SEDIMENT REDISTRIBUTION PATTERNS AND SUBAERIAL VOLUME CHANGES INDUCED BY HURRICANE IVAN: GULF ISLANDS NATIONAL SEASHORE, FLORIDA

In September 2004, Hurricane Ivan, a strong category 3 hurricane made landfall near the Florida-Alabama border. Located within the storms eastern quadrants, Gulf Islands National Seashore (GINS) located on Santa Rosa Island, experienced inundation to overwash regime level impacts, with measured surge levels approaching 4 meters, excluding contributions from run-up. This study examines the long-shore and cross-shore morphologic response to GINS induced by Ivan, within the context of subaerial sediment redistribution patterns and volumes. Analysis of pre- and post-storm Lidar survey data in conjunction with ground penetrating radar surveys, and sediment coring and trenching reveal new insights into the factors governing overwash induced dune erosion and sediment redistribution.

Inundation and overwash regime impacts at GINS yielded an average reduction in foredune elevations on the order of 1.6 m. At Ft. Pickens, along the west end of GINS, few features with pre-storm elevations above 1.5 m (NAVD) remained. The interior platform was reduced to a uniform elevation of about 1-m (NAVD), and locally, cross-shore transport resulted in 50 to 120 m of landward migration of the back-bay shoreline. However, post-storm sediment volumes indicate an overall net loss, indicating some offshore transported occurred, likely during the early building phases of the storm. Further east, higher initial foredune elevations in conjunction with greater cross-shore and long-shore continuity factors yielded a more heterogeneous morphologic response, and reduced overall net sediment losses. Barrier response to this severe storm was largely controlled by the magnitude of storm forcing, in conjunction with antecedent morphological factors. These insights can aid park managers in assessing existing resource vulnerabilities, and mitigating future storm impacts.