Geology topics
Publications and source records attributed to A. J. Williams.
Analysis of grain size distribution from RMI mosaics (Curiosity) are used to better define variations in grain size and facies in the Shaler outcrop.
Current measurements from a vector-averaging current meter (VACM) on a subsurface mooring and a benthic acoustic stress sensor (BASS) on a bottom tripod are compared to assess their relative accuracy. The instruments were deployed off northern California at a midshelf site (water depth approximately 90 m) as part of the STRESS (Sediment Transport Events on Shelves and Slopes) field program. The subsurface mooring and bottom tripod were within a few hundred meters of each other, with the BASS 5.0 m and the VACM 6.7 m above the bottom, during two tripod deployments of 49 and 32 days in the winter of 1988/89. Speed differences between the VACM and BASS current observations have a mean of 0.2 cm s −1 and a standard deviation of 1.2 cm s −1 . If the mean speed profile is logarithmic, the expected mean speed difference due to the vertical separation is about 0.4 cm s −1 . The average speed difference between the VACM and BASS increases as near-bottom wave orbital velocities get large relative to hourly averaged currents, consistent with laboratory studies of VACMs in oscillating flows. Direction differences have a mean of 1° and standard deviations of about 5° for speeds greater than 10 cm s −1 . The relative accuracy of the corresponding velocity measurements is ±2 cm s −1 (mean differences less than 0.6 cm s −1 and standard deviations of about 1 cm s −1 ). The equations used to convert VACM rotor rotation rates to current speed we based on a calibration study by Woodward and Appell rather than one based on a study by Cherriman that is routinely used at the Woods Hole Oceanographic Institution. The former yields closer agreement between the BASS and VACM speed measurements during STRESS (mean speed difference 0.2 cm s −1 versus 1.4 cm s −1 ).
Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) "C" transect as part of the 1990-1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds. ?? 1994.