Optics, Acoustics, and Stress in a Nearshore Bottom Nepheloid Layer

The goal of this research is to develop greater understanding of the how the flocculation of fine-grained sediment responds to turbulent stresses and how this packaging of sediment affects optical and acoustical properties in the water column. Objectives are to: 1. Quantify the effects of aggregation dynamics on the size distribution of particles in the bottom boundary layer; 2. Quantify how changes in particle packaging affect the optical and acoustical properties of the water column. 3. Develop models describing the associations between particle aggregation, stress, and the acoustical and optical fields.The approach is to obtain measurements that will permit comparisons of the optical and acoustical signatures of suspended particles and inferences of the particle size distribution and its temporal evolution, concurrently with fluid dynamical measurements that determine the flow field within which the particles evolve. The instrumentation is mounted on bottom tripods and includes a 9-wavelength optical attenuation and absorption meter (WetLabs ac-9, with automated regular dissolved measurement for calibration independent particulate measurements), a LISST-100 laser diffraction particle sizer (Agrawal & Pottsmith 2000), a digital floc camera (DFC) (Curran et al. 2002b), a Tracor Acoustic Profiling System (TAPS, Holliday 1987), and an array of SonTek/YSI acoustic Doppler velocimeters (ADVs). Near-simultaneous measurements with and without a filter assure high-quality particulate spectral absorption and attenuation measurements with the ac-9. The LISST-100 and floc camera together provide particulate size distributions from 2.5 micrometers to 1 centimeter. The TAPS obtains range-gated, vertical profiles of acoustical backscatter intensity at a range of frequencies between 0.3 and 3.0 MHz. The TAPS and ADVs produce acoustical measurements over a wide range of frequencies that can be used to generate particle size distributions (Holliday, 1987; Hay and Sheng, 1992).
Prepared in cooperation with Dalhousie University, Halifax, Nova Scotia; Bedford Institute of Oceanography, Dartmouth, Nova Scotia and Woods Hole Oceanographic Institition, Woods Hole, MA.