52 research outputs found
Periodicity in fields of elongating dunes
Dune fields are commonly associated with periodic patterns that are among the
most recognizable landscapes on Earth and other planetary bodies. However, in
zones of limited sediment supply, where periodic dunes elongate and align in
the direction of the resultant sand flux, there has been no attempt to explain
the emergence of such a regular pattern. Here, we show, by means of numerical
simulations, that the elongation growth mechanism does not produce a pattern
with a specific wavelength. Periodic elongating dunes appear to be a
juxtaposition of individual structures, the arrangement of which is due to
regular landforms at the border of the field acting as boundary conditions.
This includes, among others, dune patterns resulting from bed instability, or
the crestline reorganization induced by dune migration. The wavelength
selection in fields of elongating dunes therefore reflects the interdependence
of dune patterns over the course of their evolution.Comment: 7 pages, 4 figure
Coexistence of two dune growth mechanisms in a landscape-scale experiment
In landscape-scale experiments at the edge of the Gobi desert, we show that
various dune types develop simultaneously under natural wind conditions. Using
4 years of high-resolution topographic data, we demonstrate that, depending on
sand availability, the same wind regime can lead to two different dune
orientations, which reflect two independent dune growth mechanisms. As periodic
oblique dunes emerge from a sand bed and develop to 2 meters in height, we
analyze defect dynamics that drive the non-linear phase of pattern coarsening.
Starting from conical sand heaps deposited on gravels, we observe the
transition from dome to barchan and asymmetric barchan shapes. We identify a
minimum size for arm elongation and evaluate the contribution of wind reversals
to its longitudinal alignment. These experimental field observations support
existing theoretical models of dune dynamics boosting confidence in their
applicability for quantitative predictions of dune evolution under various wind
regimes and bed conditions
Methane storms as a driver of Titan's dune orientation
Titan's equatorial regions are covered by eastward propagating linear dunes.
This direction is opposite to mean surface winds simulated by Global Climate
Models (GCMs), which are oriented westward at these latitudes, similar to trade
winds on Earth. Different hypotheses have been proposed to address this
apparent contradiction, involving Saturn's gravitational tides, large scale
topography or wind statistics, but none of them can explain a global eastward
dune propagation in the equatorial band. Here we analyse the impact of
equinoctial tropical methane storms developing in the superrotating atmosphere
(i.e. the eastward winds at high altitude) on Titan's dune orientation. Using
mesoscale simulations of convective methane clouds with a GCM wind profile
featuring superrotation, we show that Titan's storms should produce fast
eastward gust fronts above the surface. Such gusts dominate the aeolian
transport, allowing dunes to extend eastward. This analysis therefore suggests
a coupling between superrotation, tropical methane storms and dune formation on
Titan. Furthermore, together with GCM predictions and analogies to some
terrestrial dune fields, this work provides a general framework explaining
several major features of Titan's dunes: linear shape, eastward propagation and
poleward divergence, and implies an equatorial origin of Titan's dune sand.Comment: Published online on Nature Geoscience on 13 April 201
Depth dependent stress revealed by aftershocks
Aftershocks are earthquakes triggered in the near field by the dynamic rupture of larger earthquake events. Here, the authors show along the San Andreas fault system that the early aftershock decay rate can be used to infer the variations of stress with depth
Un automate cellulaire de l'espace réel pour l'étude des populations de dunes
PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF
Common dependence on stress for the statistics of granular avalanches and earthquakes
International audienceBoth earthquake size-distributions and aftershock decay rates obey power laws. Recent studies have demonstrated the sensibility of their parameters to faulting properties such as focal mechanism, rupture speed or fault complexity. The faulting style dependence may be related to the magnitude of the differential stress, but no model so far has been able to reproduce this behaviour. Here we investigate the statistical properties of avalanches in a dissipative, bimodal particulate system under slow shear. We find that the event size-distribution obeys a power law only in the proximity of a critical volume fraction, whereas power-law aftershock decay rates are observed at all volume fractions accessible in the model. Then, we show that both the exponent of the event size-distribution and the time delay before the onset of the power-law aftershock decay rate are decreasing functions of the shear stress. These results are consistent with recent seismological observations of earthquake size-distribution and aftershock statistics
Condition of Occurrence of Large Man-Made Earthquakes in the Zone of Oil Production, Oklahoma
International audienceMan-made seismicity is a response of the brittle crust to fluid injection at depth and to the subsequent increase in pore-pressure and stress field perturbations. In Oklahoma, where the sharp increase in earthquake rate correlates with injection operations, we show that the earthquake-size distribution can differ significantly on the volume of injected fluid. The size distribution of M 2) may be documented for larger magnitude ranges. This change shows statistically significant positive dependence on injection activity. In addition, largest events occur at the border of the injection area at some distance from massive injection, and in the periods of steady injection rate. These observations suggest that a deficit of large induced earthquakes under conditions of high injection rate can be accompanied by an overall increase of natural seismicity along pre-existing faults in the surrounding volume, where large events are more likely to be triggered over longer space-time scales
Controls on and effects of armoring and vertical sorting in aeolian dune fields: A numerical simulation study
International audienceUnlike ripples, there are only few numerical studies on grain size segregation at the scale of dunes in aeolian environments. Here we use a cellular automaton model to analyze vertical sorting in granular mixtures under steady unidirectional flow conditions. We investigate the feedbacks between dune growth and the segregation mechanisms by varying the size of coarse grains and their proportion within the bed. We systematically observe the development of a horizontal layer of coarse grains at the top of which sorted bed forms may grow by amalgamation. The formation of such an armor layer controls the overall sediment transport and availability. The emergence of dunes and the transition from barchan to transverse dune fields depend only on the grain size distribution of the initial sediment layer. As confirmed by observation, this result indicates that armor layers should be present in most arid deserts, where they are likely to control dune morphodynamics
Migration of Reversing Dunes Against the Sand Flow Path as a Singular Expression of the Speed Up Effect
International audienceWe study the morphodynamics of reversing dunes on the gravel deposits of the alluvial fan of the Molcha river at the border between the Tibetan Plateau and the Taklamakan Desert. Independent sets of wind data show that this area of low sand availability is exposed to two prevailing winds from opposite directions and of different strengths. The predicted resultant transport direction of sand particles is westward. Nevertheless, satellite observations combined with field measurements and ground penetrating radar surveys reveal that isolated dunes a few meters high migrate eastward. This apparent dune migration paradox is resolved using numerical and analytical models that take into account the speed up effect and the continuous change in dune shape after each wind reversal. When a newly established wind hits what was before the steeper lee slope of the dune, the sand flux at the crest abruptly increases before relaxing back to a constant value as the crest migrates downwind and as the dune reaches a new steady shape. Integrated over the entire wind cycle, we find that this non linear behavior causes reversing dunes to migrate against the resultant transport direction. This migration reflects the difference in dune slope seen by irregular storm events blowing to the east and the westward wind of the daily cycle. Thus, we explore the impact of extreme events on dune morphodynamics and examine new aspects of the permanent feedback between dune topography and wind speed. We conclude that transient behaviors associated with crest reversals contribute to the observed diversity of dune patterns, even within the same area for dunes of different sizes
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