In situ observations of floc settling velocities in Glacier Bay, Alaska

Abstract

In situ floc settling velocities and diameters of particles ranging in size from 0.63 to 5.05 mm equivalent circular diameter were measured under a buoyant discharge plume by deploying a bottom-tripod-mounted Floc Camera Assembly (FCA) in Tarr Inlet, Glacier Bay, Alaska. These observations were used to estimate floc effective densities. Three results emerge from this work. First, fits of settling velocity and effective density to diameter are consistent with expressions published for other environments, suggesting that common controls on floc size and settling velocity operate across diverse marine environments. Second, the raw data show considerable scatter, with upper and lower 95% prediction intervals on settling velocity and excess density differing by about a factor of 7. Analysis of sources of error suggests that the variability is caused by differences in component-grain composition among flocs and turbulent stirring within the stilling box. Third, bin-averaged effective densities and settling velocities are highly correlated with diameter. Thus, while it is not possible, based on diameter, to predict accurately the settling velocity of a single floc, it is possible to estimate the mean settling velocity of a population of like-sized flocs.