Plate 30

Bed forms: lunate ripples

Ripples whose profile changes little along crests are called linear (plate 31); they approach the hydrodynamic equilibrium profile for a bi-dimensional flow, a flow whose configuration does not change across the direction of movement. When, on the other hand, transversal perturbations, or "side effects," exist in the flow, the bed forms acquire a three-dimensional morphology: their profile changes laterally, as in this picture. Two basic "3D" shapes exist: lunate  (or barchanoid ), and linguoid,  depending on how the crest curvature is oriented with respect to flow direction (see inset). Actually, one can be misled by "3D" morphologies when trying to identifying and measuring this direction. Remember, then, that the safer indicator  of the flow vector is the steeper side, which faces down current in any case.

The ripples are often ornamented by minor structures parallel to the current or fanning down current (spurs).