Effect of the Wave on Sediment Suspension and the Morphological Pattern of Ripple Formation

Abstract

The paper presents the evolution of the morphological structures of a fine sand bed subjected to a turbulent flow field associated with the propagation of unidirectional surface waves. In particular, starting from a flatbed, the instability of the sediment grains from the ripple formation initiation state to the quasi-equilibrium state was observed in a wave flume. During the rolling grain ripple formation stages, new ripples appeared at each trough due to the doubling transition phenomenon and the ripples migrated soon after their formation. Observations showed that the wavelength of the bed forms that appeared during the rolling grain stage was about half of that observed at the quasi-equilibrium stage. Further, the suspended sediment concentration above the ripple bed varied with the number of wave cycles and at different vertical positions of the water column—as evidenced from the present experiments. The power spectral density of the turbulent velocity fluctuations showed that the transition of the rolling grain to vortex ripples occurred between 400 and 500 cycles with signatures of distinct reduction of near-bed spectral energy at the quasi-equilibrium state. It is evident from the stress ellipsoid that until the transition from rolling grain ripples to vortex ripples occurred, the ejection and the sweeping turbulent bursting structures took the leading role in the transportation of sediment. Thereafter, gradually the outward- and inward-interacting turbulent bursting structures provided the major contribution to the Reynolds shear stresses during the different evolution stages of vortex ripples and also at the quasi-equilibrium stage.