Shape Of Dunes Research Articles

Preservation of cross‐strata due to the migration of subaqueous dunes: an experimental investigation

AbstractThis experimental investigation examined the controls on the geometry of cross‐sets formed by subaqueous dunes. A range of steady, unidirectional flow conditions spanning the field of dune existence was investigated, and aggradation rate ranged from 0 mm s−1 to 0·014 mm s−1. Data from an ultrasonic depth profiler consist of high‐resolution temporal and spatial series of bed profiles from which dune height and length, migration rate and the depth of trough scour were measured. Cross‐set thickness and length were measured from sediment peels. The size and shape of dunes from an equilibrium assemblage change continuously. Individual dunes commonly increase in height by trough scouring and, occasionally, by being caught‐up by the upstream dune. Both types of behaviour occur suddenly and irregularly in time and, hence, do not appear to depend on dunes further upstream. However, dune climbing or flattening is a typical response of dunes that disappear under the influence of the upstream dune. All types of behaviour occur at any flow velocity or aggradation rate. Successive dune‐trough trajectories, defined by dunes showing various behaviours, affect the geometry of the preserved cross‐sets. Mean cross‐set thickness/mean dune height averages 0·33 (±0·7), and mean cross‐set length/mean dune length averages 0·49 (±0·08), and both show no systematic variation with aggradation rate or flow velocity. Mean cross‐set thickness/mean cross‐set length tends to decrease with increasing flow velocity and Froude number, therefore allowing a qualitative estimation of flow conditions. Quantitative analysis of the temporal changes in the geometry and migration rate of individual dunes allows the development of a two‐dimensional stochastic model of dune migration and formation of cross‐sets. Computer realizations produced stacks of cross‐sets of comparable shape and thickness to laboratory flume observations, indicating a good empirical understanding of the variability of dune‐trough trajectories. However, interactions among dunes and aggradation rates of the order of 10−2 mm s−1 should be considered in future improved models.

Selection of dune shapes and velocities Part 1: Dynamics of sand, wind and barchans

Almost fifty years of investigations of barchan dunes morphology and dynamics is reviewed, with emphasis on the physical understanding of these objects. The characteristic quantities measured on the field (shape, size, velocity) and the physical problems they rise are presented. Then, we review the dynamical mechanisms explaining the formation and the propagation of dunes. In particular a complete and original approach of the sand transport over a flat sand bed is proposed and discussed. We conclude on open problems by outlining future research directions.

Selection of dune shapes and velocities Part 2: A two-dimensional modelling

We present in this paper a simplification of the dune model proposed by Sauermann et al. which keeps the basic mechanisms but allows analytical and parametric studies. Two kinds of purely propagative two dimensional solutions are exhibited: dunes and domes. The latter, by contrast to the former, do not present a slip face. Their shape and velocity can be predicted as a function of their size. We recover that dune profiles are not scale invariant (small dunes are flatter than the large ones), and that the inverse of the velocity grows almost linearly with the dune size. We furthermore get the existence of a critical mass below which no dune solution exists. It rises the problem of dune nucleation: how can dunes appear if any bump below this minimal mass gets eroded and disappears? The linear stability analysis of a flat sand bed shows that it is unstable at large wavelengths: dune can in fact nucleate from a small sand mass if the proto-dune is sufficiently long.

Estimating the windward slope profile of a barchan dune

AbstractA new theoretical scheme is presented to model the shape of a sand dune at equilibrium that does not require iterative calculations of the interaction between the wind flow and topography. The model is constructed by incorporating theory based on aerodynamics into a grain‐scale model that estimates the shear velocity at the dune crest through the calculation of the sand trapping efficiency of the slipface in the lee of the dune. Published field data, collected in southern Peru, California and southern Morocco, show that as a dune becomes higher the windward slope becomes steeper. For the model proposed, the wind flow over a dune was first assumed to be similar to that over a Gaussian hill. By further assuming a fixed shear velocity on the level surface, the windward slope angle and migration speed of dunes in southern Peru can be explained. To comply with all available data, some aspects are still open to further investigation. However the theoretical insight presented herein implies that the upper limit of dune height may be greater in windier environments.

Evolution and shapes of dunes

The motion of dunes and their morphology is a fascinating, largely unexplored subject. Already the barchan, the simplest moving dune, poses many questions. I will present some results of field-measurements on desert and coastal dunes. Then I will present a model which consists of three coupled equations of motion for the topography, the shear stress of the wind and the sand flux. These evolution equations are verified on the experimental data and new possibilities of simulations of dunes are put in perspective. To cite this article: H.J. Herrmann, C. R. Physique 3 (2002) 197–206.

Annual regime of bedforms, roughness and flow resistance, Lillooet River, British Columbia, BC

A field study to examine the statistical character of dune morphology and the correlation among dune morphology, discharge, and flow resistance was conducted in a meandering reach of Lillooet River, near Pemberton, British Columbia, Canada. The field season spanned the 1995/1996 hydrologic year with sample day discharge events ranging between 33 and 425 m 3 s −1. Surveys of the bed morphology along the thalweg in two dune fields (A and B) were completed using an echo sounder with chart recorder that enabled the measurement of more than 4000 dunes. Dune height ranged between 0.08 and 0.96 m, length between 2.01 and 20.99 m, and steepness between 0.02 and 0.10. Histograms of each dune shape (height, length, steepness) sample most often displayed positive skewness and non-Gaussian distributions (Gamma, Beta, and Weibull); median sample height and length histograms displayed positive skewness; and steepness was nearly Gaussian. Histograms of all dimensionless dunes (i.e., measurement divided by average measurement of sample) were Gaussian and slightly leptokurtic. Neither the height nor length of dunes measured in this investigation were successfully predicted by the empirical models of Allen [Allen, J.R.L., 1984. Developments in Sedimentology. Sedimentary Structures: Their Character and Physical Basis, 2nd edn. Elsevier, New York, vol. 30 (A and B), 1256 pp.], Fredsøe [J. Hydraul. Div., Am. Soc. Civ. Eng. 108(HY8) (1982) 932.], or Yalin [J. Hydraul. Div., Am. Soc. Civ. Eng. 90(HY5) (1964) 105.]. Least-squares regression models for dune–height relations produced here are similar to models published by other field researchers; regression models for dune length only conform to those developed elsewhere if the discharge of the study rivers was similar. The energy gradient in dune field A varied within a smaller range than in field B, enabling dune size to become more fully equilibrated with respect to flow environment. Although the average Froude numbers were much less than critical, dunes appeared to wash out towards a plane bed as discharge increased due to a change from a bedload to suspended-load dominated sediment-transport regime. Flow resistance increased most rapidly during changes in base flow and at the beginning and end of the seasonal flood; resistance tended to be smaller in field A than B, reflecting local differences in energy gradient. Flow resistance increased until a dune steepness of 0.070 was attained and then decreased. The steepness value was considered to be coincident with kolk generation [Dyer, K.R., 1986. Coastal and Estuarine Sediment Dynamics. Wiley, NY, 342 pp.], suggesting that macroturbulent flow structures play an important role in defining the roughness of dunes on a channel bed.

Saturation transients in saltation and their implications on dune shapes

We present a model for dune formation that takes saturation transients in the saltation layer into account. We show that these transients introduce a characteristic length scale that is crucial for dune formation and the macroscopic shape of dunes, albeit determined by intrinsic properties of the saltation kinetics. Finally, we demonstrate that our model can explain the scale dependence of the shape of barchan dunes revealed in recent field measurements.

BARCHAN DUNES ON THE KUISEB RIVER DELTA, NAMIBIA

ABSTRACT The shape and behaviour of the barchan dunes on the Kuiseb River delta, south of the town of Walvis Bay, and the temporal and spatial variability of those characteristics are the focus of this study. Data from a field survey undertaken in August 1999 are used to investigate a number of salient aspects of dune morphology. The Kuiseb dunes are found to exhibit many of the same morphometric characteristics as those found in other dune fields around the world, such as the strong interrelationship between dune height and horn width (r = 0.853). Abnormalities in the transverse and longitudinal profiles of the dunes are noted however, and the causative factors investigated. Grain size analysis provides an insight into the intra-dune processes that may cause horn asymmetry and the absence of a slip face. Aerial photographs taken in 1961, 1976, 1988 and 1997, in conjunction with the results of the August 1999 field survey, are used to calculate rates and directions of barchan dune movement for 58 dunes between 1961 and 1997, and for 10 dunes from 1961 through to 1999. Between 1961 and 1997 the dunes moved at an average rate of 13.15m a′1, in an approximately northwards direction ([xbar]= 0.67°N). Over this period there has however been a substantial fluctuation in the rate of movement, from a minimum mean value of 4.24 m a″′ to a maximum of 18.9m a″1. The direction of dune movement has also fluctuated considerably between mean values of 12.58°N and 338.2°N. In the past two years rates of movement appear to have been very high, at 56.1m a″1, with the dunes moving in an average direction of 355.1 °N, although the data available for this stage has considerable limitations. Composite meteorological records derived from the National Centre for Environmental Prediction (NCEP) climate model suggest that the probable cause of this fluctuation in rates of dune movement (1961–97) is a variability in the regional wind regime. Comparative plots of mean wind speed during the three subsections of this 36 year period—1961 to 1976, 1976 to 1988 and 1988 to 1997—show significant changes in conditions. The first and last periods are characterised by markedly stronger mean winds, which can explain the faster rates of dune movement. This change in mean wind speed is related to fluctuations in the strength of the South Atlantic anticyclone, also illustrated by the NCEP data.

Australian desert dunes: Wind rift or depositional origin?

New data on the source of sands found in desert alluvia and adjacent dunes have led to a revival of the suggestion that the sand ridges of the Australian dunefields, and specifically those of the Great Victoria and Simpson Deserts, are predominantly of erosional origin. In this paper, geological and morphological evidence are cited against this wind rift hypothesis. In the Great Victoria Desert several arguments put for a local derivation of dune sand are incompatible with regional and local geology. In the Simpson Desert, the distribution, shape and colour of dunes, and the topographic relation of dunes to substrates argue against an erosional origin. Long‐distance transport of sand from numerous local depocentres and under a bidirectional wind regime better accounts for the field evidence.

The shape of the barchan dunes of Southern Morocco

We present detailed shape measurements of several barchan dunes in southern Morocco, near Laâyoune. Using these data, we disprove the concept of shape invariance of barchan dunes of different sizes. Nevertheless, some parts of the barchan dune scale and we try to distinguish these from non-scaling ones. Furthermore, we point out the importance of the exact position of the brink for the overall dune shape. Finally, we propose a simple model for dune shape, using our data to determine the model parameters. Despite its simplicity, this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated. Typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described.

The shape of dunes

We present models to describe the dynamics of the surface of a dry granular bed, like a desert. These models, based on coupled differential equations can describe the formation and evolution of dunes. Field measurements have been performed to compare the numerical results. The concept of the Barchan dune as a soliton is disproved. We discuss in particular also the wind field and the microscopic coupling with the sand grains.

Simulation of the effect of wind speedup in the formation of transverse dune fields

A computer simulation model for transverse-dune-field dynamics, corresponding to a uni-directional wind regime, is developed. In a previous formulation, two distinct problems were found regarding the cross-sectional dune shape, namely the erosion in the lee of dunes and the steepness of the windward slopes. The first problem is solved by introducing no erosion in shadow zones. The second issue is overcome by introducing a wind speedup (shear velocity increase) factor, which can be accounted for by adding a term to the original transport length, which is proportional to the surface height. By incorporating these features we are able to model dunes whose individual shape and collective patterns are similar to those observed in nature. Moreover we show how the introduction of a non-linear shear-velocity-increase term leads to the reduction of dune height, and this may result in an equilibrium dune field configuration. This is thought to be because the non-linear increase of the transport length makes the sand trapping efficiency lower than unity, even for higher dunes, so that the incoming and the outgoing sand flux are in balance. To fully describe the inter-dune morphology more precise dynamics in the lee of the dune must be incorporated. Copyright © 2000 John Wiley & Sons, Ltd.

Shapes of granular surfaces

We present experimental and theoretical studies of the shape of heaps. In the simplest case one has a cone with an angle of repose. A simple 1d model is presented and solved analytically to obtain this angle from the properties of individual grains. This model is generalized to take into account segregation of different types of particles, in particular the spectacular stratification patterns. Using translational invariance one can calculate the deviations from a linear shape and obtains logarithmic tails which have also been evidenced experimentally either for heaps on a flat table or in a 2d silo. Experiments give in addition the dependence of the tails on the flux and energy of incoming grains and on the width of the silo. We will also discuss the rounding of the top of a heap and the shape of dunes, i.e. the effects of wind.

Biological control of avalanching and slope stability in the intertidal zone

Abstract Sediment structure and stability are important features of soils and sedimentary environments, and are of great importance for the maintenance of estuarine, and intertidal banks and channels. Avalanching and slope stability are critical factors controlling the shape of intertidal dunes. The role of biological activity in these processes, either in the form of vegetation and roots, animal burrows, algal mats or microbial glues, is now a forefront area of research. The objective of our research has been to quantify the effects of biological activity on the avalanching of intertidal sediments. Natural microbial and meiofaunal communities and particularly burrowing infauna such as Corophium volutator and Nereis diversicolor dramatically increase angles of avalanche and factors of safety. They do not significantly alter angles of repose. Factors of safety, duration of avalanches, and percentage increase in sediment volume, all increase with increasing angles of avalanche. Video analysis of the avalanching process shows that different slope failure mechanisms are associated with the different types of biological activity, and probably reflect specific chemical and mechanical stabilizing effects produced by the different types of biological activity. These latter include tubes, extracellular polymeric materials, and a small element of compaction. These mechanisms include rotational failure, block and wedge failure and translational failure. The significance of our results for the stability of slopes under field conditions is discussed, and it is concluded that field seeding experiments in the intertidal zone with the species that we have used would be very productive.

Turbulence structure over two‐dimensional bed forms: Implications for sediment transport

Turbulence measurements from the flow over two‐dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms. The spatial structure of the local transport rate determines the shape and stability of bed forms such as ripples and dunes, and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near‐bed flow field. The measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components. Unlike measurements in uniform boundary layers, these distributions and the higher moments of the velocity do not scale with the local shear velocity, indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux. This conclusion is supported by observations of sediment flux over a dune.

An empirical model of aeolian dune lee‐face airflow

ABSTRACTAirflow data, gathered over dunes ranging from 60‐m tall complex‐crescentic dunes to 2‐m tall simplecrescentic dunes, were used to develop an empirical model of dune lee‐face airflow for straight‐crested dunes. The nature of lee‐face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee‐face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low‐speed (0–3O% of crestal speed) back‐eddy flow, typically occur leeward of steep‐sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment.As each of the three types of lee‐face flow is defined by a range of wind speeds and directions, the nature of lee‐face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross‐strata.

Aeolian Dunes

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Aeolian dune deposits of windblown sands are important reservoirs in someparts of the world. The giant Groningen gas field in Holland and the Leman Bankgas field in the North Sea both contain Rotliegendes formation sandstones laiddown on land by winds. These types of deposits require large volumes of drysand and sufficient wind velocities to move the sand. Such conditions areusually found in desert areas (e.g., the Sahara Desert) and along coastalbeaches (e.g., Padre Island, TX). Crescent-shaped, or barchan, dunes are common(see Fig. 1). Other dune shapes are long, linear mounds, which were depositedeither transverse or longitudinal to the wind direction. The sands in the dunesare generally well sorted and range from fine to coarse sizes. However, thedeposit may also contain dry stream beds (wadis) or dry lakes where poorlysorted channels of sand and gravel are found or where thin layers of silts andclay were left. Dunes laid down on deserts may form regional sand sheetshundreds to more than 1,000 ft [300 m] thick covering hundreds of square miles. Those formed on coastal barrier islands may contain individual bars only a fewfeet thick and less than 100 ft [30 m] wide. Lateral and vertical continuity ofsands is generally good but is sometimes severely limited by postdepositionalcementation. Medium- to giant-scale cross-bedding with dips of 20 to 30 degrees[0.35 to 0.52 rad] is commonly observed (see Fig. 2). Horizontal laminae ofclay/silt deposits may be preserved as vertical barriers in some aeolianformations (see Fig. 3). Lateral communication may also be poor in somelow-permeability reservoirs, such as the Leman Bank, as evidenced by the muchlower permeabilities calculated from well tests than those from arithmeticaverages of core data. The geologic models for high- and low-quality aeoliandune reservoirs are shown in Figs. 4A and 4B, respectively. High-qualitydeposits have well-sorted to extremely well-sorted sands. They have widespreadlateral continuity with minor reductions in horizontal permeability resultingfrom cross-bedding. Vertical continuity may be good but occasionallyinterrupted by isolated, horizontal shale beds. Both continuity andpermeability can be poor in low-quality deposits. Development of production, and location and completion of wells in high-quality deposits, are relativelystraightforward. Because of the good continuity of sands, aquifer performanceand recoveries by natural waterdrive may be high. The lack of vertical barriersin high-quality deposits often requires that the wells be completed to minimizeconing of the invading fluid. In low-quality dune deposits where cementationand shale beds are frequent barriers, however, effective drainage of allregions and productivity can be severe problems. In these, an early integratedprogram of coring, logging, and well testing should be carried out indeveloping the field. Analysis of the data by teams of geophysicists, geologists, and engineers can improve understanding of reservoir continuity toaid in locating and completing additional wells and in evaluatingfluid-injection projects. After the field goes on production, surveillanceprojects. After the field goes on production, surveillance programs of periodicpressure surveys and well tests are especially programs of periodic pressuresurveys and well tests are especially important in providing data fordeveloping more-effective depletion plans for maintaining the desiredproduction rates and achieving the highest economical recovery. p. 11

Variations in wind velocity and sand transport on the windward flanks of desert sand dunes

ABSTRACTThe magnitudes of increases in wind velocity, or speed‐up factors, have been measured on the windward flanks of transverse and linear dunes of varying height. On transverse dunes, velocity speed‐up varied with dune shape and height. For linear dunes, speed‐up factors varied principally with wind direction relative to the dune, with dune shape and dune height. The main effect of velocity speed‐up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas. This is greatest for large dunes, with winds of moderate velocity blowing at a large angle to the dune. Changing ratios of base to crest sand‐transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase. On linear dunes, the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi‐directional wind regime.

Migration, modification and merging in aeolian systems and the significance of the depositional mechanisms in Permian and Triassic dune sands of Europe and North America

The main depositional mechanisms acting in aeolian systems can be assessed in two different and independent ways: first by the internal anatomical approach with analysis of the cross-stratification fabric, and second by the external morphological approach with study of shape and surface features of dunes. Anatomical investigations of various ancient aeolian systems in the Permian and Triassic of Europe and North America and an extensive literature review concerning both internal anatomy and external morphology of ancient and modern dunes reveal three main depositional mechanisms operating in aeolian systems: migration, modification and merging. Migration comprises downwind movement of isolated dunes or trains of aeolian bedforms under predominantly constant wind direction. The orientation of the interset surfaces partially depends on the angular relationship between the path of bedform climb and the depositional surface and also on subsidence. Simple systems are characterized by successive dunes of similar size, whereas compound systems originate by superimposition of smaller aeolian bedforms on larger dunes or draas. Compound migration gives rise to the hierarchy of diastems. Modification comprises overprinting of existing dunes by winds coming from directions other than those that govern migration of the original aeolian bedforms. The anatomical elements representing modification include both intrasets and extensive intersets, depending on duration and on the scale of the changes of the winds. Simple modification comprises occasional origin of either only cross-dipping erosional boundaries or both diastems and intraset laminae oriented in different directions. A special case of modification is the creation of intrasets dipping in the same direction as the foreset inclination, comprising either reactivation or relaxation surfaces, depending on whether the foreset slope is steepened or lowered in angle. Complex modification takes place under repeatedly alternating winds of changing effectiveness. Small-scale modification comprises minor alterations of existing dune types, and medium-scale modification is characterized by conversion of primary aeolian bedforms into secondary dune types. Small-scale and medium-scale modification take place during short-term to smaller-scale long-term changes of effective wind direction. Large-scale modification originates by large-scale long-term changes, accompanied by a certain subsidence, which result in burial of the older dunes by a younger train of aeolian bedforms within the same sand sea migrating obliquely to perpendicularly to the former wind direction. Very large-scale modification is induced by very large-scale long-term changes which result int deflationary erosion of older dune fields and subsequent superimposition of a new sand sea on top of the eroded remnants of the former erg. Merging comprises coalescing of bedforms with only negligible erosion during burial of the first bedform by subsequent dunes. Merging is induced mainly when migration of dunes is stopped by a morphological barrier or ceases as a consequence of weakening winds. Merging further occurs during coalescence of numerous isolated barchans in rows to parallel wavy barchanoid ridges as a consequence of increasing availability of sand. The main depositional mechanism in compound systems is migration of hierarchically superimposed bedforms with subordinate modification under predominantly unidirectional winds. In complex systems, migration, modification and merging take place in different amounts under unidirectional, bidirectional and multidirectional winds. Depending on direction and strength of both earlier and subsequent winds, complex systems are either charcterized by mainly migration with subordinate modification or by considerable modification with either conversion or dissection of older dunes. The effects of migration, modification and merging are discussed in the light of both internal anatomy of ancient cross-stratification fabrics and external morphology of recent dunes. A classification of dune types based on these mechanisms divides the aeolian bedforms into primary, secondary and tertiary types. Primary dunes originate by simple or compound migration with minor modification and comprise mainly transverse dunes. Secondary dunes originate by complex interaction of migration and modification or even predominantly by modification, with the best examples being longitudinal dunes. Tertiary dunes form by merging of transverse and longitudinal elements with varying amounts of migration and modification and are mainly represented by star dunes. The results of various anatomical case studies of ancient aeolianites indicate that simple migration was the dominant mechanism for construction of ergs in the northern hemisphere in Permian and Triassic times.

Discussion of “ Shape and Dimensions of Stationary Dunes in Rivers ” by Jørgen Fredsøe (August, 1982)

Discussion of “ <i>Shape and Dimensions of Stationary Dunes in Rivers</i> ” by Jørgen Fredsøe (August, 1982)