FGCU Sea Grant team updates modeling of Hurricane Ian impacts, plans resilience tests for Fort Myers Beach

Florida Gulf Coast University researchers told the Marine Environmental Resource Task Force on March 12 that they have run and are finalizing validation of a coastal model that reconstructs Hurricane Ian's effects on Fort Myers Beach and will use that model to test potential resilience measures.

The update came from Christopher Daley, an assistant professor of coastal geomorphology at Florida Gulf Coast University, who described lidar and ground-penetrating radar surveys, 35 airborne lidar flights, and the use of an XBeach morphodynamic model to simulate waves, surge and sediment transport. "We're using models to predict changes that will occur along the beach during extreme events, and then also looking at...what are some of the interventions we can implement," Daley said.

Why it matters: The model is intended to show where sand was eroded, where it deposited onshore and offshore, and how built features channel surge and overwash. That information is intended to help Fort Myers Beach and partner jurisdictions evaluate whether interventions such as widening the beach, constructing dunes, or adding seawalls would reduce damage in future storms.

What the team presented: Daley said the project uses XBeach and data from collaborators including DeltARES (Netherlands), NOAA, the U.S. Geological Survey and the U.S. Army Corps of Engineers. The team compared modeled post-Ian bathymetry (initialized using publicly available data from 2018) to measured post-storm surveys and found general agreement in shoreline retreat and some berm formation but also identified artifacts that stem from using the older 2018 bathymetry instead of updated 2022 pre-Ian data.

Daley described model outputs showing scour channels cut between buildings on the northern island, onshore deposition behind the primary road in several locations, and a line of sand pulled offshore in deeper water. He said the team must re-run some simulations after updating the initial bathymetry to the 2022 dataset and adding non-erodible surfaces such as roadways to improve fidelity: "We actually have to include other non erodible surfaces, especially roadways in our model. And that will help us to improve the predictions."

Planned resilience experiments: The team will test three interventions in the validated model: increasing beach width by about 50 feet, constructing a linear dune roughly 4 feet high and 15 feet wide, and that same dune combined with a seawall (the seawall represented as a non-erodible surface). They will run each intervention against multiple storm scenarios: the Hurricane Ian event used for validation, a range of 5- and 10-year storms discussed previously, and additional cases based on measured data from Hurricanes Helene and Milton.

Data and run-time notes: Daley said their year-long program included 35 lidar flights and in-situ wave and water-level measurements offshore during Helene and Milton, noting offshore wave heights and base surges of about 1.5 meters during those events. He added that each high-resolution model simulation currently takes about a day and a half to run, and that increasing grid resolution to resolve roads and buildings increases run time.

Open questions and next steps: Daley said the immediate next step is to re-initiate simulations with the 2022 pre-Ian bathymetry, include additional non-erodible surfaces in the model, complete the Ian validation, and then run the resilience experiments. He said the team plans a public workshop after the summer and an end-of-year report once the suite of simulations is complete. "After this summer we'll come back with another report. Definitely by the end of the year when all of our simulations have been completed, we will have an open workshop where we'll invite the public to...look a bit at what some of our results are like," he said.

Questions from committee members addressed event return periods and vegetation. When asked how to classify Milton, Daley estimated it as a higher-frequency extreme—"maybe something like...a 50-year event"—and confirmed vegetation will be included in dune scenarios because vegetation increases surface roughness and reduces erosion.

Provenance: The task force heard the presentation and asked questions during the March 12 meeting; the team said project documents exist and have been provided to the town.