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National Assessment of Storm-Induced Coastal Change Hazards

Hurricane Irene

Initial Assessment of Potential Coastal-Change Impacts

Generalized Storm Scenarios

Posted 08-25-2011

Hurricane landfall and associated elevated water levels, waves, and currents can lead to severe coastal change through erosion and re-deposition of sediment. 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 (storm surge and wave runup) 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 that serve as the "first line of defense" for many regions 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 landfall of 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 initial 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 were obtained from the National Oceanic and Atmospheric Administration (NOAA) storm surge model (SLOSH), which simulated the surge resulting from numerous hypothetical hurricane landfall scenarios (Category 1-5) . Predicted maximum wave heights obtained from the NOAA WaveWatch3 model 7-day forecast are used to compute wave 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 each hypothetical storm scenario.

This analysis assumes a hypothetical hurricane landfall scenario such that each location on the map would experience the most severe impacts that would be associated with the right front quadrant of the hurricane.

Ocracoke, NC to NC/VA Border

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.

Category 1

Probability of collision, overwash, and inundation under category-1 storm surge conditions.
Probability of collision, overwash, and inundation under category-1 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Approximately 20% the coast is likely to overwash in a direct landfall. Inundation of the beach system is not likely. [larger version]

Category 2

Probability of collision, overwash, and inundation under category-2 storm surge conditions.
Probability of collision, overwash, and inundation under category-2 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Over 40% of the coast is likely to overwash in a direct landfall. Inundation of the beach system is likely in few locations. [larger version]

Category 3

Probability of collision, overwash, and inundation under category-3 storm surge conditions.
Probability of collision, overwash, and inundation under category-3 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Over 60% of the coast is likely to overwash in a direct landfall. Inundation of the beach system is likely in a less than 10% of locations. [larger version]

Category 4

Probability of collision, overwash, and inundation under category-4 storm surge conditions.
Probability of collision, overwash, and inundation under category-4 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Over 70% of the coast is likely to overwash in a direct landfall. Inundation of the beach system is likely at 20% of the locations. [larger version]

Category 5

Probability of collision, overwash, and inundation under category-5 storm surge conditions.
Probability of collision, overwash, and inundation under category-5 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. In a direct landfall, over 85% of the coast is likely to overwash and over 40% of the coast is likely to experience inundation of the beach system. [larger version]

SC/NC Border to Core Banks, NC

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.

Category 1

Probability of collision, overwash, and inundation under category-1 storm surge conditions.
Probability of collision, overwash, and inundation under category-1 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Approximately 84% the coast is likely to overwash in a direct landfall. Inundation of the beach system is likely at over 30% of the locations. [larger version]

Category 2

Probability of collision, overwash, and inundation under category-2 storm surge conditions.
Probability of collision, overwash, and inundation under category-2 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Over 90% of the coast is likely to overwash in a direct landfall. Inundation of the beach system is likely at 75% of the locations. [larger version]

Category 3

Probability of collision, overwash, and inundation under category-3 storm surge conditions.
Probability of collision, overwash, and inundation under category-3 storm surge conditions. Extensive beach and dune erosion is likely for the entire coast. Over 95% of the coast is likely to overwash and 90% of the coast will experience inundation. [larger version]

Category 4

Probability of collision, overwash, and inundation under category-4 storm surge conditions.
Probability of collision, overwash, and inundation under category-4 storm surge conditions. Given a direct landfall, extensive erosion, overwash, and inundation is likely for the entire coast. [larger version]

Category 5

Probability of collision, overwash, and inundation under category-5 storm surge conditions.
Probability of collision, overwash, and inundation under category-5 storm surge conditions. Given a direct landfall, extensive erosion, overwash, and inundation is likely for the entire coast. [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 > National Assessment of Storm-Induced Coastal Change Hazards > Hurricane Irene

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Ocracoke, NC to NC/VA Border SC/NC Border to Core Banks, NC Ocracoke, NC to NC/VA Border SC/NC Border to Core Banks, NC