Dunes vs. Antidunes: Shape, Formation & Flow Regime Differences

Dunes and Antidunes: Bedforms in Sediment Transport

Both are alluvial bedforms shaped by fluid flow over sediment, but they form under opposite flow regimes and have distinct geometries.

Dunes (Lower Flow Regime)

• Shape: Asymmetric—gentle stoss (upstream) slope (10–20°), steep lee (downstream) slope (~30–34°, angle of repose).
• Flow: Subcritical (Fr < 1). Flow separates at crest; recirculation eddy on lee side.
• Sediment transport: Bedload dominant; grains avalanche down lee face.
• Migration: Downstream (crest moves downstream).
• Size: cm to 10s of m wavelength; height ~1/6–1/7 wavelength.
• Environments: Rivers, tidal channels, deserts (aeolian dunes differ in mechanics).

Antidunes (Upper Flow Regime)

• Shape: Symmetric or nearly so—in-phase with water surface. Gentle, rounded crests and troughs.
• Flow: Supercritical (Fr > 1). Standing waves on surface match bed topography.
• Sediment transport: High suspended load; bedload sheet flow.
• Migration: Upstream (crest moves upstream) or stationary.
• Size: Smaller wavelength (cm–m); lower height/wavelength ratio.
• Environments: Steep mountain streams, flash floods, alluvial fans, laboratory flumes.

Comparison Table

Dune Types (Subclassification)

• Barchan: Crescentic, horns downstream (unidirectional wind/water).
• Transverse: Straight crests ⊥ to flow.
• Parabolic: U-shaped, arms anchored (vegetated coastal).
• Star: Multi-armed, multi-directional winds.

Practical Implications

• Dunes: Increase flow resistance (Manning's n); affect flooding, navigation.
• Antidunes: Indicate flash-flood conditions; can cause sudden aggradation/degradation.
• Stratigraphy: Dune cross-sets = reservoir analogs; antidune deposits rare in rock record.