Numerical modelling of downstream migrating antidunes

A numerical model is presented that compute the geometrical dimensions and movement of downstream migrating antidunes. The model solves the Navier–Stokes equations together with the k‐epsilon turbulence model to find the water flow field over the bedforms. A two‐dimensional width‐averaged grid is used. The bed elevation changes are computed by solving the convection–diffusion equation for suspended sediments and bedload, together with the Engelund–Hansen sediment transport formula. The free surface is computed with an algorithm based on water continuity in the surface cells. Non‐orthogonal adaptive grids were used, moving vertically with the computed location of the bed and the free water surface. The numerical model was tested on data from a physical model study where regular downstream migrating antidunes had been observed. The numerical model started out with a flat bed and the trains of antidunes formed over time. Many of the physical processes observed in earlier studies were replicated by the numerical model. Four dune parameters were computed in the current tests: The antidune wavelength, height and celerity, together with the average water depth. The antidune wavelengths were best predicted with an accuracy of 3 to 8% compared with the measurements. The antidune heights were computed with a deviation of 11 to 25% compared with an empirical formula. The water depths over the antidunes were predicted with an accuracy of 3 to 9% related to the measured values. The average antidune celerity was the parameter with largest deviation: For the coarsest grid it was overpredicted with 37%. Copyright © 2017 John Wiley & Sons, Ltd.     

Implication of the flow resistance formulae on the prediction of flood wave propagation

ABSTRACT

This study examines the difference in the predictions of flood wave propagation in open channels depending on the flow resistance formulae, such as the Chézy and Manning’s equation. The celerity and diffusion coefficient are functions of the channel geometry, slope, roughness as well as the resistance formulae. The results suggest that substituting the Chézy equation with Manning’s equation results in different characteristics of flood propagation, which are consistent regardless of the cross-sectional geometry except for a circular cross-section: increasing celerity and decreasing diffusion coefficient. The celerity is more sensitive to the selection of resistance formulae than the diffusion coefficient. Geometry has a greater effect on the celerity and diffusion coefficient, and consequently on the resulting hydrographs. Manning’s equation results in a larger difference in celerity and diffusion coefficient compared to Chézy equation regardless of the water depth. Overall, this study shows that the selection of resistance formulae is important in terms of the resulting hydrographs and peak flow.

EDITOR Z.W. Kundzewicz ASSOCIATE EDITOR not assigned     

Influence of upstream inflow on wave celerity and time to equilibrium on an overland plane

Abstract

Based on the kinematic wave equations, formulae for the wave celerity along an overland plane subject to uniform rainfall excess and with a constant upstream inflow together with the corresponding average wave celerity and time to equilibrium for the entire plane are derived. The formulae are further developed in terms of both the Darcy-Weisbach resistance coefficient and the Manning resistance coefficient. By comparing the wave celerities, the average wave celerities and the time to equilibrium for planes with and without upstream inflow show that the upstream inflow causes the wave celerity and the average wave celerity to be faster and the times to equilibrium to be shorter. The effect of upstream inflow is greater with increasing inflow, but the marginal effect decreases with increasing inflow. The effect is greatest for laminar flow and least for turbulent flow. For the wave celerity, the effect is also greatest at the upstream end of the plane and least at the downstream end of the plane.     

Empirical assessment of flood wave celerity–discharge relationships at local and reach scales

ABSTRACT

An accurate comprehension of celerity (flood wave speed) dynamics is a key step for understanding flood wave propagation in rivers. We present the results of empirically estimated celerity values in 12 Brazilian rivers, and analyse the behaviour of celerity–discharge relationships (CxQ). Celerity was estimated with a reach-scale (RS) method, based on the peak travel time between stations; and with a local-scale (LS) method, based on the derivative of discharge–cross-section area relationships surveyed at gauging stations. The results indicate that the magnitudes of celerity values obtained by the methods are reasonably comparable, and can rarely be considered constant, varying with river discharge. Three reaches presented differing CxQ relationships at local and reach scales, which suggests that in situ cross-sections at gauging stations should not be extrapolated as representative of the whole reach for flood routing studies, and that CxQ relationship assessments might provide relevant insights for hydrological modelling.     

Topographic forcing of flow partition and flow structures at river bifurcations

This paper presents the predicted flow dynamics from the application of a Reynolds‐averaged Navier–Stokes model to a series of bifurcation geometries with morphologies measured during previous flume experiments. The topography of the bifurcations consists of either plane or bedform‐dominated beds which may or may not possess discordance between the two bifurcation distributaries. Numerical predictions are compared with experimental results to assess the ability of the numerical model to reproduce the division of flow into the bifurcation distributaries. The hydrodynamic model predicts: (1) diverting fluxes in the upstream channel which direct water into the distributaries; (2) super‐elevation of the free surface induced at the bifurcation edge by pressure differences; and (3) counter‐rotating secondary circulation cells which develop upstream of the apex of the bifurcation and move into the downstream channels, with water converging at the surface and diverging at the bed. When bedforms are not present, weak transversal fluxes characterize the upstream channel for almost its entire length, associated with clearly distinguishable secondary circulation cells, although these may be under‐estimated by the turbulence model used in the solution. In the bedform dominated case, the same hydrodynamic conditions were not observed, with the bifurcation influence restricted and depth scale secondary circulation cells not forming. The results also demonstrate the dominant effect bed discordance has upon flow division between the two distributaries. Finally, results indicate that in bedform dominated rivers. Consequently, we suggest that sand‐bed river bifurcations are more likely to have an influence that extends much further upstream and have a greater impact upon water distribution. This may contribute to observed morphological differences between sand‐bedded and gravel‐bedded braided river networks. Copyright © 2012 John Wiley & Sons, Ltd.     

Relative velocities of discharge and sediment waves for the River Severn,UK

Abstract

It is generally accepted that the celerity of a discharge wave exceeds that of a floodwave. The discharge wave is the initial wavefront (shown by an increase in stage at a particular site), whereas the floodwave refers to the body of water moving downstream. Yet, few studies have investigated the varying relationship between discharge and suspended sediment concentration as floods propagate downstream. This paper examines the relative velocities of the discharge and sediment waves for natural flood events on the River Severn, UK. Four monitoring stations were established within the upper 35 km reach of the River Severn (drainage basin area 380 km2). Discharge was monitored using fixed structures, and suspended sediment concentrations were monitored at similar locations using Partech IR40C turbidity meters. Results showed discharge wave celerity increased with flood magnitude, but relationships were more complex for sediment wave celerity. Sediment wave celerity was greater than discharge wave celerity, and is attributed to the dominant source of sediment, which is most probably bank erosion.     

River system discontinuities due to lateral inputs: generic styles and controls

In alluvial river systems, lateral inputs of water and/or sediment at junctions or undercut hillsides can disrupt what would otherwise be smooth downstream trends in mainstream bed elevation, channel gradient, and bed grain size. Generic styles of mainstream response to lateral inputs are investigated using a one‐dimensional sediment routing model with multiple grain size fractions. Numerical experiments isolate the effects of three para‐meters: ratio of tributary to mainstream water flux (QR), ratio of tributary to mainstream bedload flux (FR), and ratio of tributary to mainstream bedload diameter (DR). The findings are not unduly sensitive to the choice of initial conditions or to approximations made in the model. The primary distinction is between junctions that aggrade, causing local profile convexity with interrupted downstream fining, and junctions that degrade. The immediate effects of aggradation extend further upstream than downstream, whereas degradation is much more subdued and has no upstream impact. Aggradation is typical of coarse inputs (DR > 2), and degradation of fine inputs (DR < 1), but very high ratios of QR to FR also promote degradation. Both aggrading and degrading junctions can lead to a change in mainstream bed grain size well below the junction, with higher ratios of QR to FR producing a coarser distal bed. The effect of a tributary reflects the interplay between additional bed load and additional discharge to transport it. Copyright © 2006 John Wiley & Sons, Ltd.     

EXPERIMENTAL STUDY ON SEDIMENT DISTRIBUTION AT CHANNEL BIFURCATION

1 INTRODUCTION Bifurcations are typical features in alluvial rivers as well as in estuaries. In braided rivers confluence and bifurcation occur regularly (Fig.1). A bifurcation occurs when a river splits into two branches (Fig. 2). This happens when a middle bar forms in a channel or a distributary carries flow from the main river. In estuaries when the direction of tidal flow turns around an island, a confluence becomes bifurcation and vice versa. A braided river in the upstream end o…     

Sheet flow hydrodynamics over a non-uniform sand bed channel

The current study experimentally investigates the flow characteristics and temporal variations in the sheet flow profile of a non-uniform sand bed channel. Experiments were done to explore turbulent structures in the presence of a sheet flow layer with and without seepage. The turbulent events, such as stream wise velocity, Reynolds shear stresses, and turbulence intensities were found to be increasing and vertical velocity was found decreasing with a sheet layer. The presence of a sheet layer also effects the turbulent energy production and energy dissipation. All the turbulence parameters with and without a sheet layer have also been influenced by the presence of downward seepage. The rate of sheet flow movement is increased with seepage, owing to increased turbulence with seepage. The current study used wavelet analysis on temporally lagged spatial bed elevation profiles obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuation at different spatial scales. A spatial cross correlation analysis at multiple scales, based on the wavelet coefficients, has been done on these bed elevation datasets to observe the effect of downward seepage on the dynamic behavior of sheet flow at different length scales. It is found that seepage increases average bed celerity and also increases the celerity of sheet flow of similar length scales. This increase in the celerity has been hypothesized as the increase of sheet flow movement as well as the increase in turbulent parameters with seepage, which destabilizes the bed particles resulting in a disruption in the continuous propagation pattern of the sheet flow. The increase of sheet flow celerity with seepage is confirmed from the saturation level of the wavelet power spectra of the bed elevation series. The presence of seepage also affects the non-uniformity of collective sheet material.     

EXPERIMENTAL STUDY ON AGGRADATION

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EXPERIMENTAL STUDY ON RIVER TRAINING MEASURES TO CONTROL BED EROSION

IINTRODUCTIONRiversareanimportantcomponentofournatUralworld.ForthePurposeoffloodprotection,inlandshippingandlandutilization,manyrivershavebeenregulatedornarrowedinaschematic,monotonousmanner.Theriverbedandbanksaresmoothedinordertoreducetheflowresistanceandtoincreasetheflowcapacity.Thesetraditionalrivertrainingmeasures,whichcontradictnlderalevolutionlawofrivers,exertharmfuleffectsontheriverecosystemand"thealenvironmentalongtherivers.Theprimaryequilibriumofthebedloadtransportofriversaredam…     

Experimental study on flow pattern and sediment transportation at a 90°open-channel confluence

In this paper,the evolutions of flow pattern and sediment transportation at a 90° open-channel confluence with different discharge ratios (q*) of the tributary flow to the total flow were studied.The e...     

Bed evolution measurement with flowing water in morphodynamics experiments

A new approach for the profiling of movable sediment beds in laboratory experiments is presented. It couples a triangulation laser sensor and an ultrasonic level transmitter, and allows a non‐intrusive, fast and accurate measurement of bed topography without stopping the experimental runs. The distortion of the laser beam due to the refraction at the water surface is corrected by contemporaneously measuring the elevation of the water surface through the ultrasonic level transmitter and taking advantage of geometrical relations involving the water depth, distance of the sensors from the water surface, and the angles that the emitted laser beam forms with the vertical before and after refraction. Several tests, under either still‐ or flowing‐water conditions, as well as increasing/decreasing water surface elevation, were carried out to evaluate the accuracy of the measurements. These tests indicate that good‐quality measurements are obtained for flow depths in the range 0 < D < 60 mm, typical of morphodynamic laboratory experiments. Finally, two relevant applications to movable bed experiments carried out under either lagoonal or fluvial conditions are presented that show the effectiveness of the proposed profiling technique. Copyright © 2012 John Wiley & Sons, Ltd.     

Downstream fining in contrasting reaches of the sand‐bedded Yellow River

Compared to downstream fining of a gravel‐bedded river, little field evidence exists to support the process of downstream fining in large, fine sand‐bedded rivers. In fact, the typically unimodal bed sediments of these rivers are thought to produce equal mobility of coarse and fine grains that may discourage downstream fining. To investigate this topic, we drilled 200 sediment cores in the channel beds of two fine‐grained sand‐bedded reaches of the Yellow River (a desert reach and a lower reach) and identified a fine surface layer (FSL) developed over a coarse subsurface layer (CSL) in the 3‐m‐thick bed deposits. In both reaches downstream, the thickness of the FSL increased, while that of the CSL decreased. Comparison of the depth‐averaged median grain sizes of the CSL and the FSL separately in both reaches shows a distinct downstream fining dependence to the median grain size, which indicates that at a large scale of 600‐800 km, the CSL shows a significant downstream fining, but the FSL shows no significant trends in downstream variations in grain size. This result shows that fine sediment supply (<0·08 mm median grain size) from upstream, combined with lateral fine sediment inputs from tributaries and bank erosion, can cause a rapid fining of the downstream channel bed surface and can develop the FSL layer. However, in the desert reach, lateral coarse sediment supply (>0·08 mm median grain size) from wind‐borne sediments and cross‐desert tributaries can interrupt the FSL and coarsen the channel bed surface locally. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of artificial groundwater lakes on the abundance and activity of bacteria in adjacent subsurface systems

Bacterial abundances and activity, estimated by 4′,6-diamidino-2-phenylindole staining (DAPI) and the reduction of 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT), were investigated in two oligotrophic artificial groundwater lakes and the surrounding aquifers. To evaluate the effect of lake water on groundwater downstream, samples were taken from wells at different distances from the lakes, and the total number of bacteria and the number of active bacteria in these samples were compared with samples collected upstream. In addition, sterilized sandy sediments were exposed in groundwater wells to measure the number and activity of bacteria attached to particles. At one of the study sites, where the lake sediments were disturbed by dredging, total bacterial abundance and the number of respiring bacteria in the groundwater aquifer was clearly influenced by the lake water. The average bacterial abundances decreased from 2.6 ± 1.9 × 10⁵ cells ml⁻¹ in the well closest to the lake (S2) to 2.9 ± 3.8 × 10⁴ cells ml⁻¹ in the most distant one (S4), which was equivalent to cell numbers in the upstream well. The number of respiring bacteria showed a similar tendency with 1.3 ± 2.7 × 10⁴ active cells ml⁻¹ in S2 and 1.9 ± 1.5 × 10³ active cells ml⁻¹ in S4. At the second study site, which was not influenced by dredging, bacteria in the downstream wells seemed not to be affected by the lake water. The number and activity of bacteria, which colonized exposed sediments, were not significantly different in the upstream and downstream wells, indicating a minor influence of lake water on this habitat. Our results suggest that gravel-pit lakes may influence the free living bacterial assemblages in nearshore groundwater systems, but do not visibly affect numbers and activity of bacteria attached to the surface of aquifer sediments.     

ANALYTICAL HAYAMI SOLUTION FOR THE DIFFUSIVE WAVE FLOOD ROUTING PROBLEM WITH LATERAL INFLOW

The diffusive wave equation is generally used in flood routing in rivers. The two parameters of the equation, celerity and diffusivity, are usually taken as functions of the discharge. If these two parameters can be assumed to be constant without lateral inflow, the diffusive wave equation may have an analytical solution: the Hayami model. A general analytical method, based on ‘Hayami’s hypothesis, is developed here which resolves the diffusive wave flood routing equation with lateral inflow or outflow uniformly distributed over a channel reach. Flood routing parameters are then identified using observed inflow and outflow and the Hayami model used to simulate outflow. Two examples are discussed. Firstly, the prediction of the hydrograph at a downstream section on the basis of a knowledge of the hydrograph at an upstream section and the lateral inflow. The second example concerns lateral inflow identification between an upstream and a downstream section on the basis of a knowledge of hydrographs at the upstream and downstream sections. The new general Hayami model was applied to flood routing simulation and for lateral inflow identification of the River Allier in France. The major advantages of the method relate to computer simulation, real-time forecasting and control applications in examples where numerical instabilities, in the solution of the partial differential equations must be avoided.     

Transient sand transport rates after wind tunnel start-up

Wind tunnel experiments were conducted with a well mixed, flat sand bed, 5·7 m in length, to study the initial sand flux response at three different shear velocities. In some experiments, the bed was allowed to deplete without replenishment; in others, sand was fed 10·8 m upstream of the monitored cross-section. The results indicated that the transport rate increases rapidly during the first minute, and then adjusts slowly towards a steady rate. The time to reach such an equilibrium was observed to be on the order of 2–4 min in non-fed experiments and on the order of 8–9 min in fed experiments. Many factors may affect such development and bring about non-stationarity in total sand transport rate. Among these factors are differences in the natural composition of the sand bed, changes in both the topographical features of the sand bed (ripples) and its surface texture, and any artificial features that influence the adjustment between the boundary layer profile and the sand load on the wind. A useful key to the influence of each factor is obtained by noting that each has a typical and distinct ‘time constant’. The nature and relative importance of each is discussed by reference to the reported wind tunnel experiments and to the behaviour of saltation cloud numerical models. © 1998 John Wiley & Sons, Ltd.