Before and After: Coastal Change Caused by Hurricane Irma

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New high-angle oblique photos of portions of Florida’s beaches taken before and after Hurricane Irma made landfall and swept up the state show the impact of the hurricane’s storm surge and waves. These NOAA images document changes to the coast after the storm, helping the USGS fine-tune its coastal-change forecasting model.

This article is part of the August-October 2017 issue of the Sound Waves newsletter.

“The hurricane's observed impact on Florida was significantly less than the pre-storm prediction,” said Kara Doran, a USGS research scientist on the National Assessment of Coastal Change Hazards storm team. “The coastal-change forecast for Irma was based on the National Hurricane Center's predictions, which had indicated significantly more surge and greater waves along the Gulf Coast than actually occurred.”

There are eight pairs of images from the east coast, nine pairs from the southwest coast, and five pairs from the Keys.

A home on Little Gasparilla Island was swept away by Hurricane Irma’s waves and surge.

A home on Little Gasparilla Island was swept away by Hurricane Irma’s waves and surge. The beach narrowed significantly and two more homes are now more vulnerable to future storm impacts. The predicted probability of overwash for this location was 97%. Image credit: USGS

Doran and her colleagues are working on a detailed assessment of Irma’s effects on the vulnerable Florida shorelines. The team is currently comparing low-altitude, oblique aerial photos taken in 2017 to photos collected a few days after Hurricane Irma made landfall.

“High-altitude satellite images give us a big-picture view of the coastline, and that’s very useful to identify large areas of overwash, but we can’t see the dunes in those images,” Doran said. “These lower-altitude oblique photos give us a clearer view of the beach and dunes. We can see whether the storm surge and waves altered or eliminated that protective barrier, and what happened to the houses and roads.”

Waves from Hurricane Irma reached the toe of the dunes in Ponte Vedra, causing the dune face to erode and leaving a steep scarp

Waves from Hurricane Irma reached the toe of the dunes in Ponte Vedra, causing the dune face to erode and leaving a steep scarp. The predicted probability of dune erosion for this location was 58%. Photo credit: USGS

When a storm is about to strike the U.S. Atlantic or Gulf coast, the team forecasts the likelihood of coastal erosion and other changes, using a computer model that incorporates the National Hurricane Center storm-surge predictions and National Oceanic and Atmospheric Administration wave forecasts. The USGS model adds information about the beach slope and dune height to predict how high waves and surge will move up the beach.

The model forecasts three types of storm impact to the dunes that protect coastal communities: erosion, overtopping, and inundation, or flooding that reaches over and behind the dunes. After a storm has passed, the researchers test the model’s accuracy using information about the state of the dunes from before-and-after photographs and other data.

The U.S. Army Corps of Engineers, in collaboration with the USGS, is conducting a series of data-collection flights along the Florida coastlines using airborne lidar, a technology that bounces a beam of laser light at the ground to produce detailed elevation information about the surface below. Using that information, the USGS researchers can estimate the volume of sand that Hurricane Irma moved off Florida beaches, measure the height and breadth of the remaining dunes, and be ready to forecast the erosion potential of the next storm.

See more before-and-after-Hurricane Irma photos from along the Florida coastlines https://coastal.er.usgs.gov/hurricanes/irma/photo-comparisons/ and learn more about the USGS coastal-change hazard team's hurricane research at https://www.usgs.gov/centers/spcmsc/science/national-assessment-storm-induced-coastal-change-hazards.

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Sound Waves Newsletter - August-October 2017

Video cameras help forecast coastal change, USGS Monitors Huge Landslides on California's Big Sur Coast, Coastal change caused by Hurricane Irma, and more in this August-October 2017 issue of Sound Waves.