In this paper interference phenomena between particle flattening and particle rounding during the free vertical sedimentation of natural particles are investigated. Particle flattening is measured by the Corey Shape Factor (CSF), particle rounding by the Powers Index (P). Using Dietrich's equation, the effect of the flattening and the effect of the rounding on the terminal fall velocity of the particle can be studied separately. This is done by examining the influence of particle flattening and particle rounding on the Particle Sedimentation Coefficient (PSC), which is the ratio of the nominal diameter Dn of the particle to the equivalent sedimentation diameter Db. It appears that particle flattening and particle rounding are interdependent so that interference phenomena occur. In the case of positive interference, the combined effect of flattening and rounding on the PSC is larger than the sum of the effects of both parameters separately. If, on the other hand, the combined effect is smaller than the sum of the single effects, negative interference occurs.

For fine-grained particles, the more the shape of a particle differs from a sphere in one variable (flattening or rounding), the higher will be the influence of the other variable on the free vertical fall. For coarse-grained particles, the inverse occurs. For very fine-grained particles (Dn < 200 μm (air) or < 370 μm (water)) the influence of particle rounding on free vertical settling is negligibly small. For coarser grained particles, flattening is the most important parameter if the grains are strongly flattened. If not, rounding will be the governing parameter.