BARRIER ISLAND VULNERABILITY IN RESPONSE TO RECENT STORMINESS: DYNAMICS OF A LONGSHORE DIVERGENT CELL, FT. MYERS BEACH, SOUTHWEST FLORIDA

Barrier island dynamics are expected to change in response to accelerated sea-level rise and increased storminess. Altered patterns of accretion and erosion within convergent and divergent longshore cells will influence the distribution of habitat and the demands associated with the public’s use of beaches. Such concerns are great for the community of Ft. Myers Beach, Southwest Florida, where beaches are critical to the tourism-based economy. This study investigates the sedimentologic history of the island’s divergent cell, located with the center of its 11 km length, since 2004. The island experienced significant storm impacts in 2004 from Hurricanes Charley and Ivan, in 2005 from Hurricane Wilma, and in 2012 from Tropical Storm Debby. Sixteen, equally spaced beach-monitoring profiles were established over 1.5 km of shoreline. Those profiles were monitored seasonally through 2014. Digital elevation models, generated from LiDAR data obtained post-Ivan, post-Wilma, and in 2010, were used to produce historic profiles. Surveyed R-monuments and seawalls provided a consistent vertical datum; a Trimble RTK GPS unit allowed for 2-cm vertical accuracy. The medially located transect corresponds well with the divergent center. Profiles since 2004 show patterns of protracted erosion and consistently represent the narrowest beaches, indicating the position of the divergent cell has remained stable. Hurricanes Charley and Ivan were responsible for significant erosion of both the backshore and foredune. More extensive erosion occurred in front of the seawall-armored beaches in the southern half. All profiles recovered naturally between 2005 and 2010. Since 2010 and as a result of Tropical Storm Debby and numerous cold front disturbances, the divergent cell has experienced renewed erosion, while the convergent regions have received much of the translocated sand. The collective results of this study indicate the most economically important portion of Ft. Myers Beach will continue to experience shoreline retreat, necessitating renourishment. Because sands are displaced locally to neighboring convergent zones, the cost of renourishment should be mitigated. These results are currently being used to inform vulnerability modeling of the barrier island under a variety of sea-level rise and storminess scenarios.