Transport modes of marine aggregates

The transport modes of marine aggregates acquired from compacted bed material were measured in a laboratory flume. Unlike their constituent particles, marine aggregates have a lower density but are more resilient to breakup than sediments that are formed as flocs in the water column. Thus, their transport modes may be more similar to non-cohesive sediments, and as such non-cohesive transport theory may provide a first order description of their behavior. Marine aggregates were prepared with three different water contents to alter density and observed in the flume under three different flow velocities. Video monitoring revealed that the dominant transport mode of the smaller aggregates (< 1000 microns) was similar to non-cohesive particles and exhibited Rouse like profiles. Larger particles tended to move as bedload with a mean transport rate equal to half the average flow rate in the flume. While a general trend indicating aggregates with the highest water content and highest flow speeds followed Rouse like behavior, there was no clear trend based upon aggregate composition (sand, silt, clay). While further study is needed, these results suggest that, similar to non-cohesive sediments, size and density are the primary factors needed to describe the transport characteristics of marine aggregates.