The sediment bedform generated by fast, shallow flows of water with a Froude number greater than 0.8. Antidunes form beneath standing waves of water that periodically steepen, migrate, and then break upstream. The antidune bedform is characterized by shallow foresets (see cross-stratification) which dip upstream at an angle of about 10°. Their preservation potential is low, but they can be identified by low-angle (less than 10°) foresets, dipping up-current. Normally antidunes show a close association with upper-flow-regime plane beds.
Antidune
A ripple on the bed of a fluvial channel, travelling upstream. Antidunes are sediment bedforms, formed beneath standing waves by fast, shallow water flows. A critical Froude number of 0•84 may be taken as the discriminator between dunes and antidunes (Carling and Shvidchenko (2002) Sedimentol. 49, 6).