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A new U.S. Geological Survey coastal change forecast predicts sandy beaches along the northern Gulf Coast are likely to see extreme impacts from Hurricane Ida, which is expected to make landfall August 29 as a major hurricane. Sandy beaches in Louisiana and Mississippi are expected to bear the brunt of the storm’s strong waves and surge, while Alabama and Florida beaches may see moderate to minor damage.

The National Hurricane Center is currently forecasting life-threatening storm surge along parts of the Gulf coast for Ida, which upon landfall will become the 5^th named storm to strike the U.S. this year. USGS coastal change forecasts provide vital information during coastal storms that can help 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 a storm.

Waves and surge caused by Hurricane Ida are likely to cause 45% of Louisiana's and 10% of Mississippi's sandy beaches to be inundated - meaning continuously covered by ocean water. This is the most severe type of storm effect on coastal beaches, with flooding behind the dunes that may impact coastal communities. The areas currently projected to be inundated are from Marsh Island, Louisiana, to Ship Island, Mississippi. Inundation is not expected along the Alabama and Florida coastlines.

Screenshot of the Hurricane Ida coastal change forecast. (Public domain.)

“Ida has the potential to cause a lot of coastal change along the northern Gulf, especially after the region endured impacts from several large storms during the historically active 2020 hurricane season,” said oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team.

The second most severe level of coastal damage, overwash, occurs when water levels reach higher than the top of dunes. Approximately 61% of Louisiana’s beaches and 41% of Mississippi’s beaches are expected to be overwashed by Ida. About 2% of Alabama’s beaches and 1% of beaches in Florida may also be overwashed. When a beach is overwashed, sand can be pushed and deposited inland, causing significant changes to coastal landscapes and blocking roadways. Overwash can reduce the height of protective sand dunes, alter the beaches’ profile, and leave areas behind the dunes more vulnerable to future storms.

The least severe level of storm damage on sandy shorelines is erosion at the base of sand dunes. In Louisiana, 67% of coastal dunes in Louisiana are projected to be eroded by Ida. In Mississippi, 63% of dunes will likely erode along with 31% in Alabama, and 2% in Florida.

The most current forecast is always available on the USGS Coastal Change Hazards Portal.

The prediction of Ida’s 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 model starts with inputs from the NHC’s storm surge predictions and National Oceanic and Atmospheric Administration wave forecast models. It then adds detailed information about the expected landfall region’s beach slope and dune height. The predictions define “very likely” effects as those that have at least a 90% chance of taking place, based on the storm’s forecast track and intensity. 

As the USGS continues to take all appropriate preparedness actions in response to Ida, those in the storm's projected path can visit Ready.gov or Listo.gov for tips on creating emergency plans and putting together an emergency supply kit.