RIPPLES UNDER TWO-DIMENSIONAL OSCILLATORY FLOW: A TWO-DIRECTIONAL OSCILLATORY-BED EXPERIMENT

Bedforms under two-dimensional oscillatory flow with complex trajectory were examined through a laboratory experiment. The experiment employed a newly developed two-directional oscillatory bed in order to simulate relative motion between sand bed and two-dimensional oscillatory flow. The equipment generates various two-dimensional oscillations by combining horizontal x and y sinuous oscillations, which are independent each other. By using the equipment, a circular tray (100 cm in diameter, 2.5 cm in depth) filled with sediment with median diameter of 0.2 mm was oscillated in still water within a circular tank (180 cm in diameter, and 60 cm in depth). The oscillatory period was < 3s, and amplitude was < 7 cm. Typically, two-dimensional ripples, ripples with polygonal crest line (RPCL), or ladder-back ripples (LBP) were observed in each experimental run. The major and minor crests of LBR were perpendicular to oscillations. Patterned three-dimensional ripples formed when the mobility number of each oscillatory component exceeds 2, i.e., the threshold value for ripple formation from a bed with a perturbation. TPCL formed when the ratio, smaller period/larger one, of two oscillations ranged from 0.8 to 1.0. LBR occurred when the ratio of periods was less than 0.8. The size of bedforms was depended on the amplitude of oscillatory components. Ripple spacing between major and minor crests of LBR was 1/3 of corresponding oscillatory amplitude. The mean width of polygonal cells of RPCL under circular oscillation was 1/6 of amplitudes.