Straight Alluvial Channels Research Articles

Hydrodynamic instability of free river bars

In this paper, we explore the hydrodynamic instability of free river bars driven by a weakly varying turbulent flow in a straight alluvial channel with erodible bed and non-erodible banks. We employ linear stability analysis in the framework of depth-averaged formulations for the hydrodynamics and the sediment transport. A significant fraction of the sediment flux is considered to be in suspension. The analysis is performed for the alternate pattern of river bars at the leading order followed by the next order, covering the effects of flow regime. We find that the unstable region bounded by a marginal stability curve depends significantly on the shear Reynolds number, which demarcates different flow regimes, and the Shields number and the relative roughness (particle size to flow depth ratio). The results at the next order stabilize the bars with longer wavenumbers. The variations of threshold aspect ratio with Shields number and relative roughness are studied for different flow regimes. In addition, for a given Shields number and relative roughness, the diagram of threshold aspect ratio vs shear Reynolds number is explained. Unlike the conventional theories of bar instability, the analysis reveals limiting values of Shields number and relative roughness beyond which the theoretical results at the next order produce infeasible regions of instability. The limiting values of Shields number and relative roughness appear to reduce, as the shear Reynolds number increases.

Studying the arrangement of submerged vanes group in a straight alluvial channel using numerical modeling of flow and bed topography around a single vane

Submerged vanes are installed in waterways bed. These structures are usually used in groups and their main performance is to create an eddy current which leads to change in stream flow pattern and the bed topography in waterways. Performance of submerged vanes group depends on their arrangement and size. In this study using SSIIM numerical model, hydrodynamic analysis of flow pattern and bed topography around a single submerged vane in a straight channel was done and tried to estimate proper arrangement of submerged vanes group. The geometry of arrangement consisted of longitudinal distance between consecutive vanes, transverse distance of two adjacent vanes, and transverse distance of submerged vanes from the outer wall. Therefore, after verification of the results of numerical model with experimental data, flow pattern and topography of the bed around a vane with different dimensions and angles (in suggested domain) were investigated and analyzed. In this order, values of transverse and longitudinal distances of scouring hole at high-pressure and low-pressure sides of the vane from center of the vane were measured. Also values of transverse and longitudinal locations of the maximum scouring hole and maximum sedimentation downstream of the vane were determined. The results of these parameters were used to estimate the location of adjacent vanes. To reduce number of modes, the Taguchi method is used in design of simulations. Finally, mathematical relations are derived for geometry of arrangement versus vanes dimensions and angles. Comparison of the results of this research with the results of previous researchers, who have studied submerged vanes groups, showed that accuracy of the method used in this study is acceptable. The numerical results showed that the values obtained for the transverse and longitudinal distances of vanes are almost in the same range of previous laboratory studies.

3D numerical modelling of braided channel formation

A numerical model was used to compute the formation of a braided channel system. The model calculated the water flow field from the fully 3D Navier-Stokes equations on a non-orthogonal unstructured adaptive grid. The sediment transport was computed from the Engelund-Hansen formula. A free surface algorithm based on local pressure gradients was used. The model was applied to an idealized geometry of an initially straight alluvial channel, where the evolution of the braided channel system over time was computed. The complex processes and geometry for this case made it very well suited for testing the numerical model. The purpose of the study was also to explain avulsion processes of a braided river in more detail. Figures are presented with water depth, velocity, water level and secondary currents during an avulsion. The effect of the water level changes and the secondary currents are shown. The geometry, sediment size and water discharge used in the numerical model was identical to a laboratory study. Reasonable agreement was found when comparing the active braiding intensity (BIA) computed by the numerical model with measurements from the flume experiment. Parameter tests include sediment transport formula, grid size, secondary current damping and grid parameters related to wetting/drying. The results using the Engelund-Hansen formula show a higher degree of braiding than the van Rijn or Mayer-Peter Müller formula. The secondary current strength is also shown to be very important for the braiding process and the BIA values.

Experimental and numerical evidence for intrinsic nonmigrating bars in alluvial channels

Alternate bars in straight alluvial channels are migrating or nonmigrating. The currently accepted view is that they are nonmigrating if the width‐to‐depth ratio is at the value of resonance or if the bars are forced by a persistent local perturbation. We carried out 2‐D numerical computations and a long‐duration mobile‐bed flume experiment to investigate this view. We find that nonmigrating bars can also occur in straight channels without resonant width‐to‐depth ratio or steady local perturbation. They appear to be an intrinsic response of the alluvial river bed. This finding bears on explanations for meandering of alluvial rivers, for which nonmigrating bars are seen as a prerequisite. We find, however, that the intrinsic tendency of a straight channel to form meanders usually has a different origin. The identified intrinsic nonmigrating bars can only become the dominant mechanism for incipient meandering if the erodibility of the banks is very low.

Investigation on the Dimensions and Shape of a Submerged Vane for Sediment Management in Alluvial Channels

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.

Hydraulic geometry of straight alluvial channels and the principle of least action

By H.Q. HUANG, G.C. NANSON and S.D. FAGAN, Journal of Hydraulic Research, Volume 40, 2002, Issue 2, pp. 153–160

Three-Dimensional CFD Modeling of Self-Forming Meandering Channel

A three-dimensional CFD model was used to compute the formation of the meandering pattern in an initially straight alluvial channel. The numerical model was based on the finite volume method using an unstructured grid with dominantly hexahedral cells. The k-e model was used to predict turbulence and the SIMPLE method was used to compute the pressure. The sediment transport was computed as bed load in addition to solving the convection-diffusion equation for suspended sediment transport. The bed changes were calculated and the grid was altered during the computation as channel erosion and deposition caused wetting and drying. The model was tested by comparing with results from physical model studies carried out at Colorado State Univ., Fort Collins, Colo. The results showed successfully the replication of many of the meander characteristics, including secondary currents, cross-sectional profiles, meander planform, meander wavelength, downstream meander migration, and chute formation.

A stability criterion inherent in laws governing alluvial channel flow

AbstractThe stability criterion of maximum flow efficiency (MFE) has previously been found inherent in typical alluvial channel flow relationships, and this study investigates the general nature of this criterion using a wider range of flow resistance and bedload transport formulae. For straight alluvial channels, in which the effect of sediment sorting is insignificant, our detailed mathematical analysis demonstrates that a flow efficiency factor ε occurs generally as the ratio of sediment (bedload) discharge Qs to stream power Ω (γQS) in the form of $\def\endash{\hbox{--}}\def\lambdabar{{\mathchar'26\mkern-9mu\lambda}} {\varepsilon = {Q_{\rm s}}/ \Omega^{\lambdabar} (\lambdabar = 0{\cdot}86\endash 0{\cdot}88)}$. When ε is maximized (i.e. Qs is maximized or Ω is minimized), maximally efficient straight channel geometries derived from most flow resistance and bedload transport formulae are found compatible with observed bankfull hydraulic geometry relations. This study provides support for the use of the criteria of MFE, maximum sediment transporting capacity and minimum stream power for understanding the operation of alluvial rivers, and also addresses limitations to the direct application of its findings. Copyright © 2002 John Wiley & Sons, Ltd.

Hydraulic geometry of straight alluvial channels and the principle of least action

Natural rivers exhibit regular hydraulic geometry relationships for which no widely accepted explanation has been given. This paper applies the physical principle of least action to the determination of stable alluvial-channel form. For steady, uniform alluvial-channel flow, both theoretical inferences and a case study show that least action occurs when the criteria of minimum potential energy and MFE (Maximum Flow Efficiency, defined here as the maximum sediment transporting capacity per unit available stream power) are satisfied. The consistency between bankfull hydraulic geometry relationships of natural channels and those of maximally efficient or 'least action' channels identified in this study demonstrates that alluvial channels commonly adjust to a maximally efficient section. Support for the use of the extremal hypotheses of maximum sediment transporting capacity and minimum stream power is provided by illustrating that they are essentially expressions of, and hence subsumed by, the more general principle of MFE.

Modelling width adjustment in straight alluvial channels

Modelling width adjustment in straight alluvial channels

Modelling width adjustment in straight alluvial channels

Many existing numerical alluvial channel models are limited because they are unable to account for adjustments of channel width through time. In this paper, the basis for modelling width adjustment in straight river channels is discussed. Features and capabilities of selected models are discussed in terms of advances made in representing the physical processes and mechanisms involved in width adjustment, and in terms of approaches used to characterize initial and boundary conditions accurately. Validation and verification studies of existing width adjustment models are also considered. Existing models have, for the most part, not yet been subjected to rigorous evaluation of their numerical robustness, accuracy in replicating analytical solutions, accuracy in replicating laboratory data or their ability to simulate field situations. In part, these limitations reflect the lack of available detailed laboratory and field data sets required to evaluate complex models. The potential of width adjustment models for use as tools for the investigation of river channel dynamics is briefly described using two examples. © 1998 John Wiley & Sons, Ltd.

High resolution flow modelling in hydrology and geomorphology

Introduction THEORETICAL ADVANCES 1. Hydraulic modelling in hydrology and geomorphology: a review of high resolution approaches 2. Modelling wetting and drying effects over complex topography 3. The 2-D transport module SUBIEF: applications to sediment transport and water quality processes 4. Using lagrangian particle saltation observations for bedload sediment transport modelling HYDRAULIC MODELLING 5. High resolution two-dimensional spatial modelling of flow processes in a multi-thread channel 6. Numerical modelling of separated flow in river bends: model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank 7. Investigation of controls on secondary circulation in a simple confluence geometry using a three-dimensional model 8. Flood routing based on diffusion wave equation using mixing cell method 9. Investigating two-dimensional, finite element predictions of flood plain inundation using fractal generated topography 10. Integrating remote sensing observations of flood hydrology and hydraulic modelling HYDROLOGICAL MODELLING 11. An integrated physically-based model for arid region flash flood prediction capable of simulating dynamic transmission loss 12. Modelling the spatial variability in floodplain soil contamination during flood events to improve chemical mass balance estimates 13. Kinematic wave modelling of vertical movement of snowmelt water through a snowpack 14. Kinematic wave modelling of saturated basal flow in a snowpack 15. The introduction of runoff routing into large-scale hydrological models GEOMORPHOLOGICAL MODELLING 16. Numerical Modelling of floodplain hydraulics and suspended sediment transport and deposition 17. Modelling with adjustment in straight alluvial channels 18. Morphological modelling of rivers with erodible banks .

Characteristics of Self‐Formed Straight Channels

Laboratory experiments were conducted to study various aspects of the mechanics of self‐formed, stable, straight alluvial channels in the presence of bed‐load transport. The assumptions used for the formulation of the turbulent diffusion model of Parker (1978) were examined first. Velocity measurements indicated that the logarithmic law for rough walls is a valid approximation to the velocity profile, along normals to the boundary for the whole channel depth. The equivalent sand‐grain roughness, ks, is about 2d90. The experiments show that the turbulent‐diffusion model is more realistic for flow in a straight channel than the threshold‐channel model. Regime relations based on the straight‐channel model, and a resistance equation, predict accurately the center depth and the top width of a self‐formed straight channel. The shape of the bank is found to follow closely an empirically fitted exponential function that requires knowledge of only the center‐channel depth. Data obtained from other studies support the findings of the present work.

Width of Straight Alluvial Channels

The minimum width of an essentially straight stable alluvial channel transporting water with or without a bed material or wash load is related to the tractive strength and the sliding strength of the bank soils, either alluvial or residual. In addition, the variation of the bed level at the banks due to bed forms, alternate bars, and other three‐dimensional flow effects is an important factor. The variation is defined by the ratio of the maximum depth along the bank to the average depth over the bed. The maximum width is not so well defined but depends on the depositional characteristics of the suspended sediment and the development of meandering tendencies in wider channels. Use of bank soil properties to determine stable channel widths indicates that more than one width and slope are possible to carry a given water discharge with or without bed‐material load. For design, the minimum allowable width is usually the best choice.

Simulation of Bed Armoring in Alluvial Channels

A numerical procedure for simulating the temporal variation of sediment‐transport rate and corresponding variation of bed‐material composition in a straight alluvial channel has been developed. The procedure is based on a simple model for the exchange of grain sizes between a surface and a subsurface layer of bed sediment together with a standard bed‐load function. Verification was made with laboratory data. The procedure should be a useful tool for estimates of the effect of alternative flow regulation measures on a channel bed's long term stability.

Inflow Seepage Influence on Straight Alluvial Channels

A flume study was conducted to identify and analyze the effects of inflow seepage on flow in straight alluvial open channels. The effect of inflow seepage on energy slope, water surface depth, velocity, sediment transport, scour, bed forms, and resistance to flow was investigated. The results of this investigation indicated that inflow seepage increases localized mean channel velocity, energy slope, and stream power in the zone of inflow. The water surface depth decreased for subcritical flow and remained nearly constant for supercritical flow during inflow conditions. Sediment transport was slightly increased when inflow was introduced in the bed. Inflow seepage did not appear to enhance channel scour. Inflow seepage significantly enhanced bed form transformation and the bed roughness. Inflow seepage also caused dunes to become longer, flatter, and move more erratically in the reach where inflow occurred. It was concluded that inflow seepage could significantly influence the channel hydraulics, stream po...

SHEAR STRESS DISTRIBUTION ON A CONTINUOUS BOUNDARY AND WIDENING PROCESS OF A STRAIGHT ALLUVIAL CHANNEL

SHEAR STRESS DISTRIBUTION ON A CONTINUOUS BOUNDARY AND WIDENING PROCESS OF A STRAIGHT ALLUVIAL CHANNEL

A Sediment Transport Model for Straight Alluvial Channels

The paper presents a simple mathematical model for sediment transport in straight alluvial channels. The model, which is based on physical ideas related to those introduced by Bagnold (1954), was originally developed in two steps, the first describing the bed load transport (Engelund 1975) and the second accounting for the suspended load (Fredsøe and Engelund 1976). The model is assumed to have two advantages as compared with empirical models, first it is based on a description of physical processes, secondly it gives some information about the quantity and size of the sand particles in suspension and the bed particles.

On the development of dunes in erodible channels

A two-dimensional stability analysis of the flow in a straight alluvial channel has been carried out, using the vorticity transport equation. In the analysis an attempt has been made to account for the influence of gravity on bed-load transport, and this turned out to change the stability quite significantly.In the case of instability, the further growth of the dunes has been investigated using a second-order approximation, This nonlinear theory explains the experimental fact that the dunes very soon become asymmetric.

A study on meandering in straight alluvial channels

Two different types of meander patterns are found on the sediment beds of straight alluvial channels with nonerodible banks. The first type occurs when the Froude number of the flow is close to 1 and the ratio between the depth and the width of the flow is within a certain range. This pattern probably results from surface waves on the water. The second type of pattern has alternating scour holes and occurs predominantly when water is flowing between rough banks. It is shown how this pattern can be caused by the difference between the shear stresses at the two sides of the flow cross section.