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Emergence and growth of sand dunes results from the dynamic interaction between topography, wind ... more Emergence and growth of sand dunes results from the dynamic interaction between topography, wind flow and sediment transport. While feedbacks between these variables are well studied at the scale of a single and relatively small dune, the average effect of a periodic large-scale dune pattern on atmospheric flows remains poorly constrained, due to a pressing lack of data in major sand seas. Here, we compare local measurements of surface winds to the predictions of the ERA5-Land climate reanalysis at four locations in Namibia, both within and outside the giant linear dune field of the Namib Sand Sea. In the desert plains to the north of the sand sea, observations and predictions agree well. This is also the case in the interdune areas of the sand
Emergence des dunes sous des régimes de vents multidirectionnels
Les dunes éoliennes résultent de l'entraînement et du dépôt des grains de sable transportés p... more Les dunes éoliennes résultent de l'entraînement et du dépôt des grains de sable transportés par le vent. Cet écoulement s'adapte à son tour à la forme du lit sédimentaire. Ce couplage entre topographie, hydrodynamique et transport contrôle la taille, orientation et morphologie des dunes, et ce particulièrement durant la phase initiale de leur croissance. Dans cette thèse, nous mettons en évidence l'influence de certains paramètres environnementaux, tel que le régime de vents, la disponibilité en sable et les conditions aux limites du champ de dunes sur la formation des dunes. Pour cela, nous utilisons une approche théorique, couplée à des expériences de laboratoire, des simulations numériques et des études de terrain.Tout d'abord, nous validons la description de l'émergence des dunes dans les zones de fortes disponibilités sédimentaire sous forme d'instabilité linéaire. Ces dunes prennent la forme d'ondes de surface, dont l'amplitude augmente temporel...
Dune emergence in multidirectional wind regimes
Université de Paris, Oct 15, 2020
Morpho-sedimentary outlines displayed in Figures 1, S1, and S6-S15 of the article "Source-to-sink aeolian fluxes from arid landscape dynamics in the Lut Desert
Morpho-sedimentary outlines of the aeolian landforms in the Lut Desert.
Data for Table S10 of article "Source-to-sink aeolian fluxes from deserts landscape dynamics
Data for Table S10 of article "Source-to-sink aeolian fluxes from deserts landscape dynamics... more Data for Table S10 of article "Source-to-sink aeolian fluxes from deserts landscape dynamics": Exhaustive list of the 227 denudation rates in arid area compiled to estimate the median denudation rate and sediment discharge associated with the internal river system of the Lut watershed in Iran.
Coexistence of two dune growth mechanisms in a landscape-scale experiment
<p>Dune fields are recognized both by the occurrence of periodic bedforms a... more <p>Dune fields are recognized both by the occurrence of periodic bedforms and isolated dunes of different shapes and orientations. Nevertheless, there are still no field examples of whether this apparent duality results from synchronous dune growth, and on what timescales. Here, by leveling neighboring parcels of a dune field, we develop landscape-scale experiments with controlled initial and boundary conditions to test the influence of sand availability on dune formation. Starting from a flat sand bed, we observe the emergence of periodic dunes and measure for more than 3 years how they grow as they interact with each other. Over the same time period, by regularly feeding sand heaps deposited nearby on a non-erodible bed. we observe how dune shape changes, eventually leading to the elongation of isolated dunes with a different orientation. These experiments are unique by their size and duration. Under natural conditions, they show that the same wind regime can be associated with two dune growth mechanisms according to sand availability. The coexistence of these two dune growth mechanisms provides a basis for examining the diversity of dune shapes on Earth or other planetary bodies depending on local climatic conditions.</p>
Dune linear stability analysis under multidirectional wind regimes
EGU General Assembly Conference Abstracts, Apr 1, 2018
Aeolian fluxes from arid landscape dynamics in the Lut Desert (Iran)
<p>Wind-blown sand and dust emissions shape singular landscapes in ... more <p>Wind-blown sand and dust emissions shape singular landscapes in arid environments and globally impact climate, life and human activities. However, the accurate quantification of aeolian sediment fluxes are still subject to considerable uncertainties. Since extensive measurements are difficult to implement in the field, this quantification rely essentially on remote sensing data and transport laws that integrate a large number of parameters for the airflow and granular bed. However, confronted with all the sources of natural variability (wind regime, air recirculation, grain-size distribution, soil composition, <em>etc.</em>), a complete mass balance of aeolian transport remains challenging. Here we consider long time scales to smooth out such variability and integrate arid landscape dynamics into the source-to-sink assessment of aeolian mass transfers in the Lut Desert (Iran). Taking advantage of new remote sensing imagery and dating techniques, together with more accurate wind data and a deeper understanding of dune dynamics, we analyze major landforms of this desert to provide a comprehensive picture of aeolian transport on time scales from decades to millions of years. We map the modern sandflows, along which we evaluate the volume and chronology associated with the excavation of mega-yardangs upwind and the formation of giant dunes downwind. Sediment discharges deduced from long-term erosion and deposition are of the same order of magnitude <span>(10</span><sup><span>5</span></sup><span> to 10<sup>6</sup> m<sup>3</sup>yr<sup>-1</sup>) </span> as short and medium-term sand discharges derived from wind data and dune morphodynamics. At the scale of the internal aeolian sediment-routing system of the Lut, we establish an overall sediment budget constrained by the joint development of the erosional and depositional landforms. Our findings thus quantify the geomorphic controls of aeolian processes on arid landscapes at multiple length and time scales, while providing information on mass exchanges between continents and atmosphere.</p>
Spatial and temporal development of the dune instability
<p>Wind-blown sand dunes emerge due the linear instability of a flat sedime... more <p>Wind-blown sand dunes emerge due the linear instability of a flat sedimentary bed. This instability has been studied in experiments and numerical models but rarely in the field, because of the large time and length scales involved. Here we examine dune formation at the upwind margin of the White Sands Dune Field in New Mexico (USA), using 4 years of lidar topographic data to follow the spatial and temporal development of incipient dunes. Data quantify dune wavelength, growth rate, and propagation velocity and also the characteristic length scale associated with the growth process. We show that all these measurements are in quantitative agreement with predictions from linear stability analysis. This validation makes it possible to use the theory to reliably interpret dune-pattern characteristics and provide quantitative constraints on associated wind regimes and sediment properties, where direct local measurements are not available or feasible.</p><p><strong>Reference</strong>: Gadal et al., Geophys. Res. Lett. <strong>47</strong>, e2020GL088919 (2020).</p>
Source-to-sink aeolian landscape dynamics in the Lut Desert (Iran)
<div> <div> <div> <d... more <div> <div> <div> <div role="tabpanel"> <div> <div data-page-number="1" aria-label="Page 1" data-loaded="true">Over the last two decades, source-to-sink studies have provided a wealth of information on fluvial-dominated landscapes and their response to tectonic, climatic and biologic forcings. This approach is now expanding for a variety of morpho-sedimentary systems in glacial, submarine and aeolian environments, not only on Earth but also on other planetary bodies. However, works dedicated to aeolian-dominated landscapes often remain qualitative or limited to a single component of the sediment budget such as erosion or accumulation, dust or sand. Hence, the potential of source-to-sink methods is still to be exploited to bring new quantitative information on aeolian sediment-routing systems and associated landforms. The Lut Desert in Iran is nested in an endorheic basin which provides an appropriate context to develop such an aeolian source-to-sink approach. Thanks to remote sensing data and new cosmogenic dating, together with higher resolution wind data and a modern understanding of dune dynamics, we analyze the aeolian transport properties from closed depressions and mega-yardangs upwind to dune fields downwind over decades to millions of years. These erosional and depositional Quaternary landforms cover areas that geographically coincide perfectly with the present-day geometry of the aeolian sediment-routing system. Sandflows derived from modern wind data are sufficient to explain the exchange of mass from the aeolian depressions to the dune fields, providing a coherent scenario for the long-term spatial organization and temporal evolution of these features. In addition, bedform alignments predicted from the wind data are in agreement with the observed dune orientations, which suggests a stability of wind regimes and transport properties over the intermediate time scales from centuries to millennia associated with dune growth and migration. Estimates of the sand discharges associated with the developments of the erosional and depositional landforms show that only a fraction of the wind-blown sediments has accumulated in dune fields since the onset of aeolian erosion. As there is no evidence of sand evacuation through the mountain ranges surrounding the desert, the difference likely corresponds to the emission of dust into the atmosphere. Accordingly, the Lut Desert is not only an internal aeolian routing system for sand, it is also a major source of atmospheric dust leading to an overall loss of mass at the scale of the endorheic basin. Performed at the scale of a whole desert, this sediment budget reveals the full potential of source-to-sink methods to document how aeolian processes drive landscape dynamics and closely link the evolution of continental surfaces to atmospheric circulations.</div> </div> </div> </div> </div> </div><p> </p>
Source‐To‐Sink Aeolian Fluxes From Arid Landscape Dynamics in the Lut Desert
In zones of loose sand, wind-blown sand dunes emerge due the linear instability of a flat sedimen... more In zones of loose sand, wind-blown sand dunes emerge due the linear instability of a flat sedimentary bed. This instability has been studied in experiments and numerical models but rarely in the field, due to the large time and length scales involved. We examine dune formation at the upwind margin of the White Sands Dune Field in New Mexico (USA), using 4 years of lidar topographic data to follow the spatial and temporal development of incipient dunes. Data quantify dune wavelength, growth rate, and propagation velocity and also the characteristic length scale associated with the growth process. We show that all these measurements are in quantitative agreement with predictions from linear stability analysis. This validation makes it possible to use the theory to reliably interpret dune-pattern characteristics and provide quantitative constraints on associated wind regimes and sediment properties, where direct local measurements are not available or feasible.
Most terrestrial sand seas form at `horse' latitudes, where the wind direction exhibits seaso... more Most terrestrial sand seas form at `horse' latitudes, where the wind direction exhibits seasonal variation. Here, we extend the two-dimensional linear stability analysis of a flat sand bed associated with a unidirectional wind to the three-dimensional case in order to account for multidirectional wind regimes. Focusing on the simplest case of bidirectional flow regimes, we show that the transition from transverse to oblique or longitudinal patterns is controlled by the transport ratio and the divergence angle between the two flows. Our predictions agree with previous results for dune orientation, and also provide a wider range of possible alignments depending on flow strength, especially when the two winds are perpendicular, at which the transition occurs. This analysis also predicts the selected pattern wavelength, which either decreases close to the transition angle for strong winds, due to a geometric effect, or increases at low winds, when the bed slope affects the transport...
In zones of loose sand, wind-blown sand dunes emerge due the linear instability of a flat sedimen... more In zones of loose sand, wind-blown sand dunes emerge due the linear instability of a flat sedimentary bed. This instability has been studied in experiments and numerical models but rarely in the field, due to the large time and length scales involved. We examine dune formation at the upwind margin of the White Sands Dune Field in New Mexico (USA), using 4 years of lidar topographic data to follow the spatial and temporal development of incipient dunes. Data quantify dune wavelength, growth rate, and propagation velocity and also the characteristic length scale associated with the growth process. We show that all these measurements are in quantitative agreement with predictions from linear stability analysis. This validation makes it possible to use the theory to reliably interpret dune-pattern characteristics and provide quantitative constraints on associated wind regimes and sediment properties, where direct local measurements are not available or feasible.
Compared to barchan dunes, the morphodynamics of linear dunes that elongate on a non-erodible bed... more Compared to barchan dunes, the morphodynamics of linear dunes that elongate on a non-erodible bed is barely investigated by means of laboratory experiments or numerical simulations. Using a cellular automaton model, we study the elongation of a solitary linear dune from a sand source and show that it can reach a steady state. This steady state is analyzed to understand the physical processes at work along the dune. Crest reversals together with avalanche processes control the shape of transverse sections. Dune width and height decrease almost linearly with distance downstream until the minimum size for dunes. This is associated with a constant sand loss along the dune, which eventually compensates for the sediment influx and sets the dune length. This sand budget is discussed to distinguish an elongating linear dune from a barchan dune and to explain the complexity of linear dune fields in nature.
Proceedings of the National Academy of Sciences, 2021
Significance We report a landscape-scale experiment with controlled initial and boundary conditio... more Significance We report a landscape-scale experiment with controlled initial and boundary conditions to reveal incipient dune growth under natural action of wind. We measured growth rate of dunes of different wavelengths for 42 mo in a desert plot of 7,500 m2. We identify an early phase in which a clear maximum growth rate occurs for a 15-m dune spacing. Successful comparison with dune instability theory allows us to quantify the size-selection mechanism leading to the emergence of periodic dune patterns, which can be directly related to flow and sand transport properties. An experiment of this type and scale is unprecedented. Its results boost confidence in the existing theory, confirming its application to a variety of planetary landscapes from repeated aerial/orbital imagery. Modern dune fields are valuable sources of information for the large-scale analysis of terrestrial and planetary environments and atmospheres, but their study relies on understanding the small-scale dynamics ...
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Papers by Cyril Gadal