Uniform Sediment Research Articles

Experimental Study on Flow Field and Scour Pattern around T Shaped Spur Dike in a 90 Degree Bend

Spur dike is a hydraulic structure used for protection of river banks. In the past few decades, the use of spur dikes for stability of the outer banks of rivers has attracted the attention of hydraulic engineers. Setting spur dikes in the flow’s direction leads to local scour around the spur dike and changes the topography of the bed. This study experimentally measures the flow field and scour around T-shaped spur dike install in 90 degree bend. The experiments were carried out in a channel with a 90 degrees bend and with a mild curvature. Uniform sediments having an average diameter of 1.28mm were used under clear water condition. The three-dimensional Acoustic Doppler Velocimeter (ADV) and point gage were used for measuring the flow field and bed topography. For this purpose, scour test was done at the beginning of the experiment and the topography of the bed with the scour hole around the sour dike installed at the 75 degree was measured. And the three-dimensional time-averaged velocity was measured. Then the comparison of the three dimensional components of speed was made in different sections and the differences of the flow pattern along the bend was analyzed. The observations showed the effect of the spur dike and the bed’s topography in distorting the secondary flow pattern and in the formation of the subsequent secondary flow. In addition, the secondary flows are formed along the scour hole slope development. In the downstream sections of the spur dike, the power of the secondary flow increases. At the end of the bend, this changes the flow direction from the external bank towards the internal bank. In the plans, some horizontal vortices with a counter-clock-wise direction can also be observed in the upstream of the spur dike in different layers.

Study on The Angle of Repose of Nonuniform Sediment

A conception of exposure degree is used in this article to account for the hiding and exposure mechanism of nonuniform sediment transport. The force arms of particles on the bed slope are assumed to be a function of the exposure degree, which is stochastically related to the size and gradation of bed materials. Based on this conception, the formula for the angle of repose of nonuniform sediment is developed. The angle of repose of nonuniform sediment is experimentally investigated in rotation drum at the rolling motion of sediment particles. The nonuniform sediment is gotten from the mixture of two kinds of uniform sediment with different weight ratios and two experimental plans that one for changing the weight ratio of two kinds of uniform sediment from 1:9 to 9:1 and the other changing the uniform sediment size and weight ratio are used. The effect of the rotation speed and the filling degree on the angle of repose is investigated, too. Finally, the formula has been tested against a wide range of laboratory data, the prediction by the newly proposed formulas is very good.

The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration

Pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick, and are common where gas is present in near-seafloor sediments. This paper investigates how gas might lead to pockmark formation. The process is envisioned as follows: a capillary seal traps gas beneath a fine-grained sediment layer or layers, perhaps layers whose pores have been reduced in size by hydrate crystallization. Gas accumulates until its pressure is sufficient for gas to invade the seal. The seal then fails completely (a unique aspect of capillary seals), releasing a large fraction of the accumulated gas into an upward-propagating gas chimney, which displaces water like a piston as it rises. Near the seafloor the water flow causes the sediments to become “quick” (i.e., liquefied) in the sense that grain-to-grain contact is lost and the grains are suspended dynamically by the upward flow. The quickened sediment is removed by ocean-bottom currents, and a pockmark is formed. Equations that approximately describe this gas–piston–water-drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor. For uniform near-surface sediment permeability, this is a buoyancy control, not a permeability control. The rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure. Plausible estimates of these parameters suggest gas chimneys at Blake Ridge could reach the seafloor in less than a decade or more than a century, depending mainly on the permeability of the deforming near-surface sediments. Since these become quick before gas is expelled, gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation. However, detecting gas chimney growth might be a useful risk predictor. Any area underlain by a gas chimney that extends half way or more to the surface should be avoided.

Modelling dike breaching due to overtopping

Dikes along rivers protect a valley and its people and property from floods. Recent dike failures due to overtopping highlight the need to understand this damage process. Its accurate prediction is essential to develop effective emergency action plans and to design early warning systems and hazard maps. Laboratory tests of breach processes are related to scaling issues currently not understood. Therefore, a series of plane dike breach tests due to overtopping were conducted to examine model limitations. All dikes were of trapezoidal shape, consisted of uniform noncohesive sediment and neither contained a core nor a surface layer. The temporal dike breach progress was optically recorded to allow for a detailed analysis of the sediment and water surfaces.A systematic variation of both the dike dimension and the sediment diameter resulted in basic findings relative to: (1) Test repeatability; (2) Side wall effect; and (3) Scale effects. The results indicate definite minimum dimensions for both the dike height and width, sediment diameter and overtopping discharge.

Three-dimensional (3D) modeling of non-uniform sediment transport in a channel bend with unsteady flow

A fully three-dimensional non-hydrostatic model was applied to compute the water flow and morphodynamic processes in a laboratory flume. The experiments provided data for bed deformation and grain sorting processes in a 180? channel bend. The data were used to investigate the performance of different sediment transport approaches. The original van Rijn formula for uniform sediments was compared with two advanced nonuniform sediment formulas that considered the interaction between individual grain sizes by defining hiding-exposure approaches. The results showed that the default configuration of the model was able to predict bed deformations and grain sorting processes with satisfying magnitude and trend. However, the default approach considerably overestimated the bar evolution. The computed results improved significantly when applying any of the introduced hidingexposure approaches, representing better the physical mechanisms of the hiding-exposure effect. This study shows that the present numerical model is able to predict the morphodynamic bed changes in a flume of nonuniform sediments with good agreement if appropriate sediment formulations are used.

Grain size dependency in sandbank modeling for the case of uniform sediment

A 2‐D depth‐integrated stability analysis designed for marine tidal sandbanks is developed to investigate the relative influence of the grain size–dependent parameters on the simulated dynamics of the sandbanks for the case of uniform sediment. The model is applied to a real case in the North Sea. Assuming that the model is valid mainly in an area where bed load is the dominant sediment transport mode, the model taking into account the grain size dependencies gives a better prediction of sandbank occurrence than models neglecting these dependencies. All the grain size–dependent parameters are investigated separately. Focusing on the morphological time and wavelength of the sandbanks, this paper demonstrates that neglecting the grain size dependencies, especially the transport threshold, has a drastic influence on the dynamics of modeled sandbanks, especially on the sandbanks growth rate. The influence on the geometrical properties (wavelength and orientation) is less pronounced.

An experimental investigation of mechanisms involved in bed load sheet production and migration

Field measurements indicate that, in gravel bed rivers, bed load may not be a one‐to‐one response to shear stress but may instead fluctuate a great deal over time for a given flow condition. Both in flume and field experiments, these fluctuations were often associated with migration of low‐relief bed forms called bed load sheets. Whereas several studies have described bed load sheets as a consequence of grain sorting, little is known about the mechanisms responsible for their production and migration. These were investigated in flume experiments. A set of 20 experiments was conducted under constant feeding rate conditions, with mixtures of different uniform sediments and for slopes varying from 0.8 to 9%. Except for runs performed in high flow conditions, we observed periodic bed load sheet production and migration associated with fluctuations of bed slope, bed state (bed fining and paving), and bed load. Observations allowed us to conclude that bed load sheets resulted from very efficient vertical and longitudinal grain sorting that produced periodic local bed aggradation and erosion clearly observed in the upstream section of the flume. Fractional transport rates were measured in one run. Combined with the results of experiments previously conducted by authors with uniform sediments, this experiment showed that the highest (peak) solid discharges were essentially caused by the much greater mobility of the coarser gravels when transported within bed load sheets. A scenario is proposed for the mechanisms involved in bed load sheet production and migration.

Tidal sand wave formation: Influence of graded suspended sediment transport

The influence of graded suspended sediment transport on the formation of tidal sand waves is investigated. Hereto, in the framework of a linear stability analysis, we examine the time development of small‐amplitude perturbations superimposed to a flat heterogeneous seabed in a shallow tidal sea dominated by one tidal constituent. The heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom, as well as in suspension, is considered. The active layer concept is employed and hiding/exposure effects are taken into account. The mechanisms giving rise to the sediment‐sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension. On the other hand, the formation of sand waves is quantitatively, substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture. In particular, the graded suspended sediment transport largely affects the contribution to the amplification rate related to the sorting process. This leads to the growth of bed forms characterized by a smaller wavelength compared with that predicted for a uniform sediment, when the mean grain size of the sediment mixture is relatively coarse. On the other hand, longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size. Moreover, a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case. A comparison of the theoretical predictions with field observations seems to support the model, even though it cannot be considered conclusive.

Grain sorting effects on the formation of tidal sand waves

A model is developed to investigate the process which leads to the formation of sand waves in shallow tidal seas characterized by a heterogeneous sea bed composition. The main goal of the analysis is the evaluation of the effects that a graded sediment has on the formation of the bottom forms and the investigation of the sorting process induced by the growth of the bottom forms. The analysis is based on the study of the stability of the flat bed configuration, i.e. small amplitude perturbations are added to the flat bottom and a linear analysis of their time development is made. For an oscillatory tidal current dominated by one tidal constituent, the results show that the graded sediment can stabilize or destabilize the flat bottom configuration with respect to the uniform sediment case, depending on the standard deviation σ* of the grain size distribution and on the ratio between the horizontal tidal excursion and the water depth. For moderate values of , i.e. values just larger than the critical value for which the sediment is moved and sand waves appear, the presence of a sand mixture stabilizes the flat bed. On the other hand, for large values of , the mixture has a destabilizing effect. In both cases the effect that a sand mixture has on the stability of the flat bed configuration is relatively small. Moreover, for moderate values of , the fine fraction of the mixture tends to pile up at the crests of the bottom forms while the coarse fraction moves towards the troughs. For large values of , the grain size distribution depends on the value of σ*. The results are physically interpreted and provide a possible explanation of the apparently conflicting field observations of the grain size distribution along the sand wave profile, carried out in the North Sea.

Effect of Sediment Size Scaling on Physical Modeling of Bridge Pier Scour

Local pier scour experiments were performed in the laboratory to investigate the effect of relative sediment size on pier scour depth using three uniform sediment sizes and three bridge pier designs at different geometric model scales. When the data from a large number of experimental and field investigations are filtered according to a Froude number criterion, the effect of relative sediment size on dimensionless pier scour depth is brought into focus. The choice of sediment size in the laboratory model distorts the value of the ratio of pier width to sediment size in comparison with the prototype which in turn causes larger values of scour depth in the laboratory than in the field. This model distortion due to sediment size is shown to be related to the scaling of the large-scale unsteadiness of the horseshoe vortex by studying the relevant time scales of its coherent structure upstream of a bridge pier using acoustic Doppler velocimeter measurements. Observations of sediment movement, probability distributions of velocity components, and phase-averaging of velocity measured upstream of a bridge pier reveal properties of coherent motions that are discussed in terms of their contribution to the relationship between dimensionless pier scour depth and the ratio of pier width to sediment size over a large range of physical scales.

Incipient Motion of Riverbank Sediments with Outflow Seepage

Based on a force analysis, an expression is derived to describe the critical Shields number for incipient motion of uniform cohesionless sediment particles on a riverbank slope in terms of flow parameters, outflow seepage, and physical and mechanical properties of sediments. Parametric studies were conducted to investigate quantitatively the effects of hydraulic gradient of seepage, slope angle, and flow direction on the critical Shields number. The results show that the critical Shields number decreases with an increase in the hydraulic gradient. Where bank collapse is concerned, the most dangerous direction of hydraulic gradient of outflow seepage is at an angle equal to the effective internal angle of friction of the sediment mass with respect to slope surface. At a certain value of hydraulic gradient, the critical Shields number decreases with increasing slope angle. Open flow becomes more erosive when the current direction changes from horizontally parallel with the riverbank line to turning downward.

Mobile-bed aggradation and degradation in a narrow flume: Laboratory experiments and numerical simulations

Aggradation and degradation phenomena in fluvial beds having uniform sediments are explored experimentally and simulated numerically. Laboratory experiments were conducted in a narrow mobile-bed flume for different sediment sizes, flow discharges and bed slopes. The flume had a cross-section of curved wall and flat bed, where the shear stress distribution is not same on the bed and the walls. Tests were carried out in two conditions: (1) equilibrium condition, where, once the steady and uniform flow conditions were achieved for a given slope and discharge, the channel was fed with variable sediment discharges until the bed-load sediment transport achieved an equilibrium condition; and (2) non-equilibrium condition, where the sediment feeding was instantaneously stopped (degradation tests) or increased (aggradation tests) and the bed levels were measured over time. Experimental results enabled assessing the deposition and erosion rates and determining the empirical power law for the bed-load discharge. Numerical simulations were then performed with the Double Order Approximation (DORA) model, which solves the one-dimensional shallow water equations governing a free surface gradually varied unsteady flow on mobile beds. As closure relationship, three bed-load formulae were implemented in the numerical code: the Ackers, P. [1984. Sediment transport in sewers and the design implications. In: Proceedings of the International Conference on Planning, Construction, Maintenance and Operation of Sewerage Systems, Reading, England, BHRA (Cranfield), pp. 215–230.] and Meyer-Peter, E., Müller, R. [1948. Formulas for Bed-Load Transport. Report on the Second IAHR Meeting, Stockholm, Sweden, pp. 39–64.] ones and a formula empirically derived at equilibrium in the laboratory tests. The results show that the DORA model can reproduce the bed profiles over time except the local scouring at the transition between fixed and mobile bed. In particular, in the best simulations obtained by using the empirical bed-load power law, the time evolution of bed profiles is in good agreement with the measured data of aggradation and degradation tests, having small average relative errors.

Maximum Local Scour Depth at Bridge Piers under Unsteady Flow

The temporal effect of hydrograph on local scour depth is investigated under clear-water scour condition. By analyzing the characteristics of scour-depth evolutions at bridge piers for different rising hydrographs, a relation for estimating the maximum scour depth in uniform sediment is proposed. In the relation, the flow unsteadiness effect is taken into account by an unsteady flow parameter combining the peak-flow intensity and time-to-peak factors. For nonuniform sediment with d84 employed as the effective sediment size, this relation can yield reasonably good results of the maximum scour depth under rising hydrograph.

Countermeasures against Local Scouring at Bridge Piers: Slot and Combined System of Slot and Bed Sill

Results are presented and discussed from two laboratory experimental campaigns specifically designed to investigate the behavior of a slot as countermeasure against local scouring at a smooth circular bridge pier, close to threshold flow conditions of initiation of uniform sediment motion. The investigation was aimed at evaluating the effectiveness of the slot by changing its sinking depth into a sand bed, and assessing the dependence of the scour depth on different dimensionless groups. The results showed that the slot reduces the local scour at pier. The maximum reduction of the scour depth was about 30% (about 70% for both scour area and volume) in the best configurations. A combined countermeasure is also proposed and tested, consisting of a slot and a bed sill placed downstream of the pier and adjacent to it: in the best configuration, the scour reduction in front of the pier reached about 45% on average (with about 80 and 90% for scour area and volume, respectively). This last result shows that a combination of slot and bed sill may be a very effective countermeasure against local scouring at bridge piers.

Experiments in a high‐amplitude Kinoshita meandering channel: 2. Implications of bend orientation on bed morphodynamics

Experiments carried out under flat smooth bed condition in a periodic, asymmetric, Kinoshita meandering channel showed that bend skewness controls the three‐dimensional mean flow (e.g., primary and secondary flow) and turbulence structure (e.g., location of shear layers and turbulent shear stresses). In this paper, similar hydraulic conditions are used to perform movable‐bed experiments with uniform sediment size in order to study the effect of bend orientation on bed morphodynamics. Two main differences between the upstream‐ and downstream‐skewed meander configurations are found: (1) for the case of bends oriented upstream valley, the bed forms are produced just upstream of the bend apex, whereas for the case of bends oriented downstream valley they are observed around the upstream inflection point, and (2) the downstream‐skewed condition produces the deepest scour region, which is located downstream of the bend apex. A comparison with observations at two bends on the Wabash River, Illinois, shows some remarkable similarities with the bed morphology observed in the Kinoshita channel. Further analysis of the bed form celerity, bed load transport during evolution, and superimposition of migrating bed forms and their implications on bend migration are presented.

Control of Scour at Bridge Piers by a Downstream Bed Sill

Results are presented from laboratory experiments to investigate the effectiveness of bed sills as countermeasures against local scouring at a smooth circular bridge pier, for flow conditions near the threshold of uniform sediment motion. The bed sill was located downstream of the pier, and its effectiveness with the distance between pier and sill was evaluated. The dependence of the scour depth on different dimensionless groups was defined. The results showed that a bed sill placed at a short distance downstream of the pier reduces the scour depth, area, and volume. In particular, the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. The bed sill seems to take effect some time after the beginning of the test, as the scour hole downstream of the bridge pier develops sufficiently and interacts with the countermeasure.

Computer modelling of graded sediments in rivers

This paper reports on current engineering capability to simulate the behaviour of graded sediments in rivers using one-dimensional computer models. Emphasis is placed on non-equilibrium conditions when the bedload transport rate, bed elevation and composition of the bed material are changing with time. The model used is typical of those available to practicing engineers. A general description is given and comparisons are made of the model's ability to reproduce available laboratory and field data. The performance of four sediment transport equations (Ackers and White, Meyer-Peter and Muller, White and Day, Parker) in predicting graded sediment movement is assessed by comparison with the measured data. Of these equations, the first two were originally developed for application to uniform sediment and do not account for size fraction interaction, while the other two take account of sediment grading. The test simulations demonstrate that the use of the Parker equation provides the best overall agreement with the measured data, while the use of the other equations provides inadequate prediction in the case of graded sediments. It is concluded that an adequate computer simulation of long-term trends in the behaviour of graded sediments is only possible if size fraction interaction in the sediment mixtures is accounted for in the sediment transport equation used.

Experimental validation of a sediment transport two-dimensional depth-averaged numerical model using PIV and 3D Scanning technologies

This paper presents the experimental validation of a two–dimensional depth–averaged numerical model with uncoupled flow and sediment transport using the finite volume method. Tests were conducted at the Civil Engineering School of the University ofA Coruña (Spain), using laser technologies such as particle image velocimetry and a three–dimensional scanner (3D–Scanner). This device measures bed elevations through calculations that take refraction into account. The numerical model presented here solves shallow water equations in the hydraulic field and calculates, through wellknown empirical formulas, bedload transport of uniform granular sediments. A comparison between the numerical and experimental results in the longitudinal and transversal direction, as well as in the evolution of the fundamental variables lead to the validation of the numerical model. Moreover, the application of laser technologies in fluvial hydraulics, especially with promising tools such as the 3D–scanner, here presented, is another key point of this work.

Clear-Water Scour below Underwater Pipelines under Steady Flow

Experiments on clear-water scour below underwater pipelines (initially laid on the sediment bed) in uniform and nonuniform sediments under steady flow were conducted. Equilibrium scour profiles were modeled by a cubic polynomial. The experimental results are examined to describe the influence of various parameters on equilibrium scour depth. The equilibrium scour depth ds increases with increase in approach flow depth h for shallow flow depths, becoming independent of higher flow depths when h∕b>5 , where b =pipe diameter. However, the curves of scour depth versus sediment size d and Froude number Fb have a maximum value of ds ∕b=1.65 at b∕d=27 and Fb =0.6 . The influence of sediment gradation on scour depth is prominent for nonuniform sediments, which reduce scour depth to a large extent due to the formation of armor layer within the scour hole. The influence of different shaped cross sections of pipes on the scour depth was investigated, where the shape factors for circular, 45° (diagonal facing) and 90...

The influence of topography on sidescan sonar images

The backscattering signal on sidescan sonar images is to a large degree dependent on the incident angle of the acoustic beam onto the seafloor. As sidescan sonar systems are often used for mapping seafloor lithologies, it is necessary to reduce or even remove the effect of different backscattering strengths caused by varying incidence angles. This study evaluates the influence of seafloor morphology on the acoustic backscattering signal of the deep‐towed IFM‐GEOMAR DTS‐1 sidescan sonar system. Data used are from the Pacific continental slope offshore Nicaragua in a water depth between 800 and 2400 metres. There authigenic carbonate patches formed by cold fluid venting are imaged with a high backscattering level. The carbonates are often located on top or on the flanks of mound structures with a strong morphology. The specific DTS‐1 backscattering function is determined on normal, uniform seafloor sediment, and then applied in a new processing algorithm to the raw data of two test areas. The change in amplitude strength when considering seafloor morphology is calculated. The topographic influence on the backscattering signal can be quite significant when imaging mound structures. Nevertheless the high backscatter on mounds is not completely removed, leading to the conclusion that it originates from a different seafloor lithology and roughness.