Two types of plane beds under upper-flow-regime in flume experiments: evidence from grain fabric

Laboratory experiments that aimed to produce plane beds under upper-flow-regime were conducted using a flume of 15×700 cm with quartz sands of mean size of 0.72 mm (0.47 φ). Two types of plane beds were formed: (1) laminae formed and buried by the migration of low-relief asymmetric ripples near- or subcritical flow (Froude Number, Fr=0.94, Run 5), and (2) laminae formed without producing any apparent low-relief bedform but under supercritical flow ( Fr=1.98, Run 12). The mean flow velocity (83 cm/s) and the mean depth (8 cm) in Run 5 are larger than those in Run 12 (62 cm/s and 1 cm, respectively). Upcurrent grain imbrications in Run 12, however, are better developed than those in Run 5. Such an imbrication pattern may be useful to recognize the formational processes of plane beds under upper-flow-regime.