SHAPE SORTING OF FLOOD-TRANSPORTED SYNTHETIC CLASTS IN A GRAVEL-BED RIVER

An experiment was set up in a gravel-bed river to test shape-sorting of gravel-size clasts. Synthetic clasts representing four shape types were used. Spheres were entrained in larger numbers than the other shapes, and travelled furthest. Rods and blades were intermediate in mobility. Discs moved the shortest distance, and in the least numbers. The angle of repose, and therefore the pivotability of clasts in combination with lift and drag forces exerted on the clast, will determine if a clast will be entrained or not as well as the shear stress needed for the clast to maintain motion. A sphere projects relatively higher than the other shape types of equal volume and will induce a higher degree of flow separation in its wake than other shape types. Therefore larger components of lift and drag forces exerted by water flow will be present to set this shape type in motion or to maintain motion. The prolate clasts have a small angle of repose compared to discoid clasts and will roll much more readily. The imbricated orientation assumed by discoid and sometimes blade-shaped clasts is a very stable position. Very high applied shear stress will be needed to dislodge imbricated discs.