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Due to a complex three-dimensional flow pattern, the outer banks of river bends are predisposed to erosion. When endangering civil structures, preventing measures to mitigate this erosion are thus required. Vertical ribs at protection walls for scour reduction have been applied to several flood protection projects in mountain rivers; nevertheless, no systematic and intensive study has been presented so far to evaluate their effect. This paper investigates experimentally the effect of vertical ribs, placed as macroroughness elements on the outer vertical wall of a 90° laboratory channel bend. Systematic tests were performed using a wide and coarse grain-size distribution. Scour formation and velocity distribution were assessed in the channel in the presence of a macrorough outer bank, materialized in the laboratory by vertical elements placed on the outer vertical wall along the channel bend. Experiments showed that the macrorough outer bank changed considerably the bed morphology under equilibrium conditions. Maximum scour depth is considerably reduced by the vertical ribs placed at an optimal spacing on the bend outer wall. A considerable grain sorting process occurs across the cross section in the bend, which influences the scour process; differences are observed between situations with and without macrorough banks. The distribution of the time-averaged velocity field across the section shows the influence of the channel rough wall. An optimal macroroughness configuration in terms of scour reduction is discussed and proposed. It was observed that when spacing between vertical ribs is too reduced, these ribs act as uniformly distributed wall roughness, contributing to the width reduction due to the occupation of the cross section and increasing consequently the flow velocity with negative effects in scour reduction. [ABSTRACT FROM AUTHOR]

This paper presents findings from an extensive, laboratory-based study of scour at bridge abutments located in compound channels. The study considered aspects of abutment construction, including abutment and floodplain erodibility, and examined how these aspects influence scour development. Its findings show that three basic scour conditions can develop at abutments in compound channels: scour of the main-channel bed, when the floodplain is far less erodible than the bed of the main channel (Scour Condition A); scour of the floodplain around an abutment, for abutments set back on a floodplain (Scour Condition B); and scour at an abutment column exposed once an abutment is breached (Scour Condition C). These conditions may combine, depending on abutment location and site circumstances. The authors present laboratory observations and data associated with Scour Conditions A and B. The data are analyzed in terms of a formulation concept that treats Scour Conditions A and B as essentially scour at a short contraction (the bridge opening), for which flow nonuniformity and turbulence amplify scour depth associated with a long contraction. The amplification is greatest for small contractions and diminishes for larger contractions that force greater magnitude and uniformity of flow velocity through a bridge opening. [ABSTRACT FROM AUTHOR]

Spatially variable channel geometry in natural rivers produces nonuniform flow and spatial gradients in the shear stress field. The travel distance required for the flow to acquire the capacity bedload concentration and attain a new equilibrium bedload transport rate upon encountering a region of higher or lower shear stress is defined as the bedload adaptation length (Lb). Estimates of Lb are used by some numerical morphodynamic models to account for nonequilibrium bedload transport in the computation of local transport rates. However, current methods for estimating this parameter are uncertain and often crude. The authors therefore conducted experiments designed to measure Lb for a uniform sediment mixture in a laboratory flume. Instantaneous bedload transport rates were determined by counting passing sediment particles on digital imagery collected at variable distances downstream from a zero-transport boundary in a small flume. The flume was operated at three bed slopes in order to assess Lb over a range of hydraulic conditions. Bedload adaptation length was found to be about 30 8 particle diameters at a relatively low excess dimensionless shear stress (θ - θc = 0.018, where θ is the dimensionless shear stress and θ c = 0.0436 is the critical dimensionless shear stress) and about 100 ± 30 particle diameters at a moderate level of excess dimensionless shear stress (θ - θc = 0.032). The experiments failed to resolve Lb at higher shear stresses. These results support physically based models that cast Lb as an increasing function of excess shear stress. They also suggest that Lb may be small relative to the resolution of the numerical mesh used in many modeling applications. In such cases, model performance may be insensitive to the choice of any arbitrary small value of Lb. [ABSTRACT FROM AUTHOR]

The flow characteristics around two partially buried objects, a short cylinder (SC) and a truncated cone (TC) were examined experimentally and numerically. Experiments were conducted in a large tilting flume with vertical walls made of acrylic plastic and steel floor covered by a 24-m long sand pit. For each experiment, the object was placed on a flat sandy bed, the flume was filled to a water depth of 41 cm, and a steady unidirectional current was established at high Reynolds numbers while sinking of the object was tracked over time. Velocity records were taken with an acoustic Doppler velocimeter (ADV) probe at the final stage of burial and scour hole evolution. The flow structure around each object was further investigated using a model that solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with a turbulence closure model, and results were compared to the experimental measurements. Measurements and numerical simulations allowed identification of regions of high and low velocity and turbulent kinetic energy accounting for zones of potential sediment erosion and deposition that dictate the burial of the object. The turbulence closure model was shown to reproduce the experimental mean flow velocity and turbulent kinetic energy with acceptable accuracy. [ABSTRACT FROM AUTHOR]

Laboratory flume experiments were conducted to study the flow field and sediment dynamics in a mobile-bed channel bend with and without the presence of stream barbs (upstream-angled submerged groynes). This paper presents spatial distributions of the vorticity field and the turbulent Reynolds shear stresses, kinetic energy, and integral scales. The objectives of the present paper were to (1) advance understanding of the role that vorticity and turbulence play in the erosion process in channel bends with and without barbs, and (2) optimize barb design for reducing erosion along the outer bank regions of channel bends through consideration of flow-field turbulence and vorticity. Results indicate that (1) increased -vorticity occurred in the outer bank scour zone for all runs with and without barbs; (2) for the runs without barbs and with moderately sized barbs, increased streamwise-cross-stream Reynolds stress () was found to coincide with locations of scour; and (3) for the run with large barbs, all three Reynolds shear stresses increased, but was most consistently associated with scour locations. In general, local scour near the barbs was associated with increased -vorticity, tke, and nonprincipal Reynolds stresses ( and ). Furthermore, it was observed that the outer bank region (particularly between adjacent barbs) may be susceptible to increased erosion if stream barbs, due to their size and layout, generate excessive turbulence and secondary velocities. [ABSTRACT FROM AUTHOR]

The Shields and the Isbash equations for predicting the stability of rocks exposed to a turbulent flow were both proposed in 1936, and since then, both equations have been used widely in the analysis of transport thresholds for coarse sediment. These two equations were obtained using two very different approaches, but as demonstrated in this paper, the equation developed by Isbash is consistent with the relation formulated by Shields for predicting the motion of sediments either in the flume or in the field. Comparison of the two equations suggests that standard approaches tend to oversize riprap in mountain streams. [ABSTRACT FROM AUTHOR]

Sediment concentration distribution and the basic probability of sediment movement near the channel bed are two of the most important and fundamental issues in the study of sediments. Based on statistical analysis and considering the transport mechanisms, the rules of sediment concentration distribution near a channel bed are studied. Analytical expressions for the near-bed sediment concentration distribution and mean sediment concentration are derived, and the expression for the mean sediment concentration near the bed is verified by measured data, which were obtained from previous experiments. With the help of statistical theory, the expressions of basic probabilities, i.e., rolling, saltating, and suspending probabilities, for sediment movement near the bed are also derived. The expression for starting probability is verified by the measured data. The verification shows that the results from the proposed expression agree well with the measured data. This research has both theoretical and practical significance for further investigation concerning rules of bed load and suspended sediment transport. [ABSTRACT FROM AUTHOR]

Most theoretical and numerical studies employ diffusion theory to investigate suspended sediment distributions in fluvial rivers, yet diffusion-theory-based descriptions are strictly valid only in the limit of small particle inertia and low concentrations. This paper presents a transport equation for sediment in suspension based on a two-fluid model of solid/liquid two-phase flows. The transport equation is derived from the consideration of mass conservation for solid particles. The velocity of the solid phase is obtained from an asymptotic solution of the momentum conservation equation for solid phase. Moreover, this transport equation is a typical convection-diffusion equation that includes the conventional diffusion equation as a special case when particle inertia is small and concentrations are low. Retaining the essential features of two-fluid models, the transport equation is applicable to sediment-laden flows with a wide range of particle inertia and sediment concentrations. Comparisons of the results with available experimental observations are presented, and fairly good agreement is obtained. [ABSTRACT FROM AUTHOR]

By using the double-decomposition (DD) method, this study simulates transient sediment waves caused by aggradation described by a diffusion-type partial differential equation (PDE). The DD method solves the PDE by decomposing the solution function for sediment rate into a summation of M number of components, where M stands for the order of approximation. The solution was approximated by considering only the first three terms. The model satisfactorily simulated laboratory-measured aggradation bed profiles with, on average, a mean absolute error (MAE) of 0.70 cm, a root-mean-square error (RMSE) of 0.84 cm, a mean relative error (MRE) of 1.11%, and R2=0.95. The model performance was also tested by using numerical and error-function solutions. In addition, the results obtained from application of the DD solution to hypothetical field cases were found to be theoretically compatible with what may be observed in natural streams. However, sediment wave fronts in later periods of the simulation time reached equilibrium bed levels more quickly, around in the middle section of the channel. [ABSTRACT FROM AUTHOR]

This experimental study focuses the influence of bed material mobility and initial downstream water level on maximum water level and time to peak of dam-break waves. It covers horizontal bed conditions on fixed bed, sand bed, and pumice bed. Results include water surface level time evolution, maxima wave levels and time to peak. The influence of bed material mobility and downstream water level was identified and characterized, stressing the importance of using mathematical models with appropriate sediment transport formulations instead of purely hydrodynamic models to simulate dam-break waves on mobile bed channels. [ABSTRACT FROM AUTHOR]

This paper presents a two-dimensional morphological model for unsteady flow and both suspended-load and bed-load transport of multiple grain size to simulate transport of graded sediments downstream from the Three Gorges Reservoir. The model system includes a hydrodynamic module and a sediment module. The hydrodynamic module is based on the depth-averaged shallow water equations in orthogonal curvilinear coordinates. The sediment module describing nonuniform sediment transport is developed to include nonequilibrium transport processes, bed deformation, and bed material sorting. The model was calibrated using field observations through application to a 63-km-long alluvial river channel on the middle Yangtze River in China. A total of 16 size groups and a loose layer method of three sublayers were considered for the transport of the nonuniform bed materials in a long-term simulation. Predictions are compared with preliminary results of field observations and factors affecting the reliability of the simulated results are discussed. The results may be helpful to the development of more accurate simulation models in the future. [ABSTRACT FROM AUTHOR]

Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a high-resolution laboratory data set of velocity and boundary shear stress measurements and uses it to test a relatively simple, fully predictive, numerical method for determining these distributions across the cross-section of a straight channel. The measurements are made in a flume with a fairly complex cross-section that includes a simulated, cobble-roughened floodplain. The method tested is that reported by Kean and Smith in Riparian Vegetation and Fluvial Geomorphology in 2004, which is modified here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with the measurements. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of velocity and boundary shear stress across the channel. [ABSTRACT FROM AUTHOR]

Hydrodynamic separators are proprietary underground devices designed to remove floatable debris (e.g., leaves, trash, oil) and to remove suspended solids from storm-water runoff by sedimentation. They are designed for storm-water treatment in urban areas to meet tight space constraints. Limited data on the suspended solids removal performance of installed devices are available, and existing data are questionable because of the problems associated with assessment by monitoring. The objectives of our research are to: (1) investigate the feasibility and practicality of field testing to assess the performance of hydrodynamic separators as underground storm-water treatment devices; (2) evaluate the effects of sediment size and storm-water discharge on the performance of six devices from different manufacturers; and (3) develop a universal approach for predicting the performance of a device for any given application. In the field tests, a controlled and reproducible synthetic storm event containing sediment of a well defined size distribution and concentration was fed to a precleaned device. The captured sediment was then removed, dried, sieved, and weighed. To assess the performance of the devices, suspended sediment removal efficiency was related to a Péclet number, which accounts for two major processes that control performance: (1) settling of particles; and (2) turbulent diffusion or mixing of particles. After analyzing the data, all devices showed similar behavior, therefore, a three-parameter performance function was proposed for all devices. Performance functions were developed from the result of the field tests and parallel testing of two other full-scale devices in the laboratory. The performance functions can be used to determine the efficiency of the tested devices and to improve the selection and sizing of hydrodynamic separators and the assessment of their overall performance after installation. [ABSTRACT FROM AUTHOR]

Dense underflows are continuous currents that move downslope due to their density being heavier than that of the ambient water. In this work, a steady density current with a uniform velocity and concentration from a narrow sluice gate enters into a wide channel of lighter ambient fluid and moves forward downslope. Experiments varying inlet velocity and concentration and hence inlet Richardson numbers were conducted. Numerical simulations were also performed with a low-Reynolds number k–[variant_greek_epsilon] model. The results of numerical simulation agree well with the experimental data. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper concerns the quantitative evaluation of the time of bed development of alluvial meandering streams. In agreement with the prevailing approach, it is assumed that the stream centerline follows a sine-generated curve; the banks are rigid. The flow is turbulent and subcritical, and the flow width is much larger than the flow depth. The movable bed is flat at time t=0; at t=Tb, the bed reaches its equilibrium or developed state. With the aid of dimensional and physical considerations, an expression is found for the duration of bed development Tb. According to this expression, Tb is proportional to the square of the flow width B and inversely proportional to the specific volumetric bed-load rate corresponding to the channel-averaged flow. The proportionality factor is found to be a function of the initial deflection angle θ0 alone. The form of this function is investigated on the basis of a series of experimental runs carried out by the writers in a sine-generated channel having an intermediate value of θ0 (i.e., an intermediate value of sinuosity), as well as data available in the literature. [ABSTRACT FROM AUTHOR]

Sediment transport patterns in a meandering channel with instream restoration structures (vane and W-weir) have been studied. Laboratory experiments were conducted in a large-scale mobile-bed channel with graded materials under bank-full and overbank flow conditions. Bed-load samples were collected with a calibrated minisampler. Vanes, constructed against the outer bank in a meander bend, relocated the scour hole toward midchannel, thereby protecting the bank from erosion. The sediment sizes (d50,d90) in the bend became slightly more coarse and more uniform in the center of the channel. The W-weir installed immediately below a riffle section created two scour holes without affecting the upstream bed or the natural sediment transport of the channel. Predictions of bed-load transport by selected deterministic and stochastic methods showed large deviation from measurements using Helley–Smith sampler in sections downstream of the bend apex. In addition to creating local scour holes, the structures also relocated the locus of sediment transport at downstream sections. This issue should be considered when installing vanes and weirs in meandering rivers with significant bed-load transport. [ABSTRACT FROM AUTHOR]

A submerged vane is a flow-training facility mounted vertically on the channel bed to control the sediment movement in the channel cross section, and has been utilized in various applications, such as prevention of bank erosion, sediment exclusion at water intakes, and deepening channels for navigation. The performance of a submerged vane is related to its dimensions and shape. This study aims to investigate a vane’s sediment control effectiveness as a function of its size and shape, with the expectation of an optimal combination of dimensions and shape. A model for the calculation of the transverse bed profile in a cross section of a straight alluvial channel induced by a single submerged vane is developed. The model is utilized to investigate the performances for three types of vanes: (1) rectangular plates with various height and length; (2) tapered plates with linear decreasing in length from the base to the top; and (3) plates of parallelogram with the top of the plates swept forward or backward. Design guidelines and suggestions on the dimensions and shape of the vane are provided based on the results. [ABSTRACT FROM AUTHOR]

Confluence zones in rivers are common occurrences in natural rivers, and their flow structures have a great impact on sediment transport and pollutant dispersion in confluence regions. Previous studies mainly focused on two-dimensional properties of the separation zone, and the studies of the variation of the separation zone from the water surface to the bottom are limited. Based on the confluence experiments carried out in a junction flume, the detailed characteristics of the separation zone were analyzed by using the commonly used streamline and the velocity isoline first proposed in this study, respectively. The velocity isoline method is conducted by the longitudinal velocity streamline considering its great impact on flow structure. Based on the analysis of the selected data from experiments, the geometry and the tendency of the separation zone present different forms with variations of distance to the flume bottom. Further, the discharge ratio also influences the separation zone from bed bottom to surface. [ABSTRACT FROM AUTHOR]

This paper analyzes the effect of flume test section length on sediment erodibility measurements. A modular flume was constructed and experiments were conducted with two test section lengths: 0.15 and 1.10 m. The internal height and width of the flume were 0.11 and 0.13 m, respectively. A fine (7 μm) commercially available quartz sediment was used for the tests. The expectation was that the shorter flume test section would experience a significantly higher erosion rate (per unit surface area) due to its greater sensitivity to edge effects (i.e., scour) at the entrance and exit of the flume test section. However, the measured erosion rates at comparable bottom stresses were only 35% greater in the short test-section tests. These results were consistent with the lack of significant scour development at the entrance or exits of the test sections. Hence, flume test section length alone does not appear to significantly affect erodibility measurements provided edge effects (i.e., scour) are minor. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits. The governing equations are derived from mass conservation and the assumption that the amount of fine sediment deposition per unit vertical travel distance into the deposit is either constant or increases with increasing fine sediment fraction. Model results demonstrate that fine sediment accumulation decreases rapidly with depth into coarse substrate initially void of fine sediment, which is consistent with experimental observations that significant fine sediment infiltration occurs to only a shallow depth. Comparisons of the theory with flume data indicate that the model adequately reproduced the weighted-averaged fine sediment fraction values from experiments. An early model developed by Sakthivadivel and Einstein for fine sediment infiltration is in part based on the generally incorrect assumption that intragravel flow remains constant following fine sediment infiltration. Applying a correction to the Sakthivadivel and Einstein model based on alternate hypothesis that introgravel flow is driven by a constant head gives similar results as the proposed model. [ABSTRACT FROM AUTHOR]

This paper describes the flow structures that occur in a 90° junction of four open channels with supercritical flow in two orthogonal inlet channels. An experimental facility was constructed to permit the measurement of flow rates, water depths, and the positions of hydraulic jumps in the channels. The various flow patterns which appear depend on the characteristics of the incoming flows and can be classified into three main types, depending on the location and shape of the hydraulic jumps that develop. These jumps can either be normal to the flow and located in the upstream channels, or can be oblique and confined within the junction. The explanation for the existence of various flow patterns is derived from previous studies of the rapid deflection of supercritical flow. A detailed description of each flow regime is provided, with information on the surface elevations, the behavior of the hydraulic jumps, the deflections of the incoming flows, and the formation and characteristics of recirculation zones. [ABSTRACT FROM AUTHOR]

A robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors is presented. The physically based model simulates the three main processes that shape incised streams: hydraulics, sediment transport, and streambed and bank adjustments. A generalized implicit Preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals. The solution method introduces several enhancements that increase its robustness, specifically to simulate flashy flows. Transport of cohesive or cohesionless graded bed material is based on a total-load concept, and suspended and bed load transport modes are accounted for through nonequilibrium effects. The model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material. The present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds, and reports simulations of published experiments on degrading and aggrading channels with graded bed material. Description and validation of the model’s streambank erosion component and the application of the model to incised stream systems are presented elsewhere. [ABSTRACT FROM AUTHOR]

Previous studies assessing the accuracy of bed-load transport equations have considered equation performance statistically based on paired observations of measured and predicted bed-load transport rates. However, transport measurements were typically taken during low flows, biasing the assessment of equation performance toward low discharges, and because equation performance can vary with discharge, it is unclear whether previous assessments of performance apply to higher, geomorphically significant flows (e.g., the bankfull or effective discharges). Nor is it clear whether these equations can predict the effective discharge, which depends on the accuracy of the bed-load transport equation across a range of flows. Prediction of the effective discharge is particularly important in stream restoration projects, as it is frequently used as an index value for scaling channel dimensions and for designing dynamically stable channels. In this study, we consider the geomorphic performance of five bed-load transport equations at 22 gravel-bed rivers in mountain basins of the western United States. Performance is assessed in terms of the accuracy with which the equations are able to predict the effective discharge and its bed-load transport rate. We find that the median error in predicting effective discharge is near zero for all equations, indicating that effective discharge predictions may not be particularly sensitive to one’s choice of bed-load transport equation. However, the standard deviation of the prediction error differs between equations (ranging from 10% to 60%), as does their ability to predict the transport rate at the effective discharge (median errors of less than 1 to almost 2.5 orders of magnitude). A framework is presented for standardizing the transport equations to explain observed differences in performance and to explore sensitivity of effective discharge predictions. [ABSTRACT FROM AUTHOR]

Transient bed sediment waves in alluvial rivers have been described using a multitude of hydraulic formulations. These formulations are based on some form of the St. Venant equations and conservation of mass of sediment in suspension and in bed. Depending on the assumptions employed, a hierarchy of formulations is expressed. These formulations in the literature employ uncoupled, semicoupled, or fully coupled transport models treating the sediment waves as either hyperbolic (dynamic wave) or parabolic (diffusion wave). It is, however, hypothesized that the movement of bed sediment waves in alluvial rivers can be described as a kinematic wave. Kinematic wave theory employs a functional relation between sediment transport rate and concentration and a relation between flow velocity and depth. This study summarizes the hierarchy of the formulations while emphasizing the kinematic wave theory for describing transient bed sediment waves. The applicability of the theory is shown for laboratory flume data and hypothetical cases. [ABSTRACT FROM AUTHOR]

Many sediment deposits contain not only a granular but also a mineral and/or organic cohesive fraction, which has the potential to change significantly the erosion response when compared to noncohesive sediment deposits. Sediment deposits containing a low proportion of cohesive sediment display a specific, transitional behavior between cohesive and noncohesive sediments. In the past, several studies have examined partly cohesive sediment, however, the quantification of its erosion behavior is still unsatisfactory. In an attempt to advance the understanding and qualitative description of erosion and transport of mixed sediments in pipes, a comprehensive experimental investigation on the erosion behavior was designed. Tests were conducted in a flume with semicircular cross section with a diameter of 0.39 m using fine sand with a d50=0.19 mm and partly cohesive sediment deposits being a mixture of sand and clay with proportion of 3–10% by weight of clay. Tests covered a range of flow conditions with Froude numbers up to 0.6. Data on bed form geometries, friction characteristics, and sediment transport rates were collected. In steady sediment transport conditions the eroded partly cohesive bed produces an active granular top layer over an intact substrate, the thickness of which is up to ten times smaller than in the case of a granular bed. The cohesion of the substrate reduces the height and length of bed forms, bed form friction, and sediment transport rates. A comparison of the mobility of noncohesive sediments in circular pipes obtained in other studies is presented to indicate the generality of these results. [ABSTRACT FROM AUTHOR]

Results of an experimental study on clear-water scour at submerged cylindrical obstacles (circular cylinders) in uniform bed sediments under steady flow are presented. The scour depths at submerged circular cylinders are compared with the scour depths at corresponding unsubmerged cylinders (extended above the free surface of flow) of the same diameters under similar flow and bed sediment conditions. The scour depth decreases with an increase in submergence ratio. A submergence factor is introduced to determine the scour depth at a submerged cylinder from the information of the scour depth at an unsubmerged cylinder of the same diameter. In addition, the flow fields along the upstream vertical plane of symmetry of unsubmerged and submerged cylinders are presented through vector plots, which reveal that the dimension and strength of the horseshoe vortex decreases with an increase in submergence ratio. The horseshoe vortex circulations, which decrease with an increase in submergence ratio, are computed from the vorticity contours by using the Stokes theorem. [ABSTRACT FROM AUTHOR]

A negative bias in discharge measurements made with an acoustic Doppler current profiler (ADCP) is attributed to the movement of sediment on or near the streambed, and is an issue widely acknowledged by the scientific community. The integration of a differentially corrected global positioning system (DGPS) to track the movement of the ADCP can be used to avoid the systematic bias associated with a moving bed. DGPS, however, cannot provide consistently accurate positions because of multipath errors and satellite signal reception problems on waterways with dense tree canopy along the banks, in deep valleys or canyons, and near bridges. An alternative method of correcting for the moving-bed bias, based on the closure error resulting from a two-way crossing of the river, is presented. The uncertainty in the mean moving-bed velocity measured by the loop method is shown to be approximately 0.6 cm/s. For the 13 field measurements presented, the loop method resulted in corrected discharges that were within 5% of discharges measured utilizing DGPS to compensate for moving-bed conditions. [ABSTRACT FROM AUTHOR]

Existing bed-load transport formulas may overestimate the transport rate in mountain rivers by two orders of magnitude or more. Recently published field data sets provide an opportunity to take a fresh look at the bed-load transport relationship and it is hypothesized that the overestimate is due to a failure to account for the effect of a coarse surface layer of bed material inhibiting the release of fine subsurface material. Bed-load transport is determined as gs=aρ(q-qc) where q=water discharge per unit width; qc=critical value for initiation of bed material movement; ρ=water density; and a=coefficient. The gs/q relationship is typically piecewise linear, characterized by two transport phases with, respectively, low and high rates of change. Twenty-one flume and 25 field data sets were used to quantify the relationship for Phase 2. The flume data confirm the dependence of a on S1.5, where S=channel slope, in agreement with earlier studies. The field data additionally show that a varies inversely with the degree of bed armoring, given by the ratio of surface to subsurface bed material size. The finding is consistent with the hypothesis and suggests the need to account for the bed material supply limitation in the bed-load transport formula. However, the available data are not entirely sufficient to rule out an alternative dependency, or codependency, on flow resistance. The critical conditions for initiation of Phase 2 transport are also quantified as a function of bed material size and channel slope. The resulting set of equations allows a more accurate estimation of Phase 2 bed-load transport rates. However, the equations are empirical and should be restricted for use within the range of conditions used in their development, to determine mean rather than instantaneous transport rates and to determine bulk transport rates, not transport by size fraction. [ABSTRACT FROM AUTHOR]

The development of a fully three-dimensional finite volume morphodynamic model, for simulating fluid and sediment transport in curved open channels with rigid walls, is described. For flow field simulation, the Reynolds-averaged Navier–Stokes equations are solved numerically, without reliance on the assumption of hydrostatic pressure distribution, in a curvilinear nonorthogonal coordinate system. Turbulence closure is provided by either a low-Reynolds number k-ω turbulence model or the standard k-[variant_greek_epsilon] turbulence model, both of which apply a Boussinesq eddy viscosity. The sediment concentration distribution is obtained using the convection-diffusion equation and the sediment continuity equation is applied to calculate channel bed evolution, based on consideration of both bed load and suspended sediment load. The governing equations are solved in a collocated grid system. Experimental data obtained from a laboratory study of flow in an S-shaped channel are utilized to check the accuracy of the model’s hydrodynamic computations. Also, data from a different laboratory study, of equilibrium bed morphology associated with flow through 90° and 135° channel bends, are used to validate the model’s simulated bed evolution. The numerically-modeled fluid and sediment transportation show generally good agreement with the measured data. The calculated results with both turbulence models show that the low-Reynolds k-ω model better predicts flow and sediment transport through channel bends than the standard k-[variant_greek_epsilon] model. [ABSTRACT FROM AUTHOR]

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects. [ABSTRACT FROM AUTHOR]

New field data on cohesive sediment erosion is presented and discussed, with particular focus on partitioning the total erosion into resuspension and bed load. The data were obtained using a recently developed in situ flume of the National Institute of Water and Atmospheric Research, New Zealand. The erosion rate is estimated from direct measurements of bed surface elevations by acoustic sensors, whereas resuspension rate is obtained using data on sediment concentrations measured by optical backscatter sensors. The bed- load contribution to the total erosion rate is evaluated from the conservation equation for sediments. To test repeatability, the data from the in situ flume are compared with those from a previous version of the flume. The results show that comparative studies of in situ flumes and standardized deployment procedures enable direct comparison of experimental data on cohesive sediment erosion. Overall, the data show that a commonly used assumption that the erosion rate is equal to the resuspension rate is not always valid as bed load plays a significant role in cohesive sediment erosion. The data also highlight the importance of clay content and other sediment physical characteristics in the sediment mixture. [ABSTRACT FROM AUTHOR]

The upward seepage through the bed sediment downstream of an apron of a sluice gate structure is a common occurrence due to afflux of the flow level between the upstream and downstream reaches of a sluice gate. The result of an experimental investigation on the characteristics of the scour hole and the flow-field downstream of an apron due to submerged jets under the influence of upward seepage through the bed sediment is presented. Experiments were run for the conditions of submerged jets, having submergence factors from 0.99 to 1.72 and jet Froude numbers from 3.15 to 4.87, over beds of sediments (median sizes=0.8, 1.86, and 3 mm) downstream of an apron under upward seepage velocities. The characteristic lengths of the scour hole determined from the scour profiles are: the maximum equilibrium scour depth, the horizontal distance of the location of maximum scour depth from the edge of the apron, the horizontal extent of the scour hole from the edge of the apron, the dune height, and the horizontal distance of the dune crest from the edge of the apron, all of which were found to increase with an increase in the seepage velocity. Using experimental results, the time variation of the scour depth is scaled by an exponential law, where the nondimensional time scale decreases linearly with an increase in the ratio of the seepage velocity to the issuing jet velocity. The flow field in the submerged jets over both the apron and within the scour hole was detected using an acoustic Doppler velocimeter. The vertical distributions of time-averaged velocities, turbulence intensities and Reynolds stress at different streamwise distances, and the horizontal distribution of bed-shear stress are plotted for the conditions of scour holes with and without upward seepage. Vector plots of the flow field show that the rate of decay of the submerged jet decreases with an increase in the seepage velocity. The flow characteristics in the scour holes are analyzed in the context of the influence of upward seepage velocity on the decay of the velocity and turbulence intensities and the growth of the boundary layer. [ABSTRACT FROM AUTHOR]

A horizontal two-dimensional mobile bed model for simulating the formation of river-dominated deltas in the river mouth or reservoir is presented, which is composed of shallow water equations, sediment transport formula, and a sediment continuity equation. Geometry similarity of river deltas during the processes of formation is discussed. Stability analysis and sensitivity analysis of parameters in the model are analyzed, which indicates that bed configuration is sensitive to the incipient-motion criteria of bed–load particles. The effect of gravity component on the initiation of sediment movement, therefore, is recommended to be considered in the modeling. The bed configuration including the reverse slope in the longitudinal profile and concave in the transverse profiles are correctly simulated with help from the correction of incipient-motion criteria. Simulation results are verified with a series of experiments and are consistent with series geometric functions and dimensionless profiles inducted from experimental data. This reflects the great reliability of the model. Historical topographical records of two typical in-land deltas depicting their earlier developmental stages are discussed to show the usefulness of this study. [ABSTRACT FROM AUTHOR]

Some laboratory sediment-transport experiments are described in which a compound channel with a mobile-bed composed of uniform sand with a d50 of 0.88 mm was subjected to overbank flows. The main river channel was monitored to determine the effect of floodplain roughness on conveyance capacity, bed-form geometry, resistance, bed-load transport, and dune migration rate. The floodplain roughness was varied to simulate a wide range of conditions, commensurate with conditions that can occur in a natural river. For a given discharge, the main river channel bed was found to adjust itself to a quasi-equilibrium condition governed by the lateral momentum transfer between the floodplain and main channel flows and the local alluvial resistance relationship appropriate for the proportion of total flow in the main river channel. The sediment transport rate was found to reflect all these influences. The data are summarized in equation form for comparison with other experimental studies and for checking numerical river simulation models. [ABSTRACT FROM AUTHOR]

In this paper, the results of an experimental investigation of the turbulence intensity in gravel bed channels are described. The runs were carried out by measuring, with an acoustic Doppler velocimeter, the turbulence intensity profile along six verticals of a given cross section in a laboratory flume. The analysis of the measured intensity distributions has shown the existence of two different regions, above and below the tops of the roughness elements, in which different intensity profiles occur. Furthermore, the measured profiles have shown a maximum of the turbulence intensity that decreases for increasing values of the roughness height, confirming that the turbulence damping efficiency increases when the roughness elements protrude inside the flow. The applicability of Nezu’s relationship (derived for a hydraulically smooth bed) for the experimental intensity profiles above the roughness elements is positively tested. Finally a new intensity distribution for a rough bed, applicable to the whole water depth, is proposed. In this profile, two coefficients having a known physical meaning (the maximum turbulence intensity and the depth at which this maximum is located) appear. [ABSTRACT FROM AUTHOR]

Experimental results on local scour in long contractions for uniform and nonuniform sediments (gravels and sands) under clear-water scour are presented. An emphasis was given to conduct the experiments on scour in long contractions for gravels. The findings of the experiments are used to describe the effects of various parameters (obtained from dimensional analysis) on equilibrium scour depth under clear-water scour. The equilibrium scour depth increases with decrease in opening ratio and with increase in sediment size for gravels. But the curves of scour depth versus sediment size have considerable sag at the transition of sand and gravel. The scour depth decreases with increase in densimetric Froude number, for larger opening ratios, and increases with increase in approaching flow depth at lower depths. However, it becomes independent of approaching flow depth at higher flow depths. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to the formation of armor layer in the scour hole. Using the continuity and energy equations, a simple analytical model for the computation of clear-water scour depth in long contractions is developed with and without sidewall correction for contracted zone. The models agree satisfactorily with the present and other experimental data. Also, a new empirical equation of maximum equilibrium scour depth, which is based on the experimental data at the limiting stability of sediments in approaching channel under clear-water scour, is proposed. The potential predictors of the maximum equilibrium scour depth in long contractions are compared with the experimental data. The comparisons indicate that the equations given by Komura and Lim are the best predictors among those examined. [ABSTRACT FROM AUTHOR]

In a series of flume experiments using constant discharge, flow depth, and gravel feed rate, sand feed rates were varied from 0.16 to 6.1 times that of gravel. The bed slope decreased with increasing sand supply, indicating that the gravel could be transported at the same rate, along with increasing amounts of sand, at smaller shear stresses. Prediction of river response to an increase in sediment supply requires prediction of mutual changes in bed composition and transport, and therefore a transport model defined in terms of the grain size of the bed surface. A recent model provides satisfactory prediction of the experimental observations and indicates the general response of gravel beds to increased sand supply. An increase in sand supply may increase the sand content of the river bed and the mobility of gravel fractions, which can lead to bed degradation and preferential evacuation of these sediments from the river. [ABSTRACT FROM AUTHOR]

Measurements of velocity distributions, depth variation, and sediment transport have been made under bankfull and overbank flow conditions in meandering channels with a graded sand bed, using the large-scale U.K. Flood Channel Facility. The overbank conditions depend upon the relative strength of opposing secondary circulation cells generated by shear at the channel crossover and centrifugal forces around the meander bend. Generally the shear-generated secondary flow either reversed or weakened the centrifugal circulation around the next downstream bend. This led to considerable modification of the main channel bed morphology, which, in turn, altered flow distributions. Measurements of the lateral distribution of bed load were made using a 1/4-scale Helley–Smith sampler. This demonstrated that the bed load was generally concentrated within a limited width of the channel and tended to take the shortest route through the meanders. Comparisons of observed and calculated bed material load gives an indication of how secondary circulation around meanders, under both bankfull and overbank conditions, affects the predictive performance of formulas derived for predominantly one-dimensional flow. [ABSTRACT FROM AUTHOR]

A two-dimensional numerical model was developed to simulate relatively wide, shallow rivers with an erodible bed and banks composed of well-sorted, sandy materials. A moving boundary-fitted coordinate system was used to calculate water flow, bed change, and bank erosion. The cubic interpolated pseudoparticle method was used to calculate flow, which introduced little numerical diffusion. The sediment-transport equation for the streamline and transverse transport was used to estimate bed and bank evolution over time, while considering the secondary flow. Bank erosion was simulated when the gradient in the cross-sectional direction of the banks was steeper than the submerged angle of repose because of bed erosion near the banks. The numerical model reproduced the features of central bars well, such as bar growth, channel widening due to divergence of the flow around the bars, scour holes at the lee of the bars, and the increase of bar size with time. These features were in accordance with the observations for laboratory experiments. It also reproduced the features of braided rivers, such as the generation of new channels and abandonment of old channels, the bifurcation and confluence of channels, and the lateral migration of the channels. The model showed that the sediment discharge rate fluctuated with time, one of the dynamic features observed in braided channels. [ABSTRACT FROM AUTHOR]

The use of submerged vanes to divert sediment movements on seabed and open channel flows are common in many engineering projects. In the present study, experiments were conducted to investigate the characteristics of flow and sediment motion around a submerged vane in a large flume, 30 m long, 6 m wide, and 0.6 m high, paved with lightweight cylindrical plastic material with specific gravity of 1.05. The size of the uniform plastic material is 2.8 mm in diameter and 3.1 mm in height. The vane was made of 10 mm thick steel plate. The height of the vane above the mean bed ranged from one-eighth to one-third of the flow depth and its length is between 1 to 4 m. The alignment of the vane to the approach flow varied from 15° to 90°. The effectiveness of the submerged vane in sediment diversion depends on the vane alignment to the approach flow, and the height and length of the vane. The optimum skew angle to the approach flow for the purpose of sediment diversion is about 30°. The optimum vane height of two to three times the height of the bedform would be able to resist the bedload “escaping” over the bedform crest. [ABSTRACT FROM AUTHOR]

In this study, the proposed one-dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures. The equations of sediment transport, bed changes, and bed-material sorting are solved in a coupling procedure with a direct solution technique, while still decoupled from the flow model. This coupled model for sediment calculation is more stable and less likely to produce negative values for bed-material gradation than the traditional fully decoupled model. The sediment transport capacity is calculated by one of four formulas, which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport. The fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels. The tests in several cases show that the present model is capable of predicting sediment transport, bed changes, and bed-material sorting in various situations, with reasonable accuracy and reliability. [ABSTRACT FROM AUTHOR]

This paper proposes that the changes which occur in composition of the bed load during the transport of mixed-grain-size sediments are largely controlled by the distributions of critical entrainment shear stress for the various size fractions. This hypothesis is examined for a unimodal sediment mixture by calculating these distributions with a discrete particle model and using them in a probabilistic calculation of bed-load composition. The estimates of bed-load composition compare favorably with observations of fractional transport rates made in a laboratory flume for the same sediment, suggesting that the hypothesis is reasonable. The analysis provides additional insight, in terms of grain mechanics, into the processes that determine bed-load composition. These insights strongly suggest that better prediction methods will result from taking account of the variation of threshold within size fractions, something that most previous studies have neglected. [ABSTRACT FROM AUTHOR]

Presents a two-dimensional numerical model for predicting the time variation of bed deformation in alluvial channel bends. Calculation of the depth-averaged unsteady flow equations along with sediment continuity equation using the Beam and Warming alternating-direction implicit scheme; Investigation of the process of evolution and stability of bed deformation in circular bends using the model.

Presents a study that investigated the interacting mechanism between the saltating particles near the channel bed. Development of a two-dimensional continuous saltation model; Effects of particle collision; Classification of sediment transport in water.

Focuses on a study which examined the sediment dynamics that occur in channel flow over ripples. Numerical and physical details; Simulation results; Conclusions.

Presents a study which developed a mathematical model that can describe the distribution of sediment concentration from the channel bed to the water surface. Application of the rate of sediment transport; Conclusions.

Presents a study which discussed a three-dimensional model for calculating flow and sediment transport in open channels. Importance of transport calculation in river engineering; Sequence of the calculation; Conclusions.

Part I. Presents information on a study on the backfilling of dredged channels which focused on the sediment transport processes. Depth-averaged flow description; Three-dimensional flow description; Conclusions.

Focuses on a study which examined the synchronous measurements of bed-load discharge at a number of locations across the widths of Nahal Yatir and Nahal Eshtemoa, two course-grained ephemeral channels using automatic sediment transport monitoring stations. Characteristics of river channel under studied; Method used for measuring bed-load transport rates; Hydraulic control of bed-load transport rates.