Wilbert Lick

A Portable Device for Measuring Sediment Resuspension

A portable device for measuring sediment resuspension has been developed. The device consists of a cylindrical chamber inside of which a horizontal grid oscillates vertically. The sediments whose properties are to be determined are placed at the bottom of the chamber with water overlying them. The grid oscillates in the water and creates turbulence which penetrates down to the sediment-water interface and causes resuspension. The amount of material resuspended is proportional to the frequency of the grid oscillation. The device has been calibrated by comparing the concentrations of the resuspended sediment at different grid frequencies with the concentrations in an annular flume where the bottom shear stress is known. The device can be used in the laboratory for rapid and reasonably accurate measurements of resuspension. The device has also been tested on board ship and can be used in the field for rapid surveys of resuspension of undisturbed sediments.

Flocculation of fine-grained sediments due to differential settling

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.

Flocculation of Fine-Grained Lake Sediments Due to a Uniform Shear Stress

Experiments were performed to investigate the effects of fluid shear on the flocculation of fine-grained lake sediments in fresh water. In these experiments, a Couette viscometer was used to apply a uniform shear stress to a sediment suspension. The sediments were from the Detroit River inlet of Lake Erie. They were prepared such that the initial (unflocculated) size distribution contained approximately 90% of its mass in particles less than 10 μm in diameter with the average diameter being about 3.5 μm. Experiments were performed at shear stresses of 1, 2, and 4 dynes/cm2 and sediment concentrations of 50, 100, 400, and 800 mg/L, values which are characteristic of those found in the Great Lakes. Data in the form of floc size distribution as a function of time were obtained. For steady-state conditions, the median diameters of the flocs formed were typically 20 to 100 μm depending on shear stress and sediment concentration. Quantitative results for the decrease in the steady-state floc size with increasing shear stress and with increasing sediment concentration were obtained. The times required for flocculation to occur under different conditions were also determined and were typically on the order of 1 hour.

Entrainment, deposition, and transport of fine-grained sediments in lakes

Recent work on the settling, diffusion, entrainment, and deposition of fine-grained sediments in fresh water is reviewed and synthesized. Particular attention is given to the dependence of these processes on sediment properties such as particle size. The application of this knowledge to the analysis and numerical modeling of sediment transport is also discussed. Much of the work is concerned with the Great Lakes and, more specifically, with the Western Basin of Lake Erie.

Aggregation and Disaggregation of Fine-Grained Lake Sediments

Abstract The effects of fluid shear and sedimentation concentration on the aggregation, and especially disaggregation, of fine-grained sediments in lake waters continues to be an important research area. It has been shown in previous studies that the steady-state median floe size decreases as the shear stress increases and also decreases as the suspended sediment concentration increases. Here, the time rate of change of the particle size distribution as affected by aggregation and disaggregation due to fluid shear and to collisions between particles is considered from a theoretical point of view. Approximate value for the coefficients appearing in the rate equation and their dependence on floe diameter, shear stress, and density were determined. In order to explain the observed decrease in floe size as the sediment concentration increases, the theoretical analysis requires disaggregation due to three-body collisions. 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.

The Resuspension and Transport of Fine-Grained Sediments in Lake Erie

Abstract The resuspension and transport of fine-grained sediments in Lake Erie has been calculated for a variety of wind conditions. The emphasis was on the effects of major storms. Calculations were made for different constant wind speeds and wind directions and also for the November 1940 storm, one of the largest in the last century. The results indicate that major storms, despite their infrequent occurrence, are responsible for most of the resuspension and transport of fine-grained sediments in Lake Erie. The results of the calculations are also used to more quantitatively interpret geochronological data from Lake Erie.

Flocculation and its effect on the vertical transport of fine-grained sediments

Recent experimental and theoretical work on flocculation and settling speeds of flocs is reviewed. On the basis of this work, an accurate and computationally efficient model of the aggregation and disaggregation of fine-grained sediments is proposed. This model is then used to predict flocculation times and steady-state floc sizes for a wide range of environmental conditions. The predicted flocculation times are smaller, sometimes by as much as two orders of magnitude, than those predicted by mono-disperse theory. The model is also used to show that the disaggregation of flocs due to increased shear near the sediment-water interface may be a possible mechanism for the increased concentrations often observed near this interface.

Transport of Suspended Solids in the Lower Fox River

Abstract The general purpose of the present study was to study the transport of the sediments and associated contaminants in the lower Fox River and from the Fox into Green Bay. For this reason, a numerical model of the transport and fate of suspended solids in the lower Fox River has been developed and verified. The model consists of a two-dimensional, vertically-integrated, time-dependent hydrodynamic and transport model coupled with a three-dimensional, time-dependent model of the sediment bed and its properties. Settling speeds and sediment resuspension parameters needed in the model were determined from laboratory and field tests. In the description of the transport of suspended solids, three components of solids are considered, i.e., fine (zero settling speed), medium (moderate settling speed affected by flocculation), and coarse (large settling speed). It is assumed that the sediment bed is layered in the vertical direction. The properties of each layer depend on time after deposition (and therefore with depth) and composition (relative fractions of medium and coarse sediments), and the number of layers and their thicknesses can be arbitrarily specified at the beginning of the calculation. The thickness of the surface layer changes with time depending on the rates of resuspension and deposition. Calculations were made for steady flows at high, medium, and low flow rates as well as for real, time-varying flow events. In particular, two flow events were modeled in detail, the first from 22 May to 20 June 1989 (this included a once in a 10 year high flow as well as moderate to low flows) and the second from 24 March to 10 April 1989. For these events, calculated sediments concentrations at the river mouth were compared with observations. Good agreement between the calculations and observations was obtained, thereby validating the model and the description of the physical processes implied in the modeling. In particular, the presence and effect of an easily resuspendable surficial layer was demonstrated.

Effects of sediment bulk properties on erosion rates.

Considerable work has been done recently on the effects of sediment bulk properties on erosion rates. From this it is known that erosion rates depend on at least the following parameters: bulk density, average particle size, particle size distribution, mineralogy, organic content, volume of gas in the sediment, salinity of the pore waters, and time after deposition. This work is reviewed and discussed here with the purpose of presenting a quantitative overview of the effects of each of these parameters. This information is then used to demonstrate a procedure for estimating erosion rates of sediments based on a knowledge of their bulk properties.

The Entrainment and Deposition of Fine-Grained Sediments in Lake Erie

A series of entrainment and deposition experiments was performed with the general purpose of increasing our understanding of the parameters on which entrainment and deposition depend and the specific purpose of obtaining entrainment rates for a variety of sediments from the western basin of Lake Erie. The experiments were performed in an annular flume. A rotating top produced a turbulent flow which in turn exerted a turbulent shear stress on the sediments deposited on the bottom of the flume. Four different sediments from the western basin of Lake Erie were analyzed. Large variations in entrainment and deposition rates occurred and are shown to be dependent on the shear stress, water content (time after deposition), the type of sediment (grain size and mineralogy), and the manner of deposition. In the interpretation of the experiments, it is necessary to consider the frequency distribution of sediment properties as well as the average properties.