Sand Ripples Research Articles

A numerical investigation into sand grain/slope bed collision

The impact of sand grain on sandy bed surface is not only one of the main processes of near-surface aeolian sand movement, but also the main cause of sand surface damage and soil erosion. In this study, we first simulate the process of grain-bed impact and splash of sand grains with different sizes on sand bed surface using the discrete element method. Then we analyze the relationships among the number, speed and angle of splashing grains and the speed and angle of the impacting sand grain with different slopes. Finally, we present the splash function of the takeoff speed and angle of splashing grains with the incident speed and angle changing and give their probability density distributions. This research is of great importance to understand the windblown sand movement on sand dune or ripples.

Dynamic Friction Coefficient in Formulas of Bed-Load Transport Induced by Waves over Vortex Orbital Ripples

ABSTRACT Kolokythas, G.A.; Grigoriadis, D.G.E., and Dimas, A.A., 2018. Dynamic friction coefficient in formulas of bed-load transport induced by waves over vortex orbital ripples. Empirical formulas for the computation of bed sediment transport induced by wave propagation over sand ripples often depend on, among other parameters, the dynamic friction coefficient, μd, of the sand grain motion. Here, a new approach is presented for the determination of μd based on the physics of the interaction between flow and sediment transport. Specifically, the correlation of μd to the wave, ripple, and sand grain characteristics was obtained using coupled numerical simulations of the flow over the rippled bed, the bed-load transport, and the bed morphology evolution. Flow results were obtained by two different numerical methods, one for wave boundary layer flow and one for purely oscillatory flow, both based on the solution of the Navier-Stokes equations. The bed-load transport rate, which drives the bed morphology evo...

Experimental Study on 3D Scour Around an Embedded Submarine Pipeline Under Oblique Waves

Wave deformation on a sloping seabed and the incident angle of waves greatly influence local submarine pipeline scour. Most previous studies on such scour considered wave deformation but not incident wave angle. Using regular waves with an incident angle of 45°, we investigated 3D scour around an embedded submarine pipeline under oblique waves in experiments. After examining wave deformation near the pipeline at different embedment depths, we analyzed how seabed scour evolved around the pipeline. Results of experiments showed that under oblique wave action, pipeline embedment depth affected wave height on both seaside and leeside of the pipeline. A 3D scour hole occurred under the pileline when the ratio of embedment depth to pipe diameter (e/D) was less than 1/4. Different forms of sand ripples also occurred on both sides as e/D changed. The embedment depth also determined the rate at which scour developed. The scour hole evolved in two distinct stages: rapid and slack. The scouring rate on the right side of the pipeline was greater than that on the left during the rapid phase, leading to an imbalance in scouring depth between the two sides.

Simple stochastic cellular automaton model for starved beds and implications about formation of sand topographic features in terms of sand flux

A simple stochastic cellular automaton model is proposed for simulating bedload transport, especially for cases with a low transport rate and where available sediments are very sparse on substrates in a subaqueous system. Numerical simulations show that the bed type changes from sheet flow through sand patches to ripples as the amount of sand increases; this is consistent with observations in flume experiments and in the field. Without changes in external conditions, the sand flux calculated for a given amount of sand decreases over time as bedforms develop from a flat bed. This appears to be inconsistent with the general understanding that sand flux remains unchanged under the constant-fluid condition, but it is consistent with the previous experimental data. For areas of low sand abundance, the sand flux versus sand amount (flux–density relation) in the simulation shows a single peak with an abrupt decrease, followed by a long tail; this is very similar to the flux–density relation seen in automobile traffic flow. This pattern (the relation between segments of the curve and the corresponding bed states) suggests that sand sheets, sand patches, and sand ripples correspond respectively to the free-flow phase, congested phase, and jam phase of traffic flows. This implies that sand topographic features on starved beds are determined by the degree of interference between sand particles. Although the present study deals with simple cases only, this can provide a simplified but effective modeling of the more complicated sediment transport processes controlled by interference due to contact between grains, such as the pulsatory migration of grain-size bimodal mixtures with repetition of clustering and scattering.

Geological implications of acoustic imagery of the recent seabed textures in the Eastern Harbor, Alexandria

Acoustical geophysical devices depend on sound source to transmit sound waves that reflected, diffracted or scattered off the sea bed. The frequency used by each device controls the output information that related to the sea floor properties. The present study aims to get a recent mapping of the seabed texture mostly related to acoustic imagery, sediment analysis, and video images. High resolution side scan sonar has been used for mapping the seafloor of the Eastern Harbor, Alexandria, Egypt. Four different acoustic patterns were recognized: (a) the pattern of strong backscatter related to coarse sand, (b) the pattern of weak backscatter confined to mud and fine sand, (c) patches of strong and weak backscatter and d) the pattern with isolated reflections that belong to rock outcrops and boulders. The various wavelengths of sand ripples are identifiable that depend on their wavelength and amplitude. The presence of mega ripples in the front of the main EL-Boughaz outlet at the middle of Eastern Harbor indicates bed load transport of sediment during high energy conditions (storms). The current action on the sea bottom produced current ripples which are either straight or sinus. They are usually irregular-crested, showing no bifurcation.

Surface slope effects for ripple orientation on sand dunes in López crater, Terra Tyrrhena region of Mars

Abstract Ripple orientations on small sand dunes (dunes lacking substantial slip faces) at widely distributed sites across Mars have been documented using High Resolution Imaging Science Experiment (HiRISE) images, in an effort to determine the last formative aeolian sediment transport direction experienced at these locations. Howard (1977) used field measurements and first principles to derive an expression for determining how much the surface slope on a sand dune deflects the orientation of sand ripples with respect to the formative wind direction. A Digital Terrain Model derived from stereo HiRISE images was used to assess the potential deflection of ripples on sand dunes on the floor of Lopez crater on Mars. Three-quarters of the area covered by sand dunes within the DTM has a surface slope

Unique sand ripples found on Mars

Unique sand ripples found on Mars

Experimental and numerical study of Sharp’s shadow zone hypothesis on sand ripple wavelength

Abstract Despite advances in understanding processes of sand transport by saltation and reptation, which are involved in the formation of sand ripples, the mechanisms that determine the linear dependence of ripple spacing on wind speed and their relative importance are yet unknown. In a pivotal study, Sharp (1963) proposed that this linear dependence arises from the scaling of the ripples’ shadow zone – the part of the ripple devoid of particle impacts – with the wind speed. Here, we test this hypothesis by integrating wind tunnel experiments with numerical simulations of saltation. Specifically, we measured the effective shadow zone by using sand traps designed for this purpose and found a linear relationship between the shadow zone and the wind shear velocity, consistent with Sharp’s hypothesis. However, contrary to what Sharp assumed, we found that the shadow zone is not completely screened from particle impacts, which as indicated by numerical simulations is due to the wide distribution of impact angles. Nonetheless, the shadow zone can be one of the major mechanisms contributing to the linear increase of the ripple wavelength with wind speed at the nonlinear growth stage of the ripples where merging events between small ripples take place. However, for the initial stage of ripple development, when the ripple dimension is small, other mechanisms can be dominant, such as the recently suggested resonant saltation trajectory (Duran et al., 2014).

Turbulence burst over four micro-topographies in the wind tunnel

Investigating the mechanisms of wind-sand movement is essential to deeply understand sand storm and desertification process, and further to control the wind-sand hazards. It is considered that turbulence burst may have great influence on the motion of sand particles. But current theoretical and experimental studies mostly concentrate on the turbulence burst over flat surface or in the water. Sand dunes and sand ripples are basic forms of desert landscape, and the slope gradient will inevitably affect the frequency of turbulence burst. In this paper, instantaneous velocities over 4 different sand surfaces, including smooth flat bed, flat sand bed, sand bed with ripples and slope bed, are measured using one-dimensional hot wire detector. The results show that for the slope sand surface, wind field in leeward slope region is a buffering zone, which doesn't obey logarithmic function. On the flat sand surface, the distribution of turbulence intensity and Reynolds normal stress are gradually decreasing along the height of boundary layer. On the slope sand surface, the stronger turbulence behavior happens near the top of slope. Based on Mu-level method analysis, it is found that the turbulence bursts on both flat sand surface and sand ripples mainly occur near the surface, and gradually decrease with the height. Compared to flat sand surface, the burst frequency is 30%~50% higher on the sand ripples surface, which indicates that micro-topography have an influence on the turbulence burst. This work may be helpful in understanding the initiation and transportation of sand storm, as well as the desertification process.

Numerical study of shear stress distribution at sand ripple surface in wind tunnel flow

Abstract The mechanism responsible for the formation and sustainability of sand ripples sheared by a uniform air flow is not well understood, despite the significant attention that has been given to it ever since the pioneering studies of Bagnold (1941). In this study we explore ANSYS Fluent simulations of fine-scale turbulent flow structure in the vicinity of 2D sand ripples with particular emphasis on shear stress distribution at the sand bed. The flow parameters in the simulations were pertinent to the wind tunnel experiments for studying sand ripples formation. The simulations show that the shear stress at the crest is about 2.5 times larger than the shear stress at the trough and that in most of the simulations a separation bubble has been developed at the lee slope. In contrast to wind tunnel experiments the simulations show that ripples will be flattened at wind speed of 9 m/s as shear stress at the ripples surface exceeds the fluid threshold. This discrepancy between the calculations and real wind tunnel measurements are due to the important role of the saltation layer on the decrease of the shear stress at the surface. Without this effect ripples cannot grow higher and will be diminished at quite moderate winds.

A New Model for Sand-Ripple Scattering Based on SSA Method and Practical Ripple Profiles

In the former model for sand surface electromagnetic (EM) scattering, the sand ripples are generally viewed as a standard sine-shaped wave surface, which artificially deprives many inherent properties of natural sand ripples. For example, the ripples have two pretty distinct sides: Y-junctions and variable texture directions. For these reasons, these corresponding scattering models deserve doubts to final calculated results. On the other hand, the former models usually are statistical models, which are not favorable in target coupling scattering problems and synthetic aperture radar (SAR) imaging simulations. In this paper, based on the physical mechanism governing the formation of aeolian landforms, sand ripples of actual shape are generated under the discrete models. Furthermore, as the slope of the ripple surface is fully small, facet-based small-slope approximation (SSA) method is unquestionably utilized in the EM scattering studies of sand ripples. Finally, significant differences are shown between the results based on sine-shaped wave surface and practical ripple surface, which proves the necessity of the new model for EM scattering of sand ripples.

Thematic and geometric analysis of Bangpo beach based on UAV Remote Sensing

High resolution aerial photographs and digital elevation models for Bangpo beach using UAV were generated in this study to analyze the thematic and geometric characteristics of coastal features. Based on 728 aerial images acquired on September 10, 2016 by the UAV, a image mosaic at 2.2 cm spatial resolution and a digital elevation model at 4.4 cm spatial resolution were developed. This study found out that Bangpo beach consisted of intertidal zone and supratidal zone. The intertidal zone can be subdivided into lower part and upper part with distinctive geomorphological characteristics. While the lower part included sand bars and ripple marks along the coastline, the cusps and sand dunes were the major coastal features of the upper part. Part of the intertidal zone was occupied by shore platform with average slope of 0.9 degree containing various sizes of gravels. The supratidal zone slanted toward ocean with berms on the surface with an interval of 15 m. These coastal features indicated the flow intensity towards to the land and tidal effect. It validated that the UAV application in coastal research was very effective analyzing to examine coastal processes.

Quantification of layered patterns with structural anisotropy: a comparison of biological and geological systems.

Large-scale patterns evident from satellite images of aeolian landforms on Earth and other planets; those of intermediate scale in marine and terrestrial sand ripples and sediment profiles; and small-scale patterns such as lamellae in the bones of vertebrates and annuli in fish scales are each represented by layers of different thicknesses and lengths. Layered patterns are important because they form a record of the state of internal and external factors that regulate pattern formation in these geological and biological systems. It is therefore potentially possible to recognize trends, periodicities, and events in the history of the formation of these systems among the incremental sequences. Though the structures and sizes of these 2-D patterns are typically scale-free, they are also characteristically anisotropic; that is, the number of layers and their absolute thicknesses vary significantly during formation. The aim of the present work is to quantify the structure of layered patterns and to reveal similarities and differences in the processing and interpretation of layered landforms and biological systems. To reach this goal we used N-partite graph and Boolean functions to quantify the structure of layers and plot charts for “layer thickness vs. layer number” and “layer area vs. layer number”. These charts serve as a source of information about events in the history of formation of layered systems. The concept of synchronization of layer formation across a 2-D plane is introduced to develop the procedure for plotting “layer thickness vs. layer number” and “layer area vs. layer number”, which takes into account the structural anisotropy of layered patterns and increase signal-to-noise ratio in charts. Examples include landforms on Mars and Earth and incremental layers in human and iguana bones.

Geometry of Wave-Formed Orbital Ripples in Coarse Sand

Using new large-scale wave-flume experiments we examine the cross-section and planform geometry of wave-formed ripples in coarse sand (median grain size D50 = 430 μm) under high-energy shoaling and plunging random waves. We find that the ripples remain orbital for the full range of encountered conditions, even for wave forcing when in finer sand the ripple length λr is known to become independent of the near-bed orbital diameter ds (anorbital ripples). The proportionality between λr and ds is not constant, but decreases from about 0.55 for ds / D50 ≈ 1400 to about 0.27 for ds / D50 ≈ 11 , 500 . Analogously, ripple height ηr increases with ds, but the constant of proportionally decreases from about 0.08 for ds / D50 ≈ 1400 to about 0.02 for ds / D50 > 8000 . In contrast to earlier observations of coarse-grained two-dimensional wave ripples under mild wave conditions, the ripple planform changes with the wave Reynolds number from quasi two-dimensional vortex ripples, through oval mounds with ripples attached from different directions, to strongly subdued hummocky-type features. Finally, we combine our data with existing mild-wave coarse-grain ripple data to develop new equilibrium predictors for ripple length, height and steepness suitable for a wide range of wave conditions and a D50 larger than about 300 μm.

The formation and migration of sand ripples in closed conduits: Experiments with turbulent water flows

The transport of solid particles by a fluid flow is frequently found in nature and industry. Some examples are the transport of sand in rivers and hydrocarbon pipelines. When the shear stresses exerted by a fluid flow on a granular bed remain moderate, some grains are set in motion without fluidizing the bed; the moving grains form a layer, known as bed load, that moves while maintaining contact with the fixed part of the bed. Under bed load conditions, the granular bed may become unstable, generating ripples and dunes. Sand ripples are commonly observed in closed conduits and pipes such as in petroleum pipelines, sewer systems, and dredging lines. Although of importance for many scientific domains and industrial applications, the formation of ripples in closed conduits is not well understood, and the problem is still open. This paper presents an experimental study on the formation and migration of sand ripples under a turbulent closed-conduit flow and bed-load conditions. In our experiments, fully-developed turbulent water flows were imposed over a granular bed of known granulometry in a transparent channel, and bed load took place. For different water flow rates and grain diameters, the growth and migration of bedforms were filmed by a high-definition camera, and a numerical code was developed to determine the wavelength and celerity of the bedforms from the acquired images. The obtained results are compared with published stability analyses.

Preface Issue 4-2015

In their survey article on “Stochastic PDEs and lack of regularity”, Dirk Blomker and Marco Romito discuss a nonlinear partial differential equation which may serve as a phenomenological model for the growth of an amorphous surface. This equation contains an additional noise term which models fluctuations of the flow of incoming particles. The authors mention that similar equations are used to model the formation of sand or snow ripples and that their model has striking similarities to the Kadar-Parisi-Zhang (KPZ) equation. While it is possible to prove local (in time) existence of strong solutions, which are also unique, the problem of whether they are global (in time) or may blow up in finite time remains an important open problem. In one space dimension (i.e., assuming invariance with respect to the second spatial variable) the authors prove the existence of global (in time) weak solutions where, however, uniqueness is left open. Similar difficulties due to the lack of regularity arise also e.g. in the time-dependent 3D-Navier-Stokes equations. Jacques Tits is one of the most influential group theorist and algebraic geometers and received, among many other awards, in 1996 the Cantor medal of the Deutsche MathematikerVereinigung and in 2008 the Abel Prize of the Norwegian Academy of Science and Letters. Two years ago, Jacques Tits’ “Collected Works” appeared which are subject of this issue’s “Classics Revisited”-contribution. Linus Kramer highlights some of their contents grouping them according to their themes. Walter D. van Suijlekom gives a summary and his opinion of Frederic Paugam’s recent book “Towards the mathematics of quantum field theory”.

Sand Ripple Characterization Using an Extended Synthetic Aperture Sonar Model and Parallel Sampling Method

The aim of this work is to characterize the seafloor by estimating invariant sand ripple parameters from synthetic aperture sonar (SAS) imagery. Using a hierarchical Bayesian framework and a known sensing geometry, a method for estimating sand ripple frequency, amplitude, and orientation values from a single SAS image, as well as from sets of SAS imagery over an area, is presented. This is accomplished through the development of an extended model for sand ripple characterization and a Metropolis-within-Gibbs sampler to estimate sand ripple frequency, amplitude, and orientation characteristics for multiaspect high-frequency side-look sonar data. Results are presented on synthetic and measured SAS imagery that indicate the ability of the proposed method to estimate desired sand ripple characteristics.

Finite element modeling of propagation and reverberation shallow water waveguide with a variable environment

Shallow water waveguides can exhibit environmental variability in the water column, on the bottom interface and in the sediment. Finite element models provide a method to capture the effects of this variability since every element can be described by a different sound speed and density. However, fully three-dimensional finite element models are often computationally inaccessible due to extreme memory requirements. In this study, a longitudinally invariant finite element model is used to predict the reverberation from a shallow water waveguide described by environmental measurements at the Target and Reverberation Experiment 2013 conducted off the coast of the Florida panhandle. Longitudinally invariant models retain all of the fidelity of a three-dimensional model with the requirement that one geometric dimension must be invariant. Therefore, it is an ideal model for wedges and ridges. In this case, the longitudinally invariant direction describes the sand ripples. The reverberation with and without variations in sediment sound speed and density will be compared for the same bathymetry to determine the role of sediment variability in reverberation. Reverberation from along and across the sand ridges will also be examined. [Work supported by ONR, Ocean Acoustics.]

Recent near-surface wind directions inferred from mapping sand ripples on Martian dunes

The High Resolution Imaging Science Experiment (HiRISE) provides the capability to obtain orbital images of Mars that are of sufficient resolution to record wind ripple patterns on the surfaces of sand dunes. Ripple patterns provide valuable insights into aeolian erosion and deposition on Earth and Mars. In this study, we develop a systematic mapping procedure to examine sand ripple orientations and create surface process maps to evaluate the recent wind flow over the dunes, as well as the interplay of wind and dune shape. By carefully examining the morphology of the dunes and the location of grainflow and grainfall on dune slipfaces, the recent near-surface wind direction (short-term wind) can be identified. Results from the analysis of three dune fields on the floors of craters west of Hellas Basin show regional N, NW, SE, and ESE wind directions. In the three adjacent dune fields, surface process and flow maps suggest a complex wind pattern. The comparison of short-term wind with dune-constructing wind (long-term wind) shows NE and ESE winds may be persistent at least for the past thousands of years. The results also show that the orientation of inferred wind direction on linear dunes is correlated with the crestlines, which suggest that form–flow interaction may take place. The results of local wind flow documentation should improve Martian surface wind modeling and advance our understanding of sand transport, as well as the rates of sand mobility on both Mars and Earth.

Mixing efficiency of sediment and momentum above rippled beds under oscillatory flows

While the nature of the suspended load above steep, wave-induced, sand ripples is of practical importance, it also raises intriguing questions about the relative mixing efficiencies of sediment and momentum above the seabed. It has been widely accepted that the mixing efficiency of sediment is substantially greater than that of momentum. But, hitherto, this has not been explained clearly in terms of the underlying, detailed physical mechanisms which revolve around the generation and ejection of sediment-laden vortices at the ripple crest, and their subsequent advection by the flow. A two-dimensional discrete-vortex, particle-tracking research model, with the parameter settings corresponding to a well-documented laboratory experiment, is used here to represent these processes. Both the modelled and also experimental flow and concentration fields are described in detail, together with the horizontally (ripple-) averaged fields, and the cycle-mean, ripple-averaged fields. From these considerations, the ratio (β) of the sediment diffusivity to the eddy viscosity, or the inverse of the Schmidt number, is then determined. It is found that β is larger than unity, in fact between 1.3 and 3.1 for two different computational approaches (based on harmonics and exponential fitting) for the model and data. These values for β agree well with previous results reported in the literature. This research elucidates, from fundamental principles related to spatio-temporal correlations between concentration and velocity, the improved efficiency of sediment mixing compared with momentum mixing in the vortex layer above rippled beds and its key role in determining suspension profiles in such flows.