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The potential vulnerability of a particular stretch of coast can be assessed using a conceptual model that scales the impacts of storms on barrier islands.

Overview

Definition sketch showing Rhigh Rlow, Dhigh and Dlow

Definition sketch showing Rhigh Rlow, Dhigh and Dlow. The dashed lines represent the swash excursion about wave setup (solid line). (Public domain.)

The impact of a storm on a barrier island is dependent not only on the magnitude of storm characteristics, such as storm surge and waves, but also on the elevation of the barrier island at landfall, such as the line of dunes paralleling the shore that act as the coast's first line of defense. Stretches of coast with very low dunes are potentially more susceptible during storms to extreme coastal change than those with higher dunes.

The potential vulnerability of a particular stretch of coast can be assessed using a conceptual model that scales the impacts of storms on barrier islands (Sallenger, 2000). Within the model, the elevation of storm-induced water levels (Rhigh and Rlow), including storm surge, astronomical tide, and wave runup, are compared to measurements of local dune morphology such as the elevation of the dune crest and toe, (Dhigh and Dlow).

The hurricane-induced water levels (Rhigh and Rlow) are the highest reaches of the waves on the beach during a the storm. By considering these water levels relative to coastal elevations Dhigh and Dlow, the crest and base of the dune, four storm impact regimes can be defined for a specific area of the coast:

Swash: Total water levels are lower than the dune toe (Rhigh < Dlow)
Collision: Total water levels exceed the dune toe (Dlow < Rhigh < Dhigh)
Overwash: Total water levels exceed the dune crest (Rhigh > Dhigh)
Inundation: Storm surge and tide exceed the dune crest (Rlow > Dhigh)

Within each of these regimes, the nature and magnitude of coastal change are expected to be unique.

Illustration of storm impact regimes

Illustration of storm impact regimes. (Public domain.)

Swash Regime

illustration of swash regime

Swash regime. During a storm, if wave runup is confined to the beach, the beach will erode and the sand will be stored offshore. However, over the weeks and months following the storm, the sand naturally returns to the beach, restoring the beach to its original configuration. No net change to the system. (Public domain.)

Minimal Impacts of the Swash Regime

Graph of cross-shore position and elevation for beach before and after Hurricane Dennis.

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image of man and sand dune

During Hurricane Dennis (1999), at this location, wave runup was confined to the beach. The beach eroded but the dune was untouched (see photo and compared cross-section above). Most of the eroded sand returned to the beach in the weeks to months following Hurricane Dennis (1999).

Collision Regime

illustration showing collision regime

Collision Regime. If wave runup exceeds the elevation of the base of the dune, the runup will collide with the dune causing erosion and dune retreat. Unlike the temporary changes of the swash regime, this change is considered a net, or (semi-) permanent, change to the dune. Net dune erosion. (Public domain.)

Dune Erosion During the Collision Regime

Extensive dune erosion along the Outer Banks of North Carolina

Extensive dune erosion along the Outer Banks of North Carolina after Hurricane Isabel's 2003 landfall reveals the remains of a shipwreck on the beach near the town of Buxton. (Public domain.)

Below are a pair of before and after Hurricane Fran photographs show that the system was in Collision Regime, with significant dune retreat.

 July 1996, Hurricane Fran, Topsail Island, North Carolina

 July 1996, pre-Hurricane Fran, Topsail Island, North Carolina. (Public domain.)

photo with dune line annotated on photo

September 1996, post-Hurricane Fran, Topsail Island, North Carolina. (Public domain.)

Coss-sections of lidar data showing dune retreat of 20 meters during Hurricane Dennis (1999).

Above are cross-sections of lidar data showing dune retreat of 20 meters during Hurricane Dennis (1999).

Overwash Regime

illustration of overwash regime

Overwash Regime. If wave runup exceeds the elevation of the dune, or in the absence of a dune, the beach berm, the system will be overtopped, transporting sand landward. This is a net change contributing to the migration of the barrier island landward. Net onshore transport order 100 meters. (Public domain.)

Impacts of the Overwash Regime

September 1999, Hurricane Dennis, Core Banks, NC.

September 1999, Hurricane Dennis, Core Banks, NC.(Public domain.)

February 1998, Northeaster, Assateague Island, VA.

February 1998, Northeaster, Assateague Island, VA. (Public domain.)

Overwash deposits near Rodanthe, NC after Hurricane Dennis

Overwash deposits near Rodanthe, NC after Hurricane Dennis (1999), which made landfall in early September 1999. Large overwash deposits were also observed following Hurricane Dennis (1999) in Buxton, NC. (Public domain.)

Above are overwash deposits near Rodanthe, North Carolina, after Hurricane Dennis (1999). The sand was transported landward by wave runup overtopping the dune. In both photographs at right, wave runup overtopped the highest part of the system during a storm, resulting in net sand transport landward forming overwash fans.

Inundation Regime

illustrations showing partial inundation of an island

Inundation regime. If the storm surge is high and the elevation of the most seaward dune is low, the beach system can become completely subaqueous. Sand is transported landward over the island an order of magnitude farther than typical overwash of the overwash regime. Net onshore transport order 1,000 meters. (Public domain.)

illustration showing complete inundation of an island

Inundation regime. If an island is narrow and very low in elevation (no higher than the primary dune/berm), the entire island may be inundated. Net onshore transport order 1,000 meters. (Public domain.)

Catastrophic Impacts of the Inundation Regime

On August 29, 2005, Hurricane Katrina made landfall as a category-3 storm over the Mississippi Delta in Louisiana. This strong storm approached the coast with category-4 strength winds, building large waves and record-levels of storm surge. Aerial video, still photography, and laser altimetry surveys of post-storm beach conditions were collected August 31 and September 1, 2005. Comparisons of post-storm data with earlier surveys were used to show the nature, magnitude, and spatial variability of coastal changes such as beach erosion, overwash deposition, and island breaching.

Louisiana's Chandeleur Islands are an undeveloped, north-south oriented chain of barriers located approximately 110 km east of the city of New Orleans and 70 km east of Katrina's path. These low-lying barrier islands were totally stripped of sand, heavily fragmented by large waves and storm surge, and completely inundated during the storm.

photos show land loss from Hurricane Katrina

Pre- (A) and post-storm (B) photography reveal the dramatic changes that occurred along the entire stretch of islands. This loss of d. This extreme coastal change may lead to long-term, permanent changes in the barrier island morphology and may affect its future resilience in storms. (Public domain.)

First Line of Defense (Dhigh)

The vulnerability of a barrier island to storm overwash and inundation is determined, in part, by the elevation of the 'first line of defense,' or Dhigh.  On a natural beach system, this is either the first dune ridge or, in the absence of a dune, the beach berm.  For areas that have been heavily engineered with shore-parallel coastal defense structures (e.g. seawalls), the top of the structure becomes the 'first line of defense.' These features, both natural and engineered, act as a first line of defense for inland areas during storms by protecting them from waves and surge.

The spatial variability of the dune crest plays an important role in making some areas of the coast more vulnerable to hurricane-induced coastal change than other areas.  From lidar topographic surveys, detailed measurements of beach topography have been collected along the Nation's coastlines. From these high-resolution surveys, the locations and elevations of Dhigh are measured.

Measuring (Dhigh)

A quantitative method has been developed to extract the location and elevation of Dhigh from lidar surveys. The method follows a set of fixed rules and processes in order to locate Dhigh. This is an objective process, meaning that if it is repeated multiple times, the answers will always be the same. 

The complex algorithm is simplified below in order to illustrate the basic principles of this objective technique: 

Step 1:  A cross-section of lidar data is extracted from the survey, creating a profile view of the beach and dune.  In order to eliminate small variations in elevation, the profile is mathematically smoothed.

graph showing raw and smoothed data for elevation and cross-shore distance

Step 2:  Slope is calculated between adjacent data points across the width of the cross section.  Elevation peaks are identified based on changes in the direction of slope (positive to negative).

graphs of elevation and slope change with elevation peak noted

Step 3:  Using established mathematical methods (Stockdon et al., 2002), the shoreline is located.  Dhigh is the first elevation peak landward of the shoreline.

graph of elevation and cross-shore distance with Dhigh noted

In the absence of a dune, Dhigh is located at the berm crest.  The mathematical methods for locating the berm crest are nearly identical.

An alternative, more subjective method for locating the dune crest and toe is based on gridded, digital elevation models of lidar data. In this method, the location of the dune crest and toe are first digitized by a user and then refined using GIS-based algorithms. For more information on that technique, see the following publication:

Elko, N., Sallenger, A., Guy, K., Stockdon, H. and Morgan, K., 2002. Barrier Island Elevations Relevant to Potential Storm Impacts: 1. Techniques. USGS Open File Report 02-287.

Dhigh Maps

The maps below illustrate the elevation of Dhigh, or the 'first line of defense,' along the South Atlantic and Gulf coasts.   Extensive spatial variability in elevation can be observed.  Darker red shades indicate relatively low elevations and a corresponding high vulnerability to overwash and inundation.  Lighter red shades indicate high, well-developed dunes and relatively low vulnerability to overtopping and net coastal change. 

North Carolina

Map of dune elevation for coastal North Carolina. On the left side of the map are histograms showing the distribution of these 'first line of defense' elevations, one for northern North Carolina and one for southern North Carolina.

Map of dune elevation for coastal North Carolina

Graph of the elevations of the dune crest and the dune base along Cape Hatteras National Seashore. The gap in data near the top of the graph corresponds with the position of Oregon Inlet. Note the spatial variability along the coast. Lower first line of defense elevations (red) are vulnerable to overwash and inundation regimes. Coasts with lower dune base elevations (blue) are vulnerable to the collision regime and dune retreat. The dune elevation map to the right of the graph covers the same stretch of the Cape Hatteras National Seashore and the vertical scale of dune elevation corresponds to the North Carolina map.

Graph of the elevations of the dune crest and the dune base along Cape Hatteras National Seashore

South Carolina

Map of dune elevation for coastal South Carolina and Georgia. On the either side of the map are histograms showing the distribution of these 'first line of defense' elevations, one for South Carolina and one for Georgia. An additional plot of cross-sections produced from gridded lidar data illustrates differences in vulnerability along Cumberland Island National Seashore (see bottom right hand corner of map). The first line of defense elevation of the red cross-section is nearly 6 meters below the elevation of the dark green cross-section.

Map of dune elevation for coastal South Carolina and Georgia

Florida

Map of the 'first line of defense' elevations for the eastern coast of Florida. On the left of the map are histograms showing the distribution of these elevations, one for northeastern Florida, one for east central Florida, and one for southeastern Florida.

Map of the 'first line of defense' elevations for the eastern coast of Florida

Northern Gulf of Mexico

Map of Dhigh, elevations for the barrier islands in the northern Gulf of Mexico (Louisiana, Mississippi, Alabama, and Florida's Panhandle). Low elevations, less than 2 m, make Louisiana's barrier beaches vulnerable to extreme coastal change during hurricanes.

Map of Dhigh, elevations for the barrier islands in the northern Gulf of Mexico

Storm-Induced Water Levels (Rhigh and Rlow)

Estimates of hurricane-induced water levels, Rlow and Rhigh, are necessary for predicting the potential coastal change during an approaching hurricane. Rhigh is maximum water-level elevation expected during a hurricane and includes the astronomical tide, storm surge, and wave runup. Rlow is an effective still-water level during a storm and is composed of the astronomical tide, storm surge and wave setup.

Storm Surge

Modeled, maximum surge elevations, as simulated by the NOAA SLOSH model, for a category 3 hurricane making landfall

Modeled, maximum surge elevations, as simulated by the NOAA SLOSH model, for a category 3 hurricane making landfall in the Pamlico Sound basin of North Carolina at a forward speed of 15 m/s. The values shown, representing the 'maximum envelope of water,' are obtained by running several similar hypothetical storms onshore along parallel tracks (shown by black arrows). (Public domain.)

Storm surge is the temporary rise in water level due primarily to winds and pressure within a hurricane. In order to use the storm-impact scaling model in a predictive model, the elevation of surge for the five hurricane categories is modeled by the National Hurricane Center using 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 (see figure at right). The numerical model is based on linearized, depth-integrated equations of motion and continuity (Jarvinen and Lawrence, 1985). Changes in maximum surge elevations are forced by time-varying wind-stress and pressure gradient forces which depend on the hurricane's location, minimum pressure, and size measured from the eyewall out to the location of maximum winds (Jarvinen and Lawrence, 1985). The SLOSH model does not incorporate astronomical tides, wave runup or setup; however, astronomical tides are included in the final results (Houston et al., 1999). While the results are location specific, accounting for local water depths, proximity to bays and river, etc., the results are accurate to ± 20% of the calculated value (2004). Error in SLOSH values can also arise from differences between the parametric wind models which force SLOSH and the hurricane's actual wind field (Houston et al., 1999).

Wave Runup and Setup

Wave runup (R(t)) is the time-varying fluctuation of water-level elevation at the shoreline due to wave breaking (see figure below). Wave setup (η), the time-averaged water level, is the super-elevation of still water at the shoreline, again due to wave breaking. The magnitude of both runup and setup are related to offshore wave period, wave height (H), and foreshore beach slope (β). The elevation of wave runup and setup are calculated from modeled offshore wave conditions using field-data-based empirical parameterizations (Stockdon, et al., 2006). During storm conditions, the wave runup and setup can double the elevation of water levels at the coast beyond that due to storm surge alone.

Wave runup (R) is the time-varying elevation of water level at the shoreline, measured here in reference to the still water

Wave runup (R) is the time-varying elevation of water level at the shoreline, measured here in reference to the still water level (SWL). The schematic shows wave runup (R), and its components, time-averaged wave setup (η) and time-varying swash (dashed lines), as a function of wave height (H) and beach slope (β). (Public domain.)

References

NOAA, 2004. Hurricane Preparedness. National Centers for Environmental Prediction, National Hurricane Center, http://www.nhc.noaa.gov/HAW2/english/surge/slosh.shtml.

Houston, S.H., Shaffer, W.A., Powell, M.D. and Chen, J., 1999. Comparisons of HRD and SLOSH surface wind fields in hurricanes: Implications for storm surge modeling. Weather and Forecasting, 14: 671-686.

Jarvinen, B.R. and Lawrence, M.B., 1985. An evaluation of the SLOSH storm surge model. Bulletin of the American Meteorological Society, 66(11): 1408-1411.

Stockdon, H.F., R.A. Holman, P.A. Howd, and A.H. Sallenger, 2006. Empirical parameterization of setup, swash, and runup, Coastal Engineering, 53(7), pp. 573-588.