[The following is a press release from the U.S. Geological Survey.]
Although Tropical Storm Barry is expected to make landfall in Louisiana as a Category 1 hurricane on Saturday, USGS coastal change experts forecast that wind-driven waves are likely to damage beaches in Mississippi, Alabama and the western Florida Panhandle as well, with those effects being felt as early as Thursday night.
If Barry continues on the course and intensity predicted by the National Hurricane Center on Thursday, it is very likely that about one-fifth of Mississippi’s sandy beaches will be overwashed, with storm waves breaking over the dune peaks, a USGS computer model predicts. In Louisiana, about one-tenth of sandy beaches will very likely be overwashed by storm waves.
“Even though the storm is expected to make landfall in Louisiana and the storm surge is expected to be greatest there, the coastal impacts may be greater in Mississippi, Alabama and the Florida Panhandle because of large onshore-directed waves,” said research oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida.
Barry’s storm surge is forecast to be about three to six feet, centered on the Louisiana coast, but the storm is expected to generate waves of 12 to 15 feet, Doran said, with those waves beginning to strike shorelines east of Louisiana late on Thursday.
“Wave run-up is much higher for Mississippi and Alabama, and a little bit for the Florida Panhandle, because those areas are forecast to be within the storm’s northeast quadrant, which is the most powerful portion of the storm, with winds and waves directly aimed at the coast,” she said. “The main impact will probably be beach and dune erosion, with some overwash on Mississippi’s low-lying barrier islands.”
Only three percent of Alabama’s sandy beaches and none of Florida’s are very likely to be overwashed, according to Thursday’s prediction.
Overwashing 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.
The most severe category of beach damage is called inundation, when seawater completely and continuously submerges beaches and dunes. Only one percent of Louisiana’s beaches and none of the Mississippi, Alabama or Florida Panhandle beaches are predicted to be inundated, based on the current forecast for the storm.
The least severe level of storm damage is erosion at the base of the dunes. More than one-third of Louisiana’s beaches, 16 percent of Mississippi and Alabama beaches, and three percent of Florida Panhandle beaches are very likely to be eroded at their bases.
The prediction of Barry’s effects at landfall are 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.
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. They are available at high resolution for all the areas likely to be affected by storm-tides and waves from Barry, and will be updated to reflect the latest information from the National Hurricane Center and NOAA.