Bed form genesis from bed defects under unidirectional, oscillatory, and combined flows

New experimental data on bed form initiation under unidirectional, oscillatory, and combined flows are presented to gain quantitative insight into bed form genesis from artificially generated defects on a flat sediment-laden bed. Planform changes revealed in time-lapse photography allowed study of the evolution of the downstream and upstream edges of defects from their initial geometric center. Based on this temporal data set and flow velocity profiles, it was observed that combined flow bed forms share the same bed form initiation processes as unidirectional and oscillatory flows, which are reflected directly in the generation of similar geometric patterns regardless of the hydrodynamic conditions. The development of these bed defects is strongly coupled to the direction and magnitude of the shear stress applied to the bed throughout the wave cycle. Under current-dominated combined flow conditions, no defect propagation occurred in the upstream direction, despite the presence of flow reversal. In addition, spectral analysis of the evolution of the downstream and upstream edges of the defects demonstrated that (i) periodicity in the defect growth pattern scales with the wave period for pure oscillatory flows; (ii) wave-dominated combined flows possess a period-driven peak with respect to defect growth in experiments with relatively short wave periods, but this peak is absent in experiments possessing relatively long wave periods (T >15s); and (iii) current-dominated combined flows and pure unidirectional flows do not display a period-driven peak in defect growth. These results suggest that the occurrence of long-period combined flow bed forms may be underrepresented in the sedimentary record.