An experimental study of fabric development in plane-bed phases

Abstract

Results of the development of fabric in sands and sand-gravel (<31.5 mm) mix sediments deposited experimentally in plane-bed phases are presented. Experiments with particles moving over a fixed plane bed indicate that the size of substratum roughness elements (0.06–0.42 mm) does not affect the final orientation of the particles except those in the critical range (1.00–1.40 mm). Particles larger and smaller than the critical range are characterized by current-normal and current-parallel orientation patterns respectively, and those in the critical range by bimodal (Bpt) orientation patterns with current-normal and current-parallel modes. Horizontally laminated, poorly sorted, coarse sands deposited in the lower plane-bed phase are marked by Bpt orientation patterns for the coarser fractions (>0.71 mm) and current-parallel patterns for the finer fractions (<0.71 mm). Similar horizontally laminated beds deposited from the upper plane-bed phase invariably show current-parallel orientation patterns. Poorly sorted sand-gravel mixtures in the lower plane-bed phase and under partial and total bedload transport conditions usually show Bpt patterns and the current-parallel mode becomes prominent with increasing shear stress. In moderately sorted sand-gravel mixtures, current-oblique orientation patterns are frequently observed. Studies indicate that the size of the sediment fraction, the sorting of sediments, mode of transport, velocity/shear stress of flow and reorientation of particles after they come to rest on the substratum all play a significant role in determining the final orientation of particles.