Wave-driven flooding behind dunes unlikely on SE Atlantic coast, USGS predicts
This news release was updated August 3 based on the latest coastal change forecast.
To learn more about USGS’s role providing science to decision makers before, during and after Tropical Storm Isaias, visit https://www.usgs.gov/isaias.
As Tropical Storm Isaias moves towards a forecast landfall along the Carolinas, USGS coastal change experts predict that storm waves are likely to cause some erosion at the base of the dunes along nearly one-fifth of coastal beaches between Florida and Virginia. The greatest effects are predicted in South Carolina, where nearly two-thirds of sandy beaches are very likely to have some erosion.
Regionwide, only about three percent of beaches are likely to have waves overwashing the dunes, and inundation - the most severe type of beach damage - 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 Tropical Storm 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 have increased in South Carolina and North Carolina as the forecast track of Isaias has narrowed in on a landfall in these states,” said research oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida. “As the storm moves inland, storm surge and waves will increase ahead of landfall, and create the possibility of extensive beach and dune erosion.”
“It is important to remember that local conditions may vary,” Doran said. “Citizens should follow the evacuation advice of local emergency management authorities.”
The least severe level of storm damage is erosion at the base of the dunes, known as collision. In South Carolina, about 64 percent of beaches are predicted to erode at the dunes’ base. Nearly 19 percent of North Carolina’s beaches and 18 percent of Georgia’s beaches are very likely to be eroded at their bases. None of Virginia’s beaches are predicted to erode.
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.
A substantial amount of overwash is predicted in South Carolina, with about 13 percent of the state’s sandy beaches very likely to be affected by overwash. In North Carolina, two percent of beaches are predicted to overwash and in Georgia, the amount is less than one percent.
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 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.