A bedform is organized relief on a mobile sediment bed, such as a dune or an antidune. Its shape changes the overlying flow, which changes the shear stress and the sediment transport; erosion and deposition then feed back on the shape. The pattern is therefore a coupled fluid-and-grain response rather than simply a frozen pile of material.
For a small sinusoidal bed disturbance, an upstream lead of the forcing relative to bed height can favor growth, while sediment transport saturation introduces a downstream lag. The Exner equation converts the resulting flux divergence into bed-height change. Their combination determines both growth and migration; the direction of migration cannot be inferred from a bed's appearance alone. Recording a bed profile at successive times separates changing amplitude from movement of a crest, while flow measurements test the assumed bed shear response.
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