Hurricane Nate Will Affect Most Gulf Beaches, USGS Projects

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Editor’s note: This story was updated at 2 pm Saturday, October 7. For the latest graphic showing USGS predictions of the sandy coastal areas likely to be affected by Hurricane Nate, click here.

More information about USGS science in response to Hurricane Nate is at www.usgs.gov/nate

A new projection by coastal erosion experts at the U.S. Geological Survey finds Hurricane Nate could cause erosion along about 85 percent of sandy beaches from the Florida Panhandle through southeast Louisiana’s coastal barrier islands. The projection, which was updated after the National Hurricane Center’s forecast of 8 am Saturday, October 7, also shows that approximately half of the protective sand dunes along that stretch of coast will be overwashed by Nate’s storm waves. The storm’s greatest impact is likely to be along Mississippi and Alabama barrier islands, where storm waves are likely to collide with 100 percent of the islands’ coastal sand dunes.

Tropical Strom Nate
Probability of collision, or the likelihood that wave runup and storm surge will reach the dune toe, causing erosion during Hurricane Nate, issued after National Hurricane Center Advisory 8, 0800 AM EDT FRI OCT 06 2017(Public domain.)

The National Hurricane Center forecasts that Nate will come ashore in the Northern Gulf of Mexico as a Category 2 hurricane. But changes in the storm’s intensity, size or track could increase the likelihood of more significant impacts to coastal dunes and beaches, said USGS research oceanographer Dr. Joseph Long, an expert on the coastal erosion hazards posed by a full range of weather conditions from sunny days to hurricanes.

“In general, the northern Gulf of Mexico is a very vulnerable area because it has low-lying sand dunes along much of the coastline,” Long said. “Even a Category 1 or 2 storm can bring waves large enough to overtop these coastal dunes and cause coastal erosion.”

The USGS’ coastal change forecasting model uses the National Hurricane Center’s storm surge predictions and wave forecast models from the National Oceanic and Atmospheric Administration. The USGS model adds information about the beach slope and dune characteristics to predict how high waves and surge will move up the beach and whether the protective dunes will be eroded or overtopped.

The projected results are based on the assumption that the storm surge will strike at high tide, and are intended as a worst case scenario. They cover only sandy beaches and do not make any estimates of storm effects along stretches of the coast lined by seawalls, marshes or mangroves.

Beach erosion is the first level of damage a major storm can cause. Waves and surge can also overwash the top of the dune, depositing sand inland and causing significant changes to the landscape. Inundation, the most severe impact, occurs when beaches and dunes are completely and continuously submerged by the surge.

NOAA models forecast that the hurricane could generate storm waves as high as six meters (more than 19 feet) just offshore of the Mississippi River delta. Based on that forecast, combined with surge predictions from the National Hurricane Center, the USGS team projects that Mississippi and Alabama sand dunes will be heavily affected by the storm, with lesser, but still significant effects on Louisiana and Florida beaches.

The projection calls for 100 percent of Mississippi’s barrier islands to erode, 95 percent to be overtopped by storm waves and 85 percent be inundated, with flooding occurring along the primary dune line. Similarly, in Alabama, 100 percent of beaches are likely to erode, three-quarters are likely to be overwashed and inundation is projected for about one-fifth of this coast.

In Louisiana, nearly 90 percent of beaches are projected to erode, more than half to experience overwash and about 15 percent to be inundated. On the Florida panhandle, two-thirds of the beaches are projected to erode, with only 15 percent likely to be overwashed and none forecast to be inundated.

A map of these estimates is posted on the USGS’ publicly accessible Coastal Change Hazards Portal. Emergency managers can use the forecast to help identify locations where coastal impacts might be the most severe – for example, where roads may become impassable because of sand, or where storm surge and waves may put some roads underwater. As Hurricane Nate moves closer to a possible landfall on the U.S. mainland, the coastal change projections will be updated.

The USGS continues to prepare for storm impacts as Hurricane Nate approaches the Southeast coast. People potentially affected by the storm can visit http://www.ready.gov/ or http://www.listo.gov/ for emergency planning information and suggestions.