Topical Area: Sedimentology

Mark Hansen and Nancy T. DeWitt, USGS, St. Petersburg, FL

Detailed, high-resolution maps of Florida Bay mudbank elevations are needed to understand sediment dynamics and provide input into circulation models. The bathymetry of Florida Bay has not been systematically mapped in nearly 100 years, and some shallow areas have never been mapped. Due to the shallow depths of the Bay, an updated bathymetry grid (digitally derived from the survey) is critical for circulation models. An accurate, modern bathymetric survey will also provide a baseline for assessing future sedimentation rates in the Bay, and a foundation for developing a sediment budget. The objective of this research is to collect new bathymetry for all of Florida Bay, digitize the historical shoreline and bathymetric data, compare previous data to modern data, and produce maps and digital grids of historical and modern bathymetry.

The strategy is to systematically map the bathymetry in Florida Bay in order aid in the assessment of Bay sedimentation rates and to provide an accurate sea-floor surface for numerical models. The Bay is being mapped with the SANDS bathymetric system capable of measuring to within +/-10cm vertical accuracy and can collect data in water depths as shallow as 50cm. Data will be collected on a USGS 7.5 minute quad-by-quad basis proceeding westward from Blackwater Sound. Sounding trackline spacing varies depending upon the relief of the sea-floor, i.e. closer spacing in near mud banks - wider spacing in the basins. Survey data is also collected around the perimeter of all islands and mudbanks. For shallow areas not accessible by boat, we are employing satellite imagery techniques in combination with water stage data to derive mud-bank elevations. This technique will provide iso-contour information that will be used in conjunction with the bathymetry data to produce digital sea-floor grids.

To date, Florida Bay has been systemically surveyed for the area from Long Sound to Rabbit Key basin. The remaining area west of Rabbit Key to a line from Cape Sable to Long Key will be surveyed in 1998 and early 1999. Historical shoreline and hydrographic data from the 1890's for all of Florida Bay has been digitized and registered to a modern datum. Shoreline comparisons of the modern data set to the historic have been initiated in some areas. Preliminary comparisons suggest the historical shoreline data has horizontal inconsistencies in areas where there are cuts between islands but appears accurate in open coast areas. Comparison results indicate there has been significant shoreline changes (over 300m) in some areas with minimal change in others. From satellite imagery, we have successfully derived mud-bank elevations for several water stage levels. Upon completion of the bathymetric surveys, comparisons of historic to modern sea-floor maps will be made in order to help assess sedimentation rates and determine the effect of sea-level rise on the Bay. All products for this project will be completed by Oct. 1999.