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In the vicinity of a surface which is being eroded there is a region where particles arriving at and departing from the surface can collide. The frequency of collisions and the resulting motion of spherical particles has been investigated theoretically and experimentally. An expression is derived for the flux of spheres necessary for a high frequency of collisions. The directions and velocities of particles after collision have also been calculated, assuming elastic collisions. In the low-flux limit collisions do not reduce the number of particles reaching the surface. Long-exposure (ca. 1s) and high-speed (5000 frames 5 -l ) photography confirm these findings for the case of 0.6 ? 0.1 mm diameter glass spheres travelling at 13 ? 1 m S -' . These conclusions conflict with the ideas of many authors who suggest that collisions help protect the erodedsurface. The present paper shows that collisions degrade the incident beam of particles by increasing the angular divergence of the beam and creating a spectrum of velocities. Stray particles have also been observed entering the region of interest, increasing the frequency of collisions. Stray particles and changes in the incident beam are probably the causes of the observed reduction in erosion rate at high levels of flux.

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