THREE DIMENSIONAL FLOW RIPPLE PATTERNS AND THEIR INFERRED FLOW DYNAMICS ON TIDAL SAND FLATS, NETARTS BAY, OREGON

Unidirectional straight-crested, two-dimensional flow (2Df) and irregular, three-dimensional flow (3Df) ripples have been well documented on subaqueous sand flats (e.g. Allen, 1984; 1985).  Asymmetric 2Df (or continuous) ripples may initiate as Kelvin-Helmholtz waves, as their spacing conforms to the predictions of Kelvin-Helmholtz wave theory (Venditti et al, 2003).  As these ripples develop, continuous, tubular eddy currents, or vortices, occupy their lee side troughs.  The transition from 2Df ripples with troughs to 3Df ripples with scour pits occurs with the onset of secondary flows, but the geometry of 3Df ripples and the behavior of the flows under which they form are poorly understood.  The ripple forms observed in medium sand on the tide flats of Netarts Bay, OR are divisible into four basic patterns: 1) offset cosinuate; 2) offset linguoid; 3) quasilinguoid; and 4) type I and type II linguoid (figure available).  These patterns intergrade with each other, creating a wide array of forms that reflect the behavior of the lee side vortices, which are low-pressure cells that deflect flow laterally as they become progressively segmented and erosive with each morphological step, forcing the development of scour pits within the 2Df ripple troughs.  Offset cosinuate ripples form when the tubular vortices of the continuous ripples destabilize into a series of offset pinches and swells, through which flow paths respectively diverge and converge.  As the vortices continue to segment, the salient crest portions may build down flow into the arced crests of the offset linguoid form.  Alternatively, sand deposited radially from the vortices in the cosinuate scour pits may produce arced crests that force the bifurcation of laterally adjacent vortices, which consequently create bi-lobed crests to either side of the arcs, thereby producing quasilinguoid ripples.  As these bifurcated vortices develop, they grow in size and protrude downstream in elongate curves, creating rabbit-ear shaped scour pits and crests (type II linguoid) that converge around the lee sides of the mounded, heart-shaped (type I linguoid) ripples that developed from the arced crests of the quasilinguoid form.  The erosive power and stability conditions of ripple vortices are under investigation, but they appear to be a significant control on ripple geometry.