Experiments on bar formation in a straight flume 1. Uniform sediment

Laboratory experiments have been carried out in a large laboratory flume us ing a nearly uniform sand and under controlled steady flow conditions such as to ensure the development of alternate bars on a ripple and/or dune-cove red bed. The interaction between small-scale and large-scale bed forms is f ound to enhance a modulation in time and in space of the bar pattern. Moreo ver, the flow depth decrease experienced by the flow field when approaching bar fronts, leading to a progressive reduction of the dimensions of small- scale bed forms, may influence appreciably the overall flow resistance. Exp erimental values of bar wavelength and of bar celerity are compared with th e theoretical estimates obtained from a model developed within the classica l framework of linear stability analysis. In particular, the model accounts for the local variability of friction coefficient and water level, for the secondary helical flow effects, and for the influence of longitudinal slop e on sediment transport. Quantitatively satisfactory predictions of bar wav elength appear to be possible in spite of the fact that in various runs a c orrect estimation of flow resistance and flaw discharge appears to be relat ively difficult.