HR: 1330h
AN: OS23B-05    [Abstracts]
TI: (Ir)regularity of sand ripple patterns - a field observation
AU: * Xu, J
EM: jpx@usgs.gov
AF: U.S. Geological Survey, 400 Natural Bridges Dr., Santa Cruz, CA 95060 United States
AB: High-resolution photographs of sea floor were obtained from a camera mounted on a tripod that was deployed in 10 meters of water on the shelf off the Santa Clara River mouth, southern California. During the month-long (mid-January to late February, 2004) deployment the camera was only able to record high-quality images of sand ripples for a 50-hour window of relatively calm seas (and lower turbidity). During the 50-hour period the significant wave height varied between 0.8 and 2.2 m, and peak wave period between 8 and 13 seconds. Two ripple patterns were observed - (A) long-crested, 2-dimensional vortex ripples, and (B) short-crested, 3-dimensional ripples, both of which have ripple spacings between 7 and 8 cm, indicating a case of anorbital ripples whose spacings are a function of sand grain size (median diameter d0 = 0.013 cm) and independent of the hydrodynamic forcings. While the main ripple orientations in both patterns very well conformed with the wave direction that was consistently from the west, varying between 263-271 degrees, the changes of the ripple patterns during the 50 hours do not appear to be associated with wave parameters that have been shown in lab studies to determine the ripple patterns. These parameters include wave height, wave period, near-bed shear stress, and wave ellipticity. Surprisingly, the ripple patterns seem to change in association with the tidal current, whose magnitude was always less than 10 cm/s, in comparison to wave orbital velocity of 40 - 50 cm/s. The 2-D ripples were mostly observed when the tidal current directed to the south during ebb tide, and the 3-D ripples during the flood tide when the tidal current directed to the north.
DE: 3045 Seafloor morphology and bottom photography
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly