Migration of Multiple Scale Bedforms in Energetic Tidal Environments

Sub-aqueous bedforms influence sediment transport, ship navigation, wave dynamics, and coastal morphodynamics, especially near tidal inlets where strong currents interact with waves to create diverse bedform morphologies. Superposition of smaller ripples on larger bedforms has been observed in several energetic tidal environments, suggesting that this is a ubiquitous phenomenon. We observed “tidally reversing megaripples” with wavelengths from 1 to 5 m and heights of 0.1 to 0.5 m, which are larger than wave-generated ripples but smaller than dunes. Unlike dunes whose height is dependent on water depth, these bedforms have similar geometries in depths ranging from 2 to 12 m and reverse both direction and asymmetry while migrating on the dunes.

We obtained spatial and temporal observations of these multiple scale bedforms at Wasque Shoals, southeast of Martha’s Vineyard. This site has flood dominated, 1 m/s tidal currents. Data from a fixed rotary side-scan sonar depict the interaction of the megaripples with the lee-side of a migrating dune. While the dune migrates in the net direction of flood, the megaripple migration varies spatially due to a wake formed by the dune. Megaripples located in this wake converge during flood, but diverge during ebb when no wake is present. Outside the wake, the megaripples diverge during flood and converge during ebb. The net migration of the megaripples is toward the dune crest in the dune wake, but away from the crest outside the wake. This net convergence of megaripples towards the dune crest suggests that the megaripples may influence the dune height and migration. The fast, 3.2 m/day, migration of the megaripples serves as an intermediate step between the grain scale bedload transport processes and the slow, 0.5 m/day, large scale dune migration. This relationship between the multiple scale features may be an important component in coastal morphodynamic evolution in energetic tidal inlets.