Increase In Bed Shear Stress Research Articles

Bed forms in bimodal sand–gravel sediments: laboratory and field analysis

AbstractBed forms were studied in Goodwin Creek and a laboratory flume channel. The bed sediment of the field site and flume had median diameters of 8·3 (modes of 0·4, 22·6 mm) and 1·82 mm (modes of 0·5, 5·6 mm), respectively. The laboratory and field channels had similar values of bimodal parameters, ratios of flow depth to median bed material diameter, and ratios of shear stress to critical shear stress and were judged to be comparable in the transport of bed load sediment and the resulting bed forms. Three groupings of bed forms from the laboratory flume experiments (ripple‐like bed forms, bed load sheets, low‐relief bed waves) were identified using the height and period of the bed forms. For the range of flow depths and discharges investigated in the flume, bed forms became higher and longer with increasing bed shear stress. Bed forms from Goodwin Creek were similar to those from the flume with comparable ratios between bed form length, height, and flow depth. The bed forms in the flume provide a positive link between rate and size fluctuations measured in the field and the bed forms. The smaller bed forms identified were sediment starved and are not considered to be dunes, while the largest bed forms in which all of the bed material sizes were mobilized in the field and laboratory were judged to be dunes.

Bayesian analysis for erosion modelling of sediments in combined sewer systems

Previous research has confirmed that the sediments at the bed of combined sewer systems are the main source of particulate and organic pollution during rain events contributing to combined sewer overflows. However, existing urban stormwater models utilize inappropriate sediment transport formulas initially developed from alluvial hydrodynamics. Recently, a model has been formulated and profoundly assessed based on laboratory experiments to simulate the erosion of sediments in sewer pipes taking into account the increase in strength with depth in the weak layer of deposits. In order to objectively evaluate this model, this paper presents a Bayesian analysis of the model using field data collected in sewer pipes in Paris under known hydraulic conditions. The test has been performed using a MCMC sampling method for calibration and uncertainty assessment. Results demonstrate the capacity of the model to reproduce erosion as a direct response to the increase in bed shear stress. This is due to the model description of the erosional strength in the deposits and to the shape of the measured bed shear stress. However, large uncertainties in some of the model parameters suggest that the model could be over-parameterised and necessitates a large amount of informative data for its calibration.

Equal mobility of gravel in streams: The remains of the day

The concept of equal mobility of gravel transport is 2 decades old. The weak form of the hypothesis states that in order to transport the coarse half of the mean annual gravel load through a river reach at a rate equal to the fine half, the coarser material must be overrepresented on the bed surface, giving rise to a mobile‐bed armor. The degree of mobile‐bed armoring necessary to accomplish this diminishes with increasing bed shear stress. The strong form of the hypothesis states that the grain size distribution of the gravel portion of the bed load should be similar to that of the substrate and finer than that of the surface layer. Here the hypothesis is subjected to a rigorous test in the laboratory under controlled conditions which are nevertheless chosen to be as representative as possible of actual field conditions. The grain size distributions used in the experiment included both model “gravel” and model “sand.” At the end of the experiment the entire surface layer and a significant depth of substrate were removed from an entire reach of the channel and sieved. The results of the experiment confirmed the weak form of equal mobility: The surface layer was significantly coarsened compared with the bed load, even when the “sand” was excluded from the size distributions. The strong form of equal mobility was also supported by the experiment, in that the size distribution of the “gravel” substrate was nearly identical to the size distribution of the “gravel” portion of the load. The gravel substrate was, however, slightly but measurably coarser than the gravel load, a tendency that has been observed in the field in cases for which the strong form is not satisfied.

Fractal dimensions of individual flocs and floc populations in streams

The fractal dimension of an individual floc is a measure of the complexity of its external shape. Fractal dimensions can also be used to characterize floc populations, in which case the fractal dimension indicates how the shape of the smaller flocs relates to that of the larger flocs. The objective of this study is to compare the fractal dimensions of floc populations with those of individual flocs, and to evaluate how well both indicate contributions of sediment sources and reflect the nature and extent of flocculation in streams. Suspended solids were collected prior to and during snowmelt at upstream and downstream sites in two southern Ontario streams with contrasting riparian zones. An image analysis system was used to determine area, longest axis and perimeter of flocs. The area–perimeter relationship was used to calculate the fractal dimension, D, that characterizes the floc population. For each sample, the fractal dimension, Di , of the 28 to 30 largest individual flocs was determined from the perimeter–step-length relationship. Prior to snowmelt, the mean value of Di ranged from 1·19 (Cedar Creek, downstream) to 1·22 (Strawberry Creek, upstream and downstream). A comparison of the means using t-tests indicates that most samples on this day had comparable mean values of Di . During snowmelt, there was no significant change in the mean value of Di at the Cedar Creek sites. In contrast, for Strawberry Creek the mean value of Di at both sites increased significantly, from 1·22 prior to snowmelt to 1·34 during snowmelt. This increase reflects the contribution of sediment-laden overland flow to the sediment load. At three of the sampling sites, the increase in fractal dimensions was accompanied by a decreases in effective particle size, which can be explained by an increase in bed shear stress. A comparison of fractal dimensions of individual flocs in a sample with the fractal dimensions of the floc populations indicates that both fractal dimensions provide similar information about the temporal changes in sediment source contributions, about the contrasting effectiveness of the riparian buffer zones in the two basins, and about the hydraulic conditions in the streams. Nevertheless, determining the individual fractal dimensions of a set of large flocs in a sample is very time consuming. Using fractal dimensions of floc populations is therefore the preferred method to characterize suspended matter. Copyright © 2000 John Wiley & Sons, Ltd.

The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. II. Hydrodynamics and sediment transport

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine. During daylong surveys, suspended sediment concentration, bedload transport rate, water depth, flow velocity, turbulence intensity, near‐bed shear stress and water temperature were recorded over individual isolated dunes. This paper considers the hydrodynamic environment and sediment transport over the dunes. A companion paper details the sedimentology and morphology of the dunes. Flow over the flat gravel lag upstream of large dunes is more uniform than that over dunes, and gravels are rarely entrained by in‐bank discharges. Unsteady and non‐logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes, and the near‐bed flow demonstrates evidence of large‐scale, coherent low‐frequency flow structures; these may reflect stacked sequences of separated boundary layers generated by secondary dunes. However, the low‐amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides. Bed roughness and near‐bed shear stress commonly increase steadily over the stoss of dunes, but may decrease near the crest, especially where a crestal platform exists that is devoid of secondary bedforms. Bed roughness scales with the physical size of bed roughness elements. However, variability in roughness lengths is large, owing to the composite nature of bed roughness. Bedload transport over stoss slopes is spatially variable, but often shows an increase with increasing bed shear stress over the stoss. Well‐formed wakes only develop downstream of lee slopes, which are close to the angle of repose; otherwise, separation is weak, and suspension and settling of fine sediments is of little consequence to dune evolution. Wake flow is characterized by turbulence production one order of magnitude greater than over the stoss side, which may be related to vortex shedding from the dune. However, wake current speeds are extremely low and variable in direction; reverse flow is not sustained, and no retrogressive bedforms occur in the lee of large dunes. Near‐bed shear stress and bed roughness are usually low within the wake, reflecting low current speeds. Weak wake‐flow reattachment occurs at a variable distance downstream, up to several times the duneform height. A two‐dimensional numerical model for flow over dune topography, calibrated using average parameter values, provided a reasonable description of the flow upstream, and over the backs of large dunes as far as the crestal region. Wake flow could not be modelled. However, the temporal and spatial complexity of natural three‐dimensional flow over the dunes resulted in variance in parameter estimates; this variance precluded modelling of flow and bedload transport. Consequently, it was not possible to model dune evolution in a deterministic sense.

Bed Shear Stress and Scour over Bed‐Type River Intake

Laboratory experiments are carried out to determine the increase in bed shear stress and local scour over the zone of suction associated with a bed-type river intake in a channel with a mobile bed. Bed shear stress is estimated first by observing the movement of indicator grains whose threshold shear stress in uniform flow was already known, and second by observing the rate at which local scour takes place. Evaluation of the two sets of results indicates that, at the onset of scour when particles are lifted from the bed by increased bed shear stress, they do not strike the bed again until they are clear of the scour zone. The final scour depth is found to be directly proportional to the difference between the initial bed shear stress because of suction and the threshold bed shear stress for the bed grains in the presence of suction.

Hyperconcentrated Sand‐Water Mixture Flows Over a Flat Bed

Field surveys and experiments in two tilting flumes on hyperconcentrated sand‐water‐mixture (swm) flows were conducted. This paper is limited to supercritical flows over a flat bed. It is shown that an equilibrium slope can be defined at which sedimentation and erosion are in balance. For laminar flow conditions this slope is only dependent on the average sand concentration, whereas for turbulent conditions the flow rate per unit width is the main parameter. The sand concentration near the bed measured about 35% by volume for most test conditions. The vertical gradient in sand concentration decreased with increasing mean sand concentration, indicating a decrease in the damping of turbulence. The velocity distribution behaved fairly logarithmically, showing a variation in slope with varying sand concentration. A minimum value of the effective von Kármán constant was found at concentrations of about 20%. Finally, it appears plausible that at high concentrations a considerable growth of the viscous sublayer occurs, resulting in an increase in bed shear stress.

Discussion of the stability of lower stage plane beds and the absence of current ripples in coarse sands

D r P. A. A llen writes: Dr M. R. Leeder has provided a convincing argument for a separation inhibition mechanism for the stability of lower stage plane beds. Such inhibition is thought to be caused by the enhanced vertical mixing of fluid over transitional to rough boundaries, which overcomes the build up in static pressure in the lee of bed defects. This latter point is supported by the experimental flow visualization work of Grass (1971), who demonstrated an increase in bed shear stress over transitional to rough boundaries compared to those that are physically smooth. This probably corresponds to a break up of the viscous sublayer. The important experiments in the wall region of turbulent shear flows of Runstadler et al. (1963, cited in Kline et al. 1967) were made over a flat plate with a zero pressure gradient (d P /d x = 0). Kline et al . (op. cit.) later examined the effect of adverse (d P /d x >0) and favourable (d P /d x < 0) pressure gradients on processes in the inner region of the turbulent boundary layer. In the strongly favourable flow the bursting action of wall-layer streaks was almost totally suppressed. Since. bursting (ejection) appears to control the subsequent lift-up of downstream low velocity wall- streaks and the action of so-called sweeps (Corino & Brodkey 1969) originating in the logarithmic region (Offen & Kline 1975) which cause grain movement and amplification of bed defects, it is reasonable to suspect that favourable (negative) pressure gradients may inhibit propagation from these defects. Favourable pressure gradients