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As Hurricane Marco heads to a predicted landfall along the central Gulf Coast, USGS scientists are predicting that nearly half of the barrier islands and beaches across the region will see some storm-caused erosion, with effects concentrated in Louisiana and Mississippi. Across the four-state region from Alabama through Southeast Texas, other USGS scientists were in the field on Sunday, Aug. 23 setting out special instruments called storm-tide sensors that will measure Marco’s storm surge. Those instruments will stay in place to record the effects of Tropical Storm Laura if it strikes the same general area later in the week, as it is currently expected to do. 

The USGS has been closely monitoring this unusual situation, with two storms on track for landfall in the same area within days of each other. Taken together, the USGS teams’ work can help inform evacuation planning, provide information that will help document the storms’ effects for emergency managers and others, and improve future erosions and storm surge forecasting.

Coastal Change Hazards Forecast

“We are monitoring two storms that will likely make landfall in roughly the same area of coastline,” said research oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida. “Usually, there is time between storms that allows beaches to recover naturally, but in this case there won’t be any time for recovery, which makes the coast more vulnerable and forecasting more unique.”

The coastal forecast only applies to sandy beaches and barrier islands and does not include marshes, forested or sea walled shorelines. The current forecast is only for Hurricane Marco. The coastal change forecast for Tropical Storm Laura is likely to come later, when the storm’s track and intensity are clearer.

The least severe level of storm damage is erosion at the base of the dunes, known as collision. In Louisiana and Mississippi, more than half of the beaches and barrier islands – 54% in Louisiana and 56% in Mississippi - are predicted to erode at the dunes’ base. One percent or less of Alabama’s beaches are expected to be affected by collision.

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 and Mississippi, with about 46% of both states’ sandy beaches very likely to be affected by overwash. No overwash is expected in Alabama.

The most severe category of beach damage is called inundation, when seawater completely and continuously submerges beaches and dunes. Inundation is forecast to affect 9% of beaches and barrier islands in Mississippi, 3% in Louisiana, and none in Alabama.

This Coastal Change Storm Hazard Team map is current as of  2 p.m. EDT, August 23, 2020 and shows forecast beach erosion at the base of the dunes (the strip of colored bars closest to the coast), overwash (middle strip) and inundation (outer strip) from Hurricane Marco Note: The model accounts for sandy beaches and barrier islands and does not include marshes, forested or sea walled shorelines. (Public domain.)

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 Marco  is at https://marine.usgs.gov/coastalchangehazardsportal/ . It uses information from the National Hurricane Center’s surge forecasts, and will be updated whenever the hurricane center’s surge forecasts change.

Preparing to Capture Two Consecutive Storm Surges

Storm surges are increases in ocean water levels generated at sea by extreme storms. They can have devastating coastal impacts. Scientists at the USGS and elsewhere want to better understand storm surges so that forecasters can more accurately model and predict surge-related flooding, engineers can design better storm-resistant structures, and emergency responders can work more safely and effectively. 

The USGS has a national network of storm monitoring sites along the coast. These sensors can record water level and barometric pressure every 30 seconds to document the timing and magnitude of storm surge as they make landfall. 

USGS Hydrologic Technician Mark Warzecha installs a storm surge sensor in Port Lavaca, Texas. Photographer: Alex Laffoon, USGS. (Public domain.)

Anticipating the forecast paths and intensity of both Marco and Laura, USGS scientists worked fast on Sunday to deploy extra storm tide sensors at coastal locations within the expected landfall area. Eight scientists from the Lower Mississippi-Gulf Water Science Center were in the field Sunday installing 11 storm tide sensors along the Louisiana coast, three sensors in Alabama and three in Mississippi. In Southeast Texas, six scientists from the Oklahoma-Texas Water Science Center were working to install storm tide sensors at 11 locations near the Louisiana border.

The sensors are housed in steel pipes a few inches wide and about a foot long. Field crews install them on bridges, piers and other structures that have a good chance of surviving a hurricane’s storm surge. USGS scientists expect to keep these temporary sensors in place until Tropical Storm Laura has passed, to record the effects of both storms. 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: https://stn.wim.usgs.gov/fev/

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