Hill et al.: Controls on floc size in a continental shelf bottom boundary layer


Simultaneous in situ observations of floc size, waves, and currents in a continental-shelf bottom boundary layer challenge the generally accepted hypothesis that turbulence limits floc size in the sea. In September and October, 1996 and January, 1997 two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole Massachusetts. On one, a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 um, and on the other, 3 horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods were separated by approximately 150 m. Typically, maximal floc diameter stayed relatively constant around 1 mm, and it showed dependence on turbulence parameters that was significantly weaker than predicted by any model that assumes turbulence-induced stresses limit floc size. Occasionally, when waves and currents generated intense nearbed turbulence, flocs were destroyed. These precipitous decreases in maximal floc size also were not predicted by conventional models. The correlation in time between episodes of floc destruction and elevated combined wave-current stresses provides the first quantitative support for the hypothesis that floc size throughout bottom boundary layers can be controlled by breakup in the intensely sheared nearbed region. These observations demand reassessment of the forces limiting floc size in the sea, and they indicate the potential for significant simplifying assumptions in models of floc dynamics.

Paul Hill
Department of Oceanography
Dalhousie University
Halifax, Nova Scotia
Canada B3H 4J1
Phone: 902-494-2266
Fax: 902-494-3877
E-mail: Paul.Hill@Dal.Ca