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Researchers from the U.S. Geological Survey (USGS) Coastal and Marine Geology Program recently conducted two cruises aboard the research vessel Pelican in the Florida panhandle to investigate the morphology, orientation, and dynamics of ripples on the sea floor. Ripples are an important part of sediment movement on the sea floor, called bedload sediment transport, and they also contribute to flow resistance and turbulence generation in the layer of water just above the sea floor, or the bottom boundary layer. A better understanding of sediment movement on the seabed and in the bottom boundary layer can be used in many waysto predict how sediment will bury sunken objects, for example, or how and where contaminated sediment will move on the seabed.
This research is collaborative with the University of Florida; the Monterey Bay Aquarium Research Institute (MBARI); and the Office of Naval Research, which is conducting a study called "Ripples DRI" to better understand the growth, evolution, and decay of ripples on sandy sea floors in shallow water. The research is also coordinated with work by scientists from Woods Hole Oceanographic Institution and with the Sediment-Acoustics Experiment (SAX04) led by the University of Washington's Applied Physics Laboratory. The overall objective of SAX04 is to better understand acoustic detection at low grazing angles of objects, such as mines, buried in sandy marine sediment (see Sediment Acoustics Experiment - 2004).
During the cruises, conducted September 24-29 and November 6-11, we measured ripple morphology and bed-sediment size with a new instrumentation system, designed and built in collaboration with MBARI, that uses a combination of acoustical and optical techniques. Ripples were measured by using a multiple-transducer array, a sector-scanning fan-beam sonar, and a slide-projector/camera system. Bed-sediment size was measured by using a retractable bed-sediment camera known as the "poking eyeball" (see Sound Waves article "The 'Poking Eyeball'a Prototype Underwater Camera System"). All the instruments were powered and controlled through a single cable, using network communication protocols.
The passage of Hurricane Ivan just a week before the start of our field study provided an unusual opportunity to investigate the formation and endurance of ripples across the continental shelf. We observed wave-formed ripples across the continental shelf out to our deepest measurement station, at approximately 50-m depth.
Next, we will be conducting laboratory experiments under controlled wave and current conditions to investigate the orientation of ripples. These experiments will be conducted in a large flume at a unique laboratory facility at Tsukuba University in Japan.
Scientific personnel on the cruises were Carissa Carter, Hank Chezar, Daniel Hanes, Jodi Harney, Gerry Hatcher, Jessie Lacy, Kevin O'Toole, and David Rubin (USGS Coastal and Marine Geology); Craig Okuda (MBARI); Viktor Adams, Allison Penko, Sidney Schofield, and Don Slinn (University of Florida); and Hiroshi Ikeda (Tsukuba University).
in this issue:
How Sea Floor Sediment Moves
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