Alex Hurley

Particle density is fundamental for determining clearance rate of a suspension because it affects settling velocity. In most aquatic environments, however, suspended sediment is composed of loosely packed particle aggregates that cannot be sampled without disrupting the particle packing and, as a result, the particle bulk density. The goal of this study is to compare two methods that estimate particle bulk density without directly sampling suspended particles. A new, fast, but untested method (LD method) uses a Sequoia Scientific LISST 100x particle sizer, which records beam attenuation (Cp, m^-1) and particle volumes smaller than 250 µm along with a digital floc camera (DFC) used to estimate volume of particles larger than approximately 5µm.. The measured beam attenuation obtained from the LISST is proportional to mass in suspension. Volume in suspension is obtained from the LISST and DFC. Dividing mass in suspension by volume in suspension yields an estimate of the solid mass per unit volume within suspended particles. The water mass per unit volume contained within the flocculated particles can be approximated as being equal water density. Addition of solid and water masses per unit particle volume yields particle bulk density (?). A second, more accurate, but more labour-intensive method (DVC method) involves the use of a digital video camera (DVC), which measures particle size and settling velocity. With knowledge of the density and viscosity of the fluid, Stokes Law can be re-arranged to solve for particle density. These instruments were mounted to the Modified in Situ Size Settling Column Tripod (MINSSECT) and deployed in 12 metres of water at the Martha's Vineyard Coastal Observatory during approximately month-long periods in September and October 2007 and 2011. Densities from each method in 2011 are similar, while the LD Method in 2007 yields much higher densities than the DVC method. Higher densities in 2007 result from significantly lower particle volumes from the DFC. Lower DFC volumes in 2007 may reflect actual lower abundances of large particles, or they may arise from poorer image quality caused either by degraded water clarity of different particle composition.

Pages: 49
Supervisor: Paul Hill