Antidune Basics
Antidunes are in-phase bedforms that form on a sandy substrate beneath super-critical flows. Antidunes migrate upstream against the current flow, opposite the migration direction of most bedforms (such as dunes and ripples).
What does this mean? Antidunes are type of bedform (any regular, wave-like form that occurs on a channel bed). Antidunes are in-phase with the water surface above them, so the crest and trough of the antidune are matched by a crest and trough in the water's surface above them. Antidunes form in fast, shallow flows where the flow is described as super-critical. Most other bedforms, such as ripples and dunes, form in sub-critical flows that are generally deeper and slower.
The two most striking features of antidunes, that they are in-phase with the water's surface and migrate upstream, are in contrast to most other bedforms. These features result from the contrasting flow regimes (super-critical vs sub-critical) that antidunes and other bedforms form in.
Antidunes commonly occur in small streams that flow across beaches. Antidunes appear where the stream flows down the beach face toward the ocean. You will see trains of waves at the streams surface. If you look beneath the waters surface, you will notice a train of waves in the stream bed that mimics the waters surface. These bedforms are antidunes. As you watch the surface waves carefully, you will notice that they are moving upstream against the current. This upstream movement of the surface wave is caused by the upstream migration of the underlying antidune.
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