Flow Speeds

The linearity of the crests gives information about the relative strength of the flow that formed the ripples. As soon as flow speeds are high enough to form ripples, the crests of those ripples will be linear, but as flow speeds increase slightly, the crests become increasingly more irregular, until they break up to distinct bumps on the sediment bottom. If flow speeds increase even more, the ripples become unstable and are destroyed by the highest speed flows.

	As the sand grains begin to move, linear-crested ripples form.  The crests approximate straight lines, but with slight increases in flow speed, the shape of the crests becomes more irregular.  In this image, the crests are straighter (2-dimensional) in the upper part, and become more irregular (3-dimensional) toward the foreground.
	2D to 3D Symmetric Ripples.  If flow speeds increase slightly, the linear crests begin to change shape, typically into S-curved paths. The “curviness” increases with increasing flow speed.
	3D Asymmetric Ripples.   With continued increase in flow speed, the curvy crests begin to break up into irregular mounds on the sediment surface, similar to those in this photo. Notice that the asymmetry of these ripples is preserved, and that it is still possible to determine the direction of the flows that produced these ripples. Significantly though, the shape of the crests indicates that the speeds were faster than if the crests were linear.
	With increasing flow speed, the ripples become unstable. The grains are moving so fast that the ripples are washed out and the grains form a flat bed, instead of a rippled surface. This configuration is preserved in the rock record as planar stratification.
	In rare cases, at the highest flow speeds, the planar laminae give way to the formation of a final bed form, known as antidunes.  Antidunes consist of broad, upward arching laminae, that form beneath waves traveling upstream.   Thus, the bedform is quite subdued and often subtle in outcrop.