Sand Ripples Research Articles

Estimating Turbulence and Shear Stress under Regular Waves from ADV Data

The boundary layer flow determines the bottom shear stresses, which is key point for sediment transport and thereby the evolution of coastal morphology. The structure of the bottom boundary layer in coastal seas has been of interest to oceanographers for many years. In the paper Acoustic Doppler velocimeter (ADV) technique is applied to measure the bottom boundary layer under cnoidal waves in a laboratory flume with 40-m-long, 0.5-m-wide, and 0.8-m-deep.. Based on the high frequency turbulence signal collected, statistic parameters of cnoidal wave flow are calculated, compared and analyzed. The turbulent structure over plain bed and sand ripples bed are carefully studied. The turbulence intensity of near-bed velocities changes along depth of several phases in a period is analyzed. Turbulent Kinetic Energy Method (TKE Method) is used to estimate near-bed shear stress on flat and slope.

Experimental Study on Changes of Sloping Sandy Bed Profile under Breaking Waves

The wave breaking forces can exacerbate sediment transport, and lead to erosion of the seabed, coastal deformation and destruction of coastal structures. The experiment is carried out in a wave flume with a 1:30 sloping sandy seabed. A wide range of measurements from the regular wave runs are reported, including time series of wave heights, changes of bed profile. The video records are analysed to measure the time development of the seabed form and the characteristics of the orbital motion of the sand in the wave breaking region. The location and wave height at wave breaking point is measured by experiment. Formation and evolution of sand ripple and sand bar are studied under the breaking waves. It is found that effect of bed surface on wave breaking zone is more significant than wave non-breaking.

Extraction of morphometric bedform characteristics from profiling sonar datasets recorded in shallow coastal waters of China

The morphological characteristics of small-scale bedforms were measured by means of an acoustic profiling sonar on the Dafeng tidal flat, Jiangsu, in 2009, and in the Jiulong Estuary, Xiamen, in 2010, respectively. The “multi-threshold value” method was utilized to reveal the morphological undulations along which bedforms were present. Analyses of the datasets obtained show that: (1) sand ripples can have irregular shapes, and (2) changes in bedform morphology are small within a single tidal cycle but may be significant over several tidal cycles. Fractal and variogram analyses of the seabed roughness revealed the existence of a significant relationship between current speed and the fractal dimension of the seabed roughness. As current speed increases, seabed roughness increases with a trend of smaller-scale bottom structures being replaced by larger-scale structures. Furthermore, the surface of the larger-scale bottom structures can either become smooth due to the absence of smaller-scale features or become rougher due to the presence of superimposed smaller-scale structures.

Numerical simulation of saltating particles in atmospheric boundary layer over flat bed and sand ripples

In this work, we numerically simulated the saltating particles in a turbulent boundary layer over flat bed and sand ripples. By using natural sand grains in a wind tunnel, we obtained the initial conditions for the simulation and also verified the correctness of the numerical model. We carefully analyzed the numerically simulated saltating particle movement over the two sand beds, and we found the following. (1) The aeolian sand transport is a dynamic equilibrium process on both sand beds, and it took longer to reach equilibration on the sand ripples than on the flat bed. (2) According to the mass flux profile at the trough of the sand ripples, there is a maximum mass flux at about 4 cm height in the leeward section. The mass flux increases with height below 4 cm and decreases with height above 4 cm. (3) The wind profile near the surface is modified by saltating particles on the two different sand beds, and the flow field characteristics of the sand ripples are more complex than that of the flat bed.

Three-dimensional sand ripples as the product of vortex instability

Three-dimensional sand ripples can be observed under steady liquid flows in both nature and industry. Some examples are the ripples observed on the bed of rivers and in petroleum pipelines conveying sand. Although of importance, the formation of these patterns is not completely understood. There are theoretical and experimental evidence that aquatic ripples grow from two-dimensional bed instabilities, so that a straight vortex is formed just downstream of their crests. The proposition of Raudkivi (2006) [18], that three-dimensionality has its origin in a vortex instability, is employed here. This paper presents a linear stability analysis of the downstream vortex in order to obtain the transverse scales of three-dimensional ripples. The obtained wavelength is compared with experimentally observed ripples.

Genesis and morphology of cold-water coral ridges in a unidirectional current regime

Cold-water coral ridges with waveform morphology are commonly observed in mound surfaces in the Northeast Atlantic Ocean and interpreted as sediment waves overgrown by corals. This work documents the first cold-water coral ridges that are waveform but lack sand dune cores. These coral ridges are situated at the base of the Miami Terrace in the Straits of Florida (depths of 630 to 870m). They are entirely biogenic in origin and are unrelated to antecedent topography. Five lines of evidence, based on 27km2 of high-resolution geophysical, oceanographic, and ground-truthing data, support this interpretation. First, the waveform coral ridges are aligned perpendicular to a southward flowing bottom current and juxtaposed to an active sand dune field devoid of corals. Both the coral ridges and sand dunes are asymmetric in profile with respect to the current. However, the ridges have their steep sides facing the current, whereas the dunes exhibit a steep leeside. Second, the coral ridge matrix consists of very fine sand to mud particles with no bedload sand transport on the ridge flanks or troughs. In contrast, active sand ripples superimpose the juxtaposed sand dunes indicating that coarse sand is transported via traction in this domain. Third, bedload sand transport occurs in linear streaks at the shallowest part of the surveyed area. These streaks orthogonally crosscut some coral ridges, but coral colonies are not aligned along these streaks. Instead, the bedload sand smothers the coral colonies and thereby segments the ridges. Fourth, the sand dune and coral ridge acoustic responses are different. Below the dunes, there is no sub-bottom reflection due to energy attenuation. In contrast, a continuous reflection below the coral ridges proves the absence of attenuating sand cores. Fifth, this continuous sub-bottom reflection is not correlated with the coral ridge topography, indicating that antecedent topography does not control the waveform morphology and spatial distribution of the ridges. The differences in depositional profiles, sediment dynamics and features, as well as acoustic facies between the coral ridges and the juxtaposed dunes show that the coral ridges are entirely bioconstructional and not a veneer of corals covering paleo-sand dunes. This field of biogenic ridges at the Miami Terrace and the distribution of sand streaks as well as active sand dunes provide new insights how the lateral variability of current velocities and sediment transport conditions control the morphology, distribution and genesis of cold-water coral features.

Cross‐sectional profiles of sand ripples, megaripples, and dunes: a method for discriminating between formational mechanisms

ABSTRACTCross‐sectional profiles of sand ripples, megaripples, and sand dunes provide a useful tool for discriminating between formation by ripple and dune processes. Feature width, defined as the basal break in slope along the profile to either side of the crest, represents a good standard for comparison of profile attributes across more than three orders of magnitude. Aspect ratio (height/width) as a function of log width separates measurements into clusters representing differing mechanisms of formation. Scaling both height and distance for individual profiles by feature width facilitates comparison of profile shapes across three orders of magnitude in width. The data presented here should prove useful for evaluating possible mechanisms of origin for aeolian features observed remotely, including on planetary bodies. Copyright © 2012 John Wiley & Sons, Ltd.

An experimental study on erodibility of intertidal sediments in the Yellow River delta

The erodibility of intertidal sediments is an important factor affecting coastal erosion. In July and October 2008, in situ measurement of erodibility of the surficial sediment were conducted using a recirculating flume at 20 tidal flat experiment sites along the seashore of the Yellow River delta. At the same time, the characteristics of sand ripples and biogenic features on the tidal flat were observed and the physical-mechanical sediment properties such as bulk density, water content, grain size distribution, plasticity, penetration resistance, shear strength and compressibility, were measured. By field measurement, it is obtained that the critical erosion shear stress of the surficial sediment on the tidal flat varies between 0.088 Pa and 0.254 Pa. The factors influencing sediment erodibility are complicated because of physical and biological reworking after the sediment deposited. There's a positive correlation between shear strength and critical erosion shear stress. The burrowing crabs' activities changed the sediment microtopography and made the sediment have greater roughness, and that is one possible reason for the higher erodibility. The formation of scour pits on the tidal flat correlates with the heterogeneous erodibility of the surficial sediment.

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

Beneficial use of shell for constitutive materials of beach or tidal flat 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 settling 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.

Characterization of Radar Backscatter Response of Sand-Covered Surfaces at Millimeter-Wave Frequencies

Radar imaging of deserts suffers from insufficient radar backscatter at low microwave frequencies due to the low permittivity of dry sand and relatively smooth sand surface roughness. Operating at millimeter-wave (MMW) frequencies, however, rectifies this deficiency as significant radar backscatter is generated by surface and volume scattering. This is due to the fact that sand surface roughness is electrically large and signal penetration into the dry sand, which is a homogeneous mixture of air and sand particles with dimensions comparable to a fraction of a wavelength, generates considerable volume scattering. This paper investigates both surface and volume scattering from dry sand surfaces, subject to the peculiar physical properties of sand surfaces found in sand dune-covered regions. An incoherent model is proposed that characterizes the angular dependence of volume scattering from dry sand in the presence of a 1-D rippled air/sand surface. A set of indoor experiments conducted on smooth and 1-D rippled sand surfaces at Ka-band confirms that significant volume scattering is present at MMW frequencies and that the proposed model correctly captures the observed angular dependence when 1-D surface ripples are present.

Earth-like sand fluxes on Mars

Strong and sustained winds on Mars have been considered rare, on the basis of surface meteorology measurements and global circulation models, raising the question of whether the abundant dunes and evidence for wind erosion seen on the planet are a current process. Recent studies showed sand activity, but could not determine whether entire dunes were moving--implying large sand fluxes--or whether more localized and surficial changes had occurred. Here we present measurements of the migration rate of sand ripples and dune lee fronts at the Nili Patera dune field. We show that the dunes are near steady state, with their entire volumes composed of mobile sand. The dunes have unexpectedly high sand fluxes, similar, for example, to those in Victoria Valley, Antarctica, implying that rates of landscape modification on Mars and Earth are similar.

Martian sand blowing in the wind

High-resolution spacecraft images show surprisingly large rates of sand transport on Mars. This finding suggests that the planet's surface is a more active environment than previously thought. See Letter p.339 There is clear evidence for wind erosion and dune formation on the surface of Mars, but the extent of these movements and whether the processes are current have remained unclear. Analysing data from the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter, Bridges et al. measure the migration of sand ripples across the Nili Patera dune field in the Martian southern hemisphere. They find that the dunes have surprisingly large sand fluxes, equivalent to those seen in extreme desert conditions on Earth. One view of Mars has been that conditions since the end of the Hesperian period (between 1.8 billion and 3.5 billion years ago) have been fairly static, with very low erosion rates, but these findings suggest that in parts of Mars at least, there are strong gusts of sand and wind still making themselves felt.

Transverse instability of megaripples

As a result of their inherent differences in stability, sand ripples and megaripples exhibit variations in terms of their wavelengths and grain-size distributions (unimodal for sand ripples and bimodal for megaripples). While sand ripples form almost straight lines, megaripples have greater sinuosity due to their transverse instability, a property that causes small megaripple undulations to grow with time. The origin of the instability is due to variations in megaripple height, variations that do not diminish over time, and due to the inverse dependence of ripple drift velocity on the height. Thus, the taller regions of ripples will move more slowly than the adjacent, shorter portions, an outcome that promotes further perturbation growth. We show an example based on fi eld work of the transverse instability of megaripples. The instability growth rate depends on the difference between the heights of the different segments of the megaripple. In contrast to the underlying instability of megaripples, normal sand ripples are essentially stable and are not affected by transverse perturbations, instead reacting quickly to the wind, which tends to smooth ripple height irregularities. The transverse instability of megaripples derives from the composition of their crests, which comprise coarse particles that allow initial perturbations in ripple height to grow further. The results suggest a physical mechanism for the transverse instability of megaripples and new insight into the spatial patterns of sand ripples.

Study of Scour around Submarine Pipeline with a Rubber Plate or Rigid Spoiler in Wave Conditions

This paper presents results from laboratory experimental model studies investigating scour by waves around a pipeline attached to a flexible rubber plate or rigid spoiler. The rubber plate was placed between the submarine pipeline and bed, while the rigid spoiler is attached to the top of the pipe. The scour around pipelines with and without a rubber plate or a rigid spoiler under regular and irregular waves was observed and measured for a range of pipe sizes, wave amplitudes and frequencies, and the length of the plate/height of the spoiler. The experiments reveal that although the rigid spoilers can enhance the scour depth and extent (thus accelerating the self-burial of the pipe), they also have significant influence on both the upstream and downstream bed topography as sand ripples and dunes are formed. On the other hand, the rubber plates cannot only significantly increase the scour depth but also have little effect on the upstream and downstream bed. The experiments show that when the length of the plate is about 1.5 times the pipe size, it provides the optimum performance in terms of the largest scour depth while restricting the impact on nearby beds for the parameters investigated in this study.

On the vertical and temporal structure of flow and stress within the turbulent oscillatory boundary layer above evolving sand ripples

The vertical structure of flow and stress within turbulent oscillatory boundary layers above evolving sand ripples is investigated in experiments carried out using a prototype wide-band coherent Doppler profiler in an oscillating boundary facility with beds of 0.216mm median diameter sand, 10s oscillation period, and 0.9m excursion (Shields parameter=0.11). The bed evolved from an initial nominally flat state through 5cm wavelength, nearly two-dimensional ripples to 12cm wavelength, three-dimensional ripples. Average ripple height increased from 0.2cm to 1.4cm, corresponding to roughness Reynolds numbers between 68 and 7700. Average ripple steepness increased from 0.03 to 0.15. Bed elevation spectra exhibit the shift towards lower spatial frequencies observed by Davis et al. (2004) and, at higher spatial frequencies, a saturation range analogous to that in surface gravity wave spectra. Horizontal velocity profiles exhibit the phase lead and overshoot expected for oscillatory boundary layer flow. Bottom stress estimates are obtained from the acceleration defect, the Reynolds stress and the law-of-the-wall. The defect stress estimates are bounded above and below by the Reynolds stress and the law-of-the-wall estimates, respectively. The values of the bottom friction coefficient and hydraulic roughness from the defect stress estimates are consistent with results from previous work on equilibrium orbital-scale ripples, as summarized by Nielsen (1992), indicating that ripple evolution was quasi-steady.

Prediction of time-evolving sand ripples in shelf seas

The ability to predict the presence and physical properties of small-scale sand ripples on the sea bed is important for determining the bed roughness felt by currents and waves, for sediment transport applications, and for acoustic applications. A fully time-evolving model is presented for predicting the height, length and orientation of sand ripples generated by currents, waves or both, which includes processes dealing with threshold of motion, ripple wash-out and biological degradation. We believe that this is currently the only predictor to feature all these properties, and is thus more widely applicable to shelf seas than other more sophisticated, but less comprehensive, methods. Tests against a large data-set of observations using an Acoustic Ripple Profiler at a site on the east coast of England, where ripples are generated by both tidal currents and waves, showed that the predictor captures most of the main features observed. A modified criterion for the wave/current dominance in the prediction equations improved some aspects of the agreement. The properties of current-generated ripples are predicted accurately, but the wavelength, and to a lesser extent the height, of wave-generated ripples show some under-prediction. Ripple orientations are modelled in an approximate way, but the main features of the observed orientations are captured. The bio-degradation feature of the predictor could not be tested with this data-set.

Instability Theory of Sand Ripples Formed by Turbulent Shear Flows

AbstractA theory of turbulent shear flow over a sand bed is developed, addressing the instability principle of the fluid-granular bed interface leading to the formation of ripples. The Reynolds-averaged Navier-Stokes (RANS) equations and the time-averaged continuity equation are analyzed using a 1/7-power law of the time-averaged streamwise velocity and treating the curvilinear streamlines by the Boussinesq approximation. The integration of the RANS equations leads to a governing dynamical equation of flow over a mobile bed. A near-bed flow layer of 3.5 times the ripple height is considered being affected by the ripples. The dynamical equation of the mobile sand bed is based on the Exner’s sediment continuity equation in conjunction with the Meyer-Peter and Muller bed-load transport formula as modified to account for the effect of local bed slope attributable to bed forms. The coupled dynamical equations are then analyzed to estimate the parameters for the instability that results in the formation of ripp...

The nonlinear theory for sediment ripple dynamic process of straight river

There are various sand ripples in the natural world. The viewpoint of Yalin is that local disturbances result in laminar instability and in sand-ripple formation, namely, local disturbance→the instability of the laminar flow→the formation of sand ripples. Based on this viewpoint, a theoretical model of the resonant triad interaction and its nonlinear interaction with the sediment is established. The purpose of this model is to explain the formation and evolution of the sand-ripple and allow for analysis of the instability of open-channel flow caused by it and sand-ripple hydro-dynamic process. This model will not only pave a road to explore the mechanism of interaction between bed-form and turbulence, but also provide a good base for the study of aeolian sand-ripple formation.

Fractal dimension, wavelet shrinkage and anomaly detection for mine hunting

An anomaly detection approach is considered for the mine hunting in sonar imagery problem. The authors exploit previous work that used dual-tree wavelets and fractal dimension to adaptively suppress sand ripples and a matched filter as an initial detector. Here, lacunarity inspired features are extracted from the remaining false positives, again using dual-tree wavelets. A one-class support vector machine is then used to learn a decision boundary, based only on these false positives. The approach exploits the large quantities of ‘normal’ natural background data available but avoids the difficult requirement of collecting examples of targets in order to train a classifier.

Real-Time Simulation of Aeolian Sand Movement and Sand Ripple Evolution: A Method Based on the Physics of Blown Sand

Simulation and visualization of aeolian sand movement and sand ripple evolution are a challenging subject. In this paper, we propose a physically based modeling and simulating method that can be used to synthesize sandy terrain in various patterns. Our method is based on the mechanical behavior of individual sand grains, which are widely studied in the physics of blown sand. We accounted significant mechanisms of sand transportation into the sand model, such as saltation, successive saltation and collapsing, while simplified the vegetation model and wind field model to make the simulation feasible and affordable. We implemented the proposed method on the programming graphics processing unit (GPU) to get real-time simulation and rendering. Finally, we proved that our method can reflect many characteristics of sand ripple evolution through several demonstrations. We also gave several synthesized desert scenes made from the simulated height field to display its significance on application.