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St. Petersburg Coastal and Marine Science Center > Coastal Change Hazards: Hurricanes and Extreme Storms > Hurricane Irene

Coastal Change Hazards: Hurricanes and Extreme Storms

Hurricane Irene

Updated Assessment of Potential Coastal-Change Impacts

Irene-Specific Conditions

Posted 08-25-2011, last updated 08-26-2011

Hurricane landfall and associated elevated water levels, waves, and currents can lead to severe coastal change through erosion and re-deposition. The potential for coastal change due to hurricanes is predicted using a USGS-developed storm-impact scale that compares predicted elevations of hurricane-induced water levels to known elevations of coastal topography to define four coastal change regimes. These regimes describe the dominant interactions between beach morphology and storm processes and the resulting modes of coastal change along beaches and dunes that serve as the "first line of defense" for many coasts exposed to hurricanes.

The regimes include collision, which is when waves attack the base of dunes and cause dune-front erosion. Under higher surge or wave runup, waves can overtop dunes leading to overwash, dune erosion, landward dune migration, and overwash deposition on low, narrow islands. The most extreme coastal change regime is associated with inundation, where the elevation of storm surge exceeds the elevation of the primary dune or beach berm. Under these conditions the beach and dune can be severely eroded and low, narrow islands may breach.

The probability of coastal change associated with processes of collision, overwash, and inundation was assessed for areas located within the region expected to be affected by Hurricane Irene (outlined area in figure below).

Location map for focus areas.
Location map for focus areas. Click on an area to see the potential coastal change.

This assessment describes coastal change probability based on estimating the likelihood that the beach system will experience erosion and deposition patterns consistent with collision, overwash, or inundation regimes. The probabilities are estimated by calculating the difference between modeled total water levels (including storm surge and wave runup) and lidar-derived dune or berm elevations. The storm surge elevations along the open coast were obtained from the National Oceanic and Atmospheric Administration's (NOAA) probabilistic surge forecast (psurge), which is based on conditions specific to the landfalling storm. Errors in hurricane forecasts are included in order to identify probable surge levels. Rather than including the full range of storm surge probabilities, we have selected the 50% exceedance surge level to represent the best-estimate scenario. Maximum wave heights obtained from the NOAA WaveWatch3 model 7-day forecast are used to compute runup elevations.

In the maps below, red colors indicate high probability and white indicates low probability. The probabilities can be interpreted as indicating that the specified coastal change regime is very likely (probability >90%), likely (>66%), about as likely as not (33% to 66%), unlikely (<33%), and very unlikely (<10%) given the present storm forecasts.

Note that the computations assume landfall at mean astronomical tide. Landfall at high tide would lead to higher coastal change probabilities.

Delaware/Maryland/Virginia

The DelMarVa coast is bordered by thin, sandy barrier islands (except near the mainland coast of Virginia Beach) with numerous tidal inlets. Dune heights are variable (more so in the south, less so in Delaware) in this region, ranging from as low as 1 m to as high as 6-8 m. The coast includes both developed and undeveloped regions, including Assateague Island National Seashore.

Probability of collision, overwash, and inundation for Hurricane Irene - Delaware/Maryland/Virginia.
Probability of collision, overwash, and inundation for Hurricane Irene. Using current forecast information, extensive beach and dune erosion is likely for the entire coast. Much of the coast is likely to overwash (70% in VA, 40% in MD, and 2% in DE). Inundation of the beach system is likely in VA (33% of the area, all of this north of Chesapeake Bay entrance) and unlikely in MD and DE. Data are available in zipped KML files (1 MB) for DE, MD, and VA, viewable with Google Earth. [larger version]

NC/VA Border to Ocracoke, NC

The northern coast of North Carolina is bordered by thin, sandy barrier islands. Typical dune heights range between 4 to 6 m. Villages containing extensive housing and other infrastructure are interspersed with natural areas, including Cape Hatteras National Seashore and Pea Island Wildlife Refuge.

Probability of collision, overwash, and inundation for Hurricane Irene - NC/VA Border to Ocracoke, NC.
Probability of collision, overwash, and inundation for Hurricane Irene. Using current forecast information extensive beach and dune erosion is likely for the entire coast. Approximately 15% the coast is likely to overwash. Inundation of the beach system is not likely. Data also available in zipped KML file (1 MB), viewable with Google Earth. [larger version]

Core Banks, NC to NC/SC Border

The southern coast of North Carolina is bordered by thin, sandy barrier islands with numerous tidal inlets. Dune heights range from as low as a couple of meters to as high as 7-8 m. The area includes both developed and undeveloped regions.

Probability of collision, overwash, and inundation for Hurricane Irene - Core Banks, NC to NC/SC Border.
Probability of collision, overwash, and inundation for Hurricane Irene. Using current forecast information, extensive beach and dune erosion is likely for the entire coast. Approximately 71% the coast is likely to overwash. Inundation of the beach system is likely in 13% of the area. Data also available in zipped KML file (1 MB), viewable with Google Earth. [larger version]

South Carolina

The South Carolina coast is bordered by thin, sandy barrier islands with numerous tidal inlets. Dunes are typically about 3 m high, ranging from as low as 1 m to as high as 5-6 m. The coast includes both developed and undeveloped regions.

Probability of collision, overwash, and inundation for Hurricane Irene - South Carolina.
Probability of collision, overwash, and inundation for Hurricane Irene. Using current forecast information, extensive beach and dune erosion is likely for the entire coast. Approximately 87% the coast is likely to overwash. Inundation of the beach system is likely in 22% of the area. Data also available in zipped KML file (1 MB), viewable with Google Earth. [larger version]

For more information about the storm-impact scale and coastal changes expected during hurricane landfall, see the Storm-Impact Scale and Storm-Induced Coastal Change sections.

Disclaimer: This experimental product is based on research results of the USGS National Assessment of Coastal Change Hazards Project and is intended to indicate the potential for coastal change caused by storm surge and wave runup. This product is based on an analysis that simplifies the problem to include some of the most important aspects (estimated dune elevations and predicted total water levels). This product does not directly consider potential property damage or the impacts of high wind speeds and heavy rainfall. This product applies to open coast environments and does not consider potential coastal change along bays, passes, or inland lakes. The actual changes that occur during extreme storms are complex functions of a number of processes and variables including ocean waves, currents, and tides. The public should not base evacuation decisions on this product. Citizens should always heed the evacuation advice of local emergency management authorities.


Related links:

Storm-Impact Scale
USGS

Storm-Induced Coastal Change
USGS

St. Petersburg Coastal and Marine Science Center > Coastal Change Hazards: Hurricanes and Extreme Storms > Hurricane Irene

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Delaware/Maryland/Virginia Core Banks, NC to NC/SC Border South Carolina NC/VA Border to Ocracoke, NC