A GIS AND GPS STUDY OF BEACH RIDGE SYSTEMS, SAINT VINCENT ISLAND, FLORIDA

Saint Vincent Island is a 47 square kilometer barrier island located off the Big Bend of the Florida panhandle. The island contains one of the most well-developed beach ridge systems in Florida. As part of a support effort for the study sea level changes in this portion of the Gulf of Mexico we have completed a new map of these beach ridges, using modern digital infrared, false color orthophoto imaging (1999), which has a ground resolution of one meter. Beach ridges and intervening swales were entered into a GIS database using “heads up” digitizing. Over 120 separate beach ridges were identified. The beach ridges can be divided into 12 separate sets, based upon their relative age, orientation and elevation. Each ridge is believed to mark the evolving location of the island’s shoreline. Island growth was dominantly from the northeast to southwest, with some of the most recent addition being restricted to the southeast corner of the island. The primary means of identifying breach ridges from swales in the digital imaging is based upon differences in vegetation. Field checking demonstrated that beach ridges are characterized by xeric vegetation including scrub oak and live oak. Swales between ridges are characterized by slash pine, palmetto and palustine marsh. Locally, the swales are occupied by shallow, elongate ponds. Many of the ridges are closely spaced, with ridge spacing ranging from 50-100 meters.

Ashtech Pro Mark 2 survey grade GPS units were used to conduct a preliminary survey of the elevations of breach ridge sites that are being used for the study of sea level history. The measured crests of a limited number of ridges ranged from 1.9 - 4.1 meters (NAVD88). Additional elevation measurements are being made at archeological sites on the north side of the island. These sites include 1-3 meter high oyster shell middends, which are associated with the early stages (>3,000 years B.P) of the island’s history.