THE ROLE OF LEE VORTICES IN THE PATTERNS AND PROCESSES OF CURRENT RIPPLES, NETARTS BAY, OREGON

Asymmetric current ripples commonly form in subaqueous environments where water flows over fine to coarse sand. Ripple crests, troughs, and lee side vortices together reflect unidirectional current flow patterns. This study of tidal current ripples in Netarts Bay, Oregon recognizes the interplay between these elements, and integrates them in a descriptive interpretation of their genesis and subsequent morphology. Transverse, continuous ripples with nearly straight crests, troughs and lee side vortices are hallmarks of two-dimensional flow (2Df), whereas linguoid, discontinuous ripple crests with isolated scour pits and lee side vortices characterize three-dimensional flow (3Df). Linguoid ripples are bimodal; commonly intergrading between two end-members varieties: the classic bilaterally symmetrical pattern described by Bucher and Allen (here called bilateral linguoid) and newly described unilateral linguoid ripples, which splay to one side from the principal flow direction. From these patterns, I hypothesize that transverse, continuous 2Df lee side vortices behave like centrifugal air blowers, wherein axial flows balance the perimetric circulation. When transverse, continuous vortices destabilize into a series of cells with opposing directions of axial flow – thereby initiating secondary flows at the 2D-3D flow transition – the transverse crests become sinuous. With increased flow velocity, these vortex cells segment into distinct, canted vortices that attach to the sand surface at one end, and dissipate into the overriding flow at the other. Based on this observation, the wide array of ripple patterns at Netarts Bay can be descriptively classified and its flow patterns better understood. Furthermore, applying this simple, unified classification of ripple crests, troughs, and lee vortices affords a sophisticated model from which to infer the basic flow processes across ripple tidal sand flats in general.