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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Vol. 65A (1995)No. 2. (April), Pages 431-435

Washed-Out Ripples: Their Equilibrium Dimensions, Migration Rate, and Relation to Suspended-Sediment Concentration in Very Fine Sand

Jaco H. Baas (*), Herman De Koning


The transition from linguoid ripples to upper-stage plane bed under steady, uniform flow conditions in very fine sand (D50 = 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°C-equivalent depth-averaged flow velocities of 0.75 m/s and 0.91 m/s. With increasing Previous HitvelocityNext Hit in this range the 10°C-equivalent average equilibrium height of the bed forms decreases from 12 mm to zero, the 10°C-equivalent average equilibrium spacing decreases slightly from 138 mm to 103 mm, and the 10°C-equivalent average migration rate increases f om 1 mm/s to 3 mm/s.

The changes in morphology and migration rate with increasing flow Previous HitvelocityNext Hit 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 Previous HitvelocityTop 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.

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