HR: 1330h
AN: OS23B-03    [Abstracts]
TI: Observations of Sand Transport Processes Over Sorted Bedforms
AU: * Traykovski, P
EM: ptraykovski@whoi.edu
AF: Applied Ocean Physics and Engineering Dept. Woods Hole Oceanographic Institution, MS #12, Woods Hole, MA 02543
AB: High resolution sonar surveys in the past decade have revealed that complex sorted bedforms (rippled scour depressions, RSDs) are ubiquitous features on many sandy inner shelves. These features consist of alternating bands of coarse and fine sand with along-shore scales of 10 to 1000 m and across-shore scales of 100 to 5000 m. Modeling approaches for these features have ranged from rules-based approaches to sediment transport physics based approaches. However, the sediment transport processes over fine and coarse sand with combined weakly non-linear waves and mean current forcing are poorly understood. Specifically, the relative roles of bedload and suspended load forced by non-linear waves and mean currents over large ripples in coarse sand and smaller ripples, or low-relief bed conditions in fine sand are not well understood. For instance, tripod mounted rotary sonar observations have generally shown onshore ripple migration forced by non-linear wave velocities to be the dominant transport process in coarse sand. However, larger-scale sonar and grab sample surveys have shown along-shore grain size variability that is presumably forced by along-shore mean currents. Over the past three years, observations were conducted at the Martha's Vineyard Coastal Observatory RSD field in attempts to quantify the relative roles of these processes. A quadpod, with a rotary sidescan sonar and a 2-axis pencil beam sonar to measure ripple morphology, was deployed in both coarse and fine sand in successive winter seasons. The quadpod also had a 3-axis bistatic pulse coherent acoustic Doppler profiling system that is capable of measuring near-bed (within 30 cm of the seafloor) suspended and bedload transport. Bedload was estimated using spectral processing on the bed and bedload backscatter, thus the high intensity returns from the stationary bed could be separated from returns from the moving bedload in the frequency domain. Preliminary analysis of the observations revealed that in fine sand the transport is dominated by suspended load forced by wave-resuspension and mean current transport. The observations in fine sand contained many storms of varying energy thus are fairly robust. The observations in coarse sand only had 4 hours of data during moderate-energy active conditions, due to a equipment failure, thus additional data and analysis is required to determine the amount of along-shore transport and the relevant forcing processes in coarse sand during more energetic events. From the available data in coarse sand, onshore bedload and near-bed suspended load transport (within 1 cm of the seafloor) forced by non-linear waves dominated the flux, consistent with prior observations of bedload flux estimated from ripple migration.
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly