Follets Island: A Case of Unprecedented Change and Transition from Rollover to Subaqueous Shoals

Document Type

Article

Publication Date

1-18-2018

Department

Marine Science

Abstract

Follets Island, a transgressive island located on the upper Texas coast, is an ideal location to study barrier island transition from a rollover subaerial barrier to subaqueous shoals. This system also allows for an examination of coastal response to accelerated sea-level rise, storms, and sediment supply. The landward shoreline retreat rate during historical time is similar to the landward retreat rate of the bay shoreline, hence its current classification as a rollover barrier. However, the island has a limited and diminishing sand supply, which makes it even more vulnerable to erosion during storms and relative sea-level rise. Four core transects that extend from the upper shoreface to the back barrier bay are used to constrain the thickness of washover, barrier and upper shoreface deposits and to estimate sediment fluxes in the context of the overall sand budget for the island over centennial timescales. Stratigraphic architecture reveals two prominent transgressive surfaces. A lower flooding surface separates red fluvial-deltaic clay from overlying bay mud and an upper erosional surface separates back-barrier deposits from overlying shoreface and foreshore deposits.

Radiocarbon ages are used to constrain the evolution of the barrier and its long-term rate of island migration whereas 210Pb dates are used to constrain the modern sand overwash flux. Results show that significant washover sands are deposited in the bay and about twice this volume is deposited as subaerial washover deposits. The total sand washover volume shows that overwash processes account for about half of the sand produced by shoreline erosion in historical time. Our results also indicate that the historical rate of shoreline retreat is about an order of magnitude faster than the geologic rate. We estimate back-barrier accommodation space to be about three times greater than the volume of the barrier. Hence, given the current shoreline erosion and overwash flux rate, Follets Island will eventually transition from a subaerial rollover barrier to subaqueous shoals. The frequency of severe storms along the Texas coast is not believed to have varied significantly in recent time, but the rate of sea-level rise has increased approximately five-fold and sand supply to the island is minimal. This leads us to suggest that accelerated sea-level rise and diminished sand supply are the main causes of this unprecedented change.

Publication Title

Barrier Dynamis and Response to Changing Climate

First Page

147

Last Page

174

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