Bed-form patterns reflect the dynamics of the defects in the pattern. From this new hypothesis, the differential migration of defects within a field of bed forms determines how crest-line orientation responds to changing transport conditions. Predictions from a model for defect dynamics compare well to field measurements and computer simulations of bed-form orientation. This model permits quantification of the time for bed-form reorientation across a broad range of scales, from hours in the rapidly changing nearshore environment to thousands of years for linear dune fields.

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