Constancy of the relation between floe size and density in San Francisco Bay.

The size and density of fine-sediment aggregates, or floes, govern their transport and depositional properties. While the mass and volume concentrations of floes can be measured directly or by optical methods, they must be determined simultaneously to gain an accurate density measurement. Results are presented from a tidal cycle study in San Francisco Bay, where mass concentration was determined directly, and volume concentration was measured in 32 logarithmically spaced size bins by laser-diffraction methods. The relation between floe size and density is investigated assuming a constant primary particle size and fractal floe dimension. This relation is validated with measurements from several sites throughout San Francisco Bay. The constancy of this relation implies a uniform primary particle size throughout the Bay, as well as uniform aggregation/disaggregation mechanisms (which modify fractal dimension). The exception to the relation is identified during near-bed measurements, when adverted floes mix with recently resuspended floes from the bed, which typically have a higher fractal dimension than suspended floes. The constant relation for suspended floes simplifies monitoring and numerical modeling of suspended sediment in San Francisco Bay. [ABSTRACT FROM AUTHOR]