Joseph F. Atkinson

Step‐Pool Streams: Adjustment to Maximum Flow Resistance

An experiment was conducted to study the maximum flow resistance of step pool streams and the morphology of the steps formed from clastic materials. The step pool formation was qualitatively simulated to analyze numerically the formation process. A flume 4.88 mm long and .15 m wide was used and flow velocity measurements were done by electronically timing passage of salt plume down the flume. Observations showed that the natural step pool streams arranged the morphology to maximize flow resistance.

Changes in fractal dimension during aggregation

Experiments were performed to evaluate temporal changes in the fractal dimension of aggregates formed during flocculation of an initially monodisperse suspension of latex microspheres. Particle size distributions and aggregate geometrical information at different mixing times were obtained using a non-intrusive optical sampling and digital image analysis technique, under variable conditions of mixing speed, coagulant (alum) dose and particle concentration. Pixel resolution required to determine aggregate size and geometric measures including the fractal dimension is discussed and a quantitative measure of accuracy is developed. The two-dimensional fractal dimension was found to range from 1.94 to 1.48, corresponding to aggregates that are either relatively compact or loosely structured, respectively. Changes in fractal dimension are explained using a conceptual model, which describes changes in fractal dimension associated with aggregate growth and changes in aggregate structure. For aggregation of an initially monodisperse suspension, the fractal dimension was found to decrease over time in the initial stages of floc formation.

CORRECTION FACTORS IN THE DETERMINATION OF MEAN VELOCITY OF OVERLAND FLOW

The velocity of overland flow has been conventionally measured using tracers, but it is difficult to measure the mean flow velocity directly because the centroid of the tracer plume is not easily identified. Consequently, previous investigators have measured the velocity of the leading edge of the plume and multiplied it by a correction factor α to obtain an estimate of mean velocity. An alternative method is to measure the velocity of the peak concentration in the tracer plume and multiply this velocity by another correction factor β to estimate mean velocity. To investigate the controls of α and β and develop predictive models for these correction factors, 40 experiments were performed in a flume with a mobile sand bed. Multiple regression analyses reveal that both α and β vary inversely with slope and directly with Reynolds number. The derived regression equations may be used to calculate the mean velocity of other shallow overland flows, at least within the range of slope and Reynolds number for which the equations were developed. In the experiments, slope ranged from 2.7;° to 10° and Reynolds number from 1900 to 12 600.

A wind-mixed layer model for solar ponds

Abstract A computer model is described which may be used for predicting transient salinity and temperature profiles in a salt gradient solar pond. It is intended for use in modeling large surface area ponds where wind-mixing would be expected to play an important part in the dynamics of the upper layer. The formulation predicts the depth of the upper convecting zone using a mixed-layer model which incorporates the wind-mixing algorithm described by Bloss and Harleman [1,2]. This is in contrast to earlier solar pond models which have generally assumed a constant value for this layer depth. Results have been obtained for a number of 1-yr simulations of a large hypothetical pond in Richmond, Virginia, and these have been used in testing the sensitivity of the model to several of the input parameters, including the radiation term and the form of the wind-mixing algorithm. The model output is also compared with field data from an operating solar pond and good agreement is found. Results have indicated that some measures will have to be taken to counteract the mixing action due to wind stress, if the upper mixed layer depth is to be maintained at an acceptable level. The model is expected to be useful in large-scale pond design.

The influence of diffusive convection on sedimentation from buoyant plumes

Abstract Diffusive convection driven by the differential diffusion of density altering fluid properties may enhance the scavenging of particles from natural buoyant plumes. For single-phase (fluid–solute–heat) systems this phenomenon has been extensively studied because salt fingering generated at the oceanic thermocline is a major mechanism of salt transport in the oceans. However, the influence of this process on particle laden plumes, for example, fluvial plumes in lakes and estuaries volcanic clouds and seafloor hydrothermal plumes is largely unknown. In this paper, we present direct experimental measurements of the interfacial particle flux at the plume base which can be applied to predict particle scavenging from natural buoyant plumes. Particle flux is measured using a light attenuation technique employing a chain of photodiodes which average concentration over a large number of fingers. The results are in good general agreement with earlier studies based on finger velocity. Flux measurements cover a wide range of conditions from those where diffusive convection dominates to those where settling and diffusive convection are of a similar magnitude. For very small particles double diffusive (salt finger) theory is applicable to two component particulate systems as suggested by earlier studies [Green, T., 1987. The importance of double diffusion to the settling of suspended material. Sedimentology 34, 319–331]. Two component diffusive convection theory is extended to three components in order to predict particle scavenging from marine fluvial plumes which involve the diffusion of sediment, salt and heat. For larger particles which settle significantly the flux can be approximated by adding the double-diffusive and settling fluxes. A theory to predict particle transport through the lower layer and sedimentation at the bed is developed based on the observation of strong convection below the plume. Application of our theory and experimental results indicates that while double diffusion may significantly influence the longitudinal distribution and vertical sorting of deposits from lacustrine plumes, the diffusive convection process is generally insignificant in marine plumes. Observational evidence for lacustrine double diffusion based on water column measurements is presented.

Predicting sediment transport by interrill overland flow on rough surfaces

Modelling soil erosion requires an equation for predicting the sediment transport capacity by interrill overland flow on rough surfaces. The conventional practice of partitioning total shear stress into grain and form shear stress and predicting transport capacity using grain shear stress lacks rigour and is prone to underestimation. This study therefore explores the possibility that inasmuch as surface roughness affects flow hydraulic variables which, in turn, determine transport capacity, there may be one or more hydraulic variables which capture the effect of surface roughness on transport capacity suffciently well for good predictions of transport capacity to be achieved from data on these variables alone. To investigate this possibility, regression analyses were performed on data from 1506 flume experiments in which discharge, slope, water temperature, rainfall intensity, and roughness size, shape and concentration were varied. The analyses reveal that 89·8 per cent of the variance in transport capacity can be accounted for by excess flow power and flow depth. Including roughness size and concentration in the regression improves that explained variance by only 3·5 per cent. Evidently, flow depth, when used in combination with excess flow power, largely captures the effect of surface roughness on transport capacity. This finding promises to simplify greatly the task of developing a general sediment equation for interrill overland flow on rough surfaces. Copyright

Relation between grain velocity and sediment concentration in overland flow

Previous flume studies of the velocity of saltating grains have almost all measured single grains traveling in otherwise clear water. However, such measurements disregard the likelihood that grain velocities are affected by the presence of other grains. In this laboratory study of overland flow we show that grain velocity is negatively related to sediment concentration. This negative relation is attributed to two mechanisms. The first mechanism is a decrease in water velocity as sediment concentration increases. Water velocity decreases because an increasing proportion of potential energy is dissipated in transporting sediment rather than being converted to kinetic energy. The second mechanism is an increase in the dispersion of the normal component of grain momentum as grain collisions increase with sediment concentration. The dispersive stress so created opposes bed load movement and causes grain velocity to decrease. Multiple regression analyses reveal relations in the experimental data that are consistent with these two mechanisms.

Theoretical and experimental comparison of conventional and advanced solar pond performance

Abstract Numerical and physical experiments were carried out to compare the performance of two solar pond systems: (a) a conventional salt gradient solar pond (CSP) and (b) a salt gradient pond operated as an “advanced solar pond” (ASP). The main differences in the ASP, as originally proposed by Osdor[1], are an increase in overall salinity and the introduction of a stratified flowing layer near the bottom of the gradient zone. The increased salinity is meant to reduce evaporative heat loss and make up water requirements, while the additional flowing layer allows extra heat extraction and possibly higher temperatures to develop in the lower convective zone. A numerical study was performed to evaluate the salinity effect and the results show only a minor effect of increased salinity on heat collection efficiency. However, slightly higher collection temperatures are obtained, which may provide some benefit for heat engine efficiency. Physical experiments were performed to test the feasibility of constructing and maintaining the necessary flow system for the ASP and also to compare the performance of the ASP and the CSP under similar laboratory conditions. These tests showed that a stable stratified flowing layer could be maintained and that the ASP configuration produced higher efficiencies.

Spatial and Temporal Distribution of the Cyanotoxin Microcystin-LR in the Lake Ontario Ecosystem: Coastal Embayments, Rivers, Nearshore and Offshore, and Upland Lakes

ABSTRACT Cyanotoxins, a group of hepatotoxins and neurotoxins produced by cyanobacteria, pose a health risk to those who use surface waters as sources for drinking water and for recreation. Little is known about the spatial and seasonal occurrence of cyanotoxins in Lake Ontario and other lakes and ponds within its watershed. Within the embayments, ponds, rivers, creeks, shoreside, and nearshore and offshore sites of Lake Ontario, microcystin-LR concentrations were low in May, increased through the summer, and reached a peak in September before decreasing in October. Considerable variability in microcystin-LR concentrations existed between and within habitat types within the Lake Ontario ecosystem. In general, the average microcystin-LR concentration was two orders of magnitude lower in embayment (mean = 0.084 µg/L), river (mean = 0.020 µg/L), and shoreside (mean = 0.052 µg/L) sites compared to upland lakes and ponds (mean = 1.136 µg/L). Concentrations in the nearshore sites (30-m depth) and offshore sites (100-m depth) were another order of magnitude lower (mean = 0.006 µg/L) than in the creek/river, bay/pond, and shoreside habitats. Only 0.3% (2 of 581) of the samples taken in Lake Ontario coastal waters exceeded the World Health Organization (WHO) Drinking Water Guideline of 1 µg microcystin/L for humans. In contrast, 20.4% (20 of 98) of the samples taken at upland lakes and ponds within the watershed of Lake Ontario exceeded WHO Guidelines. No significant relationship between nitrate and microcystin-LR concentrations was observed in Lake Ontario even though a significant positive relationship existed between phosphorus and phycocyanin and microcystin-LR concentrations. At an upland lake site (Conesus Lake) in the Ontario watershed, the development of a littoral Microcystis population was not observed despite high nutrient loading (P and N) into the nearshore zone, well-developed nearshore populations of filamentous Spirogyra and Zygnema, the occurrence of Dreissena spp., and the known occurrence of Microcystis and microcystin production in the pelagic waters of Conesus Lake.

Effect of flow confinement on the hydrodynamics of circular impinging jets: implications for erosion assessment

The fluid dynamics of a water jet impinging on a flat surface in confined conditions were studied using particle image velocimetery. The experiments were meant to replicate conditions expected in a jet erosion test (JET) designed to assess cohesive sediment erosion parameters in field applications. High-resolution two-dimensional velocity vectors were measured in a plane passing the jet centerline including free jet, impingement, and wall jet regions within a fixed-wall box. The general flow behavior in the free jet and wall jet regions is in good agreement with the behavior of impinging jets in an unconfined environment. Results show that the entrainment coefficient, however, is lower than values in unconfined conditions, lowering lateral spreading rates. The rate of momentum transfer also increases along the axial direction since the confinement causes secondary flow and recirculation in the box. Wall shear stress is calculated based on extrapolation of Reynolds shear stress and turbulent kinetic energy, where the latter procedure provides more consistent results with expected scour hole shape under an impinging jet. This wall shear stress distribution shows higher values near jet impingement in comparison to previously reported distributions, especially as formulated for the JET under unconfined conditions. The maximum value of wall shear stress is found to be about 2.4 times greater than the commonly accepted value in the literature, and also occurs at a position closer to the impingement point. The shear stress at the impingement point is also close to its maximum value, which is consistent with the expected scour hole shape beneath an impinging jet. These findings have important implications for the use of jet impingement theory to assess sediment erosion, especially in the application of the JET.