Estimating effective density of cohesive sediment using shape factors from holographic images

To reveal the influence of shape factors on the effective density (ρ_e) of cohesive sediment, a series of laboratory experiments has been conducted under different turbulence intensities using a submersible digital holographic camera (LISST-Holo). Shape factors representing axis ratio, boundary roughness, and sturdiness of flocs exhibited characteristic variations with turbulence intensity. For turbulence intensities from 0.54 to 1.42 Pa, micro-flocs were broken into single grains, resulting in a constant axis ratio, a 13% decrease in boundary roughness, and a 16% increase in sturdiness. Out of these shape factors, ρ_e was mainly derived from the concept of sturdiness, which represents the fraction of primary particles within flocs. In fact, the 16% increase in sturdiness added 9% to ρ_e. When the flocs became sturdier, ρ_e had lower dependence on floc size due to the increase in the fraction of primary particles. Furthermore, ρ_e as a function of floc size might be affected by the response of the optical instrument. In the case of fine particles (<25 μm), not every suspended particle was reconstructed from the holographic image, inducing an underestimation of ρ_e. This is because insufficient scattering from floc edges impacts the particle detection ability. Despite the uncertainties, LISST-Holo is well suited to obtain a series of data for shape factors of flocs. These outcomes give insight into means for estimating ρ_e.