Controls on effective settling velocity of suspended sediment in the Eel River flood plume

Bulk effective settling velocities required to explain sinking losses from the Eel River flood plume off the coast of northern California are of order 0.1 mm s−1 for five different helicopter-based sampling surveys conducted in January and February 1998. These effective settling velocities exceed those expected for single-grain sinking and implicate flocculation as an important mechanism for speeding the removal of sediment from the Eel River plume. The relative constancy of effective settling velocities despite widely varying winds, waves, and currents is consistent with photographs in the plume that show little variability in floc size with total suspended sediment mass concentration, turbulent-kinetic-energy dissipation rate, elapsed time since sediment within flocs left the river mouth, or depth. These observations of floc size contrast with those made in winter 1997 during the exceptionally large New Yearʹs flood. During that event, increases of floc size with depth are evident. In 1997, higher sediment concentrations associated with the significantly larger discharge likely allowed flocs to grow substantially as they sank through the plume, whereas in 1998 low concentrations precluded significant increases in floc size with depth. These observations do not support the hypothesis that concentration controls maximal floc size; rather they indicate that the growth rate of flocs is a function of concentration. Using a published relationship between floc size and settling velocity for the Eel shelf suggests that approximately three fourths of the sediment in the plume was packaged as flocs during the 1998 floods.