Natural Bedforms Research Articles

Basal Pressure Variations Induced by a Turbulent Flow Over a Wavy Surface

Turbulent flows over wavy surfaces give rise to the formation of ripples, dunes and other natural bedforms. To predict how much sediment these flows transport, research has focused mainly on basal shear stress, which peaks upstream of the highest topography, and has largely ignored the corresponding pressure variations. In this article, we reanalyze old literature data, as well as more recent wind tunnel results, to shed a new light on pressure induced by a turbulent flow on a sinusoidal surface. While the Bernoulli effect increases the velocity above crests and reduces it in troughs, pressure exhibits variations that lag behind the topography. We extract the in-phase and in-quadrature components from streamwise pressure profiles and compare them to hydrodynamic predictions calibrated on shear stress data.

Рhysical simulation of erosion of bottom pits

The present paper is devoted to research of the erosion of large-scale sand pits in the water flow. The investigations were performed in the hydrodynamic flume with sandy bottom. To provide suitable conditions for sediment transport in the flume, the analysis of the factors leading to the motion of sediments was carried out in accordance with the Shields diagram. It was shown that the flow regime created in the laboratory channel promotes the development of natural bed forms such as ripples. Estimations of the velocity of movement of the ripples were obtained. The experiments with large sand pits on the flume bottom demonstrated that those disturb the balance of sediments and cause the reformatting of the water flow. To assess the influence of the pit configuration on the erosion process, two-dimensional triangular and trapezoidal pits were considered. It was found that the longitudinal profile of the triangular pit changes due to sediment deposition on its upper slope and erosion of the lower slope. The pit upper slope levels out and shifts forward due to the continuous flow of sediment in this region. The depth of the unevenness also decreases owing to deposition of the sediment directly on its bottom. Due to the blow of water jet to the pit lower slope, the zone of maximum erosion of the bottom surface is observed here. The bottom reformatting leads to the displacement of the pit downstream. Studies of the erosion of the trapezoidal pit have shown that its upper slope is first shifted toward the lower slope until the trapezoidal profile turns into a triangular one. The pit erosion causes also the deformation of natural forms of the channel bed and destabilization of sediment discharge. The analysis of the obtained data demonstrated that the reformation of channel bed is a durable process depending of the ratio of pit scales to the volume of sediment. The present study is useful for development of engineering solutions directed to reduction of risks caused by the interaction of sand quarries with hydraulic structures in rivers.

A Numerical Study of Non-hydrostatic Shallow Flows in Open Channels

Abstract The flow field of many practical open channel flow problems, e.g. flow over natural bed forms or hydraulic structures, is characterised by curved streamlines that result in a non-hydrostatic pressure distribution. The essential vertical details of such a flow field need to be accounted for, so as to be able to treat the complex transition between hydrostatic and non-hydrostatic flow regimes. Apparently, the shallow-water equations, which assume a mild longitudinal slope and negligible vertical acceleration, are inappropriate to analyse these types of problems. Besides, most of the current Boussinesq-type models do not consider the effects of turbulence. A novel approach, stemming from the vertical integration of the Reynolds-averaged Navier-Stokes equations, is applied herein to develop a non-hydrostatic model which includes terms accounting for the effective stresses arising from the turbulent characteristics of the flow. The feasibility of the proposed model is examined by simulating flow situations that involve non-hydrostatic pressure and/or nonuniform velocity distributions. The computational results for free-surface and bed pressure profiles exhibit good correlations with experimental data, demonstrating that the present model is capable of simulating the salient features of free-surface flows over sharply-curved overflow structures and rigid-bed dunes.

Characterising natural bedform morphology and its influence on flow

Bedforms such as dunes and ripples are ubiquitous in rivers and coastal seas, and commonly described as triangular shapes from which height and length are calculated to estimate hydrodynamic and sediment dynamic parameters. Natural bedforms, however, present a far more complicated morphology; the difference between natural bedform shape and the often assumed triangular shape is usually neglected, and how this may affect the flow is unknown. This study investigates the shapes of natural bedforms and how they influence flow and shear stress, based on four datasets extracted from earlier studies on two rivers (the Rio Parana in Argentina, and the Lower Rhine in The Netherlands). The most commonly occurring morphological elements are a sinusoidal stoss side made of one segment and a lee side made of two segments, a gently sloping upper lee side and a relatively steep (6 to 21°) slip face. A non-hydrostatic numerical model, set up using Delft3D, served to simulate the flow over fixed bedforms with various morphologies derived from the identified morphological elements. Both shear stress and turbulence increase with increasing slip face angle and are only marginally affected by the dimensions and positions of the upper and lower lee side. The average slip face angle determined from the bed profiles is 14°, over which there is no permanent flow separation. Shear stress and turbulence above natural bedforms are higher than above a flat bed but much lower than over the often assumed 30° lee side angle.

Predicting bed form roughness: the influence of lee side angle

Flow transverse bedforms (ripples and dunes) are ubiquitous in rivers and coastal seas. Local hydrodynamics and transport conditions depend on the size and geometry of these bedforms, as they constitute roughness elements at the bed. Bedform influence on flow energy must be considered for the understanding of flow dynamics, and in the development and application of numerical models. Common estimations or predictors of form roughness (friction factors) are based mostly on data of steep bedforms (with angle-of-repose lee slopes), and described by highly simplified bedform dimensions (heights and lengths). However, natural bedforms often are not steep, and differ in form and hydraulic effect relative to idealised bedforms. Based on systematic numerical model experiments, this study shows how the hydraulic effect of bedforms depends on the flow structure behind bedforms, which is determined by the bedform lee side angle, aspect ratio and relative height. Simulations reveal that flow separation behind bedform crests and, thus, a hydraulic effect is induced at lee side angles steeper than 11 to 18° depending on relative height, and that a fully developed flow separation zone exists only over bedforms with a lee side angle steeper than 24°. Furthermore, the hydraulic effect of bedforms with varying lee side angle is evaluated and a reduction function to common friction factors is proposed. A function is also developed for the Nikuradse roughness (k s), and a new equation is proposed which directly relates k s to bedform relative height, aspect ratio and lee side angle.

Flow separation and roughness lengths over large bedforms in a tidal environment: A numerical investigation

This study characterises the shape of the flow separation zone (FSZ) and wake region over large asymmetric bedforms under tidal flow conditions. High resolution bathymetry, flow velocity and turbulence data were measured along two parallel transects in a tidal channel covered with bedforms. The field data are used to verify the applicability of a numerical model for a systematic study using the Delft3D modelling system and test the model sensitivity to roughness length. Three experiments are then conducted to investigate how the FSZ size and wake extent vary depending on tidally-varying flow conditions, water levels and bathymetry. During the ebb, a large FSZ occurs over the steep lee side of each bedform. During the flood, no flow separation develops over the bedforms having a flat crest; however, a small FSZ is observed over the steepest part of the crest of some bedforms, where the slope is locally up to 15°. Over a given bedform morphology and constant water levels, no FSZ occurs for velocity magnitudes smaller than 0.1ms−1; as the flow accelerates, the FSZ reaches a stable size for velocity magnitudes greater than 0.4ms−1. The shape of the FSZ is not influenced by changes in water levels. On the other hand, variations in bed morphology, as recorded from the high-resolution bathymetry collected during the tidal cycle, influence the size and position of the FSZ: a FSZ develops only when the maximum lee side slope over a horizontal distance of 5m is greater than 10°. The height and length of the wake region are related to the length of the FSZ. The total roughness along the transect lines is an order of magnitude larger during the ebb than during the flood due to flow direction in relation to bedform asymmetry: during the ebb, roughness is created by the large bedforms because a FSZ and wake develops over the steep lee side. The results add to the understanding of hydrodynamics of natural bedforms in a tidal environment and may be used to better parameterise small-scale processes in large-scale studies.

Assessment of sedimentation processes determined from side-scan sonar surveys in the Buffalo River, New York, USA

An assessment of sedimentation processes was made using side-scan sonar records collected over a 15-year period in the Buffalo River, New York, USA. Spatial and temporal changes in natural and man-made bedforms documented sediment erosion, slumping, and sediment depositional sites. Bedforms and sediment texture were used to divide the mapped portion of the river (9 km) into four sedimentary environments, with Section 1 at the upper end and Section 4 near the mouth where the river discharges into the east end of Lake Erie. Based on annual changes in side-scan sonar records, depositional rates decrease in the downriver direction. Section 2, located 5 km from the river’s mouth, contains sedimentary furrows. The persistence of furrows in this portion of the river requires both sediment erosion and deposition under a bidirectional flow regime. Results from this study confirmed that side-scan sonar is a valuable tool in understanding sediment dynamics and can be useful in river restoration decision making.

Numerical Study of Dune Erosion Induced by Tsunami

Coastal dunes are natural bed forms caused by wind and wave actions. They run parallel to the shoreline close to the beach and thus have an important role to protect land against external force due to tsunami. To evaluate such protective effects of sand dunes, their erodible property must be considered since some part of them might be washed out during tsunami flooding. In this paper, a numericalm odelf or dune deformationi nducedb y tsunami is presented. The model is based on the Boussinesq-type equation coupled with the sediment advection equation and existing empirical formulae of sediment transport rate. The model performanc is confirmed through comparison with available laboratory data. Finally, the dune erosion induced by tsunami is discussed on the basis of the model application.

Structure of flow over alluvial bedforms: an experiment on linking field and laboratory methods

AbstractThis paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time–space averaging approach. Measurements of turbulence were obtained in a small river in Illinois, USA, over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica. Flow structure over the artificial bedform is similar to that observed in laboratory studies, but is markedly different from the flow structure over natural bedforms. These differences are most pronounced in the roughness sublayer, whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity. The double time–space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms. Mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests. However, vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms. Thus, despite the absence of flow separation, wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence. Copyright © 2006 John Wiley & Sons, Ltd.

Highest natural bed forms

Fluid flow interacts with sedimentary beds forming waves of different kinds, which are of considerable practical importance since they influence significantly the near‐bed flow, both over and below the bed, sediment transport, and wave height attenuation. We focus here on steep bed forms capable of producing flow separation. In this case, the large‐scale vorticity generated in the phenomenon of separation rules the process of friction, which appears to be practically unaffected by sediment motion. Under the crests of the bed forms, the mean shear force due to friction is balanced by the force that bed forms exert on the flow via pressure, which can be calculated from the work of Giménez‐Curto and Corniero [2002]. Bed forms grow until they have a height such that friction at their troughs is negligible, thus ceasing the motion of fluid and sediment. This condition leads to a very simple expression for the limiting steepness, which compares favorably with existing observations on bed form geometry under steady open channel flow as well as under oscillatory flow.

Equilibrium pool dimensions and sediment-sorting patterns in coarse-grained, New England channels

Pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat. An understanding of this morphology is necessary for the successful design of a reconstructed or restored channel. To better understand pool geometry and sorting characteristics of New England channels, characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions. Characterizations of channel-bed sediments were also performed at 34 of these locations. Data from 120 pools were used in least-squares, multiple-regression analysis with 19 different independent variables to develop prediction equations for pool depth, length, and exit-slope gradient. These equations were then tested using the 25 remaining pools. Eighteen different geometric variables were then used to predict sediment φ 50 values in pool centers, pool exit slopes, and riffles for the subset of 34 samples. The regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit-slope width, constriction gradient, constriction width, drainage area, upstream channel width, and the exit-slope expansion ratio, with an R 2 value of 0.53. Similarly, pool length is influenced by channel gradient, location of the channel constriction, pool width, drainage area and constriction length, with an R 2 value of 0.72. The gradient of pool exit slope is influenced by drainage area, channel gradient, and constriction gradient, with an R 2 value of 0.34. However, R 2 values for exit slope increased to 0.65 when sediment d 50 data were utilized. Using the resulting regression equations to predict dimensions for the 25 verification pools yields average residuals near zero, although moderate variations do exist. A Tukey–Kramer HSD comparison of the means for sediment φ 50 values indicates pool sediments are significantly coarser than pool exit-slope and riffle sediments for the subset of 34 pools. Regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with R 2 values that range from 0.45 to 0.62. The results suggest that pool dimensions are related to basin-wide variability and local hydraulic conditions. The findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout the pool–riffle morphology.

Turbulent Flow over Step with Rounded Edges: Experimental and Numerical Study

The study of turbulent flow over man-made obstacles and natural bed forms is an area of active research. This investigation was conducted to explore the differences between flow over a backward facing step and flow over a step with gradual transitions. Detailed measurements of longitudinal mean velocity and turbulence intensity were obtained using a one component LDV system. A turbulence-based numerical model was implemented to reproduce the measured flow variables and to explore additional flow characteristics, including Reynolds stresses and eddy viscosity. The separation that takes place in the rounded expansion plays a significant role in the flow pattern. In contrast to flow over a backward facing step, flow over a step with rounded edges shows a relatively shorter separation bubble and a slower return to the developed flow condition.

Mechanics of flow over ripples and dunes

A simple predictive model for velocity and boundary shear stress fields over two‐dimensional bedforms of finite amplitude is presented herein, and the results of that model are compared with flume data. The production of a wake due to separation of the flow near the bedform crest and the subsequent interaction of this wake region with a developing nonuniform boundary layer downstream of the flow reattachment point are treated using the method of McLean and Smith (1986). Their model is modified in order to take account of the flow response to short‐wavelength obstructions more accurately and to include the effects of wakes from upstream bedforms as these wakes coalesce in the interior. The theory is also extended to include the region between the bedform crest and the reattachment point, thereby providing a spatially complete field of velocity and shear stress. Results of the calculations are found to be in good agreement with laboratory measurements taken over immobile bedforms using a laser‐Doppler velocimeter. Our finite amplitude flow model is shown to yield accurate predictions of the fluid dynamical effects ultimately responsible for the morphology of well‐developed natural bedforms.

Compartmented Sediment Trap

A method for measuring sediment transport is presented. The measurement device is a portable, compartmented, bottom trap that is operated by burying the trap in the stream bed, waiting until the natural bedforms are reestablished, and then opening the trap during a time interval sufficient to collect a large number of moving particles. The sections of the trap allow the calculation of the fractions of the sediment that jump various lengths as they move along the bed. The jump-length distribution permits the direct calculation of the efficiency of the trap for the various grain sizes of the moving sediment. The compartmented trap was field tested in a sand-bottomed creek. The test data indicated that the trap worked well.

Possible significance of transition flow patterns in interpretation of origin of some natural bed forms

A similarity in morphology and sequence of development seems to exist between some common sedimentary structures and flow configurations encountered in transition and generation of turbulence. The question arises whether such flow instability patterns determine the embryonic configurations of bed deformation, which subsequently develop and grow, thus triggering the appearance of finite size natural bed forms.