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Experimental and theoretical investigations of the effects of salinity, fluid shear, and sediment concentration on the rates of aggregation and disaggregation of a fine‐grained sediment in estuarine waters were made. Experiments were performed in fresh water, in seawater, and in an equal mixture of fresh and sea waters. The applied shears ranged from 100 to 600 s−1, while the sediment concentrations ranged from 10 mg/L to 800 mg/L. From the experimental and theoretical results and for the above range of parameters, the following was demonstrated: (1) the steady state particle size distribution is independent of the manner in which the steady state is approached; (2) the median floc size decreases as the shear stress increases; (3) the median floc size decreases as the suspended sediment concentration increases; (4) particles flocculate faster in seawater than in fresh water; (5) flocs are smaller in seawater than in fresh water; (6) the sizes of flocs in estuarine waters seem to be weighted averages of the sizes of flocs in fresh and sea waters; (7) in order to explain the observed decrease in floc size as the sediment concentration increases, the analysis requires disaggregation due to three‐body collisions; (8) the theory does not require disaggregation due to fluid shear. For the present range of parameters, fluid shear seems to have a negligible direct effect on disaggregation, while collisions between particles (possibly due to shear but also due to differential settling and Brownian motion) are the dominant mechanism for disaggregation; and (9) the theory can accurately reproduce the experimental results of mean diameter versus time and of steady state particle size distribution.

Vertical mixing of lake sediments by tubificid oligochaetes was studied in laboratory experiments by using a radioactive (cesium 137 labeled sediment) marker horizon. Results from these experiments were used to develop and test a mathematical model describing tubificid sediment mixing as a dominantly advective process. Tubificids were found to mix sediments to a depth of 6-9 cm. The rate of tubificid sediment processing observed in the experiments agrees well with those reported by other workers. Extrapolation of laboratory results to Lake Erie shows that the feeding activity of tubificids alone may result in significant sediment mixing throughout the lake. terials such as microfossils and sediment associated radionuclides [Berger and Heath, 1969; $tockner and Lund, 1970; Arniard-Triquet, 1974; Davis, 1974a; Guianasso and $chink, 1975; Robbins and Edgington, 1975; Krezoski et al., 1978; Aller and Cochran, 1976; Schink and Guianasso, 1978]. It has also been shown that biogenic particle mixing affects the chemical diagenesis of sediments [Lee, 1970; Jernelb'v, 1970; Davis, 1974b; Davis et al., 1975; Aller, 1977, 1978]. Because of these important effects, numerous authors have conducted field and laboratory experiments to quantify bioturbation phenomena [Darwineddy diffusion equations [Goldberg and Koide, 1962; Guianasso and Schink, 1975; Aller and Cochran, 1976; Noyaki et al., 1977; Robbins et al., 1977, 1979]. Models using transport analogies other than diffusion have been proposed, but such models are neither numerous nor widely applied [Berger and Heath, 1969; Davis, 1974a; Aller, 1977; Goreau, 1977].In this paper, detailed information on the rate and depth distribution of tubificid feeding and resultant sediment mixing is presented. This information, obtained from a radioactive (cesium 137 labeled illitic clay) marker horizon experiment, is used to develop and test a mathematical description of tubificid sediment mixing. In addition, results from this and other studies are used to evaluate the significance of tubificid sediment mixing in Lake Erie. [Britt et al., 1977], it could be readily obtained in large numbers from Cleveland Harbor, and it was easily differentiated from other tubificid species found in Cleveland Harbor. T. tubilex individuals were obtained by sieving sediment collected from the western portion of Cleveland Harbor (41 ø30.05'N, 81 ø43.35'W) through a 250-/.tm mesh screen. The sieve residue was placed in shallow plastic trays and kept in an aquarium at 15øC. When required for experimentation, T. tubilex individuals were collected from the residue by hand picking with a Pasteur pipette under a stereo microscope. Sediment used as the experimental substratum was collected from a silt clay deposit in the central basin of Lake Erie approximately 4 km offshore of Ashtabula, Ohio. Macrofauna were removed from this sediment by sieving through a 250-pm mesh screen. The sediment used as the recipieni of the cesium 137 label was clay-rich material obtained by soaking partially decomposed material from ...

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The flocculation of fine‐grained particles depends on collisions due to Brownian motion, fluid shear, and differential settling. Previous experimental work on the flocculation of fine‐grained sediments has emphasized the effects of fluid shear. These effects are significant in high‐turbulence regions. However, as the turbulence and fluid shear decrease, as, for example, in open waters away from shore, differential settling becomes the dominant mechanism for flocculation. In the present article, previous work on the effects of fluid shear is reviewed. However, the emphasis is on recent experimental work on the effects of differential settling on the flocculation of fine‐grained, primarily inorganic particles. The transition in effects between situations where fluid shear is dominant and the other extreme, where differential settling is dominant, was also investigated and is discussed. The sediments used in these studies were natural bottom sediments from the Detroit River inlet to Lake Erie. The tests were initiated with disaggregated sediments and were continued as the particles aggregated and formed flocs. These flocs then grew until a steady state size distribution was reached. In order to reach a steady state the differential settling tests sometimes continued for as long as 30 days; they were done in both freshwater and seawater at sediment concentrations from 1 mg/L to 200 mg/L and with and without treatment to remove organic matter. Floe size distributions as a function of time were determined. From the experiments it is shown that the times to steady state and the steady state median diameters are much larger when differential settling is the dominant mechanism for flocculation than when fluid shear is the dominant mechanism for flocculation. It is also shown that the effects of sediment concentration and salinity are qualitatively similar; i.e., as these quantities increase, both the time to steady state and the steady state floe size decrease. Settling speeds of the flocculated particles were also measured; the settling speeds of flocs are much larger and increase more rapidly with floc diameter when produced by differential settling than when fluid shear is dominant.