NGDIR News Section--Large and powerful Hurricane Irma is likely to cause significant erosion along U.S. east coast beaches from Florida through South Carolina, according to a new projection from the U.S. Geological Survey. Strong waves and storm surge are likely to erode all sandy beaches in the three states, overtop sand dunes over three-quarters of the coast, and, in some areas, inundate areas behind the dunes. These coastal impacts are based on the National Hurricane Center's September 8 storm track forecast. Actual impacts may be very different if the track moves to the west and affects the Gulf coast.
Water levels three to four meters, or 10 to 12 feet, above normal tide levels are likely for open coast shorelines along Florida's Atlantic coast if Irma continues to track along the east coast, according to Thursday's forecast from the USGS Coastal Change Hazards Group based in St. Petersburg, Florida.
Coastal change predictions for North Carolina will become available as the storm moves closer to that part of the coast, Stockdon said.
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, dune height, sea floor topography and other features to predict how high waves and surge will move up the beach and whether the protective dunes will be 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, Stockdon said. 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 only the first level of damage a major storm can cause. Waves and surge can 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.
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.
An experimental new forecasting tool, called the Total Water Level and Coastal Change Forecast Viewer, offers projections of the timing and the amplitude of storm-induced total water levels- both waves and surge- for specific places along the Florida Atlantic coast from Miami to a point near Palm Bay, and for the state's Gulf coast from the St. Petersburg area to Marco Island. The information, which is provided at 3-hour intervals for a 6 day period, is not intended as a substitute for the National Hurricane Center's storm surge warnings, Stockdon said.
As Hurricane Irma moves closer to the U.S. mainland, the coastal change projections are subject to change. The most recent projections are available on the National Assessment of Storm-Induced Coastal Change Hazards- Hurricane Irma page.