202 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



The second exception is a phenomenon of 

 rhythm. It is probable that the rate of flume 

 traction is always affected by rhythm, just as 

 is that of stream traction, and the rhythm in 

 load implies a rhythm in efficiency of current. 

 Under certain conditions, of which the most 

 important is small slope, the rhythm is mani- 

 fested by the making of a local deposit from the 

 load, a patch of debris appearing on the bed of 

 the trough. Such a patch travels slowly down- 

 stream, being succeeded after an interval by 

 another. With suitable variation of conditions 

 the patches become more numerous, are regu- 

 larly spaced, and occupy a greater share of the 

 bed surface. They may even exceed the in- 

 terspaces in area. When wide apart they 

 are usually shaped in gentle slopes, but as their 

 ranks close they assume the profiles of dunes, 

 with steep frontal faces. 



The advance of the debris patches is like that, 

 of typical dunes, in that the upstream slopes 

 are eroded while the downstream slopes are 

 aggraded; and in this way then- travel is a 

 factor in debris transportation. It does not, 

 however, become the dominant factor, as in 



the dune phase of stream traction. There is 

 always a large share of the load which passes 

 over the deposits without being arrested and 

 continues its journey across the intervening 

 bare spaces. 



In this rhythmic process there is a combina- 

 tion of elements belonging distinctively to 

 flume traction and to stream traction, and it is 

 possible that the process constitutes a transition 

 from one system to the other; but, so far as de- 

 veloped by the experiments, it appears as a 

 phase, a rhythmic phase, of flume traction. The 

 slopes with which it was associated were much 

 flatter than those necessary to carry the same 

 load by the method of stream traction. 



TABLE OF OBSERVATIONS. 



In the following table the data are arranged 

 primarily by character of bed surface and sub- 

 ordinately by width of channel, discharge, 

 grade of debris, and slope of channel bed. 

 The characters of bed surface are illustrated in 

 Plate III. Load is given in grams per second, 

 width in feet, discharge in cubic feet per second, 

 and slope in percentage. 



TABLE 67. Observations on flume traction, showing the relation of load to slope and other conditions. 

 a. Over a surface of wood, planed and painted. 



