National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Back Barrier Erosion Monitoring at Cumberland 

Island National Seashore 

2019 Survey Report

Natural Resource Report NPS/SECN/NRR—2020/2180 
ON THE COVER 

Island margin looking south at Raccoon Key on Cumberland Island National Seashore. Photograph by J. M. Bateman 

McDonald, 2017. 
Back Barrier Erosion Monitoring at Cumberland 

Island National Seashore 

2019 Survey Report

Natural Resource Report NPS/SECN/NRR—2020/2180 

Jacob M. Bateman McDonald1,2

1 University of North Georgia

Lewis F. Rogers Institute for Environmental and Spatial Analysis

Watkins Building, Room 176

3820 Mundy Mill Road

Oakwood, GA 30566

2 National Park Service

Southeast Coast Inventory and Monitoring Network

135 Phoenix Road

Athens, GA 30605

October 2020

U.S. Department of the Interior

National Park Service

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, Colorado, publishes a range of 

reports that address natural resource topics. These reports are of interest and applicability to a broad audience in the National 

Park Service and others in natural resource management, including scientists, conservation and environmental constituencies, 

and the public. 

The Natural Resource Report Series is used to disseminate comprehensive information and analysis about natural resources and 

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All manuscripts in the series receive the appropriate level of peer review to ensure that the information is scientifically credible, 

technically accurate, appropriately written for the intended audience, and designed and published in a professional manner. 

Data in this report were collected and analyzed using methods based on established, peer-reviewed protocols and were analyzed 

and interpreted within the guidelines of the protocols.

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and policies 

of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use by the U.S. Government. 

This report is available from the Southeast Coast Network website and the Natural Resource Publications Management website. If 

you have difficulty accessing information in this publication, particularly if using assistive technology, please email irma@nps.gov.

Please cite this publication as:

Bateman McDonald, J. M. 2020. Backbarrier erosion monitoring at Cumberland Island National Seashore: 2019 survey report. 

Natural Resource Report NPS/SECN/NRR—2020/2180. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/

nrr-2279294

NPS 640/173790 October 2020

ii
iii

Contents

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Summary and Key Findings . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Cumberland Wharf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Brickhill Bluff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Plum Orchard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Dungeness Wharf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Raccoon Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Cumberland Wharf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Brickhill Bluff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Plum Orchard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Dungeness Wharf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Raccoon Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Cumberland Wharf (CW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Brickhill Bluff (BB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Plum Orchard (PB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Dungeness Wharf (DW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Raccoon Key (RK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Literature Cited . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
iv

Figures

Figure 1. Overview map of Cumberland Island National Shoreline (CUIS) with the locations of the island 

margin monitoring sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 2. Example setup for an integrated survey using the resection method to setup the total station. 

Inset shows the difference between poor and good total station/hub orientations. . . . . . . . . . . . . . . 6

Figure 3. Island margin retreat at Cumberland Wharf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 4. Schematic depiction of the process by which undercuts develop, overhangs begin to lean,

overhangs fail, and the island margin retreats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 5. Shoreline retreat at Brickhill Bluff. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 6. Shoreline retreat at Plum Orchard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 7. Shoreline retreat at Dungeness Wharf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 8. Shoreline retreat at Raccoon Key. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 9. Schematic depiction of the process by (B–C) smaller block failures develop and (D–F) large

arcuate sections of underlying peat become exposed and are then eroded (causing the margin 

to retreat). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Tables

Table 1. Summary of change in position of island margin (horizontal). . . . . . . . . . . . . . . . . . . . . . . . . . . 7
v

Summary and Key Findings 

This report provides the results of a technical assistance 

project implemented at Cumberland Island National 

Seashore (CUIS) to monitor island margin retreat in selected 

areas on the back barrier (west) side of the island. On May 

22 and 23, 2019, the Southeast Coast Network (SECN) 

conducted an island margin survey of five sites; Cumberland 

Wharf (CW), Brickhill Bluff (BB), Plum Orchard (PO), 

Dungeness Wharf (DW), and Raccoon Key (RK). These 

five sites had previously been monitored by the USGS 

from December 2011 to February 2013 to determine the 

average rate of island margin retreat at these pre-identified 

erosional hotspots (Calhoun and Riley 2016). These sites 

were resurveyed in 2017 and 2018 to determine if the rates 

of island margin retreat from 2013 to 2017 and 2017 to 

2018 (which include the effects of Hurricane Matthew 

[Oct. 7–8, 2016] and Hurricane Irma [Sept. 10–12, 2017]) 

were consistent with the rates observed from 2011 to 2013 

(McDonald and Gregory 2019). McDonald and Gregory 

(2019) found a similar pattern of gradual erosion at Raccoon 

Key and larger areas of change at the other sites with little 

change between. In addition to the areas of gradual change 

at Raccoon Key, large areas of change were also identified, 

suggesting punctuated erosion events are also influencing 

areas of the island with similar margin morphology.

The purpose of the 2019 resurvey was to determine the 

rate(s) at which the island margin retreats during a nonhurricane year. It was hypothesized that the amount and 

rate of change at the sites would be more similar to the 

results of Calhoun and Riley (2016) than to the results of 

McDonald and Gregory (2019). The 2019 resurvey found 

that the percent of retreat at all sites was much lower than 

during all previous years. The amount of island margin 

change was most similar to the change detected between 

2011 and 2013 by Calhoun and Riley (2016). The relatively 

high percent of the bank that did not change coupled with 

the fact that no major hurricane affected the island between 

the 2018 and 2019 surveys suggests that the large areas of 

change (observed during previous surveys) may be extreme 

event driven. If no major hurricanes affect the island during 

the 2019 hurricane season, an additional survey would help 

determine average non-hurricane year rate(s) of change. 

Similar processes of margin toe erosion, undercutting, 

and eventual bank failure are affecting all of the sites. As 

the island margin becomes undercut, a threshold will be 

reached and an overhang or block of bank will detach 

from the margin, causing the island margin to retreat. The 

main difference between the study sites (which creates 

the differences in the rate[s] of retreat) are the height and 

steepness of their banks, bank sediment composition, 

and type and density of vegetation cover. The main report 

provides some background on each of the study sites, a 

general overview of the methods used to collect and process 

the data, and a discussion of the results and processes 

causing these changes.

The following is a brief summary of the results for each site: 

Cumberland Wharf

• The survey was 153 meters (502 feet [ft]) in length along 

the top of the bluff.

• 55% of the island margin retreated and 20% advanced 

from its 2018 position.

• The percent of retreat is less than previous years; though 

there was a greater percent retreat than the rest of the 

sites (except for Raccoon Key).

• The maximum retreat was 1.41 meters (4.63 ft) with an 

average of 0.25 meters (0.82 ft). 

• The maximum and average retreat is similar to Calhoun 

and Riley (2016).

Brickhill Bluff

• The survey was 233 meters (764 ft) in length along the 

top of the island margin.

• 43% of the island margin retreated and 33% advanced 

from its 2018 position.

• The percent retreat is less than previous years; and 

second least among sites

• The maximum retreat was 0.92 meters (3.02 ft) with an 

average of 0.22 meters (0.72 ft). 

• Had the least amount of retreat measured by all surveys; 

the average retreat was most similar to the 2017 resurvey 

(2013–2017 annual rate).
vi

Plum Orchard

• The survey was 249 meters (816.9 ft) in length along the 

top of the island margin.

• 49% of the island margin retreated and 6% advanced 

from its 2018 position.

• The percent retreat is less than previous years; highest 

percent of area with no change in position.

• The maximum retreat was 2.06 meters (6.76 ft) with an 

average of 0.29 meters (0.95 ft).

• The maximum and average retreat is similar to Calhoun 

and Riley (2016). 

Dungeness Wharf

• The survey was 172 meters (564.3 ft) in length.

• 28% of the island margin retreated and 58% advanced 

from its 2018 position.

• Least amount of retreat

• The maximum retreat was 1.83 meters (6 ft) with an 

average of 0.33 meters (1.08 ft). 

• Maximum retreat similar to 2018 survey and average 

change most similar to Calhoun and Riley (2016).

Raccoon Key

• The survey was 400 meters (1,312 ft) in length.

• 82% of the island margin retreated and 6% advanced 

from its 2018 position.

• Slightly less percent retreat than previous surveys; most 

change amongst the sites.

• The maximum retreat was 5.41 meters (17.75 ft) with an 

average of 0.69 meters (2.26 ft). 

• Maximum similar to 2017 and 2018 and average is 

nearly half as much as 2018.
1

Introduction 

On May 22 and 23, the Southeast Coast Network (SECN) 

conducted a survey of five sites—Cumberland Wharf (CW), 

Brickhill Bluff (BB), Plum Orchard (PO), Dungeness Wharf 

(DW), and Raccoon Key (RK)—on the back barrier side of 

Cumberland Island National Seashore (Figure 1). The island 

margin at these sites ranges in morphology from high bluff 

in the north (Cumberland Wharf) to low marsh in the south 

(Raccoon Key). Brickhill Bluff and Plum Orchard are located 

on meander bends of the Brickhill River and Dungeness 

Wharf is located on Cumberland Sound near the confluence 

of the Crooked and St. Marys Rivers. All of the sites are 

affected by fluvial and tidal processes (Calhoun and Riley 

2016; McDonald and Gregory 2019).

The five sites were first surveyed by the USGS from 

December 2011 to February 2013 to determine the 

average rate of island margin retreat at these pre-identified 

erosional hotspots (Calhoun and Riley 2016). Calhoun 

and Riley (2016) observed steady low magnitude retreat 

along Raccoon Key, related to tidal influences. At the other 

sites (which are set further back from the high tide line), 

the major driving factor behind island margin retreat is 

punctuated erosions events caused by combining above 

average tide conditions with high winds which causes 

increased wave energy, and exacerbated tidal height. These 

erosional events slowly erode the toe of the bank, causing the 

banks to become undercut and unstable (Calhoun and Riley 

2016). Eventually, a threshold is reached and the undercut 

and overhanging banks will fail causing the island to retreat 

behind this failed section of the bank. 

The five sites were resurveyed in 2017 and 2018 to determine 

if the rates of island margin retreat from 2013 to 2017 

and 2017 to 2018 (which include the effects of Hurricane 

Matthew [Oct. 7–8, 2016] and Hurricane Irma [Sept. 

10–12, 2017]) were consistent with the rates observed from 

2011 to 2013 (McDonald and Gregory 2019). McDonald 

and Gregory (2019) found a pattern of erosion similar 

to Calhoun and Riley (2016) with high-frequency, lowmagnitude erosion at Raccoon Key and low-frequency, 

high-magnitude areas of change at the other sites, with 

little change between these large areas of change. In 

addition to the areas of low-magnitude change at Raccoon 

Key, a few high-magnitude areas of change were also 

identified, suggesting punctuated erosional events may 

also be influencing areas of the island with similar margin 

morphology (areas of low marsh).

This report provides the results of a technical assistance 

project implemented at Cumberland Island National 

Seashore (CUIS) to monitor island margin retreat on the 

back barrier (west) side of the island. The following sections 

provide an introduction to the island and the sites that were 

surveyed, provides a description of the methods that were 

used to conduct the survey and process the collected data, 

and presents and discusses the results of the 2019 surveys 

within the context of previous investigations. The results 

of the SECN surveys (2017–2019) are compared to the 

2011–2013 USGS surveys and an aerial photograph analysis 

conducted by Jackson (2006), for 1983–2002, to provide 

context for the island margin retreat captured by the most 

recent surveys. 
2

Figure 1. Overview map of Cumberland Island National Shoreline (CUIS) with the locations of the island 

margin monitoring sites.
3

Study Area

Cumberland Island is Georgia’s southernmost barrier 

island and is located near the Georgia/Florida border in 

Camden County, Georgia. The island is influenced by the 

Satilla River in the north, the St Mary’s River in the south, 

and the Crooked River/Cumberland River in between 

(Figure 1). Cumberland Island is composed of a central 

‘core’ of Pleistocene barrier island sediments with a narrow, 

seaward veneer of Holocene sands (Griffin 1982). The 

ocean-to-estuary profile of Cumberland Island is typical 

of large barrier islands. The ocean (east) side of the island 

is characterized by wide sandy beaches beyond which (to 

the west) are primary and secondary dunes with lower 

interdunal areas (Alber et al. 2005). The back-barrier (west) 

side of the island is characterized by salt marsh and tidal 

creeks in the south and higher bluffs in the north. In some 

areas, the island margin is being actively eroded by wave 

action coupled with fluvial processes from the tidal creeks 

and rivers that flow through and past (Calhoun and Riley 

2016). The five sites that were surveyed for this project are 

influenced by St. Andrew’s Sound (Cumberland Wharf), 

Brickhill River (Brickhill Bluff and Plum Orchard), and 

Crooked Creek, St. Mary’s River, and Cumberland Sound 

(Dungeness Wharf and Raccoon Key). The five survey 

locations were initially chosen to be representative of preidentified erosional hotspots (Jackson 2006; Calhoun and 

Riley 2016). The following sections describe the five survey 

locations in more detail.

Cumberland Wharf

The island margin at Cumberland Wharf (CW) is a bluff 

located on the north end of the park (Figure 1). The bluff 

is high (greater than 5 meters [16.4 ft]), sandy, and in many 

spots deeply undercut (approximately 1 meter [3.3 ft]). 

Large rotational slumps and undercut failures were observed 

by Calhoun and Riley (2016) as well as during the SECN 

resurveys (McDonald and Gregory 2019). The bluff face is 

exposed fine to medium sands with little organic material; 

while the top of the bluff is heavily vegetated with shrubs and 

intermittent large trees (Calhoun and Riley 2016). The base 

of the bluff is protected from erosion by an apron of eroded 

bluff sediments and slump blocks that are being actively 

eroded by wave action and fluvial processes.

Brickhill Bluff

Brickhill Bluff is located near the north end of the park 

on the cutbank side of a meander bend of Brickhill River 

(Figure 1). The island margin is an approximately 1.5 meters 

(4.9 ft) tall near vertical bank composed of very fine sand 

with a thin layer of coarser sediments and shell fragments 

near the surface (Calhoun and Riley 2016). A designated 

group campsite occupies this portion of the island and 

visitor use is contributing to island margin erosion and bank 

instability. Canopy cover is thick along the island margin but 

the understory is relatively open. In a number of places, large 

trees have been undercut and toppled onto the beach and are 

now providing some temporary bank protection (Calhoun 

and Riley 2016).

Plum Orchard

Plum Orchard is located near the center of the island on the 

cutbank side of a meander bend of Brickhill River (Figure 1). 

The island margin is 1.1 meters (3.6 ft) tall and composed of 

consolidated sands with high clay and organic content that is 

topped with a layer of coarser sediments and shell fragments 

(Calhoun and Riley 2016). A dock and boat launch are 

located near the middle of this site. There are also several 

historical structures near the island margin, one of which is 

protected by a wooden seawall. Canopy cover is high at the 

northern and southern ends of the site, while the understory 

is a well-manicured lawn. Significant undercutting of the 

banks just north of the boat launch were observed by 

McDonald and Gregory (2019).

Dungeness Wharf

Dungeness Wharf is located near the south end of the island 

on Cumberland Sound and is one of the primary points of 

drop off and embarkation for visitors (Figure 1). The island 

margin at this site is an approximately 2.8 meters (9.2 ft) 

tall near vertical bank composed of fine to medium sand 

overlain by coarser sediments and shell fragments (Calhoun 

and Riley 2016). This site is located just north of a concrete 

seawall. This site was initially chosen because Jackson (2006) 

identified a significant amount of island margin retreat to 

the north and south of the Dungeness Wharf seawall. The 

island margin is heavily vegetated in some areas and, similar 

to Brickhill Bluff, has large toppled trees from past erosion 

events that may be providing some degree of temporary 

protection to the island margin (Calhoun and Riley 2016).

Raccoon Key

Raccoon Key is located near on the south end of the island 

and is bordered by Cumberland Sound to the west (Figure 

1). The island margin is a 0.6 meters (2 ft) tall near vertical 

bank. The bank is composed of highly bioturbated marshlike sediment with high clay and organic content (Calhoun 

and Riley 2016). The upper portion of the bank has high root 

density and the bank tends to break off in smaller clumps 

when the lower unrooted portion is eroded away (see cover 

photo). One large dredge spoil mound occupies the northern 

portion of this site. A small tidal channel bisects the island 

margin where the surveys were conducted.

3

5

Methods

The methods used to survey the island margin monitoring 

sites were the same as those used by Calhoun and Riley 

(2016) and McDonald and Gregory (2019). A combination 

of real-time kinematic (RTK) GPS (global positioning 

system) and total station surveys were used to delineate the 

island margin at each site. The island margin was defined as 

the westernmost extent of the island and/or the near-vertical 

soil profile at the land-water interface that is susceptible 

to erosive forces (Calhoun and Riley 2016). At each site, 

a Trimble R10 GPS receiver was set up on the permanent 

benchmark that was used for the Calhoun and Riley (2016) 

surveys. The RTK signal from the base station was boosted 

using a Trimble TDL 450H UHF (ultra-high frequency) data 

radio to extend the RTK signal up to 5 kilometers (3.1 miles 

[mi]) from the base station. A second R10 GPS receiver was 

mounted on a survey pole in ‘rover’ mode and RTK points 

were taken where satellite coverage was sufficient. RTK 

surveys work well in open areas and under light canopy and 

was used where position solutions with horizontal error 

ranges less than 0.02 meters (0.07 ft) could be obtained. For 

the purposes of this survey, there was no vertical error cutoff 

because the purpose of the survey was to monitor horizontal 

(not vertical) island margin retreat. 

To monitor the areas that did not have sufficient satellite 

coverage to obtain a RTK solution, a total station (TS) was 

integrated into the RTK survey through re-sectioning (Figure 

2). Re-sectioning is done by obtaining a position solution 

on two points (using either RTK or TS) and then surveying 

those same points with a total station (TS is set up to make 

a triangle with the previously surveyed points) to back 

calculate the TS location. Some of the sites required multiple 

TS setups due to tidal incursion of the beach front during the 

survey and/or due to the presence of large tree falls or thick 

understory.

Once the surveys were complete, the data were downloaded 

and the Analyzing Moving Boundaries in R (AMBUR) 

package (Jackson 2010) for the statistical program R (R Core 

Team 2018) was used to determine the amount of retreat 

that occurred at each island margin site. The island margin 

position points were connected to create a line representing 

the island margin position during the survey. From these 

island margin lines, transects were cast (1 meter [3.3 ft] 

apart) perpendicular to the island margins and change 

between subsequent years was calculated by determining the 

length of the transects between the island margin lines.
6

Figure 2. Example setup for an integrated survey using the resection method to setup the total station. 

Inset shows the difference between poor and good total station/hub orientations.
7

Results and Discussion

The following section describes and discusses the results of 

the surveys that were conducted by SECN in 2019 at each 

of the five island margin monitoring sites. The results of the 

2019 surveys are compared to the 2017 and 2018 surveys 

(conducted by the SECN), the results of the USGS 2011–

2013 survey, and the aerial photograph analysis conducted 

by Jackson (2006) for 1983–2002 (Table 1). 

Table 1. Summary of change in position of island margin (horizontal). Survey results from 2019 are compared to data 

extracted from Jackson (2006) for 1983–2002, Calhoun and Riley (2016) for 2011–2013, and McDonald and Gregory 

(2019) for 2013–2017 and 2017–2018.

Parameter Cumberland 

Wharf (CW)

Brickhill 

Bluff (BB)

Plum 

Orchard 

(PO)

Dungeness 

Wharf (DW)

Raccoon 

Key (RK)

Survey Length (meters) 153 233 249 172 400

2017 Retreat (% of points indicating island margin retreat) 84 88 72 100 100

2018 Retreat (%) 100 97 73 95 90

2019 Retreat (%) 55 43 49 28 82

2019 Advance (% of points indicating island margin advance) 20 33 6 53 6

2019 No Change (%) 25 24 44 19 12

2011–2013 Maximum Retreat (meters) 1.41 1.99 1.94 2.40 2.34

2013–2017 Maximum Retreat (meters) 5.74 3.64 3.21 5.79 7.86

2018 Maximum Retreat (meters) 2.37 2.53 2.67 1.89 6.72

2019 Maximum Retreat (meters) 1.41 0.92 2.06 1.83 5.41

2019 Maximum Advance (meters) 0.49 2.38 0.96 0.51 1.52

2017 Average Retreat (meters) 1.82 1.19 0.82 2.06 2.82

2018 Average Retreat (meters) 0.98 1.05 0.47 0.74 1.16

2019 Average Retreat (meters) 0.25 0.22 0.29 0.33 0.69

2019 Average Advance (meters) 0.19 0.40 0.46 0.18 0.36

1983–2002 Annual Rate Retreat (meters/year) 1.87 0.50 0.65 1.90 1.90

2012–2013 Annual Rate Retreat (meters/year) 0.25 0.47 0.26 0.37 0.77

2013–2017 Annual Rate Retreat (meters/year) 0.43 0.28 0.19 0.47 0.64

2017 Standard Deviation Retreat (meters) 1.10 0.82 0.73 1.02 1.15

2018 Standard Deviation Retreat (meters) 0.57 0.54 0.47 0.36 1.26

2019 Standard Deviation Retreat (meters) 0.23 0.17 0.41 0.40 0.74

2019 Standard Deviation Advance (meters) 0.11 0.45 0.24 0.09 0.34
8

Cumberland Wharf (CW) 

The 2019 survey of Cumberland Wharf took place on May 

22. Though the bluff top is within range of the base station, 

canopy cover prevented RTK from being reliable (Figure 

3). High tides can have a small influence on the base of the 

island margin at this site. The survey was done during a 

falling tide, so that enough of the beach would be exposed, 

to allow the total station to be placed far enough back, to 

make the whole island margin visible with one setup. At high 

tide the beach is too small to provide adequate total station 

setup locations. Portions of the bluff are deeply undercut 

and require a spotter (i.e., total station operator) to judge the 

safety of the survey as it progresses.

Approximately 55% of the island margin (approximately 

84 meters; 276 ft) retreated and 20% of the island margin 

(30 meters; 98 ft) advanced from its 2018 position (Table 

1). The maximum amount of retreat was detected in the 

western portion of the surveyed area where a small arcuate 

portion of the island margin retreated 1.41 meters (4.63 

ft; Figure 3A-1). The maximum amount of retreat during 

the 2019 survey was equal to the maximum amount of 

retreat observed by Calhoun and Riley (2016) and 0.6 to 2.5 

times smaller than the 2018 and 2017 surveys, respectively 

(McDonald and Gregory 2019). The maximum amount of 

advance was also detected in the western portion of the 

study area (edges of outcrop, Figure 3A-2). Most of the areas 

of advance are small enough that they are likely related to 

differences in how the island margin was defined. Other 

areas of advance (e.g., Figure 3A-3), appear as though the 

edge of the bank is leaning forward. The surveyed margin 

had on average 0.25 meters (0.82 ft) of retreat and 0.19 

meters (0.62 ft) of advance. The average amount of retreat is 

equal to the annual rate of retreat observed by Calhoun and 

Riley (2016) and more than seven times less than the annual 

rate interpreted by Jackson (2006). 

Island margin retreat along this stretch of the back barrier 

is interpreted as being driven by gradual erosion of the 

toe of the bluff, followed by undercut development, and 

Figure 3. Island margin retreat at Cumberland Wharf.
9

eventual overhang failure (Figure 4). As the toe is eroded, 

the overhang will progressively get larger until it begins to 

lean forward (Figure 3A-3), a threshold is reached, an area 

of overhang breaks off, and the island margin retreats (e.g., 

Figures 3A-1, 3B [semi-continuous area of retreat], and 4D). 

The large areas that were lost between 2013 and 2017 are 

likely due to areas where trees (and their relatively large root 

wads) were part of the overhang (see Figure 9 in Calhoun 

and Riley 2016).

Figure 4. Schematic depiction of the process by which undercuts develop, overhangs begin to lean, 

overhangs fail, and the island margin retreats.

Brickhill Bluff (BB) 

Brickhill Bluff is located near the north end of the park on 

the cutbank side of a meander bend of the Brickhill River 

(Figure 1). Due to growth of the tree canopy over the survey 

benchmark, RTK GPS nor an integrated survey could be 

used. To initially setup the survey, the total station was setup 

on the benchmark and an electronic compass was used to 

estimate the azimuth to a temporary backsight point. The 

relative orientation of the survey results were corrected in 

the office using data from previous surveys. Five total station 

setups were needed because of downed trees on the beach 

preventing line-of-sight to island margin. High tides can have 

a small influence on the base of the island margin at this 

site.. Additionally, at high tide, the beach is too small and the 

large woody debris on the beach is too dense to survey large 

portions of the shoreline and a prohibitive number of total 

station setups are needed to complete the survey. 

Approximately 43% of the island margin (102 meters [335 

ft]) retreated and 33% (77 meters [253 ft]) advanced from 

its 2018 position (Table 1). The maximum amount of retreat 

was detected in the middle portion of the surveyed area 

where a small arcuate portion of the island margin retreated 

0.92 meters (3.02 ft; Figure 5B-1). The maximum amount 

of retreat during the 2019 survey was the least amount 

measured between all of the surveys. The maximum amount 

of advance was located in the southern portion of the survey 

where the bank advanced 2.38 meters (7.81 ft; Figure 5C-2). 

The surveyed margin had on average 0.22 meters (0.72 ft) 

of retreat and 0.40 meters (1.31 ft) of advance. The average 

amount of retreat was up to five times less than previously 

detected (Table 1). Additionally, the average amount of 

retreat was nearly equal to the annual amount of rate 

interpreted from the 2013–2017 survey (McDonald and 

Gregory 2018).

It appears that many of the same processes that are affecting 

Cumberland Wharf are also affecting Brickhill Bluff, though 

the bank heights are much lower. Similar to the Cumberland 

Wharf survey, most of the areas of advance are small enough 

that they are likely related to differences in how the island 

margin was defined (Figure 5A). Brickhill Bluff also appears 

to have areas with leaning banks (Figure 5B-3 and 5C-3) and 

areas where the overhang broke away from the island margin 

(Figures 5B-4 and 5C-4). The large areas of advance in the 

southern portion of the survey (Figure 5C-2), are due to 

better island margin survey coverage during 2019. The island 

margin had better survey coverage because a total station 

was setup in near proximity to this portion of the margin. 

This area was surveyed with less accuracy during the 2018 

survey due to dense vegetation cover. 
10

Figure 5. Shoreline retreat at Brickhill Bluff.
11

Plum Orchard (PB) 

Plum Orchard is located near the center of the island on the 

cutbank side of a meander bend of Brickhill River (Figure 

1). The majority of the survey was done using RTK GPS. 

One total station setup was sufficient to survey the points 

that RTK could not be used to survey (due to poor satellite 

coverage). While the high tide line is close to and can 

sometimes reach the base of the island margin, tides were 

not taken into account when surveying this site because the 

understory is managed (by mowing) and the TS was setup 

on the inland side of the margin. There is a dock and boat 

launch along this stretch of island margin (Figure 6C-1), a 

seawall protecting the margin between Figure 6A and 6B, 

and a historical structure on the edge of the island margin 

(Figure 6A-2).

Approximately 49% of the island margin (122 meters; 400 ft) 

retreated and 6% (15 meters; 49 ft) advanced from its 2018 

position (Table 1). The maximum amount of retreat was 

detected in the northern portion of the surveyed area where 

the island margin retreated 2.06 meters (6.76 ft; Figure 6A3). The maximum amount of retreat during the 2019 survey 

was similar to the maximum amount of retreat observed 

by Calhoun and Riley (2016) and 0.6 meters (1.97 ft) to 

1.2 meters (3.94 ft) less than the 2017 and 2018 surveys, 

respectively (Table 1). The maximum amount of advance was 

located in the middle portion of the survey, near the seawall, 

where the bank advanced 0.96 meters (3.15 ft; Figure 6B-4). 

The surveyed margin had on average 0.29 meters (0.95 ft) 

of retreat and 0.46 meters (1.51 ft) of advance. The average 

amount of retreat in 2019 was less than observed between 

2017 and 2018 (Table 1). The average amount of retreat was 

similar to the annual amount of retreat observed by Calhoun 

and Riley and slightly less than the 2013–2017 annual 

amount of retreat (McDonald and Gregory 2018). 

Plum Orchard had the largest area with little to no change 

(Table 1). Additionally, the majority of the ‘change’ that was 

observed at Plum Orchard is likely due to differences in how 

the island margin was defined coupled with differences in 

survey point density. The large area of advance (Figure 6B-4) 

is an artifact of the poor satellite coverage during the survey 

and the inability to conduct a more representative survey. 

It is unlikely that the seawall advanced toward the river. 

Similarly, the largest area of retreat (Figure 6A-3) might be 

due to the dense vegetation in the area and the inability to 

accurately survey this portion of the island margin using this 

methodology. Alternatively, this area of retreat is consistent 

with the progressive retreat captured by the previous surveys 

and more than likely represents actual change to the island 

margin. If this area of retreat is of management concern, any 

additional resurveys should be sure to survey this portion of 

the island margin with high survey point density to increase 

accuracy of the change estimates. Though the majority of 

the island margin did not change, there were areas that do 

appear to have retreated significantly (i.e., Figure 6B-4). 

These areas appear similar to the areas at Cumberland Wharf 

and Brickhill Bluff that developed through the process of 

bank undercutting (Figure 4). 
12

Figure 6. Shoreline retreat at Plum Orchard.
13

Dungeness Wharf (DW) 

Dungeness Wharf is located near the south end of the island 

on Cumberland Sound (Figure 1). While some of the island 

margin was measured with RTK, an integrated survey was 

required to survey the entire margin. One total station setup 

was sufficient to survey the areas that RTK could not capture. 

High tides can have a small influence on the base of the 

island margin at this site. Additionally, at high tide the beach 

is too small and the large woody debris on the beach is too 

dense to complete the survey with a total station. There is a 

seawall at the southern end of the island margin.

Approximately 28% of the island margin (48 meters [157 ft]) 

retreated and 53% (100 meters [328 ft]) advanced from its 

2018 position (Table 1). The maximum amount of retreat was 

detected in the middle portion of the surveyed area where 

the island margin retreated 1.83 meters (6 ft; Figure 7B-1). 

The maximum amount of retreat during the 2019 survey was 

similar to the maximum amount of retreat in 2018 (Table 

1). The maximum amount of advance was located in the 

northern portion of the survey where the bank advanced 

0.51 meters (1.67 ft; Figure 7A-2). The surveyed margin had 

on average 0.33 meters (1.08 ft) of retreat and 0.18 meters 

(0.59 ft) of advance. The average amount of retreat in 2019 

was two to nearly seven times less than observed in 2018 and 

2017, respectively (Table 1). The average amount of retreat 

was similar to the annual amount of retreat observed by 

Calhoun and Riley (2018) and slightly less than the 2013–

2017 annual amount of retreat (McDonald and Gregory 

2018). 

Dungeness Wharf had the smallest percent of island margin 

retreat and the highest percent of island margin advance 

(Table 1). The areas with substantial retreat (e.g., Figure 7B-1 

and 7B-3) appear to be similar to the arcuate area of retreat 

at Cumberland Wharf (Figure 3A-1). A similar process of 

development (Figure 4) is interpreted for this area of retreat 

at Dungeness Wharf. While some of the areas of advance 

might be due to bank lean (i.e., Figure 7A-2), the rest of the 

area that has ‘advanced’ is likely due to small differences in 

where the island margin was surveyed and the density of the 

survey points. 

Figure 7. Shoreline retreat at Dungeness Wharf.
14

Raccoon Key (RK) 

Raccoon Key is located near on the south end of the island 

near the confluence of Beach Creek and the St. Mary’s River 

(Figure 1). RTK was used to survey the entire site due to 

the open canopy of low marsh grasses. Tides have a large 

influence on this site. At high tide, waves often break on the 

island margin. Additionally, at high tide, the island margin 

cannot be accessed because of the deep tidal creek that lies 

between the site and the benchmark. The survey runs from a 

tidal creek in the south to halfway past a large spoil pile (just 

south of the second benchmark, Figure 8). Another (smaller) 

tidal creek runs through the middle of the surveyed margin. 

Approximately 82% of the island margin (328 meters [1076 

ft]) retreated and 6% (24 meters [79 ft]) advanced from its 

2018 position (Table 1). The maximum amount of retreat was 

observed in the southern portion of the surveyed area where 

the island margin retreated 5.41 meters (17.75 ft; Figure 8C1). The maximum amount of retreat during the 2019 survey 

was less than measured during the 2017 and 2018 surveys 

(Table 1). The maximum amount of advance was located in 

the middle portion of the survey where the bank advanced 

1.52 meters (4.99 ft; Figure 8B-2). The surveyed margin 

had on average 0.69 meters (2.26 ft) of retreat and 0.36 

meters (1.18 ft) of advance. The average amount of retreat 

in 2019 was similar to the annual rate of retreat observed by 

McDonald and Gregory (2019) and slightly less than the rate 

observed by Calhoun and Riley (2016). 

Figure 8. Shoreline retreat at Raccoon Key.
15

Raccoon Key had by far the highest percent of island margin 

retreat (Table 1). It appears two distinct processes are 

contributing to the erosion at Raccoon Key (Figure 9). The 

first process, also identified by Calhoun and Riley (2016), 

is the progressive retreat of the island margin through small 

block failures (Figure 9B–C). This process is facilitated by 

the diurnal effects of high tide waves breaking on or near 

the island margin which progressively weakens the bank 

through toe erosion and the addition of water to the bank 

through wave spray. As the wave spray drains from the 

bank face, areas of weakness (macropores) develop (Figure 

9B) along which the block failures will occur (Figure 9C). 

This process is proposed as contributing to the persistent 

relatively uniform rates of erosion along this section of the 

back barrier. The second process is proposed to be related 

to storm wash (Figure 9D–F). Similar to the process by 

which the bank failures form, storm wash draining out of the 

island margin will preferentially flow along natural zones of 

weakness and toward topographic lows (Figure 9D). In some 

areas, the rooting zone (Rz) will become detached from the 

underlying peat and a large section of vegetation will fail 

(Figure 9E). This failure will expose the less erosion-resistant 

peat to wave and storm action (Figure 9F). Once the peat 

has been eroded, the areas surrounding this arcuate loss 

of island margin will be subjected to increased stress and 

erosion rates will increase relative to undisturbed portions of 

the island margin.

Figure 9. Schematic depiction of the process by (B–C) smaller block failures develop and (D–F) large arcuate 

sections of underlying peat become exposed and are then eroded (causing the margin to retreat).

17

Conclusion

The percent of retreat observed at all sites was lower than 

during all previous years. Dungeness Wharf had the lowest 

percent of island retreat (28%) and Raccoon Key had the 

highest percent of island margin of retreat (82%). The 

maximum amount of retreat was seen at Raccoon Key (5.41 

meters [17.75 ft]) and the lowest average amount of retreat 

was observed at Brickhill Bluff (0.22 meters [0.72 ft]). The 

amount of island margin retreat was most similar to the 

change detected by Calhoun and Riley (2016) between 

2011 and 2013. Raccoon Key is retreating at the fastest 

rate (0.69 meters [2.26 ft]). This result is not surprising due 

to the influence tides have on the low marsh site. Highmagnitude areas of retreat were also identified at Raccoon 

Key (between 2013 and 2017 as well as between 2017 and 

2018), the process by which these arcuate areas of vegetation 

stripping and underlying sediment erosion need to be further 

investigated though a process was proposed in Figure 9D–F.

The relatively high percent of the bank that did not change 

(compared to previous surveys) coupled with the fact that 

no major hurricane affected the island between the 2018 and 

2019 surveys suggests that the significant areas of change are 

largely extreme event driven. If no major hurricanes affect 

the island during the 2019 hurricane season, an additional 

survey should be completed to determine an average nonhurricane rate of change. Additionally, to further determine 

the impact of hurricanes on island margin retreat, other 

potentially erosive events (e.g., storms with strong westerly 

winds during high tide) should be identified to determine 

whether the period between the 2018 and 2019 surveys was 

uncommonly quiescent, and would explain the relatively 

small amount of retreat that was detected. 

19

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Calhoun, D. L. and J. W. Riley. 2016. Spatial and temporal assessment of back-barrier erosion on Cumberland Island National 

Seashore, Georgia, 2011–2013. U.S. Geological Survey Scientific Investigations Report 2016-5071, 32 p.

Griffin, M. M. 1982. Geologic guide to Cumberland Island National Seashore. Geologic Guide 6, Prepared for the United States 

Department of Interior as part of Contract No. CX5000-8-1563, 36 p.

Jackson, Jr., C. W. 2006. Historical back-barrier shoreline change along Cumberland Island, GA 1857-2002. Prepared for 

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Jackson, Jr., C. W. 2010. Basic user guide for the AMBUR package for R, version 1.0a. Unpublished.

McDonald, J. M., and M. Brian Gregory. 2019. Back barrier erosion monitoring at Cumberland Island National Seashore: 2018 

data summary—Version 2.0. Natural Resource Data Series NPS/SECN/NRDS—2019/1201. National Park Service, Fort 

Collins, Colorado. Available at: https://irma.nps.gov/DataStore/Reference/Profile/2259083 (last accessed July 2020).

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