30 



TRANSPORTATION OF DEBEIS BY RUNNING WATER. 



leaps. Figure 9 gives an ideal conception of 

 the cross section of the zone and the distribu- 

 tion of flying grains within it. Observation 

 from the side penetrates but a short distance 

 into the cloud, the distance being least where 

 the cloud is most dense. The practical limit 

 of visual penetration may be assumed to take 

 some such form as the line AB. Thus the 





FIGURE 9. Ideal transverse section of zone of saltation at side of experi- 

 ment trough. 



tract actually studied in the work with the 

 moving field was somewhat superficial and was 

 not in strictness a vertical section of the zone. 

 The second qualification is connected with 

 turbulence. In steady flow the motion at each 

 point of a stream is constant in velocity and 

 direction. When the general velocity exceeds 

 a certain minimum, which for the streams we 

 have to consider is very small, the flow is not 

 steady, but involves eddies or vortices, which 

 as a rule move onward with the current. In 

 consequence of these eddies the course of each 

 particle of water is sinuous, and the sinuous 

 courses interweave. The flow is then said to 

 be turbulent. Usually there are both large 

 and small eddies, the minute ones being multi- 

 tudinous. As the axes of whirling movements 

 have all attitudes, the directions of motion, as 

 a rule, have upward or downward components, 

 and the suspension of particles of debris is due 

 to the upward components. Particles so small 

 that they can not come to rest on the bottom 

 are thereby lifted and relifted and kept in the 

 body of the water. Under the conditions 

 arranged for the study of saltation there 

 appeared to be no large eddies, but the zone 

 was unquestionably pervaded by small ones, 

 excited by the roughness of the bed and by the 

 differential motions of water and leaping 

 grains. With increasing strength of current 



the texture of turbulence would enlarge and 

 saltation pass into suspension. With a diver- 

 sified debris, instead of the uniform material 

 actually used, there would be phases of action 

 in which the paths of small grains were made 

 sinuous by turbulence, while those of larger 

 grains remained simple in form. The trajec- 

 tory of saltation, as described, may therefore 

 be regarded as a simple type of path which 

 combines in all proportions with the sinuous 

 type of path characterizing suspension. 



Through the entire zone of saltation motion 

 is being communicated to particles of the load 

 by the water, and there is a corresponding loss 

 of motion by the water. That loss reduces all 

 the stream's velocities but makes the greatest 

 reduction in the lower part of the zone of 

 saltation. The loss of velocity in the lower 

 strands reduces their power to cause particles 

 to leap. The greater the load the greater this 

 reduction, and thus the quantity of load is 

 automatically regulated. 



COLLECTIVE MOVEMENT. 



In the experiment used to illustrate saltation 

 the collective movement of the sand was uni- 

 form, the conditions of the experiment having 

 been adjusted to that end. But it is equallv 

 possible so to adjust them as to make the 

 collective movement rhythmic. Uniformity is 

 in fact an intermediate phase between two 

 rhythmic phases, which are of contrasted types. 

 These phases will be described. 



In another experiment a bed of sand was 

 first prepared with the surface level and 

 smooth. Over this a deep stream of water 

 was run with a current so gentle that the bed 

 was not disturbed. The strength of current 

 was gradually increased until a few grains of 

 sand began to move and then was kept steady. 

 Soon it was seen that the feeble traction did 

 not affect the whole bed, but only certain tracts, 

 and after a time a regular pattern developed 

 and the bed exhibited a system of waves and 

 hollows. As the waves grew the amount of 

 transportation increased, showing that, under 

 the given conditions, the undulating surface 

 was better adapted to traction than the plane. 

 With such waves and hollows are associated a 

 special mode of transportation, which is illus- 

 trated in figure 10. A current reaching the 

 bed at A follows the rising slope and crest of 

 the wave to C and then shoots free, to reach 



