hollow-Core Reef Enterprises Inc / Beach Consultants Inc, Norfolk, VA, 19 pp.

CSE. 1988. Beach surveys along Seabrook Island, South Carolina, through July 1988. Final Report to Seabrook Island POA;

Coastal Science & Engineering, Inc. (CSE), Columbia, SC, 31 pp + appendices.

CSE. 1989. Beach restoration and shore protection alternatives along the south end of Seabrook Island. Feasibility Study

for Seabrook Island POA. Coastal Science & Engineering, Columbia, SC, 38 pp + appendices.

CSE. 1990. Seabrook Island, South Carolina, beach nourishment project. Survey Report No. 1 for Seabrook Island POA;

CSE, Columbia, SC, 41 pp. + appendices.

CSE. 1991. Seabrook Island, South Carolina, beach nourishment project, 1990-1991. Survey Report No. 2 for Seabrook

Island POA; CSE, Columbia, SC, 37 pp + appendices.

CSE. 1992. Seabrook Island, South Carolina, beach nourishment project: performance evaluation and future needs.

Survey Report No. 3 to Seabrook Island POA; CSE, Columbia, SC, 60 pp. + Attachment I and Appendix I.

CSE. 1993. Seabrook Island, South Carolina, beach nourishment project. Survey Report No. 4 to Seabrook Island POA;

CSE, Columbia, SC, 34 pp + Appendix I.

CSE. 1994. Seabrook Island, South Carolina, beach nourishment project. Survey Report No. 5 to Seabrook Island POA;

CSE, Columbia, SC, 36 pp + Appendix I.

CSE. 1995. Seabrook Island, South Carolina, beach nourishment project. Survey Report No. 6A to Seabrook Island POA;

CSE, Columbia, SC, 19 pp + appendices.

CSE. 1995. Relocation of Captain Sams Inlet and beach restoration plan, Seabrook Island, South Carolina. Design Report,

Seabrook Island POA; CSE, Columbia, SC, 159 pp + appendices.

CSE. 1997. Captain Sams Inlet relocation project, Seabrook Island, South Carolina. Survey Report Year 1, Seabrook Island

POA; CSE (as CSE-Baird), Columbia, SC, 21 pp + app.

CSE. 1998. Seabrook Island 1996 inlet relocation. Survey Report Year 2 to Seabrook Island POA; CSE (as CSE Baird),

Columbia, SC, 22 pp + appendices.

CSE. 1999. Seabrook Island 1996 inlet relocation. Survey Report Year 3 to Seabrook Island POA; CSE (as CSE Baird),

Columbia, SC, 42 pp + appendices.

CSE. 2000. Seabrook Island 1996 inlet relocation. Survey Report Year 4 to Seabrook Island POA; CSE, Columbia, SC, 42 pp

+ appendices.


+ appendices.


+ appendices.

CSE. 2003. Seabrook Island 1996 inlet relocation. Survey Report Year 7 to Seabrook Island POA; CSE, Columbia, SC, 53 pp.

+ appendices.


+ appendices.

CSE. 2005a. Seabrook Island 1996 inlet relocation. Survey Report Year 9 to Seabrook Island POA; CSE, Columbia, SC, 59

pp + appendices.

CSE. 2005b. Kiawah Island east end erosion – opinion of probable causes and alternative strategies for management

mitigation. Memorandum Report for Town of Kiawah Island, SC; Coastal Science & Engineering, Columbia, South

Carolina, 31 pp.


CSE. 2006. Seabrook Island 1996 inlet relocation. Survey Report Year 10 to Seabrook Island POA; CSE, Columbia, SC, 55

pp + appendices.


pp + appendices.

CSE. 2007. East end erosion and beach restoration project, Kiawah Island, Charleston County, SC. Final Report for Town

of Kiawah Island, SC; Coastal Science & Engineering, Columbia, South Carolina, 54 pp + appendices.


pp + appendices.

CSE. 2009a. Seabrook Island 1996 inlet relocation. Monitoring Report Year 13 to Seabrook Island POA. CSE, Columbia, SC,

61 pp + appendices.

CSE. 2009b. Captain Sams inlet relocation project: analysis of potential impacts of inlet relocation on Kiawah spit.

Technical Report to Seabrook Island POA. CSE, Columbia, SC, 94 pp + appendices.

CSE. 2009c. Survey Report No 3 – 2006 east end erosion and beach restoration project, Kiawah Island (SC). Town of Kiawah

Island, SC; Coastal Science & Engineering, Columbia, South Carolina, 55 pp + appendices.

CSE. 2011. Survey Report No 4 – 2006 east end erosion and beach restoration project, Kiawah Island (SC). Town of Kiawah

Island, SC; CSE, Columbia, SC, 75 pp + appendices.

CSE. 2011a. Captain Sams inlet relocation project: review & analysis of alternatives. Supplementary Report 1 to USACE for

Seabrook Island POA. CSE, Columbia, SC, 27 pp.

CSE. 2011b. Captain Sams inlet relocation project: analysis of downdrift impacts. Supplementary Report 2 to USACE for

Seabrook Island POA. CSE, Columbia, SC, 33 pp.

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Columbia, SC, 116 pp plus 7 appendices.

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7.0 ACKNOWLEDGMENTS

This report was sponsored by the Seabrook Island Property Owners Association. We thank Heather

Paton (Executive Director) and Steve Hirsch (PE, Director of Engineering) for their guidance.

CSE's surveys were completed by Drew Giles and Luke Fleniken. Steven Traynum reduced and

analyzed the data with assistance by Dr. Tim Kana. Habitat mapping was developed from drone

imagery prepared by Drew Giles. Trey Hair prepared the habitat delineations based on DTMs

developed from the rectified imagery. Photographs were taken by TW Kana unless otherwise

noted. The report was written by Tim Kana with report preparation by Trey Hair and Diana

Sangster.

monitoring report — 2017 - sipoa€¦ · shows conditions in 2014 when captain sams inlet was...

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