An updated USGS coastal change forecast predicts that more than half of Louisiana’s barrier islands and beaches will be inundated - continuously submerged with flooding behind the dunes - by the storm surge from Hurricane Laura.  The USGS prediction 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 storm’s effects on sandy shorelines in Texas, Mississippi and Alabama will be less severe and extensive than initially forecast, as Laura’s forecast track narrows to a likely landfall in southwest Louisiana, said USGS oceanographer Kara Doran, who leads the team that made the new coastal erosion forecast. The prediction issued at 4 am Central Daylight Time August 26 updates an August 25 forecast for the region from East  Matagorda Bay, Texas to the Florida-Alabama border.

“The biggest change to our initial forecast is in Texas, where the percentage of dunes likely to be overwashed by storm surge has dramatically decreased,” said Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida. "The storm is stronger than predicted, but because the National Hurricane Center has narrowed down where the storm will make landfall, this makes our coastal change forecast more precise.”

Storm surges are increases in ocean water levels generated at sea by extreme storms. They can have devastating coastal impacts.

“The storm surges are likely going to be the most impactful events of this storm,” said Doran. The National Weather Service is predicting a peak storm surge of 15-20 feet in portions of western Louisiana.

Regionwide, the USGS experts predict that storm-driven waves and tides will cause some erosion of nearly half of all sandy beaches and barrier islands along the central Gulf Coast. Coastal inundation, the most serious storm surge effect on sandy shorelines, may affect more than one-fourth of all barrier islands and beaches, with these effects also most extensive in Louisiana.

The USGS coastal erosion prediction covers only sandy shorelines, such as beaches and barrier islands, and not marshes, forests or shorelines with seawalls or other armoring. Louisiana has more marshy shorelines and fewer sandy beaches than the other three states, so this prediction applies to a smaller portion of that state’s coast, but the predicted extent of overwash in Louisiana is still significant, said USGS oceanographer Kara Doran. 

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The least severe level of storm damage on sandy shorelines is erosion at the base of sand dunes, known as collision. About 81% of Louisiana’s sandy shorelines and 48% of Texas beaches and barrier islands from East Matagorda Bay to the Louisiana border are predicted to erode at the dunes’ base. About 27% of Mississippi beaches and 6% of Alabama’s beaches are projected to erode at the dunes’ base.

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 Louisiana, with about 77% of the states’ sandy beaches very likely to be affected by overwash. Texas is forecast to have 24% of its beaches overwashed and Mississippi is predicted to see 12% of beaches overwashed. Only 1% of Alabama sandy shorelines are expected to be overwashed.

The most severe category of beach damage is inundation, when seawater completely and continuously submerges beaches and dunes. The forecast calls for 55% of Louisiana sandy shorelines to be inundated, compared to 3% in Texas and none in Mississippi and Alabama.

The prediction of Laura’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 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. 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 latest coastal change forecast for Laura is at https://marine.usgs.gov/coastalchangehazardsportal/. It will be updated whenever the hurricane center’s surge forecasts change.

Over the weekend other USGS scientists from the Lower Mississippi Gulf Water Science Center and the Oklahoma-Texas Water Science Center deployed scientific instruments called storm tide sensors along the coast from southeast Texas to Alabama. The sensors will stay in place, taking readings, until after Hurricane Laura has passed. Scientists will use the information they collect to document storm surge and help improve future forecasting of storm effects. The public can see data from these sensors and other USGS instruments that track water levels in the region at the USGS Flood Event Viewer.

As the USGS continues to take all appropriate preparedness actions in response Tropical Storm Marco, 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.

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