40 research outputs found
Minimal size of a barchan dune
Barchans are dunes of high mobility which have a crescent shape and propagate
under conditions of unidirectional wind. However, sand dunes only appear above
a critical size, which scales with the saturation distance of the sand flux [P.
Hersen, S. Douady, and B. Andreotti, Phys. Rev. Lett. {\bf{89,}} 264301 (2002);
B. Andreotti, P. Claudin, and S. Douady, Eur. Phys. J. B {\bf{28,}} 321 (2002);
G. Sauermann, K. Kroy, and H. J. Herrmann, Phys. Rev. E {\bf{64,}} 31305
(2001)]. It has been suggested by P. Hersen, S. Douady, and B. Andreotti, Phys.
Rev. Lett. {\bf{89,}} 264301 (2002) that this flux fetch distance is itself
constant. Indeed, this could not explain the proto size of barchan dunes, which
often occur in coastal areas of high litoral drift, and the scale of dunes on
Mars. In the present work, we show from three dimensional calculations of sand
transport that the size and the shape of the minimal barchan dune depend on the
wind friction speed and the sand flux on the area between dunes in a field. Our
results explain the common appearance of barchans a few tens of centimeter high
which are observed along coasts. Furthermore, we find that the rate at which
grains enter saltation on Mars is one order of magnitude higher than on Earth,
and is relevant to correctly obtain the minimal dune size on Mars.Comment: 11 pages, 10 figure
Modelling formation and evolution of transverse dune fields
We model formation and evolution of transverse dune fields. In the model,
only the cross section of the dune is simulated. The only physical variable of
relevance is the dune height, from which the dune width and velocity are
determined, as well as phenomenological rules for interaction between two dunes
of different heights. We find that dune fields with no sand on the ground
between dunes are unstable, i.e. small dunes leave the higher ones behind. We
then introduce a saturation length to simulate transverse dunes on a sand bed
and show that this leads to stable dune fields with regular spacing and dune
heights. Finally, we show that our model can be used to simulate coastal dune
fields if a constant sand influx is considered, where the dune height increases
with the distance from the beach, reaching a constant value.Comment: 18 pages including 9 figure
Nontrivial temporal scaling in a Galilean stick-slip dynamics
We examine the stick-slip fluctuating response of a rough massive
non-rotating cylinder moving on a rough inclined groove which is submitted to
weak external perturbations and which is maintained well below the angle of
repose. The experiments presented here, which are reminiscent of the Galileo's
works with rolling objects on inclines, have brought in the last years
important new insights into the friction between surfaces in relative motion
and are of relevance for earthquakes, differing from classical block-spring
models by the mechanism of energy input in the system. Robust nontrivial
temporal scaling laws appearing in the dynamics of this system are reported,
and it is shown that the time-support where dissipation occurs approaches a
statistical fractal set with a fixed value of dimension. The distribution of
periods of inactivity in the intermittent motion of the cylinder is also
studied and found to be closely related to the lacunarity of a random version
of the classic triadic Cantor set on the line.Comment: 7 pages including 6 figure
The apparent roughness of a sand surface blown by wind from an analytical model of saltation
We present an analytical model of aeolian sand transport. The model
quantifies the momentum transfer from the wind to the transported sand by
providing expressions for the thickness of the saltation layer and the apparent
surface roughness. These expressions are derived from basic physical principles
and a small number of assumptions. The model further predicts the sand
transport rate (mass flux) and the impact threshold (the smallest value of the
wind shear velocity at which saltation can be sustained). We show that, in
contrast to previous studies, the present model's predictions are in very good
agreement with a range of experiments, as well as with numerical simulations of
aeolian saltation. Because of its physical basis, we anticipate that our model
will find application in studies of aeolian sand transport on both Earth and
Mars
Dunes on Pluto
The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (-0.4 to 1 kilometer) is best explained by the deposition of sand-sized (-200 to -300 micrometer) particles of methane ice in moderate winds (<10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past
The implications of sampling approach and geomorphological processes for cosmogenic Be-10 exposure dating of marine terraces
Concentrations of cosmogenic nuclides in rocks at the Earth's surface are routinely measured by AMS (accelerator mass spectrometry) to obtain exposure ages. One particular application of this technique has been to derive ages for the formation of marine terraces, thus allowing constraints to be placed on rates of tectonic uplift. However, multiple rock samples from the same terrace surface have typically shown an amount of scatter in cosmogenic nuclide concentrations in excess of analytical uncertainties, potentially undermining the confidence in the resulting uplift rates. In addition, earlier works have considered the application of bedrock samples for exposure age dating marine terraces, but little attention has been directed towards the validity of sampling clasts exposed on terrace surfaces. Here, we use Be-10 and Al-26 measurements from a flight of marine terraces in northern Chile to compare the results from pebbles with those from bedrock samples and to investigate assumptions that sample pre-exposure and burial are negligible. We examine the influence that processes of surface deflation and pebble erosion will have on cosmogenic nuclide concentrations using a Monte Carlo model, the results of which are compared to our Be-10 measurements. We find good agreement of Be-10 concentrations between bedrock and pebble samples. At the sampled location, sample burial is rare and Be-10 inheritance in pebble samples is low relative to the age of the terraces. Our modelling suggests that in arid environments, such as northern Chile, pebble erosion will be a secondary effect compared to surface deflation and that multiple periods of deflation will result in multimodal distributions of Be-10 concentrations. More broadly, our findings show that measuring multiple surface clasts from a single marine terrace reveals the geomorphological processes influencing exposure ages, and may also be used to help identify the operation of past surface processes such as episodic deflation