LONG-TERM AND STORM-RELATED CHANGES OF SHORELINES AND DUNES ON SANTA ROSA ISLAND, FLORIDA: A BASELINE FOR STUDYING THE EFFECTS OF CLIMATE CHANGE AND SEA-LEVEL RISE ON A LOW-LYING BARRIER ISLAND

Santa Rosa Island is a 70 km long barrier island located on the northwestern panhandle coast of Florida. The island is narrow (150-850 m) and has a low average elevation (2m). For most of the 19^th and 20^th centuries, 4-5 m high foredunes and 10-15 m high bay-side dunes were present on most of the island. Much of the foredune topography has been removed as a consequence of recent hurricanes. Most of the island has very limited vegetation cover. The tidal range in this region is low (~0.4m). The measured historic long-term sea-level rise near the western end of the island is 2.1 ± 0.26 mm/yr. Much of the island is undeveloped, and there has been only limited modification of the island’s topography and shoreline due to human activities. Due to the natural setting of the island and its overall low topography, it has been selected as test site for modeling the potential effects of near-term (100 years) sea-level and climate change (particularly possible increases in hurricane activity) on barrier evolution. To establish scaling information for the modeling, historical information for the island’s shoreline and topography are being analyzed. Over 20 different historic shoreline data sets are available, covering the time period 1780-2009. Sources of data include nautical charts, aerial photographs, beach profiling and Lidar. For the period 1850-1990, shoreline change varied with location and island width. Along the western portion of the island, the shoreline retreated at rates of up to 2m/yr, with an accompanying longshore growth of the western end of the island of about 6m/yr. The wide, central portion of the island had a relatively stable shoreline (<1 m/yr change); while the eastern portion of the island, adjacent to East Pass, advanced more than 1 m/yr. Following three major hurricanes, Opal (1995), Ivan (2004) and Dennis (2004), there was major retreat of the shoreline along all portions of the island. Locally, retreat rates exceeded 10 m/yr. Most of the foredunes on the island have been heavily eroded by hurricane activity, especially hurricanes Opal and Ivan. The limited presence of foredunes has allowed the development of overwash deposits along narrow portions of the island. The dramatic effects of repetitive storm events on low-lying barrier islands must be evaluated in modeling island evolution associated with rising sea levels.