Flocculation and sedimentation in the ACE basin, South Carolina

Extremely rapid deposition of fine-grained sediment has been observed near an artificially created channel between the Edisto and Ashepoo Rivers in So uth Carolina. Accumulation rates on the order of 0.05 m mo(-1) formed a reg ion of fluid mud known as the Mud Reach that was present during a study car ried out in May 1998. Subsequent surveys in March and May 1999 found no evi dence of fluid mud in this area. Image analysis of photographs of the suspe nded sediment from 1998 showed that the water column was populated by floes with a median diameter on the order of 0.3 nun which tended to remain in s uspension over the tidal cycle, and floes on the order of 0.8 mm which rema ined in the near bottom region except at maximum current speeds. In 1999, b oth the size and concentration of floes at the surface remained the same, b ut the size and abundance of floes near the bottom was reduced. Disaggregat ed inorganic grain size analysis of the suspension and samples from a core collected in the Mud Reach indicated that in 1998 the accumulation of mater ial on the bottom was the result of floc settling and that in spite of high u. values, little sorting of this material was occurring. Grain size analy sis of samples deeper in the core showed that the rapidly floc-deposited mu d was underlain by coarser, well-sorted sediment, confirming that the Mud R each was a transient feature. The observed flocculation processes may play a significant role in the trapping of fine-grained sediment throughout this region.