T160 Geological Effects of Extreme Storm Events on Small Tropical Island Nations (Posters)

Monday, 26 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

SUBSURFACE SIGNATURES OF RECENT STORM EROSION AND RECOVERY ALONG SAN SALVADOR ISLAND, THE BAHAMAS

Paper number: T160-1
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Subsurface imaging of Bahamian Holocene coastal sequences using ground-penetrating radar (GPR) reveals an important role of storm-generated topographic elements. In addition, georadar also serves as an important method for assessing the effects of recent storms by complementing surface imaging technologies (LiDAR- and drone-based mapping, ground surveys). This is largely due to the ephemeral nature of many storm-related geomorphic features (limestone clasts, berm and dune scarps, surge channels) in sediment-rich carbonate settings. We present examples of high-resolution 800 MHz GPR signatures of event features from Hurricane Joaquin (2015) along San Salvador Island four months following the storm. In a backdune swale at Sandy Hook strandplain, hyperbolic diffractions demonstrate a much greater number of buried limestone clasts compared to those still visible on the surface. Examples from other islands show the importance of landward transport of porous aeolianite blocks from nearshore platforms to sand-dominated barrier lithosomes. Most foredune scarps along San Salvador show partial recovery through aeolian ramp deposition. Pre-storm surveys at several sites (e.g., the Thumb tombolo) will form the basis for comparison with post-Joaquin accretion once the ~2-m-high scarp is healed. Where less severe erosion resulted in berm scarping and saltwater attenuation is not at issue, subsurface mapping of a scarp can complement its surface expression on nearby beach segments. Along the south-facing shoreline of French Bay, a minor depression through a vegetated foredune leads to a massive washover that traversed the main road and partially infilled a small wetland. GPR surveys of this dune gap reveal a large cut-and-fill structure (length: >10 m; width: 4-5 m; depth >1.8 m) containing landward-dipping sigmoidal-oblique reflections. This is consistent with a storm-surge channel scoured during Hurricane Joaquin (or possibly another recent storm) and infilled during its waning stages. Its radar signature is similar to inter-dune storm-surge channel fills produced by Superstorm Sandy (2012) along the New Jersey coast. Such examples of rapid erosional-depositional pulses highlight the need for real-time observations, which are often lacking due to adverse conditions associated with intense storms.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284221

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