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Summary
We show that the properties of avalanches in a
gravitationally-forced
granular layer on a rough inclined plane - a model system for rock
avalanches on a hillside - depend dramatically but in a predictable
manner on the shape (angularity) of the grains.
Measuring major characteristics
of
avalanches such as: - the dimensionless avalanche height, - the ratio of the particle and front velocities, - and the growth rate of avalanche speed with increasing avalanche size we find that they correlate well with the most basic property of the material - the angle of repose. For rough non-spherical grains (i.e. materials with a high angle of repose), avalanches are faster, bigger and overturning in the sense that individual particles have downslope speeds that exceed the front speed (see image b) as compared with avalanches of rather spherical particles that are quantitatively slower, smaller and where particles always travel slower than the front speed (see image a). We demonstrate how the dimensionless avalanche height depends on the tangent of the angle of repose in image c. Our observations are summarized in a recent paper [Phys. Rev. E 78, 011306 (2008)] and in a short communication: [Phys.Rev.Lett. 94, 208001 (2005)] 
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Movies
See below two
images of avalanches in transmitted
light (top
view) for sand. The corresponding movies can be downloaded by clicking
on the images.  Other movies with higher magnification: Movie of one avalanche (top view) for sand particles in animated gif format. Movie of one avalanche (top view) for glass beads in animated gif format. Movie of an avalanche front (side view) formed by sand particles in animated gif format or in apple Quicktime format. Movie of an avalanche front (side view) formed by spherical glass beads in animated gif format, or in Quicktime format. The avalanche height profile was obtained by detecting the reflected laser-line as it is illustrated on the images below. Click on the image to magnify. 
 A corresponding movie for sand (recorded with camera 1) can be downloaded in animated gif format. The reconstructed avalanche profile for this recording for sand is presented below (image a.) together with an other height profile obtained for glass beads (image b.). Click on the image to magnify.  |
