IN-SITU OBSERVATIONS OF FLOC SETTLING VELOCITIES IN GLACIER BAY, ALASKA

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 F loc Camera Assembly (FCA) in Tarr Inlet, Glacier Bay, Alaska. These ob servations were used to estimate floc effective densities. Three resul ts emerge from this work. First, fits of settling velocity and effecti ve density to diameter are consistent with expressions published for o ther environments, suggesting that common controls on floc size and se ttling velocity operate across diverse marine environments. Second, th e raw data show considerable scatter, with upper and lower 95% predict ion intervals on settling velocity and excess density differing by abo ut a factor of 7. Analysis of sources of error suggests that the varia bility is caused by differences in component-grain composition among f locs and turbulent stirring within the stilling box. Third, bin-averag ed effective densities and settling velocities are highly correlated w ith diameter. Thus, while it is not possible, based on diameter, to pr edict accurately the settling velocity of a single floc, it is possibl e to estimate the mean settling velocity of a population of like-sized flocs. (C) 1998 Elsevier Science B.V.