HR: 13:45h
AN: OS33A-02    [Abstracts]
TI: Morphologic Response of the North Carolina Inner Shelf to Hurricanes Isabel (September 2003) and Alex (August 2004)
AU: * Thieler, E R
EM: rthieler@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Hammar-Klose, E S
EM: ehammark@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Himmelstoss, E A
EM: ehimmelstoss@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Murray, A B
EM: abmurray@duke.edu
AF: Duke Univ, Div Earth and Ocean Sciences, Durham, NC 27708 United States
AB: The inner continental shelf between Cape Hatteras and Cape Lookout, North Carolina was mapped four times (Jun 2003, Sep 2003, Jul 2004, Aug 2004) before and after landfalling hurricanes Isabel (18 Sep 2003) and Alex (03 Aug 2004) using sidescan sonar, multibeam and interferometric swath bathymetry, and high-resolution CHIRP seismic systems. These data provide continuous coverage of the seafloor, extending from approximately the 7 m isobath near the shoreline to 10 km offshore (28-30 m water depth). Pre-Isabel imagery shows that the seafloor in this area is characterized by a variety of shore-perpendicular to shore-oblique morphologic features recently termed "sorted bedforms" in reference to the sorting of grain size that has been observed in such bedforms at many inner shelf locations. The bedforms have typical wavelengths of 100-300 m and heights up to 1 m, and extend seaward across the entire study area. Post-storm surveys used sidescan sonar and multibeam bathymetry systems to resurvey selected areas within a week following both hurricanes' landfall. On the inner shelf off Ocracoke Inlet, a 63 km² field of well-organized, low-amplitude (0.5 m), 250 m wavelength sorted bedforms was rearranged by Isabel such that coarse and fine sediments were largely dispersed across the inner shelf, in an apparent reorganization towards a larger-scale pattern adjusted to the storm flow regime. The seafloor morphology returned to its pre-Isabel state by Jul 2004. Alex was a smaller storm than Isabel; only minimal seafloor changes were noted following its passage over the study area. This suggests there is a storm-energy threshold for significant bed reworking that was exceeded by Isabel but not by Alex. These storm responses are consistent with a numerical model of seafloor bedform evolution that treats the transport of coarse and fine sediment fractions as functions of the local bed composition. Large-scale sorted patterns exhibiting the main characteristics of the natural features are a robust prediction of the model and provide insight into physical processes affecting the inner shelf during the storms.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly