Vulnerability of National Park Service Beaches to Inundation during 

a Direct Hurricane Landfall: Fire Island National Seashore

Hilary F. Stockdon and David M. Thompson

U.S. Geological Survey

FISC-St. Petersburg

600 4th Street S.

St. Petersburg, FL 33701

Abstract

Waves and storm surge associated with 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. Even at 

Fire Island National Seashore, a barrier-island coastal park in 

New York where extratropical storms (northeasters) dominate 

storm activity, the beaches are vulnerable to the powerful, sandmoving forces of hurricanes. 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 storm-induced 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 1-4 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. In our 

national coastal parks, where development is strictly managed, 

hurricanes threaten to destroy infrastructure that keeps the parks 

operational (e.g., roads, communications, park buildings) as 

well as to alter dramatically the beaches and dunes that provide 

visitors with the opportunity to 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). The longshore-variable, potential 

vulnerability of a barrier island to extreme coastal change 

during a hurricane landfall can be defined using a storm-impact 

scaling model that compares the relative elevations of barrierisland morphology and storm-induced water levels (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 Fire 

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. 

Fire Island NS is an east-west trending barrier island located in 

New York, south of Long Island (Fig. 1). The 50 km of barrierisland park include beaches, dunes, marshes, and maritime 

forest. There are 17 private communities within the boundaries 

of the park, as well as state and local parks. The beaches along 

Fire Island are most vulnerable to strong northeaster storms 

that typically batter the beaches in the fall and winter; however 

the south-facing beaches are also susceptible to the forces of 

approaching hurricanes that occasionally move into the North 

Atlantic Ocean. 

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 

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1389

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

waves, both the time-varying and time-averaged components) 

move the erosive 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 Fire Island NS 

was mapped based on an airborne lidar topographic survey 

conducted on April 29, 2007, by the USGS and NASA using 

NASA’s Experimental Advanced Airborne Research Lidar 

(EAARL). GPS-based lidar surveys provide an efficient 

method for collecting high-resolution 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). These data were 

collected approximately two weeks after a large northeaster 

storm delivered large waves and strong winds to the area. 

Because changes to dunes during storms are generally more 

long-lasting than changes on the beach face, this post-storm 

survey was used to measure the most current morphology of the 

dunes and to define the potential inundation of Fire Island. 

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 

Fire 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, 

150 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). 


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 1-4 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. 

(Note: The NOAA SLOSH model does not simulate the storm 

surge associated with Category 5 landfall in the New York basin 

or any neighboring basins.) The numerical model is based on 

linearized, depth-integrated 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 windstress 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 Fire Island NS, the maximum surge at the shoreline was 

extracted from the MOMs results for the New York model 

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

extracted at the location of the April 2007 lidar-derived, meanhigh-water (MHW, 0.46 m NAVD88) shoreline position (Fig. 4, 

right). The New York SLOSH-model grid has an approximate 

cell size of 3.4 km to 5.3 km along the shoreline of Fire 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 Fire 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 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 storminduced 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 April 2007 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, less than 1% of 

the Fire Island NS coast is vulnerable to inundation, compared 

to over 72% during a Category 4 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 5.44 m NAVD88 with substantial 

longshore variability; standard deviation, σ, = 1.32 m (Fig. 6). 

The surge is less spatially variable: alongshore averaged surge 

for a Category 1 hurricane was 2.22 m NAVD88 (σ = 0.09 m), 

and the average surge for a Category 4 storm was 6.02 m (σ = 

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

the signal along Fire Island NS, the lower dunes and berms in 

the northern 2.5 km of Otis Pike Wilderness Area (Fig. 6) make 

this area more susceptible to inundation: generally, Rlow > Dhigh

for this area for Category 2 and higher storms. 

4. Recent Shoreline Change

A USGS analysis of historic (150-yr) rates of shoreline change 

for the northeast United States, including Fire Island NS, is 

ongoing and will be published within the next year. Recent 

magnitudes of shoreline change along Fire Island have been 

calculated between a lidar survey collected in October 2005, 

following a northeaster storm, and the modern shoreline 

position as defined using the April 2007 lidar survey that was 

the basis of the inundation analysis. From each lidar survey, 

datum-based (MHW = 0.46 m) shoreline positions were 

Figure 2. Cross-shore profile of lidar data from Fire 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 1.2 km of the Fire 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 New York model grid (left) shows the variability of 

modeled surge values along the coast. Maximum, open-coast surge levels were extracted from the SLOSH grid along the 

Atlantic shoreline of Fire Island NS (right). 


Figure 5. The potential inundation, I, of the beach system at Fire Island NS for Category 1-4 hurricanes is defined as the difference between Rlow and Dhigh. applied to the data before plotting. Positive values indicate that modeled storm surge exceeds the elevation of the dune crest, suggesting that the beach 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 Fire Island NS were measured from a lidar topographic survey conducted on April 29, 2007. The surge values (Rlow) for 

extracted from the NOAA SLOSH model and represent the open-coast ‘maximum of the maximum.’ A larger version of this map can be downloaded from http://coastal.er.usgs.gov/nps-beaches/. 
extracted every 20 m along the coast using techniques described 

in Stockdon et al. (2002). 

Because two post-storm surveys were compared, the results 

are more representative of the background shoreline-change 

signal and are not intended to quantify storm-induced 

change. Horizontal movement of the shoreline, calculated as 

the difference in position between 2007 and 2005, showed 

significant longshore variability along the coast of Fire Island 

NS. The mean shoreline change was 8.76 m; however, the 

standard deviation was 21.81 m (Fig. 7). A detailed explanation 

of the patterns of shoreline change observed will require an indepth study of the wave conditions for each of the northeaster 

storms, as well as information on the typical wave conditions at 

the site. Additionally, in order to understand the storm recovery 

processes of beaches and dunes on a heavily managed coastline, 

the timing and quantities of human-induced sand movement 

(e.g., replenishments, dune rebuilding, beach scraping, etc.) 

must be considered. Only then, can the natural processes of 

island response to and recovery from a storm be separated 

from human-induced changes. Details on the magnitudes of 

shoreline change and the associated error bars for the length of 

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

Figure 7. Shoreline change for Fire Island NS as calculated between two post-northeaster lidar surveys conducted in October 2005 and April 

2007. Data were smoothed with a 100-m window. Negative values indicate erosion. Mean shoreline change over the length of the island is 

8.76 m of accretion. 

5. Discussion

The vulnerability maps detail the worst-case storm-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 Fire Island NS. The map shows areas that are most 

likely to be inundated by storm surge should the hurricane make 

landfall immediately to the west 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 

locations where the width of the beach system approaches the 

width of the island, such as the central portion 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 development of this project. Cheryl Hapke (USGS) 

provided early reviews of the map products. The lidar data were 

collected by Wayne Wright (NASA) and his EAARL team. 

We also thank Betsy Boynton, Kara Doran, Kristy Guy, Peter 

Howd, Abby Sallenger, Jolene Shirley, Katherine Serafin, and 

Charlene Sullivan for their contributions to this study. This 

work was funded by National Park Service, Natural Resource 

Program Center.

References

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1999, Comparisons of HRD and SLOSH surface wind 

fields in hurricanes: Implications for storm surge modeling: 

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english/surge/slosh.shtml, last accessed August 24, 2007.

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Stockdon, H.F., Sallenger, A.H., Holman, R.A., and Howd, 

P.A., 2007a, A simple model for the spatially-variable coastal 

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

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R.A., 2003, Longshore variability of the coastal response 

to Hurricanes Bonnie and Floyd: Proceedings, International 

Conference on Coastal Sediments 2003, World Scientific 

Publishing Corp. and East Meets West Productions, Clearwater Beach, FL.

Stockdon, H.F., Sallenger, A.H., List, J.H., and Holman, R.A., 

2002, Estimation of shoreline position and change from 

airborne scanning lidar data: Journal of Coastal Research, v. 

18, p. 502-513.

Stockdon, H.F., Thompson, D.M., and Sallenger, A.H., 2007b, 

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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