Washed-out ripples; their equilibrium dimensions, migration rate, and relation to suspended-sediment concentration in very fine sand

The transition from linguoid ripples to upper-stage plane bed under steady, uniform flow conditions in very fine sand (D ₅₀ = 0.108 mm) was studied quantitatively using a flume. The transitional ripples, referred to as washed-out ripples, are symmetrical to slightly asymmetrical bed forms with a convex-up profile and low-angle foresets in the direction of the main flow. The washed-out ripples are stable between 10 degrees C-equivalent depth-averaged flow velocities of 0.75 m/s and 0.91 m/s. With increasing velocity in this range the 10 degrees C-equivalent average equilibrium height of the bed forms decreases from 12 mm to zero, the 10 degrees C-equivalent average equilibrium spacing decreases slightly from 138 mm to 103 mm, and the 10 degrees C-equivalent average migration rate increases from 1 mm/s to 3 mm/s. The changes in morphology and migration rate with increasing flow velocity from linguoid ripples to washed-out ripples result from a spatial shift of the locus of maximum sediment flux from the bed-form crest to the downstream wake region. The shift follows the increasing concentration of sediment particles in a near-bed traction carpet above the sediment surface. With increasing flow velocity the volumetric suspended sediment concentration at 0.01 m above ripple crests increases from 0.9% at the lower boundary to 1.6% at the upper boundary of the washed-out-ripple stability field.