Initiation of Bed Forms on a Flat Sand Bed

Abstract

The results of a series of 47 bed development experiments are combined with appropriate experimental results of other authors. The generation of initial bed waves, termed sand-wavelets, on a plane sand bed is found to be instigated by the occurrence of a random pileup of sediment, this pileup subsequently growing as further sediment is trapped by the pileup. When the height of the pileup approaches the bed roughness height, further pileups are generated at preferred spacings downstream. The wavelength l of the sand-wavelets so generated is relatively insensitive to the applied bed shear stress and primarily a function of the size d g of the sediment. This wavelength can be described by the empirical formula (l/dg)R*c0.2= 10 2.5. From a theoretical viewpoint, this wavelength is approximately described by a reassessment of the flow system instability analyses of Richards. Both ripples and dunes are found to develop from the sand-wavelets.