Bommanna G. Krishnappan

Floc morphology and size distributions of cohesive sediment in steady-state flow

Fractal dimensions of particle populations of cohesive sediment were examined during deposition experiments in an annular flume at four conditions of steady-state flow (0.058, 0.123, 0.212 and 0.323Pa). Light microscopy and an image analysis system were used to determine area, longest axis and perimeter of suspended solids. Four fractal dimensions (D, D(1), D(2), D(k)) were calculated from the slopes of regression lines of the relevant variables on double log plots. The fractal dimension D, which relates the projected area (A) to the perimeter (P) of the particle (P proportional, variant A(D/2)), increased from 1.25+/-0.005 at a shear stress of 0.058Pa to a maximum of 1.36+/-0.003 at 0.121Pa then decreased to 1.34+/-0.001 at 0.323Pa. The change in D indicated that particle boundaries became more convoluted and the shape of larger particles was more irregular at higher levels of shear stress. At the highest shear stress, the observed decrease in D resulted from floc breakage due to increased particle collisions. The fractal dimension D(1), which relates the longest axis (l) to the perimeter of the particle (P proportional to l(D1)), increased from 1.00+/-0.006 at a shear stress of 0.058Pa to a maximum of 1.25+/-0.003 at 0.325Pa. The fractal dimension D(2), which relates the longest axis with the projected area of the particle (A proportional to l(D(2)), increased from 1.35+/-0.014 at a shear stress of 0.058Pa to a maximum of 1.81+/-0.005 at 0.323Pa. The observed increases in D(1) and D(2) indicate that particles became more elongated with increasing shear stress. Values of the fractal dimension D(k), resulting from the Korcaks empirical law for particle population, decreased from 3.68+/-0.002 at a shear stress of 0.058Pa to 1.33+/-0.001 at 0.323Pa and indicate that the particle size distribution changed from a population of similar sized particles at low shear to larger flocculated particles at higher levels of shear. The results show that small particle clusters (micro-flocs) are the formational units of larger flocs in the water column and the stability of larger flocs is a function of the shear stress at steady state.

Modelling of flocculation and transport of cohesive sediment from an on-stream stormwater detention pond

A new model to predict the transport characteristics of suspended sediment from an on-stream stormwater management pond is presented. It is based on Krishnappans model of flocculated settling in still water, which was extended to dynamic conditions and verified by experiments with stormwater pond sediment in a laboratory rotating flume. The model was derived from first principles, but some input parameters, such as floc properties, had to be obtained by calibration, and input parameters describing flow field properties were obtained from a kappa-epsilon turbulence model. Simulated suspended concentrations vs. time and the size distribution of the flocculated sediment, produced with the calibrated model, agreed well with flume measurements.

The effect of bed age and shear stress on the particle morphology of eroded cohesive river sediment in an annular flume

Erosion experiments were conducted in an annular flume to determine the effect of bed age and shear stress on the particle morphology (fractal dimensions D, D(1), and D(k)) of eroded cohesive river sediment. Sediment beds were deposited under low shear and left to consolidate for one, two and seven days. Fractal data and photomicrographs show particle morphology changed with shear stress and bed age. During the one-day experiment, flocs were highly branched and particle geometry became more complex with increasing shear. Microflocs present in suspension at low shear, formed larger more loosely bound flocs at moderate shear due to flocculation. At higher shear, larger flocs were less prevalent due to particle breakup. As bed age increased, less sediment was eroded and particles appeared less porous and more angular in shape for a given shear stress. Changes in floc morphology and eroded sediment mass at various shear stresses may be related to bed age-associated biostabilization of bed deposits.

Sequential erosion/deposition experiments: Demonstrating the effects of depositional history on sediment erosion

Experiments on the erosion of a bed of kaolinite were carried out in a rotating circular flume. Each experiment was carried out using the stratified bed which resulted from the previous experiment. Changes in suspended sediment concentrations during the experiments were explained by the history of the deposition. The sequence of experiments showed how the rate of erosion and the amount eroded reflected the structure of the bed and that of the individual flocs which created it. Results suggest that modelling of sediment/contaminant transport needs to account for the manner in which deposition took place.

The effect of irrigation on tile sediment transport in a headwater stream

A field-scale no-till corn plot (120 m x 90 m) located on a tile drained silt loam soil near Kintore, Ontario was irrigated with 2.5 cm of water over a 3 h period to examine the effects of irrigation on tile sediment transport in a headwater stream. Flow characteristics and the composition, concentration and size distribution of suspended solids were measured at the tile outlet, an upstream reference site and three sites located downstream of the tile drain. Results show that tile sediments at the study site are fine-grained (D50 approximately 5.0 microm) and consist primarily of quartz, anorthite/albite, dolomite and calcite. Sediment concentrations in tile effluent increased from 8 to 57 mg L(-1) after 1.5 h of irrigation and reached a maximum of 72 mg L(-1). The sediment yield from the tile drain for the irrigation event was 4.6 kg ha(-1). An unsteady, mobile boundary flow model (MOBED) was used to predict flow characteristics in the stream. According to the MOBED model, bed shear stress in the stream was approximately 6 N m(-2). This value is significantly greater than the critical shear stress for complete suspension of 1 N m(-2) for tile sediments as determined from laboratory experiments using a rotating circular flume. Grain size distributions of suspended solids in the stream were close to the dispersed size distribution because of the high shear stress in the receiving stream.

Upgrading the North Toronto Combined Sewer Overflow (CSO) Storage and Treatment Facility

Traditional CSO storage facilities can be environmentally upgraded by optimising their hydraulics and implementing chemically aided settling. In the case study presented for the North Toronto CSO facility, a number of structural measures serving to enhance the facility treatment rate and inducing favourable settling conditions in the settling basins were addressed by means of physical and computer modelling. A physical scale model was effective for establishing the hydraulic performance of the facility (flow rates, water levels, and changes in these parameters) and verifying a CFD model. The CFD model simulated well the hydraulic phenomena in the facility and will be used in the next study stage for particle tracking. With respect to chemically aided settling, the settleability of CSOs can be assessed well by elutriation testing. The choice of a coagulant/flocculant depends on the characteristics of the wastewater treated, including pH, dissolved organic matter contents and characteristics, and temperature. Jar testing offers the best way for selecting the most suitable chemical and its dosage. Finally, the solids removals in excess of those required by a provincial (Ontario) CSO control directive (primary treatment equivalency) were achieved at the facility studied with flow-proportional polymer dosing of 6–8 mg/L and the surface load rates of 15 m/h. Further refinements of settling conditions in the facility (by flow conditioning baffles), dosing (switching to solids flux proportional dosing) and higher surface loading rates are planned and will be assessed by both CFD modelling and field observations.

Vertical and lateral distribution of fine-grained particulates in prairie and cordilleran rivers: Sampling implications for water quality programs

The fine sediment fraction, usually <63 μm, is generally regarded as significant for water quality issues. This fraction is usually presumed to be evenly distributed in the vertical column. Using period of record data for three prairie rivers and three cordilleran river sites, and midstream data from the Mackenzie River, we examine the degree to which near-surface samples of silt and clay are representative of the vertical and cross section. These data are available only for high flow conditions. Surface samples of silt + clay tend to underestimate the vertical mean concentration by less than 10%; also, 89% of the surface data at five of the six sampled sites are within ±15% of the vertical mean concentration. The individual vertical distributions of clay and silt display inconsistent and variable patterns of concentration with depth and can include large excursions within individual profiles. Our data do not indicate that large, deep rivers behave differently from shallow ones. There is no evidence of increasing homogenization of silt + clay across the section as discharge increases. The data indicate typical errors that may be expected if surface samples are used to characterize the water column at high discharges. A theoretical basis for the observations is provided together with a discussion of assumptions which render theory imprecise for fine-grained cohesive sediment.