Detailed Bathymetric Surveys Offshore Santa Rosa Island, FL: Before and After Hurricane Ivan (September 16, 2004)

Abstract

Two weeks before Hurricane Ivan reworked the shores and nearshore seafloor of the northeastern Gulf of Mexico, swath bathymetry surveys were conducted with high-frequency (300 and 455 kHz) multibeam echosounders in three areas offshore Santa Rosa Island, FL, an 80-km barrier island extending west from Destin to Pensacola Bay, FL. These surveys were repeated in late October 2004, six weeks after the passage of the hurricane, allowing for quantitative pre- and posthurricane seabed comparisons. Bathymetric difference maps (0.2-0.3-m grid cells) show that sediment accretion exceeded 1 m in areas near the 6-7-m isobaths, where a submerged longshore bar was formed below the breaker zone of large storm waves. Accretion of sediment continued seaward tapering off near the 11-12-m isobaths, with evidence of slight seabed erosion (0.1-0.2 m) seaward of this boundary. Between the 6- and 12-m contour lines, the increase in sediment volume is about 279 000 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Grab samples obtained in the area by Vaughan [IEEE J. Ocean. Eng., vol. 34, no. 4, pp. 495-515, 2009] support the hypothesis that the added material is fine sand eroded from the beach and dunes on Santa Rosa Island by the overwash and inundation associated with Ivan's storm surge and eventually deposited offshore by storm-surge ebb currents. Two-dimensional bottom roughness power density spectra computed from colocated east-west (EW) bathymetry swaths near the 12-m isobath show a post-Ivan threefold increase in root mean square (rms) roughness over the [0.104, 0.495] m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> spatial wave number band. Bottom roughness spectrograms computed along individual north-south (NS) survey track lines perpendicular to the shoreline and extending 10 km offshore indicate that Ivan-induced waves and currents reworked the seabed to water depths of at least 22 m, with a twofold to fourfold increase in rms roughness over the [0.023, 0.156] m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> spatial wave number band.