Size and settling velocities of cohesive flocs and suspended sediment aggregates in a trailing suction hopper dredge plume

摘要

A field experiment was conducted to quantify settling velocities, aggregate states, and flocculation within a hopper dredge plume. Particular interest was in determining the abundance of dense, bed aggregates suspended from the consolidated bed during dredging. A suspended sediment plume from the hopper dredge Essayons was sampled for a period of 90 min after dredging. Settling velocities and suspended particle sizes were quantified through sampling with the Particle Imaging Camera System (PICS) and automated image processing routines. The sediment plume was identified and a profiling instrumentation frame was positioned within the plume using Acoustic Doppler Current Profiler (ADCP) backscatter. Results indicated that suspended bed aggregates (defined by densities of 1200-1800 kg m~(?3)) represented 0.2-0.5 of total suspended mass, and flocs (densities<1200 kg m~(?3)) represented 0.5-0.8 of total suspended mass. The peak diameter of bed aggregates and flocs occurred near 90 and 200 μm, respectively, corresponding to peak settling velocities of about 1 mm s~(?1) in each case. Floc settling velocities increased with particle size d~(1.1), while bed aggregate settling velocity increased like d~(1.3).