Sand Ripples Research Articles

Mid- to High-Frequency Acoustic Penetration and Propagation Measurements in a Sandy Sediment

During the recent 2004 sediment acoustics experiment (SAX04), a buried hydrophone array was deployed in a sandy sediment near Fort Walton Beach, FL. The array was used to measure both the acoustic penetration into the sediment and sound speed and attenuation within the sediment while a smaller, diver-deployed array was also used to measure sound speed and attenuation. Both of these systems had been deployed previously during the 1999 Sediment Acoustics Experiment (SAX99). In that experiment, the buried array was used to make measurements in the 11-50-kHz range while the diver-deployed array made measurements in the 80-260-kHz range. For the SAX04 deployment, the frequency range for the measurements using the buried array was lowered to 2 kHz. The diver-deployed array was also modified to cover the 40-260-kHz range. Unlike the SAX99 deployment, there were no obvious sand ripples at the SAX04 buried array site at the time of the measurements. To examine the role of sand ripples in acoustic penetration over this new frequency range, artificial ripple fields were created. For the high frequencies, the penetration was consistent with the model predictions using small-roughness perturbation theory as in SAX99. As the frequency of the incident acoustic field decreased, the evanescent field became the dominant penetration mechanism. The sound speed measured using the buried array exhibits dispersion consistent with the Biot theory while the measured attenuation exceeds the theory predictions at frequencies above 200 kHz.

Small-scale, patchy distributions of infauna in hydrodynamically mobile continental shelf sands: Do ripple crests and troughs support different communities?

Spatial variability of infauna with respect to distribution of topographic habitat features was examined in hydrodynamically mobile sandy sediments on the inner continental shelf off New Jersey, USA (39° 27.69′ N, 74° 15.81′W). Sediment cores for infauna were taken by SCUBA divers at multiple spatial scales over time at 12-m depth in the LEO-15 research area on Beach Haven Ridge. Crests, troughs and less consistently flanks of sand ripples 5–15-cm in height, were characterized by different infaunal community patterns at spatial scales of centimeters to kilometers on several sampling dates. Overall, infaunal community differences among ripple crests, troughs, and/or flanks within areas <1-m 2 were greater than those found for each of these habitats (i.e., either crests, troughs, or flanks) that were separated by distances of 2 m–4 km. Infaunal density and species richness were consistently higher in troughs compared to crests. Indirect measures of food resources such as particulate organic carbon, chl a, and pheophytin were associated with ripple crests and troughs. Troughs contained significantly higher levels of particulate organic carbon (~1.2 times higher) associated with finer sediments, compared with crests and flanks. Various combinations of taxa had higher densities in either crests or troughs of sand ripples depending on date, and the relative abundances of three taxa, the deposit-feeding polychaete Polygordius jouinae, the suspension-feeding surfclam Spisula solidissima, and predatory nemerteans were important in distinguishing between crests and troughs on most dates. Thus, a priori knowledge of whether a benthic sample comes from a crest or trough helped to explain small-scale infaunal patchiness in relatively homogeneous, subtidal sandy sediments. Consideration of such topographic features in sampling designs can help in explaining variation in species’ distributions at several spatial and temporal scales.

Lagrangian drift near a wavy boundary in a viscous oscillating flow

The formation of sand ripples in oscillating flows is thought to be due to a steady streaming current which near the bottom is towards the crests. We present quantitative observations of this mean flow over self-formed and artificial ripples, by observing the displacement of a coloured filament after a number of oscillations in the simple situation of viscous Couette flow. Confusingly, the filament moves in the ‘wrong’ direction, because it follows the Lagrangian mean flow. We calculate the Lagrangian mean flow. A complication is that the amplitudes of the oscillations in the experiments are not small. We compare the predictions with the experimental observations of displacements of the filament, showing good agreement.

Transverse Aeolian Ridges on Mars: First results from HiRISE images

Three images obtained by the High Resolution Imaging Science Experiment (HiRISE) were analyzed for the information they could provide regarding Transverse Aeolian Ridges (TARs) on Mars. TARs from five locations in a HiRISE image of the floor of Ius Chasma show remarkably symmetric (cross-sectional) profiles, with average slopes for the entire feature of ~ 15°; these results apply to TARs that span an order of magnitude in wavelength and a factor of 6 in height. A HiRISE image of Gamboa impact crater in the northern lowlands shows low albedo sand patches < 2 m high that are covered with sand ripples, surrounded by larger TAR-like ripples that are very similar in profile to surveyed granule ripples on Earth. TARs in a HiRISE image from Terra Sirenum, in the cratered southern highlands, are comparable in height to those in Ius Chasma, but many have tapered extensions that are more consistent with them being erosional remnants rather than the result of extension of the TAR by deposition from the tapered end. The new observations generally support a reversing transverse dune origin for TARs with heights ≥ 1 m, and a granule ripple origin for TAR-like ripples with heights ≤ 0.5 m.

风成沙波纹和沙丘的动力行为分析

Three dimensional quantitative simulations on the formation and evolution of aeolian sand ripples and sand dunes are presented. The simulations successfully replayed the self-reparation behavior of sand ripples and the collision behavior of sand dunes. The simulation results revealed the dominant physical laws governing the self-reparation behavior of sand ripples and Y-junctions, and the phenomena that two aeolian barchan dunes collide with each other in a manner similar to solitons or solitary waves. The results indicate that the self-reparation time of sand ripple will reduce with the frictional wind speed increasing, and the existence of Y-junction and the increase of sand diameter will extend the self-reparation process of sand ripple. The collision’s consequence between sand dunes is related with the two sand dunes relative scales and the angle between the line lock of center of dunes and wind direction. When the scales of two sand dunes are close and the angle between the line lock of center of dunes and wind direction is small, the solitary waves behavior will appear.

Separation vortices and pattern formation

In this paper examples are given of the importance of flow separation for fluid patterns at moderate Reynolds numbers—both in the stationary and in the time-dependent domain. In the case of circular hydraulic jumps, it has been shown recently that it is possible to generalise the Prandtl–Karman–Pohlhausen approach to stationary boundary layers with free surfaces going through separation, and thus obtain a quantitative theory of the simplest type of hydraulic jump, where a single separation vortex is present outside the jump. A second type of jump, where an additional roller appears at the surface, cannot be captured by this approach and has not been given an adequate theoretical description. Such a model is needed to describe “polygonal” hydraulic jumps, which occur by spontaneous symmetry breaking of the latter state. Time-dependent separation is of importance in the formation of sand ripples under oscillatory flow, where the separation vortices become very strong. In this case no simple theory exists for the determination of the location and strengths of separation vortices over a wavy bottom of arbitrary profile. We have, however, recently suggested an amplitude equation describing the long-time evolution of the sand ripple pattern, which has the surprising features that it breaks the local sand conservation and has long-range interaction, features that can be underpinned by experiments. Very similar vortex dynamics takes place around oscillating structures such as wings and fins. Here, we present results for the vortex patterns behind a flapping foil in a flowing soap film, which shows the interaction and competition between the vortices shed from the round leading edge (like the von Karman vortex street) and those created at the sharp trailing edge.

Seasonal patterns in permeable sediment and water-column biogeochemical properties on the inner shelf of the Middle Atlantic Bight

Abstract This study examines the responsiveness of permeable shelf sediments to seasonal factors affecting cycles of organic matter deposition and erosion. At a site on the inner continental shelf within the Middle Atlantic Bight (39°27′N, 74°14′W; water depth 13 m), sediment inventories of chlorophyll, phaeopigments, and fine particles enriched in organic matter are reported from SCUBA diver-collected cores retrieved during six cruises conducted over 2 years. These measurements provide little evidence for substantial seasonal variation in rates of organic matter accumulation, in spite of the potential for the sinking of large phytoplankton blooms associated with summer upwelling/downwelling events, or for wave orbital motion to cause both phytodetritus and benthic algae to be trapped in sand ripples. Sediment permeability, porosity, grain size, and extracellular proteolytic activity in both coarse and fine fractions showed little seasonality. Instead, a negative correlation between chlorophyll in surface sediments and bottom-water pools ( r 2 =0.56) was observed. Furthermore, overlays of other water-column properties with results of a principal component analysis of sediment variables suggest benthic–pelagic biogeochemical cycling is forced primarily by sediment resuspension events. Wave and current observations made during this study and during previous long-term experiments in the study area show that sediment resuspension stems from bottom stresses produced by both persistent irregular wind-waves and by very intense storm waves that occur in all seasons.

Nonlinear mechanism of bed load transport

From the group movement of the bed load within the bottom layer, details of the nonlinear dynamic characteristics of bed load movement are discussed in this paper. Whether the sediment is initiated into motion corresponds to whether the constant term in the equation is equal to zero. If constant term is zero and no dispersive force is considered, the equation represents the traditional Shields initiation curve, and if constant term is zero without the dispersive force being considered, then a new Shields curve which is much lower than the traditional one is got. The fixed point of the equation corresponds to the equilibrium sediment transport of bed load. In the mutation analysis, we have found that the inflection point is the demarcation point of breaking. In theory, the breaking point corresponds to the dividing boundary line, across which the bed form changes from flat bed to sand ripple or sand dune. Compared with the experimental data of Chatou Hydraulic Lab in France, the conclusions are verified.

Formation of aeolian ripples and sand sorting

We present a continuous model capable of demonstrating some salient features of aeolian sand ripples: the realistic asymmetric ripple shape, coarsening of the ripple field at the nonlinear stage of ripple growth, saturation of ripple growth for homogeneous sand, typical size segregation of sand, and formation of armoring layers of coarse particles on ripple crests and windward slopes if the sand is inhomogeneous.

Experimental Study on Bed Load Transport of Shell fragment-Mixed Sand under Waves

Beneficial use of shell for constitutive materials of beach is proposed instead of its disposal. Since the shape of shell fragment is oblate, the knowledge on littoral drift of sand with spherical shape could not be applied directly for understanding of its behavior under waves. In this study, critical shields number and fall velocity of shell fragment are evaluated changing oblate shapes of the fragments, and the characteristics of bed load transport of shell and shell-mixed sand beds are examined by a movable-bed experiment. From the experiments, the effects of oblateness on the increase of resistance to wave and current are elucidated. It is found that the interaction of sand and shell on their movements has an effect on the bed - load rate changing the height of sand ripples.

The study of removal (detachment) and transitional regions of wind erosion upon ground indicator (case study: Khartouran Erg).

The result of wind dynamic (capacity and potential) is transport of soil or sediment particles and subsequent formative desert landforms based on the severity of erosion or sedimentation. The Khartouran region, because of its geology, geomorphology and climatology conditions is a special place in the arid region on the closed basin of Dasht-e-kavir. It is evident that the ecosystem balance of the region is affected by anthropogenic exploitation and wind activity, so this area has a geoecological structure with special features. The wind erosion process in the Khartouran area can be categorized as origin, transport and sedimentation sites. In this research, the studied area was about 200000 ha, southwest of the town of Sabzevar covering sand origin and transport areas. The studies were carried out based on topographic, geologic maps, landsat images, aerial photographs and field observation. Based on these documents, the area was divided into a 9 km2 (3?3 km) grid and identity points of each grid were determined (214 points). On each identity point, indices of wind erosion (deflation, abrasion and accumulation), were collected. The results have shown two geomorphologic units (pediment playa) in project area and the geomorphologic facies (landforms) such as desert pavement, small yardang (kalotak), rebdou, nebka and sand ripple have been found by aeolian transport of sand. Based on ground indicators, determined removal and transit zones. Based on studies and results can conclude that these regions were affected by wind erosion in the past more than of current time and khartouran Erg is a result of wind activity in the past. Currently, the rate of sand flux and the balance between erosion and sedimentation are dominant factors in shaping geomorphologic facies (Aeolian landforms). Key words: Khartouran Erg; Deflation; Abrasion; Wind erosion; Removal zone; Transit zone; Sand dunes; Wind erosion indicators

Pattern formation in a granular medium excited by waves in a flume

This work deals with the dynamics of sand ripples which occur on a flat sandy sea bed excited with an oscillatory flow in a wave flume. The bathymetry of the bed is reported since the first stages of ripple formation with an optical technique. A statistical approach is developed to study the dynamical evolution of ripples. With an identical mobility number and different Reynolds number, two tests leading to 2D and 3D pattern formation are compared. Two stages are detected during ripple formation independently of pattern morphology. However, pattern tridimensionality can be detected with statistical tools. 2D and 3D equilibrium ripple geometric data are compared with the literature.

Aeolian Megaripples: Mathematical Model and Numerical Simulations

Aeolian sand ripples are a common feature on sandy deserts and beaches. Standard aeolian ripples have wavelengths of a few centimeters and amplitudes of a few millimeters. Sometimes, much larger ripples are observed. These are known by different names, such as ridges, granule ripples, gravel ripples, or megaripples. Aeolian megaripples are composed of a mixture of coarse and fine noncohesive material; a bimodal distribution of particle sizes is thought to be necessary for large, ripplelike bed forms to develop. Here, we discuss the extension of a recent model for the dynamics of aeolian megaripples. The model assumes that both the saltation and reptation fluxes are modulated by the emergent bed forms. We present a linear stability analysis of the full model as well as the results of numerical simulations. The modeled megaripples evolve through interactions between ripples of different sizes. The formation of megaripples is therefore another example of a self-organizing geomorphological system.

Amplitude equation and long-range interactions in underwater sand ripples in one dimension

We present an amplitude equation for sand ripples under oscillatory flow in a situation where the sand is moving in a narrow channel and the height profile is practically one dimensional. The equation has the form ht = - epsilon(h-h)+((hx)2-1)hxx-hxxxx+delta((hx)2)xx which, due to the first term, is neither completely local (it has long-range coupling through the average height h) nor has local sand conservation. We argue that this is reasonable and show that the equation compares well with experimental observations in narrow channels. We focus in particular on the so-called doubling transition, a secondary instability caused by the sudden decrease in the amplitude of the water motion, leading to the appearance of a new ripple in each trough. This transition is well reproduced for sufficiently large delta (asymmetry between trough and crest). We finally present surprising experimental results showing that long-range coupling is indeed seen in the initial details of the doubling transition, where in fact two small ripples are initially formed, followed by global symmetry breaking removing one of them.

Statistics of synthetic aperture sonar image resolution degradation.

Synthetic aperture sonar (SAS) image statistics can often be characterized by a probability density function with a heavier tail than the expected Rayleigh distribution. The K‐distribution shape parameter can be used as a metric of non‐Rayleighness, with physical ties to both seafloor properties and sonar parameters. Recent results have shown that increasing the resolution cell size, or degrading the resolution of the image, produces images with statistics tending toward Rayleigh (i.e., a higher K‐distribution shape parameter). In general, a doubling of the resolution cell area results in a doubling of the K‐distribution shape parameter. A caveat to this generalization is the orientation of the sonar system to any features that may exist on the seafloor (i.e., sand ripples). In such a situation, image statistics may continue to be significantly non‐Rayleigh for certain orientations in spite of resolution degradation. SAS images of seafloors with various bottom types and feature orientations collected with an AUV by the Naval Surface Warfare Center Panama City Division have been postprocessed to analyze changes in image statistics as the image resolution is degraded. Results of the image degradation on statistics will be discussed. [Work supported by ONR Grant Nos. N00014‐04‐1‐0013 and 1N00014‐06‐1‐0245, and Code 32.]

Large-eddy simulation of formation of three-dimensional aeolian sand ripples in a turbulent field

With the method of large-eddy simulation, the equation of spherule motion and the method of immersed boundary condition, numerical simulations of three-dimensional turbulent aeolian motion and the formation of sand ripples under three-dimensional turbulent wind and the mutual actions of saltation and creeping motion were carried out. The resulting sand ripples have the form that is flat on the upwind side and steep on the leeward, which is identical to the sand ripples in nature. We also realized the self-restoration process of three-dimensional sand ripples, which shows the correctness of the method of numerical simulation and the models of saltation and creeping. Finally, We analyzed the influence of sand ripples on the three-dimensional turbulent wind field, and found that due to the appearance and development of sand ripples, in the normal direction of ground there exists stronger energy exchange, and moreover, there is close correspondence between the forms of sand ripples and the vorticity close to the ground surface.

Oscillation-induced sand ripples in a circular geometry

This study deals with the observation of sand ripples in a circular geometry under oscillatory flow. We characterize the observed patterns as a function of the excitation parameters. We report the time evolution of the corrugated front invading the flat bed. These experiments reveal unambiguously, because of the gradient of shear stress, the existence of two separated thresholds: one for grain motion and the other for the appearance of ripples. In addition, we display the phase diagram of this instability as a function of the Froude number and a Reynolds number.

Prediction of sand ripple geometry under waves using an artificial neural network

The length and height of a sand ripple in the seabed are the two basic parameters used to estimate the bottom shear stress and predict the transport of sand by wave action. These values are currently obtained with the help of many empirical equations. A different estimation method, in the form of an artificial neural network, is presented in this paper. The network is trained by measurements collected in the laboratory and in-situ under different forcing conditions. Validation of the present neural network results with different measurements shows that the new method can predict the ripple length and height much more accurately than the conventional empirical equations.

Wind‐driven particle mobility on Mars: Insights from Mars Exploration Rover observations at “El Dorado” and surroundings at Gusev Crater

The ripple field known as “El Dorado” was a unique stop on Spirit's traverse where dust‐raising, active mafic sand ripples and larger inactive coarse‐grained ripples interact, illuminating several long‐standing issues of Martian dust mobility, sand mobility, and the origin of transverse aeolian ridges. Strong regional wind events endured by Spirit caused perceptible migration of ripple crests in deposits SSE of El Dorado, erasure of tracks in sandy areas, and changes to dust mantling the site. Localized thermal vortices swept across El Dorado, leaving paths of reduced dust but without perceptibly damaging nearly cohesionless sandy ripple crests. From orbit, winds responsible for frequently raising clay‐sized dust into the atmosphere do not seem to significantly affect dunes composed of (more easily entrained) sand‐sized particles, a long‐standing paradox. This disparity between dust mobilization and sand mobilization on Mars is due largely to two factors: (1) dust occurs on the surface as fragile, low‐density, sand‐sized aggregates that are easily entrained and disrupted, compared with clay‐sized air fall particles; and (2) induration of regolith is pervasive. Light‐toned bed forms investigated at Gusev are coarse‐grained ripples, an interpretation we propose for many of the smallest linear, light‐toned bed forms of uncertain origin seen in high‐resolution orbital images across Mars. On Earth, wind can organize bimodal or poorly sorted loose sediment into coarse‐grained ripples. Coarse‐grained ripples could be relatively common on Mars because development of durable, well‐sorted sediments analogous to terrestrial aeolian quartz sand deposits is restricted by the lack of free quartz and limited hydraulic sediment processing.

Two Dimensional Wavelet Coefficient Statistics for Sea Bottom Classification

In this paper we examine the classification of different seafloors based on the analysis of images obtained by side-scan sonar. For this purpose, we apply various two dimensional multilevel wavelet decomposition schemes on images obtained from three different seafloor types, i.e., sand ripples, rocks and sands, and then we examine the statistics of the corresponding wavelet coefficients. The observed Probability Density Functions (pdf) are modeled using various theoretical distributions such as the Alpha Stable, Sum of Gaussians, Log-normal. The parameters of the fit are subsequently used to classify the side scan sonar images according to well known cluster analysis techniques. The use of the energy of the wavelet coefficients as a tool for side scan sonar image classification is also evaluated. A new unsupervised classification scheme based on the pdf fitting parameters is proposed.