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Coastal and Marine Geology Program
Origin, Geologic History, and Morphodynamics of Egmont Key, Hillsborough County, Florida

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Introduction
Agenda
Processes
Framework
Morphodynamics
Attendees
Contact:
Chief Scientist
Jennifer A. Kling, Department of Geology, University of South Florida, Tampa, Florida
Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, Florida

site map

Figure 1: Site map of Egmont Key.

Egmont Key is located on the ebb-tidal delta at the mouth of Tampa Bay amid the west central Florida barrier island chain (Figure 1). Many studies have been conducted along the west Florida coast, however investigation of Egmont Key has been minimal. The primary objective of this study is to use the stratigraphic and historical records of Egmont Key to identify changes in the island s morphology through time and to reconstruct the origin and geologic history of the island. Data obtained in this study will also be useful in determining the role, if any, that dredging of the ebb tidal delta for beach nourishment plays in contributing to the erosion of Egmont Key.

The proposed investigation of Egmont Key will be conducted with two general approaches. First, the prior history of the island will be examined through vibracores and historical map analysis and secondly, the current dynamics of the island s coast will be studied by monitoring modern processes and responses. These approaches will provide appropriate data to enable assessment of long-term changes on the island and details of the processes currently modifying the shoreline of Egmont Key.

map of shoreline changes

Figure 2: Map of Egmont's shoreline changes dating back to 1877 (Terry, 1995).

The study of Egmont Key s long-term history involves the collection, examination, and interpretation of sediment cores from the island. Ten to twelve vibracores will be taken on Egmont Key using the method of Lanesky et al. (1976). The more recent history of changes on Egmont Key will be determined through a time-series analysis of maps and aerial photographs. Considerable effort will be directed toward a thorough examination of the first accurate maps (1881) and subsequent aerial photographs which extend back to 1926. Shoreline changes will be mapped for all available photos and the area of change for each time period will be determined by digitizing the shoreline positions. In addition, shallow subtidal and intertidal sand bars will be mapped. The shoreline changes and sediment bodies will be incorporated into a sediment budget for the island which will be used to determine if there has been an overall loss or gain in sediment on Egmont Key over the time period involved; about a century. Preliminary analysis indicates that the gulfward side of Egmont Key has been eroding throughout historical times while the shoreline on the landward side of the island has remained stable (Figure 2).

The other primary objective of the project is directed toward an understanding of the present conditions on the island. This time-series monitoring program will involve the collection of data on changes along the Egmont Key shoreline as well as recording current, wave, and weather data. Beach profiles will serve as the monitoring system for shoreline change. In total, 13 beach monuments are being profiled each month starting in January of 1996 and concluding in January of 1997 (Figure 3). Additional profiles will be conducted as deemed necessary due to storm events. Initial profiles indicate that significant erosion or shoreline modification is occurring monthly on Egmont Key. This is shown in (Figure 4) which is a profile taken from the monument R2 as seen in (Figure 3). (Figure 4) compares monthly profiles from January, February, March and April.
aerial photograph

Figure 3: Aerial photograph (3/94) of Egmont Key showing locations of monuments that are available for beach profile monitoring. All monuments except DNR 6 and DNR 14 will be utilized in this study.

profile data

Figure 4: Profile data of R2 from January-April 1996.

References

  • Davis, R.A., Jr., Hine, A.C., and Shinn, E.A., 1992, Holocene coastal development on the Florida Peninsula, in Fletcher, C.H., III, and Wehmiller, J.F., eds., Quaternary Coasts of the United States: Marine and Lacustrine Systems, SEPM Special Publication, No. 48, p. 193-212.
  • Davis, R.A., 1985, Coastal Morphodynamics: Egmont Key to Anclote Key, in Geology of the Barrier Island and Marsh-Dominated coasts, West-Central Florida, ed. R.A. Davis, A.C. Hine, and D.F. Belknap, Univ. South Fla., p. 6-26.
  • Lanesky, D.E., Logan, B.W., Brown, R.G., and Hine, A.C., 1979, A new approach to portable vibracoring underwater and on land, Journal of Sedimentary Petrology, v 49., p. 654-657.
  • Terry, James, 1992, Egmont Key Shoreline Study, Pinellas County, Florida, Public works G.I.S. Map.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Morphodynamics > Origin, Geologic History, and Morphodynamics of Egmont Key, Hillsborough County, Florida


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