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by Vic Hines, Hilary Stockdon, and Hannah Hamilton

The probability of hurricane-induced coastal change on sandy beaches from Florida to New York has been assessed for the first time in two U.S. Geological Survey (USGS) studies released on July 1, 2013.

The two reports—one assessing the coastline from Florida to North Carolina, the other from Virginia to New York—can function as part of a “virtual toolkit” for U.S. Atlantic coast community planners and emergency managers as they make decisions on how to best address coastline vulnerabilities. The new publications complement a 2012 report on the hurricane vulnerability of the U.S. Gulf of Mexico coast.

The new reports show that during even the weakest hurricane, a category 1 with winds between 74 and 95 miles per hour, 89 percent of the dune-backed beaches from Florida to New York are very likely (probability exceeds 90 percent) to experience dune erosion during a direct landfall. But scientists involved say the strength of the studies is in their ability to predict coastal change in specific areas.

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An associated online mapping tool, based on a USGS state-of-the-art model, will allow community planners and emergency managers to focus on a specific storm category and see the predicted coastal change in their area. The tool incorporates findings from the two new reports on the Atlantic coast as well as the 2012 report on the Gulf of Mexico coast. The information it provides can help planners and managers with decisions ranging from changing building codes and choosing locations for new construction to determining the best evacuation routes for future storms.

“The USGS has been working on identifying storm-driven coastal-change hazards for more than a decade,” said Hilary Stockdon, co-author of the studies and research oceanographer with the USGS St. Petersburg Coastal and Marine Science Center in St. Petersburg, Florida. “The data collected and modeling capabilities developed during that period are what’s enabled us to complete these regional assessments of predicted coastal change, providing key information to decision makers working to build more resilient communities and take actions to protect lives and property before storms hit.”

For the entire Atlantic coast study area, the modeling also shows that during a category 1 hurricane, storm waves are expected to increase water levels at the shoreline by approximately 150 percent above storm-surge levels. For example, in a category 1 storm with a surge of 2 meters (about 6 feet), waves would raise water levels at the shoreline by an additional 3 meters (10 feet), for a total water elevation of 5 meters (16 feet). Results show that waves play a significant role in elevating water levels during lower category storms, whereas storm surge is the major contributor to high water levels in stronger storms.

In an assessment of Atlantic coast dune heights from Florida to New York, the researchers found the southeast coasts, because of their lower dune elevations, more likely to experience overwash, or the landward movement of beach sand, than coastlines farther north. Dune heights from Delaware to New York are 1.4 meters (4.6 feet) higher, on average, than the dunes from Maryland south to Florida. Of the beaches studied, those on the South Carolina coast, where average dune elevations are only 2.9 meters (9.5 feet), are the most vulnerable to overwash. Ninety-six percent of coastal locations in the State are likely to undergo overwash if a category 1 hurricane makes landfall there.

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Before Hurricane Sandy, dunes on New York’s south shore were among the highest on the Atlantic coast, and so during a category 1 storm, only 9 percent of these coastal areas were likely to experience overwash. Still, these high dunes were vulnerable to extreme erosion during a category 1 hurricane, with 76 percent of the dunes very likely to be eroded. Such erosion was observed during Hurricane Sandy, which made landfall as an extra-tropical cyclone but pounded the beach with hurricane-strength waves and surge. The protective sand dunes along barrier islands in New Jersey and New York were completely eroded in places, increasing vulnerability to more extreme erosion during future storms.

But vulnerability doesn’t just come down to dune height; scientists credit the continuity of the dunes as being one of the key reasons coastal vulnerabilities vary greatly along the coast.

“Large areas of the South Carolina coast are very likely to erode during hurricanes due to long, continuous stretches of low dunes. In other areas, such as Delaware and New York, the mix of high and low dune elevations creates a more complex picture of vulnerabilities, where relatively safe areas are adjacent to areas that are likely to be inundated,” said Stockdon.

“Inundation” is a process by which an entire beach system is submerged and, in extreme cases, can result in island breaching. Only 9 percent of the entire Atlantic coast study area is very likely to be inundated in a category 1 storm, although the percent is significantly higher in some regions. If a category 1 storm makes landfall on the South Carolina coast, for example, 34 percent of the beaches and dunes there are very likely to be inundated.

Beaches serve as a natural buffer between the ocean and inland communities, ecosystems, and natural resources. However, these dynamic environments move and change in response to winds, waves, and currents. During extreme storms, changes to beaches can be large, and the results are sometimes catastrophic. Lives may be lost, communities destroyed, and millions of dollars spent on rebuilding.

These reports and the online mapping tool can serve as an important resource for coastal planners and emergency managers as they work to protect their communities from future storms.

The 2013 Atlantic coast publications and the 2012 Gulf of Mexico publication are available online. Their full citations are:

Doran, K.S., Stockdon, H.F., Sopkin, K.L., Thompson, D.M., and Plant, N.G., 2013, National assessment of hurricane-induced coastal erosion hazards—Mid-Atlantic coast: U.S. Geological Survey Open-File Report 2013–1131, 28 p. https://doi.org/10.3133/ofr20131131

Stockdon, H.F., Doran, K.J., Thompson, D.M., Sopkin, K.L., and Plant, N.G., 2013, National assessment of hurricane-induced coastal erosion hazards—Southeast Atlantic coast: U.S. Geological Survey Open-File Report 2013–1130, 28 p. https://doi.org/10.3133/ofr20131130

Stockdon, H.F., Doran, K.J., Thompson, D.M., Sopkin, K.L., Plant, N.G., and Sallenger, A.H., 2012, National assessment of hurricane-induced coastal erosion hazards—Gulf of Mexico: U.S. Geological Survey Open-File Report 2012–1084, 51 p. https://doi.org/10.3133/ofr20121084