Graduation Year

2016

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Geology

Major Professor

Ping Wang, Ph.D.

Committee Member

Albert C. Hine, Ph.D.

Committee Member

Lori D. Collins, Ph.D.

Keywords

Beach Processes, Shoreline Erosion, Coastal Morphology, Coastal Management, Shore Protection, Coastal Geology

Abstract

Egmont Key, located at the mouth of Tampa Bay, is part of a dynamic system with many interrelated natural and anthropogenic factors influencing its morphodynamics. This study started in August 2012. During the 3-year period until August 2015, 28 beach profile transects were established and surveyed 10 times. Seventeen historical aerial images from 1942 to 2013 were geo-rectified and analyzed. Three hundred and fourteen sediment samples were procured from the navigation channel dredge area and the beach nourishment area and analyzed for grain size. A numerical wave model was established to simulate the nearshore wave field. The overall goals of this study are to understand the complex morphodynamics of Egmont Key and to evaluate the shore-protection efforts.

The overall area of the Egmont Key has reduced 52% from 2.1 km2 in 1942 to 1.o km2 in 2002. The area loss was mostly caused by beach erosion along the Gulf-facing beach. The island-area reduction from 1942 to 2002 was largely linear. Two periods of accelerated area loss from 1978-1984 and 1999-2002 can be related to dredging of the Egmont Channel and the disposal of dredged materials along the channel. Concerning the relatively high mud content in the borrow area for the 2014 nourishment, a large amount of the fine sediment was lost at a temporal scale of hours to days during the dredging and beach nourishment construction processes. Some of the mud was deposited outside the surf zone at water depths of 2 m or greater. This mud became eroded naturally by energetic conditions at a temporal scale of months. Beach erosion and accretion along the Gulf-facing beach can be related qualitatively to tidal flow patterns. Numerical wave modeling shows that the transverse bars offshore Egmont Key have a moderate influence on the wave field, leading to slightly different wave heights along the shoreline. However, there is no clear relationship between the nearshore wave conditions and the erosion/accretion patterns. The severe shoreline erosion has exposed various fort structures at the shoreline and in the nearshore zone. These structures function as detached breakwaters or groins and have localized influence on the beach state.

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