Transition from wavelets to ripples in a laboratory flume with a diverging channel

An experimental investigation on the initiation and development of bed forms on a bed of fine silica sand was conducted under alluvial flow conditions in a laboratory flume with a diverging channel.The main aims of the study were to assess:I)the steepness of bed forms in the transition stage of development;and ii)the threshold height of wavelets(nt)that triggered the start of ripple development.Detailed bed profile measurements were carried out using an acoustic Doppler probe,traversed longitudinally over the sediment bed at various experimentation times.The bed form dimensions were extracted from such bed profile records and analysed for the wavelet,transition and equilibrium stages.It was found that the steepness of ripples in the transition and equilibrium stages were similar,confirming predictions of previous mathematical model simulations.A lognormal distribution fitted the wavelet length data.The wavelet threshold height was estimated as ηt≈7 mm,or y ≈80 in wall units.Such a height magnitude suggested that ripple development could be triggered by the wavelets reaching the outer flow zone of a turbulent boundary layer.The ηt value obtained corresponded generally to the intersection point between two predictive equations for bed form dimensions.A formulation was developed to predict ηt as a function of the sediment grain size,which was confirmed for the fine sand used in this study.