THE MORPHOLOGY AND DYNAMICS OF LOW AMPLITUDE BEDWAVES UPON UPPER STAGE PLANE BEDS AND THE PRESERVATION OF PLANAR LAMINAE

Experimental studies of subcritical, unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves. Flow depth was 0.11 m, mean flow velocities were 0.86-1.0 m s-1, shear velocities were 0.058-0.71 m s-1 and dimensionless shear stresses were 0.56-0.87. Bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height (mainly 2-6 mm), from 0.7 to 1.3 m in wavelength and have mean celerities of 10 mm s-1. Flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs. High resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds. Lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point; individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced. Laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave.