Shoreline Analysis and Barrier Island Dynamics: Decadal Scale Patterns from Cedar Island, Virginia

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Shoreline Analysis and Barrier Island Dynamics: Decadal Scale Patterns fromCedar Island, VirginiaAuthor(s): Stephanie H. Nebel, Arthur C. Trembanis, and Donald C. BarberSource: Journal of Coastal Research, 28(2):332-341. 2012.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/JCOASTRES-D-10-00144.1URL: http://www.bioone.org/doi/full/10.2112/JCOASTRES-D-10-00144.1

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Shoreline Analysis and Barrier Island Dynamics: DecadalScale Patterns from Cedar Island, Virginia

Stephanie H. Nebel{, Arthur C. Trembanis{, and Donald C. Barber{

{Department of Geological SciencesUniversity of Delaware255 Academy Street, Penny HallNewark, DE 19716, [email protected]@udel.edu

{Department of GeologyBryn Mawr College101 N. Merion AvenueBryn Mawr, PA 19010, [email protected]

ABSTRACT

Nebel, S.H.; Trembanis, A.C., and Barber, D.C., 2012. Shoreline analysis and barrier island dynamics: Decadal scalepatterns from Cedar Island, Virginia. Journal of Coastal Research, 28(2), 332–341. West Palm Beach (Florida), ISSN0749-0208.

Aerial photography, topographic maps, high-resolution satellite imagery, and global positioning system (GPS) data werecompiled in ArcMAPTM and analyzed using the Digital Shoreline Analysis System (DSAS) to determine decadal trends ofshoreline movement on Cedar Island, Virginia. Shoreline retreat rates for Cedar Island had an alongshore average of24.1 m/y based on simple endpoint analysis (1852–2007), while the short-term (1994–2007) retreat rates increased to212.6 m/y. Retreat statistics were further calculated for the time intervals 1852–1910 (25.1 m/y), 1910–62 (23.5 m/y),1962–80 (23.9 m/y), 1980–94 (26.5 m/y), 1994–2002 (212.4 m/y), and 2002–06 (213.8 m/y). This analysis indicates thatretreat of the Cedar Island shoreline has been accelerating with a notable increase in rate within the years 1980 to 1994.Additionally, the shoreline data confirms that the orientation of the Cedar Island shoreline has rotated through time.

ADDITIONAL INDEX WORDS: Barrier islands, erosion rate, Virginia, coastal retreat, shoreline mapping,Wachapreague, Eastern Shore.

INTRODUCTION

Barrier islands, defined by Komar (1976) as ‘‘an unconsoli-

dated elongate body of sand or gravel lying above the high-tide

level and separated from the mainland by a lagoon or marsh,’’

front much of the U.S. Eastern and Gulf coast shoreline. These

fragile settings are subject to degradation from both natural

and anthropogenic processes (Davis, 1994; Leont’yev and

Nikiforov, 1973). The physiography of barrier islands reflects

a delicate balance among sea level, sand supply, waves, wind,

and tidal currents (Davis, 1994; Robertson, Zhang, and Whit-

man, 2007). The study of how barrier islands have moved in the

recent past (,150 y) will improve understanding of how these

features might behave in the future. It is particularly

important to quantify and understand barrier island move-

ment given global climate change and associated sea-level rise

(Cooper and Pilkey, 2004; Leatherman, 1983; Zhang, Douglas,

and Leatherman, 2004). Given that major population centers

are located along the coast, it is pivotal to understand the long-

term trends that drive shoreline movement.

Cedar Island is an uninhabited barrier island located on the

southern end of the Delmarva Peninsula in Virginia (Figure 1).

The island is part of the larger Virginia Barrier Island chain

and has historically been transgressing landward at a rapid

rate. The retreat rates reported for the Virginia Barriers have

been much higher than the retreat rates reported for the

remainder of the Mid-Atlantic Barrier Islands (Dolan et al.,

1979b). Furthermore, the undeveloped nature of the Virginia

coastline provides a unique opportunity to study barrier

islands in a setting that is virtually absent of human influence.

The high retreat rate of Cedar Island means that the observed

signal of shoreline change far exceeds both noise of natural

variability and observational/measurement errors. These

characteristics were exploited in the present study, allowing

us to document an acceleration of the island’s shoreline retreat

rate in the latter part of the 20th century AD. The results from

Cedar Island are compared to a similar study conducted on

neighboring Parramore Island (Figure 1) by Richardson and

McBride (2007).

STUDY AREA

The Virginia Barrier Islands are classified as mixed-energy

and tide-dominated islands (Oertel, Allen, and Foyle, 2008). The

coast is mesotidal (mean tidal range ,2 m) and has an annual

mean wave height of 0.55 m (Davis and Fox, 1978; Demarest

and Leatherman, 1985; Fenster and Dolan, 1996; Oertel, Allen,

and Foyle, 2008). The islands are short in length because of the

prevalence of tidal inlets. These inlets overlie paleochannels and

remain relatively stable, and the islands are found in former

interfluves (Fenster and Dolan, 1996; Halsey, 1979).

Cedar Island (Figure 1) is approximately 11.5 km long, lies at

an oblique angle relative to the mainland shoreline, and is

DOI: 10.2112/JCOASTRES-D-10-00144.1 received 22 September 2010;accepted in revision 28 January 2011.Published Pre-print online 17 June 2011.’ Coastal Education & Research Foundation 2012

Journal of Coastal Research 28 2 332–341 West Palm Beach, Florida March 2012

bounded to the north by Metompkin Inlet and to the south by

Wachapreague Inlet. The northern two-thirds of the island

have a well-developed back barrier salt marsh, while the

southern one-third lacks marsh and directly abuts the lagoon

(Gaunt, 1991). An ephemeral inlet located about 2.4 km north

of Wachapreague Inlet and just south of the marsh-backed

portion of the island opens and closes on fairly frequent time

scales (Newman and Munsart, 1968). The majority of the island

is less than 3 m above sea level (Newman and Munsart, 1968).

As a result of this low relief, the island displays extensive

washover caused by hurricanes, tropical storms, and nor’ea-

sters (Gaunt, 1991; Newman and Munsart, 1968). Recent

relative sea level rise estimated from local and regional tide

gauges is 3.1 mm/y (Gaunt, 1991).

Parramore Island, located just south of Cedar Island

(Figure 1), is 12.9 km long and is bounded to the north by

Wachapreague Inlet and to the south by Quinby Inlet.

Parramore is a drumstick-shaped barrier with a wide northern

end (Oertel and Kraft, 1994). The island is at maximum 9.1 m

above sea level (Newman and Munsart, 1968; Rice, Niedoroda,

and Pratt, 1976). In contrast to Cedar Island, Parramore has a

well-developed maritime forest at its northern end consisting of

conifers and hardwood trees (Shao et al., 1998). The southern

end of Parramore is low in profile and characterized by storm

washover features (Richardson and McBride, 2007). Italian

Ridge, a NE trending dune ridge, is located in the northern

section of the island. The cross-bedded sands of the ridge are

thought to be Holocene deposits, perhaps formed during an

earlier period of island progradation (Newman and Munsart,

1968).

PREVIOUS WORK

Generally, sediments that supply the U.S. East Coast

beaches are introduced to the coastal system from inland by

river discharge, are carried in from offshore sources by wave

action, or can be delivered as longshore drift after being

reworked from eroding areas of the coast. Sediments derived

from upland areas generally do not make it to the mid-

Atlantic coast because of estuarine trapping or sediment

trapping by dams (Honeycutt and Krantz, 2003). There is no

documented headland to supply the study area; therefore,

sediments supplying the Virginia Barrier Islands appear to

be derived primarily from the reworking of relict offshore

deposits (Demarest and Leatherman, 1985). Sediments

derived from erosion of the shoreface come from Holocene

and pre-Holocene sedimentary units (Demarest and Leather-

man, 1985).

A concave erosion arc termed Chincoteague Bight (Oertel,

Allen, and Foyle, 2008) extends 35 km in length from the

northern end of Wallops Island to the southern end of Cedar

Island. There is a 10-km landward offset in the shorelines

between the southern end of Assateague Island and the

northern end of Wallops Island. The shoreline of Parramore

Island is offset to the east (seaward) from the Cedar Island

shoreline by approximately 1 km, marking the southern

termination of the Chincoteague Bight erosion arc (Oertel,

Allen, and Foyle, 2008). Erosion of the islands located in the

Chincoteague Bight (Wallops to Cedar) has been thought to

provide sediment to the downdrift islands located south of the

arc (Rice and Leatherman, 1983).

METHODS

Shoreline Data Sources

The Cedar Island shorelines for years 1852, 1910, 1959–62,

and 1980 were delineated using National Oceanographic

Service (NOS) topographic sheets (T-Sheets). The 1994

shoreline was digitized using the digital orthophoto quadrangle

from the United States Geological Survery (USGS), the 2002

shoreline was digitized from orthophoto mosaics from the

Virginia Geographic Information Network, and the 2007

shoreline was digitized from a set of georeferenced aerial

photographs using the high water line (HWL) as a reference

point.

The HWL is defined as ‘‘the landward extent of the last

high tide’’ and was utilized by early NOS topographers to

delineate shorelines (Crowell, Leatherman, and Buckley,

1991). National Oceanographic Service T-Sheets list the

shoreline as mean high water level (MHWL); however, early

surveyors used an approximation of the MHWL as this line

averages the HWL over a 19-y period. Nevertheless, the

difference between the MHWL and the HWL is often small

Figure 1. Cedar and Parramore Islands, part of the greater Virginia

Barrier Islands.

Shoreline Analysis, Cedar Island 333

Journal of Coastal Research, Vol. 28, No. 2, 2012

(Crowell, Leatherman, and Buckley, 1991). The HWL was

the most visible indicator of shoreline in the aerial photos

and was chosen for consistency for the different survey types

(aerial photos, satellite imagery, global positioning system

[GPS] shoreline surveys). On aerial photographs, HWL is the

line on the beach demarcated by color change (Crowell,

Leatherman, and Buckley, 1991).

Differential Global Position System (DGPS) shoreline sur-

veys were conducted on Cedar Island on June 28, 2007, using a

backpack-mounted GPS unit. Surveys were performed by

tracing the HWL on foot during the bottom half of the tide

cycle using a RangerTM TCSe Data Collector and TrimbleH

ProXRTM receiver. The GPS data were postprocessed and

differentially corrected using the Pathfinder Office (v. 3.10)

software package.

IKONOS satellite imagery was gathered for the study area

on July 2, 2006. This imagery covered Cedar Island, and the

shoreline was digitized from the georeferenced imagery using

the HWL as a reference for the shoreline.

All shoreline data were imported and analyzed using the

geographic information system ArcViewTM (Esri). For the

majority of the earlier datasets, the date of initial survey was

not known. Digital Shoreline Analysis System (DSAS) requires

that a date be associated with each shoreline shapefile in order

to calculate rate of change statistics. The earliest date possible

for the given year (January 1) was chosen when the shoreline

survey date was not known.

Error

Shoreline error was determined using estimates of error

from the original source (e.g., T-Sheet, aerial photo, satellite

imagery, GPS shoreline survey). For the earlier shorelines

(1852, 1910, 1959–62, 1980, and 1994) we used error estimates

calculated by Crowell, Leatherman, and Buckley (1991;

Table 1). An unknown sketching error was introduced by

NOS surveyors when connecting the HWL between surveyed

points. This sketching error is estimated to be small on many

maps (Crowell, Leatherman, and Buckley, 1991). It is impor-

tant to note that the error estimates are conservative, and that

error, in most cases, is lower than reported by Crowell,

Leatherman, and Buckley (1991). The error of the 2002

aerial-photo dataset was provided by the Virginia Geographic

Information Network. The positional error of the 2006

IKONOS satellite imagery was provided by eMap international

(data provider). These accuracy measurements are summa-

rized by year and data type in Table 2.

To translate these error measurements reported in m to an

error rate in m/y, we assumed the worst-case error estimate

for the earliest and latest shoreline we were analyzing and

divided this by the time elapsed. For example, to calculate an

error rate for the time period 1852–1910, we assume an error

for the 1852 shoreline of 8.9 m and an error for the 1910

shoreline of 8.4 m. The sum of this error, representing the

displacement of the earliest shoreline seaward by 8.9 m and

the latest shoreline landward by 8.4 m, was divided by the

time elapsed to yield an error measurement in m/y. This

method was repeated for all shoreline time intervals and is

summarized in Table 3.

The Digital Shoreline Analysis System

We evaluated Cedar Island shoreline data using the DSAS

version 3.0 developed by Thieler et al. (2005). Digital shoreline

analysis system is a freely available extension that runs in

ArcViewTM, a geographic information system (GIS). This

extension calculates rate of change statistics from shapefiles,

which are vector files with geospatial attributes. For the

purpose of this study, this extension has been used in order to

calculate rate of change statistics from shoreline data.

The modern day mainland shoreline (on landward side of the

lagoon) was used as the baseline for the DSAS rate of change

calculations. Transects were cast seaward every 25 m along the

baseline. We follow standard convention wherein negative

shoreline retreat values indicate shoreline erosion, while

positive values indicate shoreline accretion. After studying

the initial shoreline retreat rates, we decided to exclude two

separate segments of the Cedar Island shoreline from our long-

term retreat rate study. First, spit accretion and progradation

has extended the north end of Cedar Island into Metompkin

Inlet during the study period, thereby yielding anomalously

high shoreline accretion rates (transect number 1–92)

(Figure 2). Furthermore, while this section of the island was

connected to Cedar Island during our initial (2006) field survey,

by the time of our return survey (June 2007) an inlet had

formed, and this northern spit had been severed from Cedar

Island. The second island segment from which transects were

excluded from our retreat-rate calculations is the area of an

ephemeral inlet located approximately 3 km north of Wacha-

preague Inlet (transect number 341–369) (Figure 2). The local

inlet dynamics (opening, closing, and migrating) make both of

these areas prone to abnormally high erosion and accretion

rates that would overwhelm the rates calculated elsewhere

along the island. For comparison we provide the results of

calculations both with and without transects within these

ephemeral inlet zones, but we omit them from discussion of our

overall findings.

Shorelines were analyzed by applying the DSAS method

over varying timescales. Initially, all the shoreline data (1852–

2007) were compiled and analyzed together. Long-term (1852–

2007) and short-term (1994–2007) retreat rates were also

calculated. Each distinct interval was also analyzed separately

(1852–1910, 1910–62, 1962–80, 1980–94, 1994–2002, and

2002–06) in order to determine whether the erosion rates have

accelerated or decelerated between the individual time spans.

The shoreline statistics from 2006–07 were not calculated

because of the large error (610.2 m/y) associated with the

dataset. The IKONOS satellite imagery (2006) has the greatest

Table 1. Error estimates for geospatial data reported by Crowell,

Leatherman, and Buckley (1991).

Data Type Estimated Error

T-Sheets, 1844–1880 8.9 m + sketching error

T-Sheets, 1880–1940 8.4 m + sketching error

Recent maps using aerial photos 6.1 m + inaccurate location of HWL

Aerial photos, 1840–present 7.5–7.7 m + inaccurate location of

HWL

334 Nebel, Trembanis, and Barber


source error (610 m). The 2007 GPS shoreline has a small

position error of 60.2 m. Additionally, statistics were calcu-

lated on the 1962–2007 shoreline data to further investigate

the hypothesis of accelerated shoreline retreat during the

recent past.

RESULTS

Long-Term (1852–2007) and Short-Term (1994–2007)Shoreline Retreat on Cedar Island

The average long-term (1852–2007) retreat rates for the

Cedar Island shoreline are provided in Table 4 both with and

without the retreat rates at the anomalous northern and south-

central portions of the island, as noted previously in the

Methods section. Results calculated using both the endpoint

rate (EPR) and linear regression rate (LRR) are provided in

Table 4 and are plotted against alongshore position in

Figure 3. The short-term (1994–2007) EPR and LRR values

for Cedar Island are given in Table 5, and these values are

plotted alongshore for the entire island in Figure 4.

Endpoint Rates for Individual Time Intervals

The EPR for the individual time intervals are presented in

Table 6, and the alongshore EPR values for the individual time

steps are shown in Figure 5. It is important to note that the

EPR is the only statistic calculated by DSAS if there are only

two shorelines in the analysis. The shoreline retreat rate

accelerates notably during the years 1980–2002 (Table 6;

Figure 5), and EPR values after 1994 are roughly triple those

calculated for the pre-1980 time period. This acceleration is

further confirmed in Table 7, in which the shoreline retreat

data is partitioned into roughly equal time periods (1852–1910,

1910–62, 1962–2007). Erosion hotspots, shoreline segments

that are prone to higher erosion and/or accretion rates than

surrounding areas, also are visible in Figure 5. In particular,

the hotspot near the north end of Cedar Island occurs

throughout the 154-y study period.

DISCUSSION

Island Erosion Patterns

Previous studies of the erosion patterns of the Virginia Barrier

Islands have subdivided the Virginia shoreline into three

distinct segments based on observed erosion patterns (Leather-

man, 1983; Rice and Leatherman, 1983). These previous studies

observed that the shorelines from Wallops to Cedar Island

underwent parallel beach retreat. The islands from Parramore

to Wreck displayed rotational instability where one end of the

island was prone to erosion while the opposite end underwent

accretion (Rice and Leatherman, 1983). The islands south of

Wreck were found to undergo nonparallel retreat, in which

different islands in this southern group eroded at different rates.

The findings presented here indicate that Cedar Island has

transitioned from a state of parallel beach retreat to a state

characterized by counterclockwise shoreline rotation. The

shoreline data from Cedar Island (1952–2006) shows this

rotation (Figure 6). This trend was further observed when we

averaged the endpoint rate over long (1852–2007) and short

time scales (1994–2007) every 1500 m on Cedar Island

(Figure 7). The highest erosion rates were observed at the

north end of Cedar Island and in the area of the ephemeral

inlet, while the lowest rates were observed just north of the

ephemeral inlet and on the southern end of the island. Given

the data presented in Figure 7, it appears that separate

counterclockwise rotation is occurring on north and south

Cedar Island. While the rates were higher over the short-term

time period, this trend is also observed in the long-term data as

well. However, the rates calculated in section 4, just north of

the ephemeral inlet, were similar at 25.9 (short term) and 25.4

Table 2. Shoreline dataset information.

Shoreline Name Date of Collection Source Notes

Cedar_1852 01/01/1852 NOS T-Sheet N Actual date of survey not known

N Accuracy: 68.9 m1


N Accuracy: 68.4 m1

Cedar_1959_1962 01/01/1962 NOS T-Sheet N Actual date of survey not known

N Survey spanned several years, therefore

latest date was reported in DSAS

N Accuracy: 66.1 m1


N Accuracy: 66.1 m1

Cedar_1994 01/01/1994 Aerial photo N Actual date of flight not known

N Accuracy: 67.6 m1

N Resolution: 1 m

Cedar_2002 01/01/2002 Aerial photo N Actual date of flight not known

N Accuracy: 63.7 m3

N Resolution: 0.6 m3

Cedar_shore2006jul 07/02/2006 IKONOS Satellite N Accuracy: 610 m2

N Resolution: 1 m

Cedar_6_28_07 06/28/2007 GPS survey N Accuracy: 620 cm

Accuracy estimate sources:1 Crowell, Leatherman, and Buckley, 1991.2 B.A. McCarty, personal communication.3 S. Blankenship, personal communication.



(long term) m/y. The northern spit on Cedar Island was

excluded from this analysis because it accreted after 1994,

thereby yielding the same average retreat values for the long-

and short-term time intervals.

Parramore Island has likewise switched erosion patterns

since 1974. Richardson and McBride (2007) found that

Parramore Island now displays parallel beach retreat as

opposed to the rotational instability that Rice and Leather-

man (1983) observed. Tide-dominated barriers exhibit

rotational retreat behavior more frequently than wave-

dominated barrier islands (Hayes, 1979). The switching

between rotational- and parallel-shoreline retreat behav-

iors that we observe may be facilitated by the marginally

tide-dominated, mixed-energy regime of the Virginia Bar-

rier Islands.

Long-Term Shoreline Retreat on Cedar Island

The long-term (1852–2007) retreat rates calculated for Cedar

Island in this study are similar to those calculated in previous

studies. In this study, the long-term EPR ranged between

24.1 m/y (all shoreline data) and 24.9 m/y (omitting anoma-

lous shoreline segments). These values closely agree with the

Cedar Island retreat rates of about 24 to 25 m/y calculated for

the period 1852–1974 by Dolan, Hayden, and Jones (1979a),

Leatherman, Rice, and Goldsmith (1982), and Rice and

Leatherman (1983). Similarly, Gaunt (1991) found that Cedar

Island had retreated at a rate of 24.38 m/y within the years

1910–86, and Rice, Niedoroda, and Pratt (1976) reported an

average retreat rate of 25.12 m/y for Cedar Island within the

years 1852–1968.

Gaunt (1991) observed that the shore-normal width of Cedar

Island narrowed by 2.1 m/y between 1910 and 1986, such that

the subaerial extent of the island decreased by 32% over that

76-y interval, apparently attributable to lost barrier sediment

volume. The rate of thinning accelerated from 0.6 m/y during

the 1910–62 interval up to 4.0 m/y within the years 1962–86.

Gaunt (1991) attributed the post-1962 changes, which were

most notable at the northern end of Cedar Island, to changes on

Metompkin Island just to the north. Rates of thinning were not

directly measured in this study; however, given the availability

of digital aerial imagery, these rates could be calculated in the

future. The increase in the Cedar Island erosion rate within the

years 1994–2007 indicates that this system continues to lose

sediment, yet storm washover processes that move sediment

toward the marsh are also active.

Short-Term Shoreline Retreat on Cedar Island

This work indicates that the Cedar Island shoreline retreat

rate was much more rapid (212.6 m/y) in the last decade and a

Table 3. Error estimate by time period.

Time Period

Time Elapsed

(y)

Source Error,

Earliest

Shoreline (m)

Source Error,

Later

Shoreline (m)

Total Error

over Time

Elapsed (m/y)

1852–1910 58 8.9 8.4 60.29

1910–1962 52 8.4 6.1 60.27

1962–1980 18 6.1 6.1 60.67

1980–1994 14 6.1 7.6 60.97

1994–2002 8 7.6 3.7 61.41

2002–2006 4 3.7 10 63.45

1852–2007 155 8.9 0.2 60.05

1994–2007 12 7.6 0.2 60.65

1959/62–2007 45 6.1 0.2 60.14

Figure 2. Cedar Island map depicting the areas influenced by inlet

dynamics and spit growth (hatched box). The retreat rates calculated by

transects that fell within the hatched boxes were removed. The rates

excluding those in the hatched boxes are presented in the Results section

under the heading ‘‘Without Transects 1–92 and 341–369.’’ The 1852 and

2007 shorelines are included for reference.

Table 4. Long-term (1852–2007) shoreline statistics calculated for

Cedar Island.

All Values

Without Transects,

1–92 and 341–369

Potential

Error

EPR 24.1 m/y 24.9 m/y 60.05 m/y

Standard

deviation of

EPR

16.3 m/y 3.0 m/y

LRR 25.3 m/yr 24.7 m/y 60.05 m/y

Standard

deviation of

LRR

7.2 m/y 3.3 m/y

R2 for LRR 0.844 0.897



half (1994–2007) than the average rate (24.6 m/yr) over the

entire study period (1852–2007). This increased erosion rate

over the short term might be attributable to an increase in

tropical storm activity in the region beginning in the 1970s

(Riggs and Ames, 2007). Storms contribute to barrier island

erosion, yet following the storm sediment begins to accrete,

often after a few tidal cycles (List, Farris, and Sullivan, 2006).

This process is slow, requiring several years to accrete back to

the prestorm position (Morton, Paine, and Gibeaut, 1994). If a

second storm impacts the area before it is able to fully recover,

erosion will continue.

The short-term rates of erosion on neighboring Parramore

Island also accelerated in recent decades, though to a lesser

degree than those for Cedar Island (Richardson and McBride,

2007). The long-term (1952–2007) rates of erosion on Parra-

more Island were approximately 23.6 m/y, and this rate more

than doubled over the short term (1994–2007) to 28.8 m/y

(Richardson and McBride, 2007).

Effect of Inlet Breaching on Downdrift Shorelines

The opening and closing of the ephemeral inlet located on

southern Cedar Island may have an effect on erosion and

Figure 3. Long-term (1852–2007) endpoint rate and linear regression

rates alongshore for Cedar Island. The hatched boxes represent the rates

that were removed from the second analysis (Transects 1–92 and 341–369).

Note the northern end of Cedar Island accreted north into Metompkin Inlet

during this time period, therefore, high accretion rates (up to +178 m) were

recorded at the northern end of the island. The graph showing the

alongshore EPR was truncated at 40 m and therefore does not show these

high rates.

Table 5. Short-term (1994–2007) shoreline statistics calculated for

Cedar Island.

All Values

Without

Transects, 1–92

and 341–369 Potential Error

EPR 212.6 m/y 212.6 m/y 60.65 m/y

Standard

deviation of

EPR

19.6 m/y 8.9 m/y

LRR 214.0 m/y 212.8 m/y 60.65 m/y

Standard

deviation of

LRR

12.0 m/y 8.9 m/y

R2 for LRR 0.865 0.916

Figure 4. Short-term (1994–2007) endpoint rate and linear regression

rates alongshore for Cedar Island. The hatched boxes represent the rates

that were removed from the second analysis (Transects 1–92 and 341–369).

Note the northern end of Cedar Island accreted north into Metompkin Inlet

during this time period, therefore, high accretion rates (up to +178 m) were

recorded at the northern end of the island. The graph showing the

alongshore EPR was truncated at 40 m and therefore does not show these

high rates.



accretion rates on Northern Parramore Island located downdrift

of Cedar Island. The northern end of Parramore Island, which

historically has been fairly stable, eroded to the south between

1955 and 1963, occurring contemporaneously with the breaching

of the ephemeral inlet on southern Cedar Island (Rice,

Niedoroda, and Pratt, 1976). The increased erosion rates,

however, may also be attributable to storms. Storms like the

Ash Wednesday Nor’easter might have caused an increased tidal

discharge through Wachapreague Inlet during this time period,

eroding Parramore Island (Rice, Niedoroda, and Pratt, 1976).

The natural opening of the ephemeral inlet on southern

Cedar Island is believed to have resulted from a January 1956

storm; this inlet subsequently closed in 1961 (Rice, Niedoroda,

and Pratt, 1976). This inlet was reopened by the May 1962

storm but closed quickly thereafter in 1963 (Rice, Niedoroda,

and Pratt, 1976). This inlet was open during our initial field

survey in July 2006 and closed during the time between our

surveys in September 2006 and June 2007. The shoreline data

further indicates that this inlet was open in 1994 and 2002, but

whether it remained open during this time period or underwent

a series of openings and closings is unknown.

The 1994 data (Figure 6) shows that the Cedar Island

shoreline was roughly straight. The inlet open in 1994 was

located just south of the marshbacked portion of the island.

Eight years later in 2002, the inlet was located about 750 m

south of the position of the 1994 inlet. The shoreline just south

of the northern marsh-backed portion of the island had

retreated at a higher rate than the northern shoreline. This

trend is further captured in the short-term data presented in

Figure 7. Section 4 (marsh-backed portion of the island) eroded

at a rate of 25.4 m/y over the short term, while section 5

(ephemeral inlet) eroded at a rate of 223.0 m/y. Section 6, just

south of the ephemeral inlet, had a slightly higher erosion rate

of 223.3 m/yr. Given that the overall longshore current

direction in this area is to the south (Leatherman, Rice, and

Goldsmith, 1982), sediment may be traveling through the

ephemeral inlet, starving the coastline just to the south.

Cedar Island Variability Alongshore

The retreat rates on Cedar Island are highly variable

alongshore, and the shoreline is subject to differential erosion

patterns (Figures 3, 4, 5, and 7). The long-term retreat rates

(Figure 3) for Cedar Island show that the island, for the most

part, eroded during 1852–2007. The highest erosion rates

(approximately 26 to 28 m/y) are found on the northern and

south-central portion (in the area of the ephemeral inlet) of the

island (Figure 7). The erosion rates in the middle and southern

end of the island (sections 4 and 7, Figure 7) are lower

(approximately 25.4 m/y).

The short-term retreat rates (Figure 4) show higher vari-

ability in erosion rates within 1994–2007. Furthermore, the

erosion rates over the short term are much higher than the

erosion rates over the long term. The highest erosion rates

during the short-term study period were, once again, found to

the north, just south of the accreted spit (224.6 m/y) and in the

region of the ephemeral inlet (223.0 and 223.3 m/y). The

opening of the ephemeral inlet on South Cedar Island

contributes to the high erosion rates observed to the south.

Over the short term the center of the island (section 4,

Figure 7) displayed near identical retreat rates to those

Table 6. Endpoint retreat rates for the individual time intervals.

Time Interval

All Values Without Transects, 1–92 and 341–369

Potential Error (m/y)EPR (m/y) Standard Deviation (m/y) EPR (m/y) Standard Deviation (m/y)

1852–1910 24.1 63.5 25.1 62.7 60.29

1910–1962 23.0 61.9 23.5 61.5 60.27

1962–1980 23.8 64.3 23.9 64.1 60.67

1980–1994 26.3 67.8 26.5 67.2 60.97

1994–2002 215.6 619.4 212.4 612.9 61.41

2002–2006 211.4 618.1 213.8 68.9 63.45

Figure 5. Earlier alongshore endpoint retreat (top) in comparison to the

more recent endpoint retreat (bottom). Acceleration of shoreline retreat in

the recent past is visible, as are erosion and accretion ‘‘hotspots.’’



calculated over the long term (25.9 and 25.4 m/y, respective-

ly). The southern end of Cedar Island has lengthened and

shortened over the study period perhaps as a result of inlet

dynamics. High accretion rates at the northern end of Cedar

Island (Figures 3–5) document the accretion of the northern

spit into Metompkin Inlet occurring during 1994–2007.

Figure 5 documents the alongshore retreat rates for Cedar

Island over the individual time periods (1852–1910, 1910–62,

1962–80, 1980–94, 1994–2002, 2002–06). The rates presented

in these two graphs show the acceleration of shoreline retreat

on Cedar Island since 1980. A long-term erosion hotspot, which

represents the flattening of the northern shoreline, has existed

on the north end of Cedar Island throughout the 154-y study

period. The 1910 and 1962 shoreline (Figure 6) is curved and

offset to the east from the trend of the remainder of the

shoreline. Additionally, the alongshore retreat data show that

the north and south end of Cedar Island is prone to higher rates

of retreat when compared to the center of the island.

Changes on Cedar Island have been linked to shoreline

changes occurring on Metompkin Island to the north (Rice,

Niedoroda, and Pratt, 1976). Given that Metompkin Island lies

updrift and the prominent longshore current direction in this

area is to the south (Leatherman, Rice, and Goldsmith, 1982),

an interruption in this sediment flow patterns to the north

would affect islands that lie to the south. For example, within

the years 1942–86, the highest retreat rate (24.0 m/yr) on

Cedar Island was measured on the north end (Gaunt, 1991). A

noted acceleration of erosion on northern Cedar after 1962

occurred contemporaneously with an inlet breach on Metomp-

kin Island. Longshore sediment transport was interrupted

leading to the increased retreat rate observed on Northern

Cedar Island (Gaunt, 1991).

Factors Contributing to High Erosion Rates

Cedar and Parramore Islands are retreating at a rate that is

anomalously high for the mid-Atlantic shoreline. Cedar Island’s

Table 7. Temporally portioned shoreline retreat rates.

Time Period Years Elapsed EPR (m/y) LRR

1852–1910 58 24.1 Not calculated

1910–1962 52 23.0 Not calculated

1962–2007 45 27.7 27.15 m/y

Figure 6. Cedar Island shoreline data derived from aerial photos, GPS

shoreline surveys, T-Sheets, and high-resolution satellite images. A

rainbow color ramp is used, so the earliest shorelines are shown in

‘‘hotter’’ colors (red/orange), and the most recent shorelines are shown in

‘‘cooler’’ colors (blue/purple). The rotation of the Cedar Island shoreline is

evident in these data, particularly on the southern part of the island. A

color version of this figure is available in the online journal.

Figure 7. Average long- and short-term erosion and accretion rates

(reported by section, indicated in map to the left) calculated for Cedar

Island in this study. Short-term and long-term rates are indicated in black

and gray, respectively. Section retreat rates were calculated using the

average EPR over 1500 m spacing along Cedar Island.



long-term retreat rate was calculated in this study to be

approximately 24 to 25 m/y. Furthermore Cobb Island, located

at the southern end of the Virginia Barrier Island chain, has

eroded at a rate of 23 to 26 m/y over a 150-y study period

(Oertel et al., 1989). In comparison to Virginia, the long-term

(1800–2000) retreat rate for coastal North Carolina, located

directly south of the Virginia Barrier Islands, was calculated to

be approximately 20.7 m/y (Morton and Miller, 2005). There are

clearly factors contributing to the high rates of erosion

measured throughout the Virginia Barrier Island chain.

As previously stated, the Chincoteague Bight is an erosion

arc that extends from Wallops to Cedar Island, Virginia. The

section of coast from Wallops Island to Cedar Island is

underlain by silty clay; this could explain the increased erosion

rate (Dolan, Hayden, and Lins, 1980). Fishing Point, found on

the southern end of Assateague Island just north of Wallops

Island, acts as a natural sediment trap. This feature has grown

at a rate of 6.5 km/century, and, as a result, the islands south of

Assateague Island erode because of the removal of sediment

from the natural littoral system (Rice and Leatherman, 1983).

The sediment starvation resulting from the trapping of

sediment at Fishing Point results in accelerated erosion on

the northern Virginia Barrier Islands (Wallops to Cedar

Island). The central and southern islands receive the sediment

eroded from the northern islands and therefore retreat at a

lesser rate (Rice and Leatherman, 1983). This observation

could explain why the retreat rates calculated in this study for

Cedar Island are higher than those observed by Richardson

and McBride (2007) on Parramore Island. Cedar Island, located

within Chincoteague Bight, is sediment starved. The erosion of

Cedar Island provides sediment to the islands located to the

south and they, therefore, erode at a slower rate.

CONCLUSIONS

Cedar Island, Virginia, is a transgressive barrier island

located on the eastern shore. Shoreline data indicate that

Cedar Island has been eroding and retreating landward at

different rates over the 155-y study period (1852–2007). The

long-term shoreline data (1852–2007) indicate that Cedar

Island retreated at a rate of 24.1 m/y. The retreat rate over

the short-term time scale (1994–2007) accelerated to

212.6 m/y. The average long- and short-term erosion rates

at 1500 m intervals along Cedar Island are summarized in

Figure 7. The data presented in this figure represents the

average EPR along seven sections of Cedar Island. These

data show that the highest erosion rates on Cedar Island are

recorded at the northern end of the island near Metompkin

Inlet and in the area of the ephemeral inlet to the south.

Sediment travels through the inlets and is trapped in the

lagoon behind the island, and then the areas to the south

erode.

The data from Cedar Island agree with a shoreline change

study on neighboring Parramore Island, which is located just

south of Cedar Island. Richardson and McBride (2007)

observed that Parramore Island has also been retreating. The

long-term (1852–2006) retreat rate on Parramore Island was

23.6 m/y while the short-term (1998–2006) retreat rate

accelerated to 28.8 m/yr.

The large-scale alongshore spatial patterns of retreat

observed on Cedar Island by Rice and Leatherman (1983)

have changed. Rice and Leatherman (1983) observed that

Cedar Island displayed parallel-beach retreat, while Parra-

more Island displayed a rotational instability. The shoreline

data for this study show that Cedar Island has developed a

dual rotational instability, wherein the northern and

southern sections of the island rotate counterclockwise.

Likewise, Richardson and McBride (2007) found that the

Parramore Island shoreline is now undergoing parallel-

beach retreat.

In general, the causes of shoreline change on barrier islands

and in coastal environments are multifaceted and complex.

Future work in the area will hopefully categorize sediment

found offshore of Cedar Island to determine if onshore erosion

patterns are affected by offshore sediment sources.

ACKNOWLEDGMENTS

We thank the University of Delaware Research Foundation

for supporting this project. We would also like to thank Donna

Milligan and Scott Hardaway at Virginia Institute of Marine

Science (VIMS) for sharing their shoreline data, and Mark

Luckenbach, P.G. Ross, Sean Fate, and the staff at the VIMS

Eastern Shore Lab for sharing their equipment and expertise.

Additionally, we would like to thank Nathan Maier, Elizabeth

McCarty, Adam Skarke, and Hilary Stevens for their help

collecting data in the field.

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Shoreline Analysis and Barrier Island Dynamics: Decadal Scale Patterns from Cedar Island, Virginia