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1.
Gerald Herrling Marius Becker Alice Lefebvre Anna Zorndt Knut Krämer Christian Winter 《地球表面变化过程与地形》2021,46(11):2211-2228
The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments. 相似文献
2.
Active bed forms of three major classes are formed in Terek lower reaches during summer floods. They include ripples, dunes of the first order, and dunes of the second order (from smaller to larger), which commonly form an incomplete hierarchy. The morphology of the bed forms is essentially stochastic and can be adequately described by probability distribution functions of bed form characteristics for some narrow ranges of hydraulic flow characteristics. At the same time, the mean values of bed form morphometric characteristics (length, height, and asymmetry) show stable relationships with flow velocity and depth. The celerity of active dunes can be adequately described by a modified Snishchenko–Kopaliani formula. The channelforming sediments that move as bed forms in Terek lower reaches account for 7% of sediment transport rate of all channel-forming sediments, a value near the lower limit for rivers with sand alluvium. 相似文献
3.
J. R. L. Allen 《地球表面变化过程与地形》1976,1(4):361-374
In natural environments, where unsteady flows prevail, the common one-way bed forms (ripples, dunes, antidunes, parting lineations on plane beds) show delayed responses to changes of flow. A dual classification may then be necessary for the features: 1. Geometric, with no implication of flow conditions. 2. Geometric-hydraulic, in which the shape of the observed forms is combined with the character of the concurrent flow. The delayed responses arise because, under the simplest steady-state equilibrium conditions, the bed forms behave deterministically as populations, which take average dimensions controlled by the bed-material and/or flow conditions. The response of the bed to a change of flow is a rearrangement of the particles forming the bed in an attempt to meet the new conditions. The rate of rearrangement, however, is controlled by the sediment transport rate, in turn determined by the flow conditions, and so is finite. Two specific mechanisms of change of opposing tendency are quantifiable: 1. Creation-destruction of forms, where the new forms are better adjusted than predecessors. 2. The imperfect modification of existing forms during their life-spans. Both rates may be controlled by a combination of flow variables, the attributes of the bed forms, and ‘constants’ specific to each kind of form. A better knowledge of the unsteady responses of these bed forms should increase our understanding of and control over river and tidal systems and may provide models for a better appreciation of still larger features in landscapes and waterscapes. 相似文献
4.
Flow and sedimentary processes in the meandering river South Esk,Glen Clova,Scotland 总被引：1，自引：0，他引：1
Channel geometry, flow and sedimentation in a meander bend of the River South Esk were studied from bankfull stages (January–February) to low water stages (May) in 1974. Bed topography varied little over the study period, showing a typical pool and ripple geometry. Variation of mean depth and velocity with discharge differed from section to section around the bend, due primarily to locally varying flow resistance with stage. The flow pattern for all stages was dominated by a single spiral over the point bar, with a development zone at the bend entrance. Deviation of bed shear stress from the mean flow direction was in general accord with theory, especially for high stages. The use of a uniform longitudinal water surface slope in the calculation of bed shear stress is not justified because of a complicated water surface topography, also such calculated shear may not represent effective bed shear on grains, as it accounts also for energy losses associated with secondary flows. Dunes covered much of the bar at high stages, with increasing proportions of ripples, sand ribbons and lower phase plane beds at low stages. Local flow resistance generally decreases from dunes, diminished and ripple-backed dunes, ripples, sand ribbons to plane beds, and bed forms are predicted quite well by the stream power-grain size scheme. Mean size, sorting and skewness of sediment over the bed changes little with stage. In general, size decreases, sorting improves and skewness changes from positive to negative from the talweg to the inner bank, and in the downstream direction. Allen's (1970a, b) force balance equation for moving bed load particles is supported for bankfull stage, with some reservations, and textural characteristics are explained by progressive sorting in the direction of sediment transport. Large-scale trough cross stratification (with some flat bedding) formed at high stage by dunes (and lower phase plane beds) dominates the point bar sediments. Alternations of fine-medium sand (often cross-laminated) and vegetation-rich layers result from periodic deposition on the grassed upper bar surface. Fining upwards sequences produced by lateral channel migration are modified by a coarsening upward subsequence in the upstream bar region where spiral flow is developing from the bend upstream. 相似文献
5.
Large asymmetric bedforms known as dunes commonly dominate the bed of sand rivers. Due to the turbulence generation over their stoss and lee sides, dunes are of central importance in predicting hydraulic roughness and water levels. During floods in steep alluvial rivers, dunes are observed to grow rapidly as flow strength increases, undergoing an unstable transition regime, after which they are washed out in what is called upper stage plane bed. This transition of dunes to upper stage plane bed is associated with high transport of bed sediment in suspension and large decrease in bedform roughness. In the present study, we aim to improve the prediction of dune development and dune transition to upper stage plane bed by introducing the transport of suspended sediment in an existing dune evolution model. In addition, flume experiments are carried out to investigate dune development under bed load and suspended load dominated transport regimes, and to get insight in the time scales related to the transition of dunes to upper stage plane bed. Simulations with the extended model including the transport of suspended sediment show significant improvement in the prediction of equilibrium dune parameters (e.g. dune height, dune length, dune steepness, dune migration rate, dune lee side slope) both under bed load dominant and suspended load dominant transport regimes. The chosen modeling approach also allows us to model the transition of dunes to upper stage plane bed which was not possible with the original dune evolution model. The extended model predicts change in the dune shapes as was observed in the flume experiments with decreasing dune heights and dune lee slopes. Furthermore, the time scale of dune transition to upper stage plane bed was quite well predicted by the extended model. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
6.
《国际泥沙研究》2020,35(2):171-179
One of the important issues in water transport and sewer systems is determining the flow resistance and roughness coefficient.An accurate estimation of the roughness coefficient is a substantial issue in the design and operation of hydraulic structures such as sewer pipes,the calculation of water depth and flow velocity,and the accurate characterization of energy losses.The current study,applies two kernel based approaches [Support Vector Machine(SVM) and Gaussian Process Regression(GPR)] to develop roughness coefficient models for sewer pipes.In the modeling process,two types of sewer bed conditions were considered:loose bed and rigid bed.In order to develop the models,different input combinations were considered under three scenarios(Scenario 1:based on hydraulic characteristics,Scenarios2 and 3:based on hydraulic and sediment characteristics with and without considering sediment concentration as input).The results proved the capability of the kernel based approaches in prediction of the roughness coefficient and it was found that for prediction of this parameter in sewer pipes Scenario 3 performed better than Scenarios 1 and 2.Also,the sensitivity analysis results showed that Dgr(Dimensionless particle number) for a rigid bed and w_b/y(ratio of deposited bed width,w_b,to flow depth,y) for a loose bed had the most significant impact on the modeling process. 相似文献
7.
Dune bedforms and salt‐wedge intrusions are common features in many estuaries with sand beds, and yet little is known about the interactions between the two. Flow visualization with an echosounder and velocity measurements with an acoustic Doppler current profiler over areas of flat‐bed and sand dunes in the highly‐stratified Fraser River estuary, Canada, were used to examine the effect of dunes on interfacial mixing. As the salt‐wedge migrates upstream over the flat‐bed, mixing is restricted to the lower portion of the water column. However, as the salt‐wedge migrates into the dune field from the flat bed, there is a dramatic change in the flow, and large internal in‐phase waves develop over each of the larger dunes, with water from the salt‐wedge reaching the surface of the estuary. The friction Richardson number shows that bed friction is more important in interfacial mixing over the dunes than over the flat‐bed, and a plot of internal Froude Number versus obstacle (dune) height shows that the salt‐wedge flow over the dunes is mainly supercritical. Such bedforms can be expected to cause similar effects in interfacial mixing in other estuaries and sediment‐laden density currents, and may thus be influential in fluid mixing and sediment transport. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
8.
Michel Lapointe 《地球表面变化过程与地形》1992,17(3):253-270
In a much quoted paper, Jackson (1976) hypothesized that turbulent [bursting] motions such as those documented in laboratory boundary layers play a major role in alluvial sediment suspension. To date, the hypothesis remained largely untested, due to difficulties in monitoring turbulent suspension in rivers. This study provides field data documenting burst-like turbulent motions over a sandy bed channel and quantifying the role of these motions in sand suspension. The data were collected in a 10 m deep channel of the Fraser River near Mission, British Columbia, Canada. Turbulent fluctuations of both flow components, downstream and normal to the bed, along with the output of an optical suspended sediment sensor, were monitored 1 m above the river bed. Typical flow velocities averaged 0·9 ms−1 at the sensors, where mean suspended sediment concentrations were 500 mgl−1; decimetre height small dunes on the backs of larger, half-metre amplitude dunes covered the channel bed in the area. Brief but intense, burst-like [ejection and inrush] events were identified in the flow records, where they are responsible for a high degree of [intermittency] in shear stress over the dunes: 80 per cent of the turbulent momentum exchange across the 1 m level can be ascribed to such brief (3-8 s duration) events, active under 12 per cent of the time. In addition, the record of fluctuating sediment concentrations reveals these burst-like motions to be highly effective in vertically mixing suspended sediment and thus, ultimately, in maintaining suspended sediment transport above the dune bed. The bulk (60 and 90 per cent in two deployments) of the vertical sediment mixing was accomplished by intense events active some 10 per cent of the time. No discrete recurrence timescale for these ‘burst-like’ mixing events is evident, however. Rather, a continuous variation of return periods was observed as a function of the magnitude of vertical mixing event considered. To that extent, conceptual models of sediment transport in terms of burst events with a predictable recurrence such as proposed by Jackson (1976) may be misleading. 相似文献
9.
Basically, sand dunes are patterns resulting from the coupling of hydrodynamic and sediment transport. Once grains move, they modify the surface topography which in turns modifies the flow. This important feedback mechanism lies at the core of continuous dune modelling. Here we present an updated review of such a model for aeolian dunes, including important modifications to improve its predicting power. For instance, we add a more realistic wind model and provide a self‐consistent set of parameters independently validated. As an example, we are able to simulate realistic barchan dunes, which are the basic solution of the model in the condition of unidirectional flow and scarce sediments. From the simulation, we extract new relations describing the morphology and dynamics of barchans that compare very well with existing field data. Next, we revisit the problem of the stability of barchan dunes and argue that they are intrinsically unstable bed‐forms. Finally, we perform more complex simulations: first, a barchan dune under variable wind strength and, second, barchan dune fields under different boundary conditions. The latter has important implications for the problem of the genesis of barchan dunes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
10.
Flume experiments were conducted on different bed stages across the ripple–dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two‐dimensional rippled bed. With further increase in velocity, two‐dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple–dune transition to occur is imposed on the bed surface, these non‐equilibrium linguoid ripples are further transformed into larger, two‐dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two‐dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three‐dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
11.
Robert J. Bialik Mikołaj Karpiński Agnieszka Rajwa Bartłomiej Luks Paweł M. Rowiński 《Acta Geophysica》2014,62(6):1413-1434
This paper presents results of a field investigation conducted to examine the bed sediment, riverbed morphology and flow structure over dunes in natural and regulated channels. Field measurements using an acoustic Doppler current profiler (ADCP) have been carried out on two parts of lowland Wilga River in Poland. It is shown that the bedforms with a low angle of lee side develop more frequently than asymmetrical dunes with high lee-side angles, which are mostly associated with the occurrence of local scours and river meanders. Wavenumber analysis of bed elevation confirms the existence of scaling region in the longitudinal wavenumber spectrum, with “?3” scaling exponents for the natural and regulated channels as well. Moreover, the results of flow velocity field are presented in the form of a 2-D streamwise-vertical vector field, showing several similarities to previous laboratory and field investigations conducted on much deeper rivers than the Wilga. The experimental campaign and methods used to obtain the results are also presented briefly. In addition, a short database of fluvial dunes statistics is provided. 相似文献
12.
13.
P.S. Pauw S.E.A.T.M. Van der Zee A. Leijnse J.R. Delsman P.G.B. De Louw W.J. De Lange G.H.P. Oude Essink 《Ground water》2015,53(5):771-781
Groundwater‐surface water (GW‐SW) interaction in numerical groundwater flow models is generally simulated using a Cauchy boundary condition, which relates the flow between the surface water and the groundwater to the product of the head difference between the node and the surface water level, and a coefficient, often referred to as the “conductance.” Previous studies have shown that in models with a low grid resolution, the resistance to GW‐SW interaction below the surface water bed should often be accounted for in the parameterization of the conductance, in addition to the resistance across the surface water bed. Three conductance expressions that take this resistance into account were investigated: two that were presented by Mehl and Hill (2010) and the one that was presented by De Lange (1999). Their accuracy in low‐resolution models regarding salt and water fluxes to a dense drainage network in a confined aquifer system was determined. For a wide range of hydrogeological conditions, the influence of (1) variable groundwater density; (2) vertical grid discretization; and (3) simulation of both ditches and tile drains in a single model cell was investigated. The results indicate that the conductance expression of De Lange (1999) should be used in similar hydrogeological conditions as considered in this paper, as it is better taking into account the resistance to flow below the surface water bed. For the cases that were considered, the influence of variable groundwater density and vertical grid discretization on the accuracy of the conductance expression of De Lange (1999) is small. 相似文献
14.
On the basis of experiments carried out in flume with a wavy bed with vegetation cover, flow velocity, turbulence intensities and Reynolds stress distributions are investigated. The wavy bed was similar to dune in this study. The fixed artificial dunes were constructed over the bed and artificial vegetation put over them in a laboratory flume. An Acoustic Doppler Velocimeter and spatially-averaged method were applied to determine turbulent flow components and shear velocity. Results were compared with a gravel bedform. It was observed that vegetation cover influences considerably the flow structure and displays clearly the flow separation and reattachment point. The law of the wall was not valid within the vegetation cover, but it was fitted well to the zone above the vegetation cover within the inner layer. For a wavy bed having the same dimensions, shear velocity and friction factor over vegetation cover are 1.7 and 2.6 times of those for the gravel bedform, respectively. The results of laboratory study were compared with those of river study. 相似文献
15.
DEVELOPMENT OF STEP-POOL SEQUENCE AND ITS EFFECTS IN RESISTANCE AND STREAM BED STABILITY 总被引：8，自引：0，他引：8
Zhao-YinWANG JiangXU ChangzhiLI 《国际泥沙研究》2004,19(3):161-171
Experiments were conducted and field investigations were performed to study the development of step-pool sequence and its effects on resistance to the flow and stream bed stability. Step-pool sequence develops in incised channels as a result of streambed erosion, which is compared with sand dunes and armor layer of the role in resistance and streambed protection. The tight interlocking of particles in steps gives them an inherent stability which only extreme floods are likely to disturb. That stability suggests that step-pools are a valid equilibrium form, especially when coupled with their apparent regularity form and their role in satisfying the extreme condition of resistance maximization. The development degree of step-pools, SP, is proportional to the streambed slope. If the incoming sediment load is equal to or more than the sediment-carrying capacity of the flow, there is no bed erosion and thence there are no step-pools. If the flow depth increases and is over the step-height the resistance caused by the step-pool sequence will be greatly reduced. The rote of energy dissipation by step-pools is a function of SP. The higher is SP,, the larger is the rate of energy dissipation. The step-pool sequence increases the resistance and flow depth, reduces the shear stress of the flow and protects the streambed from erosion. Moreover, step-pool sequence provides ecologically sound habitats for aquatic bio-community as well. 相似文献
16.
This paper addresses the development of a flow region associated with turbulence and stress characteristics over a series of 2-D asymmetric dunes placed successively at the flume surface. Experiments were conducted over twelve asymmetric dunes of mean length 32 cm, crest height 3 cm and the dune width almost as wide as width of the flume, using 3-D Micro-ADV at the Indian Statistical Institute, Calcutta. The variations of turbulence statistics along the flow affected by the wavy bottom roughness have been studied. Quadrant decomposition of the instantaneous Reynolds shear stress has been adopted to calculate the contribution of ejection and sweeping events in shear stress generation. The relative dominance of two events are found to contribute in a cyclic manner (spatially) in the near bed region, whereas such phenomenon seems to be disappeared towards the main flow. 相似文献
17.
Effects of silt loading on turbulence and sand transport 总被引：2，自引：1，他引：1
The transport of bed material and fluid turbulence are affected by many factors,including the fine sediment load being carried in a channel.Current research has focused on sand-sized particles introduced to gravel beds,while the effect of silt load on sand transport has received less attention. Experiments on the effects of silt load,in concentrations 0-26,900 mg l-1,on sand transport were performed with a recirculating laboratory flume using three different sand bed configurations:ripples （Fr=0.24）,dunes（Fr=0.34）,and dunes（Fr=0.48）.Three Acoustic Doppler Velocimeters were arranged to measure flow and turbulence quantities simultaneously in one vertical.Sand transport did not change in a consistent manner with increasing silt load,increasing up to 4,000 mg l-1 for dunes （Fr=0.48） and up to 2,000 mg l-1 for dunes（Fr=0.34） and then declining to near the clear water case with increasing silt concentrations.Silt addition for the ripple case caused a relatively small change in sand transport,decreasing with added silt up to approximately 2,000 mg l-1 and then increasing as silt went up to approximately 10,000 mg l-1.Dunes（Fr=0.48） decreased in length and height as silt increased,while dunes（Fr=0.34） did not show a consistent trend.A clear trend of decreasing Reynolds stress with increasing silt concentration was observed in the ripple case,with a 33% reduction in near-bed Reynolds stress caused by an 8,900 mg l-1 concentration of silt. 相似文献
18.
Andries J. Paarlberg C. Marjolein Dohmen‐Janssen Suzanne J. M. H. Hulscher Paul Termes Ralph Schielen 《地球表面变化过程与地形》2010,35(15):1854-1866
This paper presents an approach to incorporate time‐dependent dune evolution in the determination of bed roughness coefficients applied in hydraulic models. Dune roughness is calculated by using the process‐based dune evolution model of Paarlberg et al. ( 2009 ) and the empirical dune roughness predictor of Van Rijn ( 1984 ). The approach is illustrated by applying it to a river of simple geometry in the 1‐D hydraulic model SOBEK for two different flood wave shapes. Calculated dune heights clearly show a dependency on rate of change in discharge with time: dunes grow to larger heights for a flood wave with a smaller rate of change. Bed roughness coefficients computed using the new approach can be up to 10% higher than roughness coefficients based on calibration, with the largest differences at low flows. As a result of this larger bed roughness, computed water depths can be up to 15% larger at low flow. The new approach helps to reduce uncertainties in bed roughness coefficients of flow models, especially for river systems with strong variations in discharge with time. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
19.
WANG Guixian WANG Xingkui GUO Jiachao Professor Sediment Research Laboratory Dept. of Hydraulic Engineering Tsinghua University Beijing China. Associate Professor Dept. of Hydraulic Engineering Tsinghua University. Assistant Dept. of Hydraulic Engineering Tsinghua University. 《国际泥沙研究》1993,(3)
I. INTRODUCTIONResistance to flow in alluvial streams is one of the crucial problems in fluvial hydraulics. Despite that large amount of studies have been conducted for centuries, no perfectmethod describing and calculating the resistance to flow in alluvial streams is availablehll now. Investigation on bed forms and resistance properties of light--weight bed materials is even more scarce. USWES (1936) has conducted systematic experiments onlight--weight materials in a flume & m long an… 相似文献
20.
《国际泥沙研究》2021,36(6):687-695
Environmental flows are generally characterized by complex bed morphology and high current speeds. Such configurations favor the formation of vortex structures that strongly affect hydrody-namics and sediment transport. Large-Eddy Simulation (LES) enables investigation of the dynam-ics of the largest turbulence scales and, thanks to enhanced calculation resources, has now become applicable for simulating environmental flows. In this paper, a LES approach is developed in a CFD code (TELEMAC-3D), which was originally developed to simulate free surface flows using RANS methods. The present developments involve implementing subgrid models, boundary con-ditions and numerical schemes suitable for LES. The LES version of TELEMAC-3D was validated by comparing results on the model with experimental data for flow past a cylinder. Then, the model was applied to a test case representing flow over dunes. After validating the hydrodynamics, the model was used to assess the bottom shear stress, using both a RANS and a LES approach. Com-parison highlighted the potential contribution of LES to investigating the hydrodynamic forces acting on the bottom. 相似文献