The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Gary Kocurek - One of the best experts on this subject based on the ideXlab platform.
-
aeolian Dune accommodation space for holocene wadi channel avulsion strata wahiba Dune field oman
Sedimentary Geology, 2020Co-Authors: Gary Kocurek, Robin Westerman, Caroline Hern, Dominic Tatum, H M Rajapara, A K SinghviAbstract:Abstract Geomorphic evolution of the Wahiba Dune Field, Oman, during the Quaternary has occurred within a set of boundary conditions that include climatic forcing of fluvial, aeolian and eustatic cycles within an active tectonic basin. Because of basin down-warping and sediment transport into the basin, evolution of the geomorphic surface has been accompanied by the generation of a distinctive stratigraphic record. The coupled geomorphic and stratigraphic record of the northeastern portion of the Dune field illustrates wadi-aeolian interactions, in which a channel avulsion, likely initiated during a flood, scoured through the interDune corridor between linear Dunes. InterDune outcrops (7 m thick) consist of a lower interval interpreted as deposited by ephemeral fluvial flow, but an upper interval consists of six fining-upward units, each of which is interpreted to represent a flood event that culminated in ponding followed by desiccation. Luminescence dating indicates that the channel remained open for 2–3 ka during the Holocene, but ground-penetrating radar imaging shows that Dunes encroached into the channel between floods and suggests that the transition from ephemeral flow to ponding resulted from Dune damming. Maximum channel width and length are unknown, but width was greater than the current interDune area, and a speculative extended channel course is identified. Subsequently, interDune strata and linear Dunes were buried by crescentic Dunes sourced by an influx of sand with wadi affinity. The resultant complex stratigraphic architecture illustrates the role of existing surface topography in providing local geomorphic accommodation space for short-lived, concentrated patterns of sedimentation.
-
Dune deformation in a multi directional wind regime white sands Dune field new mexico
Earth Surface Processes and Landforms, 2015Co-Authors: Anine Pedersen, Gary Kocurek, David Mohrig, Virginia SmithAbstract:As with most Dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic Dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR-derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand-transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of Dune brinklines, overall Dune behavior consists of crest-normal migration to the NE, but also along-crest migration of Dune sinuosity and stoss superimposed Dunes to the SE. The SW winds are transverse to Dune orientations and cause most forward migration. The NNW winds cause along-crest migration of Dune sinuosity and stoss bedforms, as well as SE migration of NE-trending Dune terminations. The SSE winds cause ephemeral Dune deformation, especially crestal slipface reversals. The Dunes deform with migration because of differences in Dune-segment size, and differences in the lee-face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms Dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence-angle control on Dune deformation and lee-face stratification types allows for an idealized model for White Sands Dunes. Copyright © 2015 John Wiley & Sons, Ltd.
-
Dune deformation in a multi‐directional wind regime: White Sands Dune Field, New Mexico
Earth Surface Processes and Landforms, 2015Co-Authors: Anine Pedersen, Gary Kocurek, David Mohrig, Virginia SmithAbstract:As with most Dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic Dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR-derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand-transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of Dune brinklines, overall Dune behavior consists of crest-normal migration to the NE, but also along-crest migration of Dune sinuosity and stoss superimposed Dunes to the SE. The SW winds are transverse to Dune orientations and cause most forward migration. The NNW winds cause along-crest migration of Dune sinuosity and stoss bedforms, as well as SE migration of NE-trending Dune terminations. The SSE winds cause ephemeral Dune deformation, especially crestal slipface reversals. The Dunes deform with migration because of differences in Dune-segment size, and differences in the lee-face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms Dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence-angle control on Dune deformation and lee-face stratification types allows for an idealized model for White Sands Dunes. Copyright © 2015 John Wiley & Sons, Ltd.
-
aeolian Dune interactions and Dune field pattern formation white sands Dune field new mexico
Sedimentology, 2010Co-Authors: Ryan C. Ewing, Gary KocurekAbstract:Pattern formation is a fundamental aspect of self-organization in fields of bedforms. Time-series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian Dunes at White Sands, New Mexico, interact and the Dune pattern organizes in systematically similar ways as wind ripples and subaqueous Dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off-centre collision; (vi) defect creation; and (vii) Dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off-centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of Dunes, crest length, defect density, crest spacing and Dune height), Dune migration rates, and the type and frequency of Dune interactions within a 3500 m box transect from the upwind margin to the core of the Dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo-lake topography. Pattern evolution is most strongly coupled to the pattern parameters of Dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the Dunes become further apart and have fewer defects.
-
Aeolian Dune interactions and Dune‐field pattern formation: White Sands Dune Field, New Mexico
Sedimentology, 2010Co-Authors: Ryan C. Ewing, Gary KocurekAbstract:Pattern formation is a fundamental aspect of self-organization in fields of bedforms. Time-series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian Dunes at White Sands, New Mexico, interact and the Dune pattern organizes in systematically similar ways as wind ripples and subaqueous Dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off-centre collision; (vi) defect creation; and (vii) Dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off-centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of Dunes, crest length, defect density, crest spacing and Dune height), Dune migration rates, and the type and frequency of Dune interactions within a 3500 m box transect from the upwind margin to the core of the Dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo-lake topography. Pattern evolution is most strongly coupled to the pattern parameters of Dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the Dunes become further apart and have fewer defects.
Suleyman Naqshband - One of the best experts on this subject based on the ideXlab platform.
-
Modeling river Dune development and Dune transition to upper stage plane bed
Earth Surface Processes and Landforms, 2015Co-Authors: Suleyman Naqshband, Olav Van Duin, Jan S. Ribberink, Suzanne J.m.h. HulscherAbstract:Large asymmetric bedforms known as Dunes commonly dominate the bed of sand rivers. Due to the turbulence generation over their stoss and lee sides, Dunes are of central importance in predicting hydraulic roughness and water levels. During floods in steep alluvial rivers, Dunes are observed to grow rapidly as flow strength increases, undergoing an unstable transition regime, after which they are washed out in what is called upper stage plane bed. This transition of Dunes to upper stage plane bed is associated with high transport of bed sediment in suspension and large decrease in bedform roughness. In the present study, we aim to improve the prediction of Dune development and Dune transition to upper stage plane bed by introducing the transport of suspended sediment in an existing Dune evolution model. In addition, flume experiments are carried out to investigate Dune development under bed load and suspended load dominated transport regimes, and to get insight in the time scales related to the transition of Dunes to upper stage plane bed. Simulations with the extended model including the transport of suspended sediment show significant improvement in the prediction of equilibrium Dune parameters (e.g. Dune height, Dune length, Dune steepness, Dune migration rate, Dune lee side slope) both under bed load dominant and suspended load dominant transport regimes. The chosen modeling approach also allows us to model the transition of Dunes to upper stage plane bed which was not possible with the original Dune evolution model. The extended model predicts change in the Dune shapes as was observed in the flume experiments with decreasing Dune heights and Dune lee slopes. Furthermore, the time scale of Dune transition to upper stage plane bed was quite well predicted by the extended model
-
Understanding river Dune splitting through flume experiments and analysis of a Dune evolution model
Earth Surface Processes and Landforms, 2014Co-Authors: Jord Jurriaan Warmink, Suleyman Naqshband, Andries Paarlberg, Catarine M. Dohmen-janssen, A.p.p. Termes, Jord Lansink, Olav Van Duin, Suzanne J.m.h. HulscherAbstract:Forecasts of water level during river floods require accurate predictions of the evolution of river Dune dimensions, because the hydraulic roughness of the main channel is largely determined by the bed morphology. River Dune dimensions are controlled by processes like merging and splitting of Dunes. Particularly the process of Dune splitting is still poorly understood and – as a result – not yet included in operational Dune evolution models. In the current paper, the process of Dune splitting is investigated by carrying out laboratory experiments and by means of a sensitivity analysis using a numerical Dune evolution model. In the numerical model, we introduced superimposed TRIAS ripples (i.e. triangular asymmetric stoss side-ripples) on the stoss sides of underlying Dunes as soon as these stoss sides exceed a certain critical length. Simulations with the model including Dune splitting showed that predictions of equilibrium Dune characteristics were significantly improved compared to the model without Dune splitting. As Dune splitting is implemented in a parameterized way, the computational cost remains low which means that Dune evolution can be calculated on the timescale of a flood wave. Subsequently, we used this model to study the mechanism of Dune splitting. Literature showed that the initiation of a strong flow separation zone behind a superimposed bedform is one of the main mechanisms behind Dune splitting. The flume experiments indicated that besides its height also the lee side slope of the superimposed bedform is an important factor to determine the strength of the flow separation zone and therefore is an important aspect in Dune splitting. The sensitivity analysis of the Dune evolution model showed that a minimum stoss side length was required to develop a strong flow separation zone
-
Morphodynamics of river Dunes Suspended sediment transport along mobile Dunes and Dune development towards upper stage plane bed
1Co-Authors: Suleyman NaqshbandAbstract:Dunes are the most common bedforms present in nearly all fluvial channels. Because of their dimensions, Dunes are of central importance in predicting flow roughness and water levels. During floods in several rivers, Dunes are observed to undergo an unstable transition regime after which they are washed out in what is called the upper stage plane bed. This morphological evolution of Dunes to upper stage plane bed is the strongest bedform adjustment during time-varying flows and is associated with a significant change in hydraulic roughness and water levels. This thesis aims to obtain a better understanding and quantitative data of the flow and sediment transport mechanisms controlling the Dune morphology and Dune transition to upper stage plane beds.
Suzanne J.m.h. Hulscher - One of the best experts on this subject based on the ideXlab platform.
-
Modeling river Dune development and Dune transition to upper stage plane bed
Earth Surface Processes and Landforms, 2015Co-Authors: Suleyman Naqshband, Olav Van Duin, Jan S. Ribberink, Suzanne J.m.h. HulscherAbstract:Large asymmetric bedforms known as Dunes commonly dominate the bed of sand rivers. Due to the turbulence generation over their stoss and lee sides, Dunes are of central importance in predicting hydraulic roughness and water levels. During floods in steep alluvial rivers, Dunes are observed to grow rapidly as flow strength increases, undergoing an unstable transition regime, after which they are washed out in what is called upper stage plane bed. This transition of Dunes to upper stage plane bed is associated with high transport of bed sediment in suspension and large decrease in bedform roughness. In the present study, we aim to improve the prediction of Dune development and Dune transition to upper stage plane bed by introducing the transport of suspended sediment in an existing Dune evolution model. In addition, flume experiments are carried out to investigate Dune development under bed load and suspended load dominated transport regimes, and to get insight in the time scales related to the transition of Dunes to upper stage plane bed. Simulations with the extended model including the transport of suspended sediment show significant improvement in the prediction of equilibrium Dune parameters (e.g. Dune height, Dune length, Dune steepness, Dune migration rate, Dune lee side slope) both under bed load dominant and suspended load dominant transport regimes. The chosen modeling approach also allows us to model the transition of Dunes to upper stage plane bed which was not possible with the original Dune evolution model. The extended model predicts change in the Dune shapes as was observed in the flume experiments with decreasing Dune heights and Dune lee slopes. Furthermore, the time scale of Dune transition to upper stage plane bed was quite well predicted by the extended model
-
River Dune predictions: Comparison between a parameterized Dune model and a cellular automation Dune model
2014Co-Authors: J.m. Seuren, O.j.m. Van Duin, Jord Jurriaan Warmink, M.a.f. Knaapen, Suzanne J.m.h. HulscherAbstract:River Dunes are of great importance for the determination of water levels, especially during flood events. They have a large influence on the hydraulic roughness and thereby on water levels. In addition, Dune formation could affect the navigability of rivers and propagation of Dunes could uncover pipelines or other constructions beneath the river bed. Because fast calculations are essential during an upcoming flood event, there is a need for fast model predictions. The focus of this research is on a parameterized Dune model and the cellular automaton Dune model (CA model) HR Wallingford is experimenting with. Both models are relatively fast in their calculations but have a fundamentally different approach to predict river Dunes. This research reveals the performance of these two models tested under various conditions. The main objective of this research is: “To compare the performance of the cellular automaton Dune model and the parameterized Dune model for the prediction of Dune dimensions, migration rates and sediment transport in equilibrium state, under flume conditions, similar to low-land river situations like the River Rhine (the Netherlands).” The first step in this research was the preparation of the CA model for comparison with the parameterized Dune model. A sensitivity analysis provided insight in the behaviour of the input parameters used to adjust the model: A length scale was added by assuming a fixed domain and defining the model parameters in a unit of distance instead of a number of cells. Sediment transport was determined by counting all moving slabs and used to implement a time scale. Finally, the input parameters of the model were linked to the flow characteristics. After these adjustments, the model was calibrated using the same data as used for the calibration of the parameterized Dune model. The second step was the comparison of the parameterized Dune model and the CA model using a data set containing sixteen experiments. Research has shown that the parameterized Dune model is reliable for prediction of Dune dimensions, although it seems limited to experiments with a slope between 11*10-4 and 22*10-4. The parameterized Dune model overestimates migration with approximately a factor of three. The CA model is tested for the first time in the way as presented in this thesis, by adding time and length scales to the model. Results seem promising and show predictions that are reasonable for five experiments; however in general the predictions are slightly underestimated. The CA model underestimates the migration with approximately a factor of three. In this research a non-dimensional CA model is made dimensional. The model has potential and recommended improvements are: a) linking the shear velocity to flow characteristics and b) adding an equilibrium state.
-
Understanding river Dune splitting through flume experiments and analysis of a Dune evolution model
Earth Surface Processes and Landforms, 2014Co-Authors: Jord Jurriaan Warmink, Suleyman Naqshband, Andries Paarlberg, Catarine M. Dohmen-janssen, A.p.p. Termes, Jord Lansink, Olav Van Duin, Suzanne J.m.h. HulscherAbstract:Forecasts of water level during river floods require accurate predictions of the evolution of river Dune dimensions, because the hydraulic roughness of the main channel is largely determined by the bed morphology. River Dune dimensions are controlled by processes like merging and splitting of Dunes. Particularly the process of Dune splitting is still poorly understood and – as a result – not yet included in operational Dune evolution models. In the current paper, the process of Dune splitting is investigated by carrying out laboratory experiments and by means of a sensitivity analysis using a numerical Dune evolution model. In the numerical model, we introduced superimposed TRIAS ripples (i.e. triangular asymmetric stoss side-ripples) on the stoss sides of underlying Dunes as soon as these stoss sides exceed a certain critical length. Simulations with the model including Dune splitting showed that predictions of equilibrium Dune characteristics were significantly improved compared to the model without Dune splitting. As Dune splitting is implemented in a parameterized way, the computational cost remains low which means that Dune evolution can be calculated on the timescale of a flood wave. Subsequently, we used this model to study the mechanism of Dune splitting. Literature showed that the initiation of a strong flow separation zone behind a superimposed bedform is one of the main mechanisms behind Dune splitting. The flume experiments indicated that besides its height also the lee side slope of the superimposed bedform is an important factor to determine the strength of the flow separation zone and therefore is an important aspect in Dune splitting. The sensitivity analysis of the Dune evolution model showed that a minimum stoss side length was required to develop a strong flow separation zone
-
Modelling river Dune splitting
2007Co-Authors: Catarine M. Dohmen-janssen, Andries Paarlberg, Jord Lansink, Suzanne J.m.h. HulscherAbstract:River Dunes are submerged bed forms on the bottom of alluvial channels. They form as a result of the complex interaction of water flow and sediment. River Dunes influence the water level, by creating additional bed roughness as a result of form drag and can form a threat to shipping activities. Therefore they deserve the attention of river basin managers. The relevance of physical modelling in contrast to the use of equilibrium predictors or empirical models lies in the applicability in extreme events. Equilibrium predictors do not consider the time component of Dune development. Hysteresis for Dune height is observed to be strong during a flood wave, which makes the use of equilibrium predictors inaccurate. Empirical modelling depends on calibration with data. The availability of data for extreme events is limited or absent, so the need for physical modelling is strong. The Dune Development (DuDe) model is developed to describe river Dune evolution by combining two-dimensional vertical (2DV) flow equations with a sediment transport formula using a parameterization of flow separation to avoid complex turbulence modeling inside the flow separation zone. Dune growth, migration and merging of river Dunes is described well by the DuDe model. Model simulations have shown that the model did not predict equilibrium dimensions. Dunes did not reach a finite length and height. Therefore, flume experiments have been conducted in Auckland (New Zealand) to observe the process of Dune development, especially regarding Dune splitting. Dune splitting in the form of initiation of superposed sand wavelets is seen as the mechanism to obtain equilibrium Dune dimensions. Superposed sand wavelets develop a flow separation zone and can decrease the migration rate of underlying Dunes and even cease migration when they scour the crest of the underlying Dune. Therefore the implementation of superposed sand wavelets is seen as the solution to improve the prediction of equilibrium Dune dimensions.
-
Modelling river Dune development
2005Co-Authors: Andries Paarlberg, H.j.t. Weerts, Catarine M. Dohmen-janssen, I.l Ritsema, Suzanne J.m.h. Hulscher, A.g. Van Os, A.p.p. TermesAbstract:Since river Dunes influence flow resistance, predictions of Dune dimensions are required to make accurate water level predictions. A model approach to simulate developing river Dunes is presented. The model is set-up to be appropriate, i.e. as simple as possible, but with sufficient accuracy for management purposes. Model results so far are promising. During floods, Dunes develop on the river bed as a result of the interaction between flow and sediment transport. In the River Rhine, Dunes can become 1-2 m in height and up to 50 m in length in a water depth of about 10 m. The dynamic behaviour of Dunes is not yet completely understood. In our model, we include the processes that we think are most important for Dune development, namely: the flow and sediment transport over Dunes, and the formation of a wake behind the Dunes.
Virginia Smith - One of the best experts on this subject based on the ideXlab platform.
-
Dune deformation in a multi directional wind regime white sands Dune field new mexico
Earth Surface Processes and Landforms, 2015Co-Authors: Anine Pedersen, Gary Kocurek, David Mohrig, Virginia SmithAbstract:As with most Dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic Dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR-derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand-transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of Dune brinklines, overall Dune behavior consists of crest-normal migration to the NE, but also along-crest migration of Dune sinuosity and stoss superimposed Dunes to the SE. The SW winds are transverse to Dune orientations and cause most forward migration. The NNW winds cause along-crest migration of Dune sinuosity and stoss bedforms, as well as SE migration of NE-trending Dune terminations. The SSE winds cause ephemeral Dune deformation, especially crestal slipface reversals. The Dunes deform with migration because of differences in Dune-segment size, and differences in the lee-face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms Dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence-angle control on Dune deformation and lee-face stratification types allows for an idealized model for White Sands Dunes. Copyright © 2015 John Wiley & Sons, Ltd.
-
Dune deformation in a multi‐directional wind regime: White Sands Dune Field, New Mexico
Earth Surface Processes and Landforms, 2015Co-Authors: Anine Pedersen, Gary Kocurek, David Mohrig, Virginia SmithAbstract:As with most Dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic Dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR-derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand-transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of Dune brinklines, overall Dune behavior consists of crest-normal migration to the NE, but also along-crest migration of Dune sinuosity and stoss superimposed Dunes to the SE. The SW winds are transverse to Dune orientations and cause most forward migration. The NNW winds cause along-crest migration of Dune sinuosity and stoss bedforms, as well as SE migration of NE-trending Dune terminations. The SSE winds cause ephemeral Dune deformation, especially crestal slipface reversals. The Dunes deform with migration because of differences in Dune-segment size, and differences in the lee-face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms Dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence-angle control on Dune deformation and lee-face stratification types allows for an idealized model for White Sands Dunes. Copyright © 2015 John Wiley & Sons, Ltd.
Hans J. Herrmann - One of the best experts on this subject based on the ideXlab platform.
-
A continuous model for sand Dunes: Review, new developments and application to barchan Dunes and barchan Dune fields
Earth Surface Processes and Landforms, 2010Co-Authors: Orencio Duran, Eric J R Parteli, Hans J. HerrmannAbstract:Basically, sand Dunes are patterns resulting from the coupling of hydrodynamic and sediment transport. Once grains move, they modify the surface topography which in turns modifies the flow. This important feedback mechanism lies at the core of continuous Dune modelling. Here we present an updated review of such a model for aeolian Dunes, including important modifications to improve its predicting power. For instance, we add a more realistic wind model and provide a self-consistent set of parameters independently validated. As an example, we are able to simulate realistic barchan Dunes, which are the basic solution of the model in the condition of unidirectional flow and scarce sediments. From the simulation, we extract new relations describing the morphology and dynamics of barchans that compare very well with existing field data. Next, we revisit the problem of the stability of barchan Dunes and argue that they are intrinsically unstable bed-forms. Finally, we perform more complex simulations: first, a barchan Dune under variable wind strength and, second, barchan Dune fields under different boundary conditions. The latter has important implications for the problem of the genesis of barchan Dunes. Copyright © 2010 John Wiley & Sons, Ltd.
-
Dune formation under bimodal winds
Proceedings of the National Academy of Sciences of the United States of America, 2009Co-Authors: Eric J R Parteli, Orencio Duran, Haim Tsoar, Veit Schwämmle, Hans J. HerrmannAbstract:The study of Dune morphology represents a valuable tool in the investigation of planetary wind systems—the primary factor controlling the Dune shape is the wind directionality. However, our understanding of Dune formation is still limited to the simplest situation of unidirectional winds: There is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions. Here we present the calculation of sand transport under bimodal winds using a Dune model that is extended to account for more than one wind direction. Our calculations show that Dunes align longitudinally to the resultant wind trend if the angle θw between the wind directions is larger than 90°. Under high sand availability, linear seif Dunes are obtained, the intriguing meandering shape of which is found to be controlled by the Dune height and by the time the wind lasts at each one of the two wind directions. Unusual Dune shapes including the “wedge Dunes” observed on Mars appear within a wide spectrum of bimodal Dune morphologies under low sand availability.
-
modelling the formation of residual Dune ridges behind barchan Dunes in north east brazil
Sedimentology, 2009Co-Authors: Noam Levin, Haim Tsoar, Hans J. Herrmann, Luis Parente Maia, Vanda ClaudinosalesAbstract:Residual Dune ridges are often formed by vegetation growing along a line some distance upwind of the lower stoss slope of migrating Dunes. This process is common in areas where vegetation germinates along the edge of the water during the rainy period when the water level is higher and interDune areas are flooded. The phenomenon occurs on a large scale in North-east Brazil, because of the rise and fall in groundwater level at the end of the rainy season. Each residual Dune ridge corresponds to the position of the Dune during the wet period in each year. Therefore, variations in the distance between these residual Dune ridges could be used potentially to monitor climatic fluctuations in rainfall and wind. To examine the potential use of these residual Dune ridges for the reconstruction of past climatic fluctuations, a model that simulates them under varying conditions of wind, rainfall and evaporation rates was formulated. The model was tested for sensitivity to climatic variability in North-east Brazil and validated against residual Dune ridge displacements as measured in the field and from high spatial resolution satellite images. Based on the results, it is concluded that residual Dune ridges may not form in North-east Brazil in years which are exceptionally
-
The Dune size distribution and scaling relations of barchan Dune fields
Granular Matter, 2008Co-Authors: Orencio Duran, Veit Schwämmle, Pedro G. Lind, Hans J. HerrmannAbstract:Barchan Dunes emerge as a collective phenomena involving the generation of thousands of them in so called barchan Dune fields. By measuring the size and position of Dunes in Moroccan barchan Dune fields, we find that these Dunes tend to distribute uniformly in space and follow an unique size distribution function. We introduce an analytical mean-field approach to show that this empirical size distribution emerges from the interplay of Dune collisions and sand flux balance, the two simplest mechanisms for size selection. The analytical model also predicts a scaling relation between the fundamental macroscopic properties characterizing a Dune field, namely the inter-Dune spacing and the first and second moments of the Dune size distribution.
-
Dune whitening and inter-Dune freshwater ponds in NE Brazil
CATENA, 2007Co-Authors: Noam Levin, Haim Tsoar, Luis Parente Maia, Vanda Claudino Sales, Hans J. HerrmannAbstract:The changes in color of sand Dunes from white to yellow or red is often interpreted to signify their age or their source materials. In this study we demonstrate the effect seasonal inter-Dune freshwater ponds have on the bleaching of the color of sand Dunes by iron reduction in the anaerobic conditions they create. By combining spectral measurements of field samples of Dune sand together with the analysis of satellite images covering three Dune fields in NE Brazil (Lencois Maranhenses, Jericoacoara and Canoa Quebrada) we demonstrate its existence in the field. In areas where ponds cover 41% of the Dune field (as in Lencois Maranhenses) an almost total bleaching in the color of the sand occurs after which the Dunes remain white with no relation to distance from the coastline. In areas with less ponds (e.g. Jericoacoara or Canoa Quebrada) Dune whitening is less active, and there are areas where Dune rubification occurs. As this process may have occurred in other Dune fields during past climate conditions that may have been different from current ones, interpretations of Dunes age based on their color should be handled with care.
