Ripple and Dune Phases in a Narrowly graded Sand

Observations of flow resistance and bed-form dimensions were made for flow, over a narrowly graded sand, having a median diameter of 0.49 mm; the purpose of the narrow grading was to accentuate any changes which occur in the transition region between the ripple and dune bed regimes of flow. The test runs were spaced at small intervals of discharge over these flow regimes and the observations showed that there is an unstable region between ripples and dunes where the friction factor falls quite rapidly as discharge increases. A comparison was made between bed-form dimensions as determined by subjective interpretation of bed profile traces and wave lengths deduced from spectral density and auto-correlation diagrams. Wave lengths corresponding to peaks on the spectral density were compared with the results of other investigators on a dimensionless basis and showed a similar range of values in all cases, despite widely varying experimental conditions. A comparison was also made between depth of flow and channel width as parameters related to bed-form wave length.