A laboratory examination of floc characteristics with regard to turbulent shearing

Turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters, and their respective aggregate break-up. This study examines the inter-relationships between flee characteristics o ver increasing turbidity (80-200 mg l(-1)) and turbulent shear (0-0.6 N m(- 2)) environments, by the use of a laboratory flume within which a suspensio n can be sheared at a controlled rate, and with an unintrusive macro-lens m iniature video camera mounted in a viewing port on the flume channel wall. The camera enables the direct simultaneous measurement of both flee size an d settling velocity, from which accurate estimates of flee effective densit y and porosity can be made. Measurements were made 120 s after the induced turbulence has ceased. The instrument has an upper viewing turbidity limit of 210 mg l(-1), and a lower resolution of 20 mu m. The sediment was collec ted from the inter-tidal mudflats at Weir Quay on the Tamar Estuary in Devo n, Southwest England. The results indicated that increasing turbidity at lo w shear levels encouraged flee growth, but the effect of the increasing tur bulent shear (0.35 N m-2) together with increasing concentration in suspens ion causes disruption rather than enhancing the flocculation process. At sh ears up to 0.35 N m(-2), the largest size and settling velocity flocs were produced at high concentrations, whereas above 0.35 N m(-2) disruption caus ed smaller flocs at higher concentrations. The use of algorithms which were based either on a single flee characteristic (i.e., size or settling veloc ity) or a mean fractal dimension, were seen not to accurately approximate t he experimental data. A multiple regression analysis of the experimental da ta produced the following formula, based on mean values of the 20 largest f locs sampled under each of the imposed environmental conditions (referred t o as max20size mean values): settling velocity = 0.301 - 0.00337 rms of the gradient in turbulent velocity fluctuations - 0.000606 SPM + 0.00335 flee size, which proved to be the most accurate representation with an R-2 value of 0.95. A similar formula was determined for the average value of the fou r fastest settling flocs within each sample-group (max4W(S)). This highligh ts the importance of modelling algorithms that are developed from data that take into account effective density variations (i.e., simultaneous size an d settling velocity measurement). (C) 1999 Elsevier Science B.V. All rights reserved.