GEOLOGIC CONTROLS ON COASTAL DUNE MORPHOLOGY AND BARRIER ISLAND RESPONSE TO HURRICANES

The response of a barrier island to an extreme storm depends on the surge elevation relative to the height and extent of coastal dune development, which can exhibit considerable variability alongshore. The morphology of the primary and secondary dunes along Santa Rosa Island in northwest Florida were characterized before and after Hurricane Ivan using empirical orthogonal function analysis, and cross-correlation analysis was used to identify statistically significant correlations with the offshore bathymetry, island width and historical rates of shoreline retreat. Results suggest that areas with little to no dune development before Hurricane Ivan were associated with a narrow island and the trough of a shore-oblique bar on the inner-shelf. Overwash penetration tended to be larger in these areas and island breaching was common, leaving the surface close to the watertable and covered by a lag of shell and gravel. In contrast, large foredunes are associated with the widest sections of the island (the sound-side cuspate headlands) and the crest of a shore-oblique bar. As a consequence, these areas have smaller overwash penetration distances and during an extreme storm most sediment is deposited on the shoreface. This alongshore variation in dune morphology and island response was coherent with the rates of island retreat. Areas with smaller dunes had larger retreat rates, whereas areas with smaller dunes had smaller rates of shoreline retreat. It is argued that the alongshore pattern of dune morphology and island response to a hurricane is geologically controlled and that areas where the island was breached will continue to be areas of concern. It is further argued that the cuspate headlands develop through alongshore sediment transport of sediment deposited by overwash in Santa Rosa Island, which reinforces the alongshore patterns described.