Seasonal size distributions of suspended solids in a stormwater managementpond

Three seasonal surveys of suspended solids were carried out in an on-stream stormwater management pond, by means of a submersible laser particle size analyser. Size distributions were measured at up to 17 points in the pond, and water samples collected at the same locations were analysed for primary particles aggregated in flocs. Observed suspended solids were mostly compo sed of flocs, with maximum sizes ranging from 30 to 212 mu m for winter and summer surveys, respectively. Using a relationship defining the flee densi ty as a function of flee size and Stokes' equation for settling, an empiric al relationship expressing the flee fall velocity as a function of floc siz e was produced. This relationship indicates that naturally formed flocs in the size range from 5 to 15 mu m would settle faster than both smaller prim ary particles of higher density, and somewhat larger flocs of lower density , which are however susceptible to break up by turbulence. (C) 1999 IAWQ Pu blished by Elsevier Science Ltd. All rights reserved.