THE INFLUENCE OF STORM FREQUENCY ON SHORELINE STABILITY, SANTA ROSA ISLAND, FLORIDA

Santa Rosa Island is one of the longest barrier islands in the northern Gulf of Mexico. The island extends 75 kilometers from Destin to Pensacola, Florida. The island is narrow, with an average width of 500 meters and average elevation of 2 meters. During the 19th and much of the 20th century prominent foredunes, ranging as high as 7 meters, helped to keep the island in near-equilibrium sedimentologically with only limited shoreline erosion and some periods of shoreline growth. Major hurricanes in the late 20th and early 21st century, including Opal (1995), Ivan (2004) and Dennis (2005) have destroyed much of the foredune system and significant erosion has taken place along the entire shoreline of Santa Rosa Island. We have used the USGS Digital Shoreline Mapping and Analysis System (DSAS) to analyze all available shoreline data and evaluate the changes in the shoreline history of the island. More than 25 separate shoreline positions have been derived from USCGS T-sheets, aerial photographs and lidar mapping. The data cover the time period 1870-2010. A five-decade period of relative quiescence (1934-1994) during the mid-twentieth century resulted in modest advance (~1 meter/year) of the island’s eastern coastline and a near stable shoreline for the western portion of the island. Several major hurricanes and topical storms that passed over or near the island in the period 1995-2005 produced very significant erosion along the entire length of the island, with an average net erosion of 60 meters (~4-6 meters/year).

In contrast to the historic stability of the island’s foreshore, the western end of the island grew rapidly during the period 1854-1965 with an average growth of 6,500 square meters per year or approximately 6 meters/year of shoreline advance. Increased dredging of Pensacola Pass (1960-present) has apparently stabilized the island’s western terminus. The eastern end of the island was shortened by 4 kilometers in 1928 when extensive flooding produced by an extratropical storm created the current inlet (East Pass) between the island and the mainland. The historic shoreline data underscore the dominant influence of storm frequency in determining coastal change in this region