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 conducte d in January and February 1998. These effective settling velocities exceed those expected for single-grain sinking and implicate flocculation as an im portant mechanism for speeding the removal of sediment from the Eel River p lume. The relative constancy of effective settling velocities despite widel y varying winds, waves, and currents is consistent with photographs in the plume that show little variability in flee size with total suspended sedime nt mass concentration, turbulent-kinetic-energy dissipation rate, elapsed t ime since sediment within flocs left the river mouth, or depth. These obser vations of floc size contrast with those made in winter 1997 during the exc eptionally large New Year's flood. During that event, increases of flee siz e with depth are evident. In 1997, higher sediment concentrations associate d with the significantly larger discharge likely allowed flocs to grow subs tantially as they sank through the plume, whereas in 1998 low concentration s precluded significant increases in flee size with depth. These observatio ns do not support the hypothesis that concentration controls maximal flee s ize; rather they indicate that the growth rate of flocs is a function of co ncentration. Using a published relationship between flee 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. (C) 20 00 Elsevier Science Ltd. All rights reserved.