RIPPLES UNDER LABORATORY PARTIALLY STANDING WAVES

The cross-shore variations of ripple geometry under partially standing waves, which are generated by the interference between incident waves and those reflected from seashore, were experimentally examined. A wave-flume with 20 m long, 0.5 m wide, and 0.6 m deep was employed in the test. Ripples were developed from a horizontal flat sand bed, which was made with quartz sand with median grain diameter of 0.2 mm. Five experimental runs were performed with the same conditions but wave reflection coefficient as follows: water depth h = 0.30 m, wave period T = 1.2 s, incident wave height H_I = 85 mm, and reflection coefficient R_c = 0.03–0.52. Under partially standing waves, the local maximum velocities, bed shear stresses, and orbital diameter are the largest at nodes, and they are the smallest at antinodes; their difference increases with larger R_c value. The largest ripples developed rapidly under nodes, and the smallest ones slowly under antinodes. In the case of R_c = 0.52, ripples did not appear under antinodes within the test duration. The local-mean ripple spacings measured in each run approximately correspond with those predicted by Nielsen’s (1979, 1981) empirical formula. Especially, the ripple spacings under nodes and anti-nodes were accurately predicted. The present result indicates that periodic cross-shore distribution of ripple wavelength provides a clue to estimate the intensity of wave reflection.