Shoreface ravinement evolution tracked by repeat geophysical surveys following Hurricane Ike, Bolivar Peninsula, Texas, 2008–2013

Abstract

We have investigated the impact of Hurricane Ike on the shoreface of the Bolivar Peninsula, Texas, using three near-surface marine geophysical surveys during a five-year span following the storm, which made landfall in September of 2008. Multibeam bathymetry, sidescan backscatter, and CHIRP subbottom reflection data were collected during each survey. The first survey, in November, 2008, discovered the presence of an erosional, landward-facing scarp across a broad of the barrier spit shoreface at approximately 3.5-m water depth. The erosion incised into a shallow reflection horizon interpreted as the Holocene shoreface ravinement, exposing lag deposits that were truncated at the scarp and forming a thin high-backscatter anomaly. Up to 1.0 m of material was excavated shoreward of the scarp, including up to 0.5 m of shoreface sand above the shoreface ravinement, and another 0.5 m of indeterminate lithology below, consisting of easily erodible material. A second survey, in May 2010, found that the scarp had migrated seaward by up to 100 m and the depth of erosion indicated by the scarp had reduced to approximately [Formula: see text]. In a May 2013 survey, the scarp was no longer in evidence. The initial erosional event is interpreted to be caused by the ebb flow of the storm surge across the peninsula compounded by high-wave orbital velocities in the waning storm. This erosional event and its subsequent evolution demonstrate an impact on the shoreface lasting for years post-storm, as the shoreface gradually adjusted to a new equilibrium profile. These results document that an advancement of a shoreface ravinement can be caused by a single large event (e.g., tropical storm or hurricane) in this microtidal, relatively low wave energy setting. This process, tracked with near-surface geophysical techniques, was a key but seldom-observed component of the back stepping of barrier islands during sea-level rise.