Flume Experiments on Bedforms and Structures at the Dune‐Bed Transition

Abstract

A tiltable, recirculating flume, 18 m long and 76 cm wide, was used to investigate bedforms and structures near the transition between dunes and a plane bed for a moderately sorted coarse sand. At Froude numbers ranging from about 0·4 to 1·0, three dune types developed: (1) asymmetrical (triangular) dunes, (2) convex (symmetrical) dunes and (3) humpback dunes. Asymmetrical dunes had gentle, long stoss sides and steep, short lee sides, and contained cross-stratification with a maximum dip of about 30–35°. Flow separation and avalanching were strongly developed to the lee of these dunes. Convex dunes formed when the bed was thin and had longitudinal profiles that were convex-upwards, with stoss and lee sides of equal steepness. Internal cross-beds were likewise convex and formed from draping of sediment over the lee sides rather than from avalanching. Humpback dunes were the most distinctive bedforms in that on each dune profile the point of maximum elevation was offset from the top of the foreset (avalanche) slope. Immediately downstream from this maximum point, low-angle topset bedding merged uninterruptedly into steep foreset beds and these into bottomsets, producing sigmoidal bedding inside each dune. Although foreset slopes were much shorter in humpback dunes than in asymmetrical dunes, their steepness remained about 30–35° right up to the change to a plane bed at a Froude number of about 1·1. For one other run a plane bed also formed, but at a Froude number of about 1·7, a rather high value for a plane bed just beyond the dune bed phase. This second plane bed may be that which occurs at Froude numbers larger than those for in-phase waves.