Flume Experiments on the Transition from Ripples to Lower Flat Bed with Increasing Sand Size

Abstract

ABSTRACT Observations were made in a small flume with three sizes of very well sorted quartz sand at constant mean flow depth to study in detail the relationship between ripples in finer sands and the lower-flat-bed mode of transport at similar flow velocities in coarser sands. A graph of mean flow velocity vs. sediment size for a flow depth of 7 cm plotted from the flume runs shows that in the size interval from 0.6 mm to 0.7 mm the lower flat bed intervenes between ripples at lower flow velocities and dunes at higher flow velocities, so that the sequence of bed configurations in these sizes is more complicated than for either finer or coarser sands. The ripple field pinches out with increasing size at about 0.7 mm. There is a triple point involving ripples, dunes, and lower flat bed at about 0.6 mm.