ACOUSTIC MEASUREMENTS OF THE SPATIAL AND TEMPORAL STRUCTURE OF THE NEAR-BOTTOM BOUNDARY-LAYER IN THE 1990-1991 STRESS EXPERIMENT

As part of the 1990-1991 Sediment TRansport Events on Shelves and Slop es (STRESS) experiment, a 5 MHz Acoustic BackScatter System (ABSS) was deployed in 90 m of water to measure vertical profiles of near-bottom suspended sediment concentration. By looking at the vertical profile of concentration from 0 to 50 cm above bottom (cmab) with 1 cm vertica l resolution, the ABSS was able to examine the detailed structure of t he bottom boundary layer created by combined wave and current stresses . The acoustic profiles clearly showed the wave-current boundary layer , which extends to (order) 10 cmab. The profiles also showed evidence of an ''intermediate'' boundary layer, also influenced by combined wav e and current stresses, just above the wave-current boundary layer. Th is paper examines the boundary-layer structure by comparing acoustic d ata obtained by the authors to a I-D eddy viscosity model formulation. Specifically, these data are compared to a simple extension of the Gr ant-Glenn-Madsen model formulation. Also of interest is the appearance of apparently 3-D ''advective plume'' structures in these data. This is an interesting feature in a site which was initially chosen to be a good example of (temporally averaged) I-D bottom boundary-layer dynam ics. Computer modeling and sector-scanning sonar images are presented to justify the plausibility of observing 3-D structure at the STRESS s ite. (C) 1997 Elsevier Science Ltd.