D 



SYMBOLS AND DEFINITIONS 



amplitude of the water motion, one half the orbital 

 diameter (2a) 



average grain diameter, equivalent to D50 



D a descriptive grain diameter, meaning that m percent of 



the sand, by weight, is composed of grains with 



diameters less than D 

 m 



f bottom friction factor, defined by equation (23) 



g acceleration due to gravity 



h depth of water 



H height of surface wave (trough to crest) 



k surface wave number, = 2tt/L 



L surface wavelength 



n the number of periods, or cycles, since ripples first 

 appeared on a flat bed, or since the start of an 

 experiment with a bed previously rippled 



N the number of turns of a nut setting the stroke of 



the pistons and, hence, a measure of the amplitude of 

 the water motion (a = 0.4576 N centimeters) 



r a dimensionless measure of the grain size, defined by 

 equation (2) 



T period of the sinusoidal water motion 



T T for the initiation of grain motion on a flat bed 



T T for the initiation of ripples on a flat bed 



U amplitude, or greatest magnitude, of the water velocity 

 above the boundary layer 



U U for the initiation of grain motion on a flat bed 



U U for the initiation of ripples on a flat bed 



^2 dimensionless parameter defined by equation (7) 



n ripple height (see Fig. 1) 



' 8 



