HR: 1330h
AN: OS43A-18    [Abstracts]
TI: Side-Scan Sonar Observations off Santa Rosa Island, Florida Before and After Hurricane Ivan, September 2004
AU: * Vaughan, C
EM: chad.vaughan@nrlssc.navy.mil
AF: Naval Research Laboratory, Code 7431 Bldg 1005, Stennis Space Center, MS 39529 United States
AU: Briggs, K B
EM: kevin.briggs@nrlssc.navy.mil
AF: Naval Research Laboratory, Code 7431 Bldg 1005, Stennis Space Center, MS 39529 United States
AU: Richardson, M D
EM: mike.richardson@nrlssc.navy.mil
AF: Naval Research Laboratory, Code 7431 Bldg 1005, Stennis Space Center, MS 39529 United States
AB: A 6-km² area immediately offshore of Santa Rosa Island, Florida in 5-15-m water depth was surveyed with a 300-kHz side-scan sonar as part of the Coastal Dynamics in Heterogeneous Sedimentary Environments research initiative funded by the Office of Naval Research. Side-scan sonar observations were made five days before and five days after Hurricane Ivan made landfall approximately 100 km to the west of the study location in September 2004. The sonar observations made prior to the hurricane indicate a mostly sand bottom that included areas of uniform, small-scale wind-wave ripple morphology occurring mostly in the deeper regions of the western half of the survey area. The ripple orientation is shore-normal, with the crests and troughs essentially extending E-W. The ripple morphology is less prominent in the eastern half of the surveyed area and not apparent in the more shallow areas, consisting mostly of little apparent bottom structure. Imagery also show two distinct areas of low backscatter trending NW-SE through the survey area that appear to be mud swales. The obvious morphology change as a result of Hurricane Ivan was the widespread formation of low-backscatter patches that give the imagery a mottled texture. These patches vary in size and shape, but 25 m² is typical. Diver observations in an area about 5 km to the east of the study site confirmed the existence of mud deposits, or mud-filled depressions. These mud deposits range in thickness from thin, 2-cm layers up to 15-cm deposits. Outflow from the lagoonal areas behind barrier islands after the passage of the hurricane likely supplied the mud that pooled in bathymetric depressions. Also, post-hurricane sonographs show more prominent wave ripples with larger crestal spacings and amplitude, but similar to pre-hurricane sonographs, ripples were less uniform to non-existent in the shallow areas. The mud swale formations remained and are better defined in the imagery after the hurricane. Particle-size analysis of 30 grab samples collected in the survey area post-hurricane indicate a medium sand throughout, except in two locations where mud deposits were collected in the grab sample.
DE: 3022 Marine sediments--processes and transport
DE: 3045 Seafloor morphology and bottom photography
DE: 4219 Continental shelf processes
DE: 4546 Nearshore processes
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly