St. Petersburg Coastal and Marine Science Center
Sandy beaches provide a natural barrier between the ocean and inland communities, ecosystems, and resources. However, these dynamic environments move and change in response to winds, waves, and currents. During an extreme storm, these changes can be substantial and sometimes catastrophic. High waves and storm surge act together to erode beaches and inundate low-lying lands, putting inland communities at risk. A decade of USGS research on storm-driven coastal change hazards has provided the data and modeling capabilities needed to identify areas of our coastline that are likely to experience extreme and potentially hazardous erosion during an extreme storm.
The assessment methodology is based on a storm-impact scaling model that uses observations of beach morphology combined with sophisticated hydrodynamic models to predict how the coast will respond to the direct landfall of extreme storms. Storm-induced water levels, due to both surge and waves, are compared to beach and dune elevations to determine the probabilities of three types of coastal change (1) collision - occurs when the dune toe is eroded by waves and surge, (2) overwash - occurs when the sand is transported landward over the beach and dune by waves and surge, and (3) inundation - occurs when the beach and dune are completely and continuously submerged by surge and wave setup. As new beach morphology observations and storm predictions become available, the assessments will be updated to describe how coastal vulnerability to storms will vary in the future.
The National Assessment of Coastal Change Hazards Portal allows online access to the data and tools that enable users to apply coastal change hazard assessments to their specific needs.
Hurricanes are tropical cyclones that form over tropical or subtropical waters in the North Atlantic Ocean, generally travel to the north, and can impact the United States along the Atlantic and Gulf Coasts. Hurricanes are categorized from 1 to 5 using the Saffir-Simpson Hurricane Wind Scale. A category 1 hurricane has wind speeds of 33 to 42 meters per second (m/s), while a category 5 hurricane has wind speeds greater than 70 m/s. Due to high wind speeds, hurricanes cause large waves and storm surge to impact the coast. Surge and waves from all categories of hurricanes can be expected to erode beaches and dunes and potentially overtop dunes to flood the area behind the dune.
In general, lower dune elevations along the Gulf of Mexico and southeast Atlantic coasts make these regions more vulnerable to erosion hazards during hurricanes. Average dune elevations along the shores of the Gulf of Mexico are just 2.9 meters (m) high, making approximately 72% of these beaches very likely (probability > 90%) to experience extreme erosion due to overwash in the direct landfall of a category 1 storm. By contrast, dunes along the mid-Atlantic coast are, on average, over 2 m higher than dunes along the Gulf of Mexico coast and are less likely to experience overwash or inundation. In the mid-Atlantic, only 29% of the beaches are very likely (probability > 90%) to experience overwash in the direct landfall of a category 1 storm. However, this area also has the most variability in dune size and shape. This leads to a corresponding variability in the vulnerability of the beaches, placing areas highly likely to erode adjacent to more stable locations. The northeast Atlantic coast has the highest average dune elevation of 4.8 m, about 1 m higher than that in the southeast Atlantic. However, these two regions have very similar probabilities for coastal change because water levels in the northeast Atlantic tend to be nearly 1 m higher for category 1-4 storms. Another regional difference is found in the relative role of waves and storm surge in increasing coastal erosion vulnerability during low category hurricanes. Because surge is smaller in the Gulf of Mexico and southeast Atlantic regions, waves play a larger role in elevating shoreline water levels than they do in the mid- and northeast Atlantic. During a category 1 hurricane in the Gulf of Mexico or southeast Atlantic, the contribution of waves to storm-induced extreme water levels is nearly twice that of surge.
|Percentage of sandy beaches very likely (probability > 0.9) to experience erosion associated with collision, overwash, and inundation during category 1-5 hurricane landfall.
|Hurricane Intensity Category|
|collision (dune erosion)|
|US Gulf of Mexico||99||100||100||100||100|
|US Southeast Atlantic||89||95||96||96||96|
|US Northeast Atlantic||97||99||99||100||-|
|US Gulf of Mexico||72||83||91||96||98|
|US Southeast Atlantic||47||66||78||87||92|
|US Northeast Atlantic||52||66||75||82||-|
|US Gulf of Mexico||29||57||72||80||88|
|US Southeast Atlantic||12||36||51||64||75|
|US Northeast Atlantic||14||36||52||65||-|
*Category 5 hurricane conditions were not simulated for the mid-Atlantic or Northeast Atlantic regions. See the reports listed at the bottom of the page for detailed analysis and data access.
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National Assessment of Hurricane-Induced Coastal Erosion Hazards: Gulf of Mexico - Open-File Report 2012–1084
National Assessment of Hurricane-Induced Coastal Erosion Hazards: Southeast Atlantic - Open-File Report 2013–1130
National Assessment of Hurricane-Induced Coastal Erosion Hazards: Mid-Atlantic - Open-File Report 2013–1131
National Assessment of Hurricane-Induced Coastal Erosion Hazards: Northeast Atlantic - Open-File Report 2014–1243
Nor’easters are extratropical storms that often impact the mid- and northeast Atlantic coast of the United States, and are so named because winds hitting the coast come from a northeasterly direction. Nor’easters usually develop along the mid-Atlantic coast of the U.S. and move north or northeastward. While wind speeds during nor’easters generally don’t reach hurricane wind speeds (> 33 m/s), nor’easters are slower moving, affect the coast for a longer duration and can cause significant coastal change due to the frequency and duration of this type of storm event. For the purposes of this assessment, a classification system for storm strength is defined based on wind speed: here, a class 1 nor’easter has wind speeds ranging from 6 to 12 m/s, a class 2 nor’easter has wind speeds ranging from 12 to 18 m/s and a class 3 nor’easter has wind speeds in excess of 18 m/s.
The mean dune elevation along the northeast Atlantic coast is 4.9 m compared to the mean dune elevation of 4.2 m in the mid-Atlantic. However, wave runup and surge tend to be about 50% higher along the northeast Atlantic coast than along the mid-Atlantic coast, leading to greater vulnerability to erosion due to collision and overwash. Wave runup is the dominant hydrodynamic process, comprising roughly 75% of the extreme water level at the coast for all 3 nor’easter classes. Inundation of dunes during nor’easters is very unlikely; even during the strongest class of storm, only 6% of the beaches and dunes along the mid- and northeast Atlantic coast are expected to flood. During a class 3 nor’easter, collision is expected along the majority of the coast and overwash is likely to occur for roughly a third of the coastline.
|Percentage of sandy beaches very likely (probability > 0.9) to experience erosion associated with collision, overwash, and inundation during class 1-3 nor’easter impact.
|Nor’easter Intensity Class|
|collision (dune erosion)|
|US Northeast Atlantic||70||88||95|
|US Northeast Atlantic||12||25||45|
|US Northeast Atlantic||1||2||6|
National Assessment of Nor'easter-Induced Coastal Erosion Hazards: Mid- and Northeast Atlantic Coast - Open-File Report 2015–1154
Research of forecasting the impact of storms on beaches along the US West Coast is currently on-going. See the West Coast Storms Research page for more detail.