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.

Sorted bedforms off Western Long Island, New York, USA: Asymmetrical morphology and twelve‐year migration record

“…In stratigraphic analysis, this unconformity, or ravinement, usually separates overlying ‘modern’ marine sands from underlying Holocene estuarine sediments (typically best preserved in palaeo‐channels, e.g. Nordfjord et al ., ; Liu et al ., , ) or Pleistocene or older sediments (Swift & Thorne, ; Goff, ; Goff et al ., ,b).…”

“…This study focuses on the morphology of the transgressive ravinement (the erosional surface created by landward migration of the shoreface caused by sea‐level rise; e.g. Schwab et al ., ; Goff et al ., ,b), the thickness of the overlying modern (i.e. at the sea floor) marine sand unit, as well as cross‐sectional seismic profiles for sorted bedform stratigraphy.…”

The impact of Hurricane Sandy on the shoreface and inner shelf of Fire Island, New York: Large bedform migration but limited erosion

“…documented by Goff et al (2014) at Bolivar Peninsula occurred in only ~4 m water depth. In 547 contrast, the Fire Island shoreface extends to ~13-16 m water depth, and the FIE and FIW 548 surveys did not reach shallower than ~10-12 m. Therefore, it is possible that a Sandy-driven 549 erosional event, similar to the Ike-driven Bolivar Peninsula event, occurred in water depth 550 shallower than we were able to map offshore of Fire Island.…”

“…However, the modern shoreface 551 sand wedge is also much thicker seaward of Fire Island than on Bolivar Peninsula. Given the 552 trends suggested by Figures 12 and 15, the shoreface wedge will be meters thick at such shallow 553 depths, compared with ~0.5 m or less seen along the lower Bolivar shoreface (Rodriguez et al, 554 2001;Goff et al, 2014). Therefore, an erosional event along the Fire Island shoreface of similar 555 magnitude and at similar depth to that of the Bolivar shoreface would not contribute to evolution 556 of the transgressive ravinement.…”

“…But no such erosional event was observed along the Fire Island shoreface after 543 Sandy; the modern sand bedforms are intact, though migrated. 544A critical difference between the two settings is that the Bolivar Peninsula shoreface is very 545 shallow compared to Fire Island, extending only to ~5-6 m water depth; the erosional event 546documented by Goff et al (2014) at Bolivar Peninsula occurred in only ~4 m water depth. In 547 contrast, the Fire Island shoreface extends to ~13-16 m water depth, and the FIE and FIW 548 surveys did not reach shallower than ~10-12 m. Therefore, it is possible that a Sandy-driven 549 erosional event, similar to the Ike-driven Bolivar Peninsula event, occurred in water depth 550 shallower than we were able to map offshore of Fire Island.…”