SURVIVAL AND DESTRUCTION OF COASTAL DUNES ALONG THE NORTHWESTERN FLORIDA COAST DURING THE IMPACT OF HURRICANE IVAN IN 2004

This study examines the severe impact to coastal dunes along the northwestern Florida coast by a large strong category 3 hurricane, Ivan, via comparison of pre- and post-storm ground surveys, aerial and ground photos, and airborne LIDAR (Light Detecting and Ranging) surveys. Based on pre-Ivan LIDAR survey, the elevation of berm and backbeach is typically 2.0 m above MSL. Incipient dunes range from 2.5 to 10 m above MSL, or 0.5 to 8.0 m above the beach. The hummocky dunes developed over recent washover platforms are typically less than 4.0 m above MSL. The densely vegetated established dune fields are composed of dunes of less than 7.0 m high and extensive intra-dune wetland of less than 1.0 m above MSL. The entire northwestern Florida coast was impacted by Ivan. The above landscape was substantially changed including the destruction of over 70% of the incipient and hummocky dunes and infilling of a large portion of low-lying wetlands. The degree of storm-induced morphology change depends not only on the intensity and duration of the storm but also on the antecedent morphological characteristics of the barrier island. Dune survival is controlled by: 1) hurricane characteristics, including intensity, duration, and frequency, and 2) morphological parameters including width of the barrier island, height and width of the dunefields, vegetation type, distance of the dunes to the ocean, and continuity of the dunefields. Comparisons of pre- and post-Ivan cross-island LIDAR profiles indicate that at most locations, more sand was eroded from the sub-aerial portion of the barrier island, e.g., beach and dune, than that deposited as washover lobes. This suggests a net sand loss to the offshore region. Evidence of sand moving alongshore related to the oblique orientation of the dunes was also identified. Under inundation regime, the sub-aerial sediment deficit may be accounted for by the sub-aqueous sedimentation in the back-barrier bay.