45237-GB8
Sediment Transport Processes on the Continental Shelf of Northwest Florida
Funding from the American Chemical Society Petroleum Research Fund supported the construction and deployment of a dedicated submersible instrument platform to monitor the hydrodynamics (waves and currents) on the inner and mid-continental shelf of northwest Florida. Specifically the platform was needed to characterize the patterns of sediment transport and bed response on the continental shelf to extreme storms relative to the entire range of wave and current forcing. While it is recognized that storm waves are responsible for a large part of the sediment transport and bed reworking on the continental shelf. However, in the low-energy environment of northwest Florida it is assumed that shelf processes have been relatively unimportant to the development of the continental shelf since the last lowstand. The recent increase in extreme storm events within the region with the warming of Atlantic sea-surface temperatures suggests that the shelf experiences wave conditions on decadal or longer timescales capable of entraining and transporting sediment on the seafloor of the inner- and mid-shelf. Because exploratory wells have recently revealed the existence of gas reserves and interest in drilling in federal water off the Florida Panhandle has grown (OCS Study MMS 2004-021), data from the instrumented platform will provide an important scientific tool that can be used in the current debate over the utilization of Outer Continental Shelf (OCS) oil and natural gas resources in northwest Florida. An improved understanding of sediment transport on the inner and mid-continental shelf is also important for the identification of hazards to structures placed or anchored on the shelf and to in the assessment of how these structures and related activity will impact the local sedimentary environment. Funding in the first year was partly used to fund a graduate student to develop the instrument platform and to complete a geophysical survey of the field site. The geophysical survey included a 100 kHz sidescan-sonar system, a high- resolution marine sediment imaging instrument (Stratabox) and a hull-mounted fathometer. Transects were spaced every 500 m, extending into the Gulf of Mexico to a distance of 1 km and into Santa Rosa Sound to a maximum distance of 1 km. The bathymetry of the area turned out to be more complex than originally believed. A series of offshore sinusoidal ridges and troughs oriented northwest-southeast were observed along the entire length of the island. While these features were also described by Hyne and Goodell (1967), little is known about how these shore-transverse ridges developed except that they appear to be aligned with DeSoto Canyon to the east of the study area. The location of these ridges was found to be an important control on the height and extent of the foredunes on the adjacent barrier island. Sections of the coast with little to no dune development and heavy storm damage during Hurricane Ivan were observed in the narrowest portions of the island (between headlands) and along the western edge of the transverse ridges. In contrast, large foredunes and the backbarrier dunes were observed at the widest sections of the island (the cuspate headlands), at the crest of the transverse ridges. Since the ridges were related to dune morphology and hurricane impact on the island, they were also found to be coherent with recent and historical rates of shoreline erosion. The genesis of the transverse ridges is not clear. It is reasonable to expect that they were formed during the alongshore growth of the barrier island, but it is also possible that they were formed or at least are modified by tropical storms and hurricanes. In either case, it appears that the inner-shelf stratigraphy and morphology is dominated by shoreface and shelf processes. Based on the geophysical survey we have been able to identify a number of deployment sites in relation to the transverse ridges, including but not limited to the crest and trough. The instrument platform and instrument capabilities were first tested during a field study conducted in May 2008. An earlier deployment was postponed as I and my research program moved from the University of West Florida to Texas A&M University. The depth of the first deployment was only 20 m and the duration only 1 month so that the installation and extraction of the sensor by divers and buoys could be tested. Permanent deployment of the revised instrument pod is planned for May 2009.
