Saltating Sand Grains Research Articles

Observations of collisions of saltating grains with a granular bed from high‐speed cine‐film

ABSTRACTHigh‐speed photography was used to record saltating sand grains colliding with a horizontal, noncohesive bed of similarly sized grains. Impacting grain/bed interaction is discussed in general. The process, as observed from the films, is then described in terms of the apparent bed contact length (ABCL) and various parameters of the impacting grains and any ejected grains. Examples are given of typical behaviour of bed grains in response to impacting grains of different sizes. Saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments, so that previously buried grains become available for entrainment. This process is discussed in relation to the potential release of dust particles into the airflow.

Effect of saltation bombardment on the entrainment of dust by wind

Saltation is the wind‐driven, hopping motion of sand‐sized particles across an erodible surface. This mode of motion not only transports sand and similar materials in its own right but can also initiate (through bombardment of the surface) the entrainment and subsequent transport by suspension of smaller dust particles. In this paper, we report a wind tunnel study of the effect of saltation bombardment on dust entrainment. The technique is to allow sand grains to saltate from an upwind sand source onto a bed of dust particles. The experiment confirms that the ejection of dust particles by saltation bombardment (as opposed to detachment of dust particles by aerodynamic forces) is the principal mechanism for the natural entrainment of dust by wind. The data are used to examine the dependence of the dust emission flux Fd (mass per unit ground area per unit time) upon the friction velocity u*; it is found that Fd is closely proportional to the streamwise flux of saltating sand grains, which in turn is approximately proportional to u*3 At a given u*, Fd increases as the size of the bombarding sand grains increases. On the basis of the hypothesis that Fd is proportional to the kinetic energy flux of the saltating sand grains, we derive theoretically the result that Fd scales with the streamwise saltating sand grain flux and thence approximately with u*3, as observed in this experiment.

The overshoot and equilibration of saltation

Downwind of the leading edge of a saltating surface, the saltation process develops toward an eventual stream wise equilibrium. The nature of this development is investigated in a portable wind tunnel by measuring the vertically integrated streamwise flux Q of saltating sand grains as a function of distance x from the leading edge of the erodible surface, at wind speeds between 8 and 13 m s−1. It is found that Q(x) increases from zero at x = 0 to a maximum value at x around 7 m, before decreasing toward an eventual equilibrium value (about half the maximum value) when x is greater than about 15 m. These results are in qualitative agreement with two recent numerical simulations of the approach of saltation to equilibrium. The experiment indicates the minimum length of wind tunnel required for studying the saltation process in its equilibrium state, and the order of magnitude of the measurement errors if shorter tunnels must be used.

The grain-bed impact process in aeolian saltation

We report the results of impact experiments in which high velocity steel spheres (BBs) were directed against a loose bed of similar particles. The purpose of these experiments is to shed some light on the collision processes which occur when saltating sand grains driven by the wind strike the bed. The scattered particles fall into two categories: a single high energy rebound which scatters quasi-specularly, and a number of low energy recoils. The high energy rebound is identified with the “successive saltation” particle of Rumpel, and the low energy recoils are interpreted as creeping, or reptating particles. These observations provide information on the “splash function” of Ungar and Haff, which describes the response of a bed to grain impact and which plays a central role in the theory of saltation.

Collisions in aeolian saltation

Velocity vectors of saltating sand grains immediately before and after collision with a bed of mineralogically similar particles are looked at statistically. A cine-camera is used to film a controlled number of impacting grains in a wind tunnel. As a result, individual collisions can be examined.

Small Scale Aeolian Bedforms

ABSTRACT This paper presents results from one hundred and twenty samples which were collected from sand ripples in the Algerian Sahara. Ripples were found that are transitional between small aeolian ripples (wavelength 1 cm-20 cm) formed in fine unimodal sand, and larger ripples (wave length 20 cm-20 m) formed in coarse bimodal sand; these are commonly distinguished as two distinct types. The formation of the first type is usually explained by a grain impact mechanism in which the ripple wavelength corresponds to the mean path length of the saltating sand grains. A mathematical model is described which simulates the saltation mechanism of bimodal sand mixtures; the spread of grain impact velocities is taken into account by means of an empirically determined rebound probability matrix. Results f computations using this model are presented which demonstrate that the mechanism of saltation can also account for the largest ripples. These results which explain the presence of the transitional ripples, are attributed to the improved rebound of the fine fraction off the coarse fraction in bimodal sand.

Ventifact Sculpture by Windblown Dust

Detailed observations of features of natural ventifacts plus wind-tunnel studies of rocks, minerals, and artificially prepared mineral aggregates indicate how aerodynamic principles may be used as a basis for correlating both macroscopic and microscopic features on ventifacts with directions and kinds of responsible air movements. Special attention is directed to the shaping of bottom surfaces and to vortex movements of wind. Both field and laboratory data appear to support the hypothesis that many of the features of ventifacts which are attributable to wind erosion have been produced in response to collisions by suspended dust particles rather than by saltating sand grains.

COHESION OF SOIL PARTICLES AND THE INTRINSIC RESISTANCE OF SIMPLE SOIL SYSTEMS TO WIND EROSION

SummaryIn a simple dry soil system in which erosion is taking place by virtue of the impact of saltating sand grains, the intrinsic variables affecting soil resistance can be investigated with the aid of a mathematical model. The soil erodibility depends on the cohesiveness of the soil which is indicated by the tensile strength. This is directly proportional to the packing density of the system, to the co‐ordination number of the particles, and to the inter‐particle bond strength. Tensile strength is inversely proportional to the cube of the particle diameter. In a cohesive material any decrease in the particle size produces a great increase in the tensile strength and hence the mechanical stability is improved, but because the particles are lighter they are easily eroded once they are disturbed.

Formation of Small Ventifacts

Sharp edges of small, late Pleistocene, wind-eroded pebbles (or ventifacts) embedded in bedrock at Ocean Cove, California, have an average trend oblique to inferred Pleistocene wind direction. This and other features suggest that abrasion was produced chiefly by a stream of suspended or partly suspended particles following local deflections of the wind stream. That such material would be capable of producing ventifact shapes is suggested by previous studies on wind flow over obstacles. Consideration of wind-flow deflection suggests that currently accepted concepts of ventifact abrasion by saltating sand grains must be modified, and it further suggests that source and size distribution of wind-transported abrasive material, more than size of ventifacts, may determine whether abrasion will result largely from scour by suspended material or from impact by saltating grains. Occurrence of three- and four-edged ventifacts strengthens the view that orientation of faces may be partly controlled by original pebble...