Graduation Year

2024

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

John Bishop, Ph.D.

Committee Member

Thomas Juster, Ph.D.

Keywords

beach erosion, beach profiles, coastal resiliency, longshore current, nearshore sediment transport, shore protection

Abstract

This study examines the long-term performance of repeated beach nourishment over an approximately 35-year period at two barrier islands, Sand Key and Treasure Island along the west-central Florida coast, USA. Five cycles of beach nourishment on Sand Key and six on Treasure Island were conducted during the study period. Three erosional hotspots were identified along a combined 14-km stretch of the nourished beach. The processes that cause the erosional hotspot were examined through numerical modeling of nearshore wave and current fields.

The most recent three nourishments since 2006 were monitored systematically by this study. Beach profiles were surveyed bimonthly to quarterly on Sand Key and Treasure Island, including 74 profiles on Sand Key (R-55 to R-124) and 17 on Treasure Island (R-127 to R-143). Profiles were spaced ~300-m apart. The 14-km stretch was divided into six beach segments based on beach dynamics, including three erosional hotspots: North Sand Key, Headland, and Sunset Beach, one gap in the nourishment at Belleair Shores, and two typical erosive beaches: Indian Rocks, and South Sand Key. The previous two nourishments before 2006 on Sand Key and three on Treasure Island were also analyzed based on existing data collected by other studies. A numerical model was constructed to depict the causes of erosional hotspots using the Coastal Modeling System (CMS), developed by the US Army Engineer Research Development Center. Wave fields and flow fields driven by both waves and tides were computed for the analysis of erosional hotspots.

The repeated nourishment maintained a minimum 10-m dry beach at all the studied segments. The current design at Sunset Beach erosional hotspot is insufficient because the dry beach becomes progressively narrower after each renourishment cycle, despite constructing the beach farther seaward each cycle. The gap in the nourishment on Sand Key did not gain any significant dry beach width due to the lack of a mechanism to retain sand. The nourishment cycles successfully compensated for the existing sand deficit but did not eliminate the deficit. Therefore, the repeated nourishment served as a maintenance strategy while the cause of erosion was not fundamentally altered.

The three identified erosional hotspots are caused by a gradient in longshore sediment transport, although the causes and magnitude of the transport gradient are different. At North Sand Key, the aggressive transport gradient is caused by wave refraction around the Clearwater Pass ebb delta under northerly approaching waves. Under southerly approaching waves, the transport gradient can be attributed to a depleted sand supply from the non-nourished beach to the south. At the broad Sand Key headland, the elevated erosion is caused by relatively higher waves. However, both wave height and longshore current are relatively uniform along this stretch. The lack of a local transport gradient results in a less aggressive erosional hotspot as compared to North Sand Key. At Sunset Beach, the southward increasing wave height and longshore current occur under both northerly and southerly incident waves. The wave shadowing by the shallow John’s Pass ebb delta results in an aggressive southward increasing transport gradient under northerly incident waves, while under southerly incident waves, a mild southward increasing gradient is caused by significant wave refraction and a southwest facing shoreline. Sunset Beach has an abundance of sand supply from the north. However, the sand supply did not fundamentally change the trend of erosion.

Download

Included in

Geology Commons

Share

COinS