III. INITIATION OF GRAIN MOTION 



Determination of the Condition. 



The 24 experiments starting with a leveled bed yielded 24 observa- 

 tions of the initiation of grain motion, the first stage in the 

 development of ripples. These observations, as do others of the type 

 in the literature, have a somewhat subjective character. Determination 

 of the condition of incipient motion is a perception of some combina- 

 tion of the number of grains moving and how far they move. As U is 

 increased slowly and steadily, eventually a few sand grains are seen 

 to rock or tumble from their nests, twice each period, presumably near 

 times of maximum stress. As U continues to increase, grain motion 

 intensifies, more or less steadily, until all the surface grains are 

 in motion. Between these stages U typically increases around 20 to 

 40 percent. The determination of "incipient motion" is made somewhere 

 in this range, and is seldom further defined. Manohar (1955) identifies 

 conditions of "initial" and "general" grain motion, in laminar and tur- 

 bulent boundary layers, separated by increments in U of about 13 and 

 10 percent. Carstens, Neilson, and Altinbilek (1969) identify incipient 

 motion with the motion of 10 percent of the surface grains. In this 

 study, incipient motion is defined by the motion of from 10 to 20 

 grains per square centimeter combined with a perception of rapid 

 intensification of grain motion with increasing U. Since moving grains 

 dislodge other grains and set them in motion and may progressively 

 loosen the surface, a long slow rate of increase in U may produce a 

 greater intensity of grain motion, at a given U, than does a short 

 quick rate. Possibilities for personal bias are obvious, and may 

 produce significant differences between observers. However, the 

 observations of a single observer following set procedures should be 

 repeatable enough to reveal trends. 



2. Effects of Surface Compression . 



A different, impersonal, possible source of scatter in observations 

 of incipient motion is the degree of compaction of the surface grains. 

 Experiments 52 and 54 with the 0.55-millimeter sand (Table A-1) were 

 designed to test this effect. A tamper 13 by 32 centimeters, was fixed 

 to a rigid frame such that with the frame resting on the tops of the 

 channel walls the tamper rested on the sand bed surface as usually 

 leveled. Before each of the two compression experiments, the scraper 

 of the leveler was elevated a small distance, 6, which left the surface 

 of the sand bed 6 above its usual level. The tamper was then carefully 

 placed upon the submerged leveled sand surface leaving a gap 6 between 

 its frame and the tops of the channel walls. Forcing the frame down 

 to make contact pressed the sand surface down a distance 6 and so 

 defined a compaction of the surface grains. Since the tamper was small. 



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