Uniform Sand Bed Research Articles

Variations in clear water scour geometry at piers of different effective widths

The equilibrium scour geometry around a circular pier protruding vertically on a scoured channel bed was experimentally investigated. In order to conduct a comparative study, measurements within an equilibrium scour hole at a square pier, an equilateral triangular pier, and a flat plate with sides, facing the approaching flow, equaling the width of the circular pier were taken. Flow in an equilibrium scour hole under a clear water regime was considered for 4 separate tests. Tests were carried out for approaching flow having undisturbed constant flow depth greater than the pier width with a depth-averaged approaching flow velocity of about 68.2% representing the critical velocity of the uniform sand bed. It was observed that all the equilibrium dimensionless scour parameters, namely scour depth, length, width, area, and volume, depend on the effective pier width equal to the diameter of the smallest circumscribing circle of the pier. When the effective pier diameter increases and approaching flow depth is constant, scour parameters increase. Based on the results, some empirical equations are proposed for the scour depth, scour length, scour width, scour area, and scour volume. These equations may be utilized to estimate scour geometry in future studies.

Prediction of equilibrium scour time around long abutments

Scouring around abutments and piers is a significant problem in bridge failure. The main objective of this study is to estimate the equilibrium time around long vertical abutments using genetic programming (GP). The data from a large number of clear-water scour experiments in uniform sand beds were collected for GP modelling techniques. To predict the equilibrium scour time, three independent parameters were selected using sensitivity analysis: y/L, U/Uc and L/d50. The results based on GP were compared with those of the artificial neural networks (ANNs) and empirical methods. Statistical tests indicate that, although the ANN method produced better results compared with GP and non-linear regression techniques, the GP equation is more useful for practical purposes.

Assessment of portable traveling pluviator to prepare reconstituted sand specimens

Air pluviation method is widely adopted for preparation of large, uniform and repeatable sand beds of desired densities for laboratory studies to simulate in-situ conditions and obtain test results which are highly reliable. This paper presents details of a portable traveling pluviator recently developed for model sand bed preparation. The pluviator essentially consisted of a hopper, orifice plates for varying deposition intensity, combination of flexible and rigid tubes for smooth travel of material, and a set of diffuser sieves to obtain uniformity of pluviated sand bed. It was observed that sand beds of lower relative density can be achieved by controlling height of fall, whereas, denser sand beds could be obtained by controlling deposition intensity. Uniformity of pluviated sand beds was evaluated using cone penetration test and at lower relative densities minor variation in density was observed with depth. With increase in relative density of sand bed higher repeatability of uniform pluviation was achieved.

Erosion of sand beds by obliquely impinging plane turbulent air jets

This paper presents the results of an experimental study of the erosion of a cohesionless sand bed by obliquely impinging plane turbulent air jets. The jet, with velocity of 20.1-49.2 m/s and width at the nozzle, 2b o, of 4.7 mm, impinged on the bed at angles varying from 15 to 60°. The uniform sand bed had a mean particle size of 1.2 or 2.38 mm. Several characteristic dimensions of the scour hole were measured and found to depend mainly on the impingement angle and the erosion parameter F o/(H/2b o)0.5, where F o is the densimetric Froude number, and H is the impingement distance. Correlations were developed for the main characteristics of erosion at asymptotic state. As well, comparisons were made with the erosion of sand beds by obliquely impinging submerged plane water jets. The differences in the scour produced by the air and water jets were found to be significant.

Liquefaction and Drainage in Stratified Soil

Remediation schemes for the treatment of liquefiable ground, such as vertical drains, have evolved over the last 30 years largely as a result of consideration of uniform sand beds. However, many liquefiable sites also contain thin layers of silts and clays which will act as an obstacle to fluid dissipating from liquefying sands, such that the performance and requirements of a remediation scheme will be different. In particular, water films may be produced. This paper presents results from a series of dynamic centrifuge experiments on vertical drains in level liquefiable sand deposits. Centrifuge tests were conducted both with and without surface fine layers. The drains prevented sand boiling, and the thin surface fine layers did not significantly affect excess pore pressure response. A further centrifuge test on a thin layer of low-permeability fines separating two liquefiable sandy layers, showed evidence suggesting that material boiled from the lower sand to the upper sand in the absence of a drain through the fines. All of the experiments suggest that the presence of unblocked vertical drains prevent water film formation.

Dynamic behavior of bed material around bridge pier under water pressure variation

Bridge damage is commonly occurring due to sudden attack of flood flows or storm waves. Failure of bridges under such a condition is closely related to the dynamic stability of the bed material as well as to the formation of local scour. The mechanisms of the dynamic behavior of bed material and local scour around bridge pier subjected to water pressure variation are presented in this paper. An experimental program was carried out using a cylindrical pier with uniform sand bed under wave action and under sudden water pressure variation. Pore water pressure at different levels of bed material and local scour around the pier were observed. Experimental results show that an excess pore water pressure develops under wave action, which reduces the effective stress of the bed material. The results also reveal that the excess pore water pressure developed under sudden water pressure drop liquefies the sediment bed and, hence, the scour depth increases considerably at a stage of liquefaction occurs.

Comparison of Methods for Predicting Incipient Motion for Sand Beds

A comparison of four methods for predicting the incipient motion conditions of a uniform sand bed is presented. The four methods are: (1) the Shields diagram, (2) an empirical approach, (3) a method derived from resolution of rotational forces, and (4) a simplified resolution of rotational forces with a variable lift force coefficient. The four methods are used to predict the incipient motion conditions for 97 experimental runs taken from seven independent experimental flume studies. The effectiveness of predicting depth averaged incipient motion velocity for each of the four methods are compared. The simplified resolution of rotational forces model (4) and Shields method (1) were most successful in predicting the incipient motion velocity [R2=0.77, 0.74 and root mean square error (RMSE)=0.18, 0.15, respectively]. The slope of line of best fit for plots illustrating predicted versus measured incipient motion velocity were similar (slope=0.63, 0.65, respectively), illustrating that both methods provide a s...

Initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds: experiments and modelling

Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory flows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional flows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are ‘unworked’ at the start of the experiments.The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined. The pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data. The measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form Φ = αDγ(Di/D50)β (after Li & Komar, 1986), where α, γ and β are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship.The entrainment of grains is found to be easier beneath unidirectional flows than combined flows, in accordance with previous authors’ findings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near‐threshold flows.

Time Scale for Local Scour at Bridge Piers

The temporal development of clear-water local scour depth at cylindrical bridge piers in uniform sand beds is considered. New data are presented and used to quantify the influence of flow duration on the depth of local scour. An equilibrium time scale (t*) is defined. The data show that both t* and the equilibrium scour depth (dse) are subject to similar influences of flow and sediment parameters, as might be expected because they are inherently interdependent. A method is given for determination of the time for development of dse for a given pier, sediment, and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole. The results show that the scour depth after 10% of the time to equilibrium is between about 50% and 80% of the equilibrium scour depth, depending on the approach flow velocity.

Time Scale for Local Scour at Bridge Piers

The temporal development of clear-water local scour depth at cylindrical bridge piers in uniform sand beds is considered. New data are presented and used to quantify the influence of flow duration on the depth of local scour. An equilibrium time scale (t*) is defined. The data show that both t* and the equilibrium scour depth (dse) are subject to similar influences of flow and sediment parameters, as might be expected because they are inherently interdependent. A method is given for determination of the time for development of dse for a given pier, sediment, and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole. The results show that the scour depth after 10% of the time to equilibrium is between about 50% and 80% of the equilibrium scour depth, depending on the approach flow velocity. of local scour d se is rapidly attained in live-bed conditions, but rather more slowly in clear-water conditions (Fig. 1). Clear- water scour occurs for mean flow velocities up to the threshold velocity for bed sediment entrainment, i.e., V # Vc, while live- bed scour occurs for V > Vc. The maximum equilibrium scour depth dse)max occurs at V = Vc. In armored cobble or cohesive sediment bed streams, multiple flood events may be required before the maximum clear-water scour is reached. This may take many years. The equilibrium scour depth in live-bed con- ditions fluctuates due to the effects of bed form migration. The dashed lines in Fig. 1 represent the temporal average scour depth under live-bed conditions. The diagram also shows the time taken, te, for the equilibrium scour depth to develop. The equilibrium time, te, is the focus of this paper. It increases rapidly with flow velocity under clear-water conditions, but then decreases rapidly for live-bed scour. Existing equations for depth of local scour at bridge piers give the equilibrium depth and are therefore conservative re- garding temporal effects. For the live-bed conditions that typ- ically pertain in floods, equilibrium scour depths are appro- priate. However, where clear-water scour conditions exist, the equilibrium depth of scour may be overly conservative. Peak flood flows may last only a number of hours or a few days in the field, and short floods have insufficient time to generate equilibrium depths. For example, bridge piers situated on the floodplain may be wet for periods of less than one day during a flood; typically, clear-water conditions pertain at such sites. The actual scour may be only a small fraction of the equilib- rium scour depth, which could take weeks to fully develop. Johnson and McCuen (1991) developed an analytical model to simulate the temporal process of local scour at piers. The model was applied to a hypothetical bridge pier using a gen- erated sequence of flood flows over a 75 year period, the in- dividual storms being of 24 h duration. At the end of the 75 year period, the scour depth was still increasing.

FLOW FIELD AND SCOURING EFFECTS OF STEADY AND PULSATING JETS IMPINGING ON A MOVABLE BED

The flow field and erosion pattern of continuous and pulsating impinging jets has been studied experimentally. The mean velocity field as well as turbulent and pulsation components, wall pressure and wall shear stress distributions were investigated in an axisymmetric air jet impinging upon a rough wall. The results show how harmonic velocity pulsations at the nozzle affect both the mean and the turbulent flow field. Corresponding erosion tests with a vertical water jet impinging upon a uniform sand bed exhibited under certain conditions a greatly enhanced erosion rate. The increase in erosion capacity due to pulsation seems to be due to both the changes in the mean flow field and to the greatly increased lateral correlation of the velocity field in the impinging region.