Vulnerability of National Park Service Beaches to Inundation during a 

Direct Hurricane Landfall: Cumberland Island National Seashore

Hilary F. Stockdon, David M. Thompson, and Laura A. Fauver

U.S. Geological Survey

FISC-St. Petersburg

600 4th Street S.

St. Petersburg, FL 33701

Abstract

Cumberland Island National Seashore, a barrier-island coastal 

park in Georgia, is vulnerable to the powerful, sand-moving 

forces of hurricanes. Waves and storm surge associated with 

these strong tropical storms are part of the natural process of 

barrier-island evolution and can cause extensive morphologic 

changes in coastal parks, leading to reduced visitor accessibility 

and enjoyment. The vulnerability of park beaches to inundation, 

and associated extreme coastal change, during a direct hurricane 

landfall can be assessed by comparing the elevations of storminduced mean-water levels (storm surge) to the elevations of 

the crest of the sand dune that defines the beach system. Maps 

detailing the inundation potential for Category 1-5 hurricanes 

can be used by park managers to determine the relative 

vulnerability of various barrier-island parks and to assess which 

areas of a particular park are more susceptible to inundation and 

extreme coastal changes.

1. Introduction

Along much of the East Coast of the United States, hurricanes 

have been responsible for some of the most dramatic changes to 

our coastal environments – from the creation of large overwash 

deposits to the opening of new inlets. Strong winds associated 

with these tropical storms bring large waves and storm surge 

that force significant changes on fragile barrier islands, where 

the balance between island stability and rising water levels is 

maintained by dynamic changes in beach morphology. On 

developed barrier islands, hurricane-induced coastal change 

makes local infrastructure more vulnerable to failure. Even 

in our national coastal parks, where development is strictly 

managed, hurricanes threaten to destroy infrastructure that keeps 

the parks operational (e.g., unpaved roads, communications, 

park buildings) as well as to alter dramatically the beaches 

and dunes that provide visitors with the opportunity to observe 

wildlife and experience the interaction of the land and the sea.

The impact of a hurricane on a beach has been shown to be 

highly variable over both large and small stretches of coast 

(Stockdon et al., 2003) One section of beach may be largely 

unaffected by a storm, while an adjacent area experiences 

extreme coastal change associated with island overwash 

or breaching. This spatially variable response to storms is 

partly due to longshore variability of the pre-storm beach 

morphology combined with variability in the offshore physical 

forcing (Stockdon et al., 2007a). Using a storm-impact scaling 

model that compares the relative elevations of barrier-island 

morphology and storm-induced water levels, the potential 

vulnerability of a barrier island to extreme coastal change during 

a hurricane landfall can be defined (Sallenger, 2000). 

A complete and comprehensive management plan preparing for 

a hurricane landfall, as well as plans for post-hurricane recovery, 

should be based, in part, on an assessment of what areas of the 

coast are most vulnerable. In order to help several of our coastal 

National Parks prepare for a possible hurricane landfall, the 

USGS, at the request of the National Park Service (NPS), has 

prepared an analysis of the vulnerability of Cumberland Island 

National Seashore (NS) beaches to inundation during a direct 

landfall. The results can be used to assess what areas of the 

park are most susceptible to extreme coastal change during a 

hurricane. These findings can also be compared to those from 

other NPS coastal parks to determine, in a relative sense, which 

parks are most at risk during strong tropical storms. 

Cumberland Island NS is located in southern Georgia between 

the St. Mary’s River to the south and St. Andrews Sound and 

the Satilla River to the north (Fig. 1). The 30 km of coastal 

park include wide beaches, marshes, and maritime forest and 

are composed of two uninhabited barrier islands, Cumberland 

Island and Little Cumberland Island (private property). The 

width of Cumberland Island is extremely variable, ranging from 

approximately 5.5 km in the northern portion of the island to less 

than 1 km toward the southern end. The east-facing beaches are 

open to the Atlantic Ocean and are most susceptible to the forces 

of approaching hurricanes. 

2. Methods

2.1 Storm-Impact Scaling Model

A simple storm-impact scale that compares elevations of the 

most seaward sand dune to elevations of hurricane-induced 

water levels can be used to define the likely impact regime for 

an approaching hurricane (Sallenger, 2000). During a storm, 

the combined effects of 1) the astronomical tide, 2) storm surge 

(elevated water levels associated with the large winds and low 

pressures of a hurricane), and 3) wave runup (the super-elevation 

of the water surface at the shoreline due to waves, both the 

time-varying and time-averaged components) move the erosive 

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1387

October 2007
Figure 1. Location map for Cumberland Island National Seashore.

forces of the storm higher on the beach face than during typical 

wave conditions. The total elevation of these three parameters 

defines the maximum water level (Rhigh) attained during a storm, 

while the storm-induced mean-water level (Rlow) can be defined 

by only storm surge and wave setup. These forces may reach the 

elevation of the base (Dlow) and crest (Dhigh) of the most seaward 

sand dunes that define the landward limits of the beach system 

and represent the first line of defense for a barrier island in an 

approaching storm. Using these parameters, four storm-impact 

regimes, or thresholds for coastal change, maybe defined—

swash (Rhigh < Dlow), collision (Rhigh > Dlow), overwash (Rhigh > 

Dhigh), and inundation (Rlow > Dhigh) —to provide a framework for 

examining the general types and relative magnitudes of coastal 

change that are likely to occur during hurricanes (Sallenger, 

2000; Stockdon et al., 2007a). 

Here we focus on the most extreme of the four impact regimes, 

inundation, which occurs when the storm-induced meanwater level (Rlow) exceeds the elevation of the crest of the most 

seaward sand dune (Dhigh). Within this regime, the beach system 

(foredune ridge and beach) is completely submerged, and net 

landward transport of sediment is likely to occur (Sallenger, 


2000). Typically, larger magnitudes of shoreline retreat and 

beach erosion will occur when the beach is inundated due to the 

transport of sand in both the offshore and onshore directions 

(Stockdon et al., 2007a). 

The predictive accuracy of the scaling model was tested by 

hindcasting the likely impact of Hurricanes Bonnie and Floyd 

(Stockdon et al., 2007a) and of Hurricane Ivan (Stockdon et al., 

2007b). For Hurricane Ivan, the overall hindcast accuracy of the 

model in predicting one of the four regime types was 68%. The 

sensitivity of the model within the inundation regime was 51% 

(Stockdon et al., 2007b). Underprediction errors were likely 

due to profile evolution of the low-lying barriers during the 

landfall of this Category 4 storm. In order to use this model in a 

predictive mode to examine the potential for inundation during 

a future hurricane landfall, accurate estimates of both the dune 

parameters and the expected hurricane-induced mean-water level 

are needed.

2.2 Dune Elevation

The morphology of the beach and dunes at Cumberland Island 

NS was mapped based on an airborne lidar topographic survey 

conducted on January 16, 17, and 18, 2006, by the U.S. Army 

Corps of Engineers (USACE) CHARTS system. GPS-based 

lidar surveys provide an efficient method for collecting highresolution data of subaerial topography with sufficient accuracy 

(root-mean-square vertical accuracy = 15 cm) to resolve the 

spatial details of sand-dune elevation and position (Sallenger et 

al., 2003).

The elevation of the foredune crest (or, in the absence of a 

dune, the beach berm) was extracted every 20 m along the coast 

of Cumberland Island NS from cross-shore profiles of lidar 

topography (Fig. 2). An automatic algorithm was used to select 

the peak of the most seaward dune within a prescribed beach 

width (here, 175 m). The results were then manually checked 

to ensure the extraction of a consistent feature defining the 

landward extent of the beach system (Fig. 3). The cross-island 

width of Cumberland Island is very large in some areas of the 

park, allowing for large inland dune complexes. The stormimpact scaling model is focused on the vulnerability of the beach 

system, which is defined by the location of the most seaward 

dune. Accordingly, interior dunes, regardless of their elevation, 

were not considered in this analysis. 

2.3 Storm Surge

In this analysis, Rlow is represented only by the storm surge. 

Wave setup was not considered because predictions of wave 

conditions (height and period) for a generic hurricane of each 

category are not currently available. The predicted elevations of 

storm surge for Saffir-Simpson Category 1-5 hurricanes were 

extracted from the NOAA SLOSH (Sea, Lake, and Overland 

Surges from Hurricanes) model, a real-time forecast model 

for hurricane-induced water levels for the Gulf and Atlantic 

Coasts. The numerical model is based on linearized, depthintegrated equations of motion and continuity (Jarvinen and 

Lawrence, 1985). Storm surge is modeled by simulating the 

conditions of each category storm approaching the coast from 

different angles and at varying speeds. Changes in maximum 

surge elevations are forced by time-varying wind-stress and 

pressure-gradient forces that depend on hurricane location, 

minimum pressure, and size measured from the eyewall out 

to the location of maximum winds (Jarvinen and Lawrence, 

1985). The maximum surge within each grid cell is defined as 

the Maximum of the Maximum Envelope of Water (MOMs) 

and represents the worst-case, localized surge that will occur 

for landfall in a given location, not what would occur along the 

entire coast for a single storm. The results are location specific, 

accounting for local water depths, proximity to bays and rivers, 

etc., and are accurate to ± 20% of the calculated value (NOAA, 

2007). Errors in the SLOSH model can arise from differences 

between the parametric wind models, which force SLOSH, and 

the actual hurricane wind field (Houston et al., 1999), as well 

as discrepancies between the coarse model grid and the real 

topography and bathymetry over which the storm will travel.

For Cumberland Island NS, the maximum surge at the shoreline 

was extracted from the MOMs results for the Brunswick model 

grid (Fig. 4, left). Maximum, open-coast surge values were 

extracted at the location of the January 2006 lidar-derived, 

mean-high-water (MHW, 0.68 m NAVD88) shoreline position 

(Fig. 4, right). The Brunswick SLOSH-model grid has an 

approximate cell size of 1.4 km to 1.8 km along the shoreline 

of Cumberland Island NS. In locations where grid resolution 

resulted in anomalously low values because the barrier island 

was not fully resolved, the most nearshore grid cell representing 

conditions seaward of the barrier island was chosen.

3. Inundation Vulnerability

The potential inundation, I, of the beach system was defined 

every 20 m along Cumberland Island NS by calculating the 

difference between the elevations of SLOSH-modeled storm 

surge (Rlow) for each hurricane category and of lidar-measured 

dune crests (Dhigh) (Fig. 5). Negative values (blues) indicate that 

Figure 2. Cross-shore profile of lidar data from Cumberland Island 

NS showing the location of the most seaward sand dune. The 

ocean is located to the right.
Figure 3. The extracted crest of the dune line for approximately 1.3 km of the Cumberland Island NS coast, 

superimposed on the lidar topography. The arrow refers to the location of the profile shown in Figure 2.

Figure 4. SLOSH-model results for a Category 3 hurricane for the Brunswick model grid (left) shows the variability of modeled surge 

values along the coast. Maximum, open-coast surge levels for Category 1-5 hurricanes were extracted from the SLOSH grid along the 

Atlantic shoreline of Cumberland Island NS (right). 
water levels are predicted to be lower than the dune crest and 

that a particular section of beach is not likely to be inundated 

during the direct landfall of a hurricane. Positive values (reds) 

signify areas where the beach is more likely to be inundated 

by storm surge. These estimates assume landfall at mean 

astronomical tide and do not include the effects of wave setup, 

which, during strong storms (Category 3 and above), may 

increase the storm-induced mean-water level by more than 30% 

above that due to storm surge alone. This would increase the 

vulnerability to inundation. Additionally, the maps represent 

the vulnerability of the beach system as it was at the time of the 

January 2006 lidar survey. Major changes to beach morphology, 

such as those caused by large storms and hurricanes, may change 

the future vulnerability of this stretch of coast. 

For the direct landfall of a Category 1 storm, 10% of the 

Cumberland Island coast is vulnerable to inundation, compared 

to over 97% during a Category 5 storm. The vulnerability for a 

single category storm is highly variable along the length of the 

park due to spatial variations in the height of the frontal dune. 

The mean elevation of Dhigh is 4.19 m NAVD88 with substantial 

longshore variability, standard deviation, σ, = 1.49 m (Fig. 6). 

The surge is much less spatially variable: alongshore averaged 

surge for a Category 1 hurricane was 2.99 m NAVD88 (σ = 0.09 

m), and the average surge for a Category 5 storm was 7.27 m (σ

= 0.30 m) (Fig. 6). While the variability of I and Dhigh dominate 

the signal along Cumberland Island, the lower dunes on the 

northern end (6.5 km) and central third of the park (Fig. 6) make 

these locations more susceptible to inundation: generally, Rlow > 

Dhigh for these areas for Category 2 and higher storms. 

4. Rates of Recent Shoreline Change

Detailed analysis of the historic (150-yr) rates of shoreline 

change, calculated using the horizontal position of the MHW 

shoreline from four time periods (mid-1800s, early 1920s, early 

1970s, and 1999), along Cumberland Island NS shows that over 

80% of the shoreline along the two islands is accreting with a 

5.5-km stretch of erosion located ~3 km north of Stafford Beach 

(Morton and Miller, 2005). The short-term shoreline-change 

rate calculated over an approximate 27-yr period shows a similar 

pattern of erosion and accretion; however, the northern 3.5 km 

of Cumberland Island were also found to be eroding. Details of 

the calculations and specific rates of change for both historic and

short-term periods can be found in Miller et al. (2005). 

Rates of recent shoreline change were calculated between the 

October 1999 shoreline used in the Morton and Miller (2005) 

analysis and the modern shoreline position as defined using the 

January 2006 lidar survey that was the basis of the inundation 

analysis. From each lidar survey, datum-based (MHW = 0.68 

m) shoreline positions were extracted every 20 m along the coast 

using techniques described in Stockdon et al. (2002). Horizontal 

movement of the shoreline, calculated as the difference in 

position between 1999 and 2006, showed accretion over 80% of 

this 30-km stretch of coast (Fig. 7). The largest magnitudes of 

change were observed along the short (2.5 km) beach of Little 

Cumberland Island and at adjacent inlets. Excluding change 

on Little Cumberland Island, the mean shoreline change over 

the 6.25-yr time period was 12.40 m and the standard deviation 

was 19.50 m. The magnitude of shoreline change ranged from 

120.70 m of accretion to 105.31 m of shoreline retreat. The 

mean, short-term-change rate over the 6.25-yr period was 1.98 

m/yr (σ = 3.10 m/yr), again excluding the inlet effects near 

Little Cumberland Island. The pattern of this short-term-change 

rate was consistent with the long-term (~150 yr) rate found by 

Morton and Miller (2005): the system is dominated by accretion 

with a 5-km stretch of eroding coast located just north of the 

center of the longshore extent of the park (Fig. 7). Detailed rates 

of change and associated error bars for the length of the park can 

be found at http://coastal.er.usgs.gov/nps-beaches/. 

5. Discussion

The vulnerability maps detail the worst-case surge scenario for 

each location along the coast. For open-coast barrier islands, 

maximum surge typically occurs to the right of landfall under 

the eyewall and decreases with distance away from the center 

of the storm. Consequently, the worst case for any given 

storm is localized and will not occur along the entire coast of 

Cumberland Island NS. The map shows areas that are most 

likely to be inundated by storm surge should the hurricane make 

landfall immediately to the south of that particular location. 

If the beach system is inundated, the dune will be overtopped 

by storm surge. Strong waves and currents may transport large 

amounts of sand landward across the island. These types of 

changes have been shown to be more long-lasting because 

sand removed from the beach face does not typically return 

in the years following storm landfall (Stockdon et al., 2007a). 

In the southern portion of the park, where the width of the 

beach system approaches the width of the island, the currents 

associated with inundation may cross the island and create new 

or expose relict inlets.

Acknowledgments

We thank Rebecca Beavers and Mark Borrelli (NPS) for their 

assistance in the development of this project. John Fry, Tony 

Curtis, and Linda York (NPS) provided early reviews of the 

map products. The lidar data were collected by Jeff Lillycrop 

(USACE) and his CHARTS team. We also thank Betsy 

Boynton, Kara Doran, Kristy Guy, Peter Howd, Abby Sallenger, 

Jolene Shirley, and Charlene Sullivan (USGS) for their 

contributions to this study. This work was funded by National 

Park Service, Natural Resource Program Center.

References

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Comparisons of HRD and SLOSH surface wind fields in hurricanes: 

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Jarvinen, B.R., and Lawrence, M.B., 1985, An evaluation of the 

SLOSH storm surge model: Bulletin of the American Meteorological Society, v. 66, p. 1408-1411.
Figure 5. The potential inundation, I, of the beach system at Cumberland Island NS for Category 1-5 hurricanes is defined as the 

difference between Rlow and Dhigh. A 200-m smoothing window was applied to the data before plotting. Positive values indicate that 

modeled storm surge exceeds the elevation of the dune crest, suggesting that the beach system is more vulnerable to inundation and the 

associated extreme coastal changes. A larger version of this map can be downloaded from http://coastal.er.usgs.gov/nps-beaches/.
Figure 6. Dune elevations (Dhigh) for Cumberland Island NS were measured from a lidar topographic survey conducted on January 16, 17, 

and 18, 2006. The surge values (Rlow) for Category 1-5 hurricanes were extracted from the NOAA SLOSH model and represent the opencoast ‘maximum of the maximum.’ A larger version of this map can be downloaded from http://coastal.er.usgs.gov/nps-beaches/. 
Figure 7. Short-term (6.25-yr) rates of shoreline change for Cumberland Island NS. Data were smoothed with a 

100-m window. Negative values indicate erosion. Away from the effects of inlets, the mean rate is 1.98 m/yr. 

Miller, T.L., Morton, R.A., and Sallenger, A.H., 2005, National Assessment of Shoreline Change: A GIS Compilation of Vector Shorelines 

and Associated Change Data for the U.S. Southeast Atlantic Coast: 

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FL, 40 pp.

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slosh.shtml, last accessed August 24, 2007.

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and Stockdon, H., 2003, Evaluation of airborne scanning lidar for 

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2007a, A simple model for the spatially-variable coastal response to 

hurricanes: Marine Geology, v. 238, p. 1-20.

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Longshore variability of the coastal response to Hurricanes Bonnie 

and Floyd: Proceedings, International Conference on Coastal Sediments 2003, Clearwater Beach, FL, CD-ROM.

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For more information:

For the USGS:

Hilary F. Stockdon

U.S. Geological Survey 

Florida Integrated Science Center–St. Petersburg

600 4th Street S.

St. Petersburg, FL 33701

Tel: (727) 803-8747

Email: hstockdon@usgs.gov

For the NPS-GRD:

Mark Borrelli

National Park Service

Natural Resource Program Center

Geologic Resources Division (GRD)

P.O. Box 25287

Denver, CO 80225-0287

Tel: (303) 969-2171