Particles in the bottom boundary layer: Concentration and size dynamics through events

The dynamics of particles is examined using two laser diffraction instruments, LISST-100, mounted on a tripod. These sensors measured optical transmission and the area and volume distribution of particles. Particle size and concentration data from these instruments are examined together with the mean stress estimates obtained from a nearby tripod using a Benthic Acoustic Stress Sensors array velocimeter. The following three significant results are presented: (1) the size distribution of particles varies widely in response to forcing conditions; (2) in a surprise observation during Hurricane Edouard, despite a decrease in optical transmission that would ordinarily be interpreted as an increasing sediment load, a dramatic decrease in suspended sizes revealed an actual reduction in suspended volume; and (3) there is a positive correlation between median particle size in the current boundary layer and mean current stress during combined wave current storm events. This final result indicates that after the initial breakup of flocculates during a storm event the size dynamics of the boundary layer is no longer controlled by aggregation/disaggregation processes but is controlled by a balance between gravitational settling and upward diffusion.