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As Hurricane Isaias moves towards the Southeastern Atlantic Coast, USGS coastal change experts predict that storm waves kicked up by the Category 1 hurricane are likely to cause some erosion at the base of the dunes along about 11 percent of coastal beaches between Florida and Virginia. However, only about one percent of beaches in the region are likely to have waves overwashing the dunes, and inundation - the most severe type of beach erosion - is not predicted anywhere in the region.

USGS coastal change forecasts can help local emergency management officials decide which areas to evacuate, where and when to close coastal roads, and where to position clean-up equipment in advance of the storm. The latest coastal change forecast for Hurricane Isaias is at https://marine.usgs.gov/coastalchangehazardsportal/ui/alias/isaias2020 . It uses information from the National Hurricane Center’s surge forecasts, and will be updated whenever the hurricane center’s surge forecasts change.

“Predicted coastal change impacts from Isaias are not as severe as one might expect, due to the forecast track of the storm away from land and its relatively fast forward motion,” said research oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida. “Large waves will impact the entire US Atlantic coastline, but without large storm surge, these waves will likely not overtop the protective dunes.

“It is important to remember that local conditions may vary,” Doran said. “Citizens should follow the evacuation advice of local emergency management authorities.”

This Coastal Change Storm Hazard Team map was created August 1, 2020 and shows forecast beach erosion (the strip of colored bars closest to the coast), overwash (middle strip) and inundation (outer strip) effects of Hurricane Isaias. For possible updates, see https://marine.usgs.gov/coastalchangehazardsportal/ui/alias/isaias2020 Credit: USGS, Public domain.​​​​​​​​​​​​​​

The least severe level of storm damage is erosion at the base of the dunes, known as collision. Nearly 23 percent of Georgia’s beaches are very likely to be eroded at their bases. In South Carolina about 18 percent of beaches are predicted to erode at the dunes’ base. That effect is predicted for 11 percent of Florida’s beaches, three percent of North Carolina’s beaches, and none of Virginia’s beaches.

Overwash is the middle range of potential storm damage. As waves and surge reach higher than the top of the dune, overwash can transport large amounts of sand across coastal environments, depositing sand inland and causing significant changes to the landscape. Overwash can reduce the height of the coast’s protective line of sand dunes, alter the beaches’ profile, and leave areas behind the dunes more vulnerable to future storms.

South Carolina is the only state where a substantial amount of overwash is predicted, with about six percent of the state’s sandy beaches very likely to be affected by overwash. In North Carolina and Georgia, less than one percent of beaches are predicted to overwash.

The most severe category of beach damage is called inundation, when seawater completely and continuously submerges beaches and dunes. None of the Southeast Atlantic beaches are predicted to be inundated by storm waves from Isaias.

The prediction of Isaias’ effects at landfall is based on results of the USGS Coastal Change Forecast model, which has been in use since 2011 and is continually being improved. The Coastal Change Forecast model starts with inputs from the National Hurricane Center’s storm surge predictions and National Oceanic and Atmospheric Administration wave forecast models. The USGS model then adds detailed information about the forecasted landfall region’s beach slope and dune height. It predicts how waves and surge will move up the beach, and whether the protective dunes will be overtopped.

Because the waves used in the USGS model are offshore in deep water, the model forecasts the storms’ effects on coastal, seaward-facing sandy beaches, not estuarine shorelines. The USGS and its research partners are working on developing similar forecasting capacity for other types of shorelines.

The predictions define “very likely” effects as those that have at least a 90 percent chance of taking place, based on the storm’s forecast track and intensity. The predictions are available at high resolution for all the areas likely to be affected by storm-tides and waves from Isaias. They will be updated to reflect the latest information from the National Hurricane Center and NOAA.

The most up-to-date forecasts for potential coastal change predictions are at https://marine.usgs.gov/coastalchangehazardsportal/

The USGS continues to take all appropriate preparedness and response actions as Isaias approaches the Atlantic coast. People potentially affected by the storm can visit http://www.ready.gov/ or http://www.listo.gov/ for tips on creating emergency plans and putting together an emergency supply kit.