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 o f gravel-size clasts. Synthetic clasts representing four shape types w ere used. Spheres were entrained in larger numbers than the other shap es, and travelled furthest. Rods and blades were intermediate in mobil ity. Discs moved the shortest distance, and in the least numbers. The angle of repose, and therefore the pivotability of clasts in combinati on 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 tha n the other shape types of equal volume and will induce a higher degre e of flow separation in its wake than other shape types. Therefore lar ger components of lift and drag forces exerted by water flow will be p resent to set this shape type in motion or to maintain motion. The pro late clasts have a small angle of repose compared to discoid clasts an d will roll much more readily. The imbricated orientation assumed by d iscoid and sometimes blade-shaped clasts is a very stable position. Ve ry high applied shear stress will be needed to dislodge imbricated dis cs.