Evaluation of gravel sphericity and roundness based on surface-area measurement with a laser scanner

Sphericity and roundness have been employed in sedimentological and geomorphological studies to represent the gross shape of a gravel particle. The original complex definitions of gravel sphericity and roundness led to the use of visual charts or simplified parameters. Although the accurate derivation of sphericity requires the surface area of a gravel particle, manual measurement of the area is extremely difficult. To obtain the surface area and to calculate sphericity based on the original definition, 3D digital models of gravel particles were constructed using a laser scanner. The scanned gravel samples, collected from a riverbed in central Japan, include various shapes from angular to well-rounded. The obtained true sphericity values show an unexpectedly high correlation with Krumbein's roundness, despite that sphericity and roundness have been regarded as different concepts. Such a high correlation may reduce the value of one of the parameters. As a spherical particle of gravel rarely occurs in nature, a new parameter, a modification of the true sphericity, is proposed to express the deviation of gravel shape from an ellipsoid with given axis lengths. The parameter has a very weak correlation with roundness and gravel size, suggesting its independence and potential usefulness. The surface area data also permit the estimation of gravel roundness without using a visual chart.