Author(s): Yarko Nino; Manuel Barahona
Linked Author(s): Yarko Niño
Keywords: No Keywords
Abstract: Results of an experimental study on the formation and evolution of underwater barchan-ripples are presented. The experiments were conducted in a flume with a fixed, smooth bed. Sediment transport was provided by recirculating a given amount of sand into the flume. Initially, sand grains move along low-speed streaks of the near-wall flow. Later, and under certain flow conditions, barchan-ripples develop freely over the fixed bed, from the accumulation of sand grains into patches resulting from the transverse interaction of wall-streaks. Barchan-ripples evolve as self-organized flow-sediment structures with a crescent shape, that migrate in the downstream direction. Their celerity decreases as the ripple height increases. Their size is variable for a given flow condition, however their shape is approximately self-similar. As the amount of recirculating sediment is increased, barchan-ripples are more easily formed. When a sufficiently large number of ripples are present in the flume, the interaction between them creates a pattern with a characteristic wavelength. Two types of patterns were observed: ripple trains, where bedforms align one after the other, and alternate ripples, where the center of one structure is located downstream from one of the legs of the upstream structure. Results of flow visualizations indicate that sediment transport over the bedforms is controlled by quasi-periodic events, related to the existence of coherent structures of the flow, that are able to entrain sediment into suspension, thus transporting the grains downstream and away from the flow-sediment structure.
Year: 1997