Bed forms: ripples, dunes in alluvial streams

Flow over an alluvial (sand) bed forms regular bed features called bed forms. As the flow strength increases the sequence is: plane bed (no motion), ripples, dunes, transition (washed-out dunes), plane bed with motion, then antidunes.

Ripples

• Small, triangular bed forms with a gentle upstream slope and a steep downstream (lee) slope.
• Wavelength is small (typically less than 0.6 m) and height is a few centimetres (less than about 0.06 m); the size depends on the grain size and not on the depth of flow.
• They occur in fine sand (grain size below about 0.6 mm) at low shear stress and low Froude number, just above incipient motion.
• Grains are rolled up the gentle slope, fall into the separation zone behind the crest and are deposited on the lee face, so the ripples migrate downstream.
• The water surface is almost undisturbed. They add form roughness.

Dunes

• Larger bed forms of similar triangular shape with a long gentle upstream slope and steep lee face, but with wavelength and height related to the depth of flow (wavelength about 5 to 7 times the depth, height up to about one third of the depth).
• They occur at higher flow strength than ripples, in sand of any size, with subcritical flow ($Fr < 1$).
• Flow separates at the crest and a strong eddy forms in the lee. The water surface is out of phase with the bed (the water surface is lowest above the dune crest).
• They move downstream at a slow rate and give a large form resistance (high roughness and a large increase in flow depth).

 Ripples:   /\/\/\/\/\/\    small, regular
 Dunes:    __/\___/\___/\   large, depth-related; flow separation
 flow ---->