Measurement and prediction of wave-generated suborbital ripples

Experiments in a large-scale wave flume using regular and irregular waves with periods between 4 s and 6 s and heights between 0.2 m and 1.55 m have examined the formation of wave-generated ripples using sediment beds composed of four grain sizes (D-50 = 0.349 mm, 0.329 mm, 0.220 mm, and 0.162 mm) in a water depth of approximately 4 m. Estimates of wave-generated ripple height, eta, and wavelength, lambda, were obtained using zero down-crossing analyses of bed profiles measured by acoustic means along a 4-m transect normal to the ripple crests. Further information pertaining to lambda was obtained from bed images obtained using scanning sonar. The analyses reported here focus on the sequence bedforms that evolved in response to stepwise increases and decreases in wave height. Results show that ripples for the most part are suborbital in nature and do not conform well to empirical equations used frequently to predict eta and lambda values in the field. On the basis of the present data, two new equations for prediction of eta and lambda are obtained and their use in field situations where hydrodynamic and sedimentary conditions favor development of suborbital bedforms is recommended.