Ripples under partially standing waves for different reflection coefficients: a laboratory experiment
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A series of wave-flume experiments was performed to examine ripple geometry, development, and migration under partially standing waves for five different intensities of wave reflection. The present experiments showed the cross-shore variation of the development and geometry of bedforms corresponding to the position of nodes and antinodes. The ripple field tended to initiate under nodes and spread from nodes to antinodes. The local-mean ripple spacings were the largest under nodes and the smallest under antinodes. The difference of the ripple development and the ripple spacing between nodes and antinodes became more pronounced with increasing the intensity of wave reflection. The local-mean ripple spacings in the present experiments correspond approximately with those predicted by Nielsen's (1979, 1981) empirical formula. The trend of sediment transport predicted by ripple migration is consistent with the bed profiles at the end of each run. The geometry, development, and migration of ripples are a possible clue for understanding the cross-shore variation in local hydraulic conditions and mass sediment transport under partially standing waves.Keywords:
Ripple marks
Reflection
Flume
Standing wave
Bedform
Wave flume
Abstract Laboratory experiments were conducted to investigate the formation of river bedforms under sediment supply‐limited conditions, i.e. when a motionless substratum is bared by the dynamics of the mobile sediments. Three series of experiments were organized in a laboratory flume by fixing all the hydrodynamic and morphodynamic parameters but varying the thickness of the initial layer of mobile sediments which covers the rigid bottom of the flume. At the end of all the experiments, which lasted for the same amount of time, the formation of transverse sand dunes was observed. For decreasing , the rigid bottom of the flume was bared progressively earlier during the experiment and the measurements showed a clear tendency of the bedforms to lengthen, i.e. to increase their crest‐to‐crest distance. Moreover, under strong supply limitation, the two‐dimensional transverse dunes turned into three‐dimensional barchanoid forms and into isolated barchan dunes characterized by an abrupt reduction in bedform heights. A two‐dimensional Fourier analysis of the bottom profile was performed, providing the amplitude of the main streamwise and spanwise harmonic components of the bottom morphology as a function of . © 2019 John Wiley & Sons, Ltd.
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Bedform
Crest
Froude number
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The process of clear-water scour is very usual phenomena in natural river.So far,most of the previous studies mainly aim at the calculation of bed armoring layer and process of sediment transport with the action of the flow scour.However,there has been very little attention paid to the flow structure,in particular,the changes of flow structure among different armoring levels for the limitation of monitoring setups.The stable bedform and flow structure on the clear-water scour in gravel-bed flume have been measured by means of acoustic doppler velocimeter(Sontec-ADV) and Nikon-DTM.The results show there is a close relationship among flow,bedload transport,bed material and bedform.The distribution of coarse surface on the armoring layer was affected by the initial bed and its armoring degree with the closely experimental condition.The mean velocity profile may be changed from the logarithmic velocity distribution to the S-shaped curve along relative water depth direction because of the changes of impacted factors,in addition,there is a very good interrelationship between the maximum of turbulence kinetic energy(TKE) and the minimum of mean velocity,but the transitional region and detail location were strongly related to the experimental conditions of flow,sediment and bedform.
Bedform
Flume
Acoustic Doppler velocimetry
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Flume
Wave flume
Smoothed Particle Hydrodynamics
Numerical models
Coastal engineering
Rogue wave
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Flume
Bedform
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Bedform
Flume
Dimensionless quantity
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ABSTRACT Saline currents (SCs) have commonly been used to model muddy turbidity currents (TCs) in a laboratory. However, little is known about the limitations of this proxying, in particular when concerning processes and products related to their sedimentologic and stratigraphic imprints. The present study is aimed at investigating experimental hydraulic and sedimentological conditions and processes involved in the generation and development of bedforms by both SCs and TCs, when similar input conditions are applied (discharge, slope, densimetric Froude number). In all performed runs reported herein, only ripples were observed to form, and were identified and classified using known criteria such as their dimensions, near-bed shear stresses (), shear velocities (), and grain Reynolds values (Re*). Turbidity currents were observed to deposit sediments carried by the flow predominantly in the upstream section of the flume, increasing bed slope and thus increasing , , and near-bed concentrations (cb). This resulted in longer-wavelength bedforms compared to those generated by SCs under similar input conditions in those sections of the flume. On the other hand, along the downstream sections of the flume, bed slopes were observed to remain similar for all experiments, and both types of currents showed similar vertical distribution of velocities, concentrations, and stable stratification. Measured bedform wavelengths and heights were slightly higher when generated by SCs, due to the coarser bed material observed in SCs (which influenced the increase in near-bed turbulent intensities). Moreover, TCs presented a slight decrease in turbulence intensities due to their observed high near-bed suspended-sediment concentration. Spatial and temporal changes in several hydraulic parameters in both SCs and TCs highlight the role of sediment suspension in modifying turbulent processes and vertical stratification of these flows, depending on their concentrations. Both hydraulic and sedimentologic observations of this study support in principle the assumption that SCs can be used experimentally as a surrogate for diluted TCs to reproduce bedforms classified as ripples, as long as both type of currents reach similar hydraulic and sediment-transport conditions, in particular for depth-averaged concentration Cvol < 1% and near-bed concentration cb < 2%.
Bedform
Flume
Froude number
Turbidity current
Ripple marks
Settling
Hydraulic jump
Stratification (seeds)
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The dam break wave is a kind of broken wave, which can destroy the downstream structures. To preliminarily investigate the mechanism of the dam break wave, a numerical model was established in this study. This paper introduce the method of the numerical model and the set up of the numerical flume. The dam break numerical flume includes a reservoir, a sluice gate, a test flume, and a structure. The dam break wave was generated by suddenly lift the sluice gate, so the water in the reservoir rushed into the test flume. The generation and propagation of the dam break in the flume was simulated and observed. The time history of the pressure impact on the structure was recorded. Results show that, the dam break wave was generated by reservoir and sluice gate. In the test flume, the dam break wave stabilized, propagated, and finally engulfed the whole structure. The time history of the pressure experienced a significant peak, fluctuated line, and a quasi-steady line.
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Wave flume
Sluice
Dam break
Lift (data mining)
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Natural levees can be observed worldwide in nearly all river systems characterized by frequent flooding in combination with the transport of suspended bed material. Several parameters and processes have been suggested to explain the formation and the highly variable geometry of natural levees. However, the effect of bedforms migrating in the main-channel has not been amongst the studied parameters and processes. To fill this gap, this study investigates the relationship between bedforms and natural levee formation by flume experiments. Experiments were conducted in a 2 m wide and 30 m long sediment recirculating flume, monitoring the bed level elevation in the main channel by ultrasonic sensors as well as recording the levee development on the floodplain by a camera. The effect of different bed configurations on the developed levees is compared, and discussed, showing that bedforms could enhance levee formation.
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Flume
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An experimental investigation of bedforms generated by the currents is carried out in a flume. Tests with different flow conditions are performed on sand beds. Figures and data obtained in the study are analyzed and the mechanism of ripple formation is discussed as well in this paper.
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Bedform
Ripple marks
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