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.