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Paul Hill 1 , Timothy Milligan 2 , Kristian Curran 1 , and Brent Law 2

OPTICS, ACOUSTICS, AND STRESS IN A NEARSHORE BOTTOM NEPHLOID LAYER (2005) The role of in situ particle size distribution. Paul Hill 1 , Timothy Milligan 2 , Kristian Curran 1 , and Brent Law 2 1 Department of Oceanography, Dalhousie University,

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Paul Hill 1 , Timothy Milligan 2 , Kristian Curran 1 , and Brent Law 2

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  1. OPTICS, ACOUSTICS, AND STRESS IN A NEARSHORE BOTTOM NEPHLOID LAYER (2005)The role of in situ particle size distribution Paul Hill1, Timothy Milligan2, Kristian Curran1, and Brent Law2 1 Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J1 2 Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, B2Y 4A2

  2. Goals of the Study • In a coastal bottom nephloid layer, measure the suspended size distribution of particles 1 µm to 1 cm in diameter. • Link the suspended particle size distribution to the optical and acoustical properties of the water column.

  3. Alter the Digital Floc Camera (DFC) orientation to prevent accumulation of sediment on the camera housing. Deploy the INSSECT. Deploy optics, acoustics, particle sizing and flow velocity instruments simultaneously. Objectives for 2005

  4. Digital Floc Camera (DFC) orientation OASIS 2004 OASIS 2005 In 2005, images were captured every 15 minutes from September 3 to 22, 2005.

  5. INSSECT deployment The deployment of INSSECT was not successful in the energetic coastal environment offshore of Martha’s Vineyard. Wave pumping prevented determination of floc settling velocity by the Digital Video Camera. Sediment resuspension/deposition overwhelmed the carousel sediment trap system and prevented estimation of the flux.

  6. DFC spectra: Hurricane Ophelia D50 = 552 µm September 17, 2005 at 1200 h D50 = 143 µm September 18, 2005 at 0400 h

  7. D50 during Hurricane Ophelia D50 estimated from DFC size spectra versus environmental variables.

  8. Entropy analysis on DFC spectra:Hurricane Ophelia

  9. Merging DFC and LISST-B spectra On average, the LISST-B underestimates the DFC volume concentration in the overlapping size classes by a factor of 50.

  10. Entropy analysis on merged spectra:First two weeks of deployment

  11. Waves (m) Single Grains (<36 µm) Microflocs (36 - 133 µm) Macroflocs (>133 µm) Suspension composition Suspension composition estimated from merged size spectra. Area Volume

  12. Flocs are not spheres How do flocs of similar cross-sectional area, but different shape and composition, affect DFC and LISST-B spectra?

  13. Next steps in data analysis • Merge the DFC size spectra with spectra from the LISST-B, LISST-C, and LISST-Floc. • Estimate mass spectra assuming effective floc density (Khelifa and Hill, 2005. Journal of Hydraulic Research, IAHR, Accepted May 2005). • Characterize the variability of the merged size spectra for the entire deployment period using the Entropy Analysis approach. • Compare size spectra variability to environmental variables (e.g. total boundary shear stress).

  14. Questions • What DFC and LISST arrangement best characterizes the in situ size spectra? • Do the LISST-Floc and DFC show good agreement in the larger size classes? • What variables control variation in the size spectra through time (e.g. local resuspension/deposition or advection caused by tide and waves)? • Can we constrain the timescales of floc formation, breakup, and deposition?

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