CHAPTER XII. FLUME TRACTION. 



THE OBSERVATIONS. 



SCOPE. 



That which distinguishes flume traction from 

 stream traction is the fixity of the channel bed. 

 In stream traction the shapes of the bed are 

 adjusted to the rhythms of the mode of trans- 

 portation, and its texture is that of the debris 

 in transit. In flume traction the bed is unre- 

 sponsive, but its texture, being independently 

 determined, has an important influence on the 

 mode of transportation. The experiments 

 were arranged to determine the influences of 

 different textures of bed on mode of traction 

 and capacity for traction and were otherwise 

 varied in respect to slope, width, discharge, 

 and the character of debris transported. 



GRADES OF DEBRIS. 



The material transported in the experiments 

 included most of the grades already described 

 (see Table 1 and Plate II) and also several 

 mixtures not previously mentioned. In order 

 conveniently to show the relations of the mix- 

 tures to their components, all the grades of 

 debris used in the flume experiments are listed 

 below in Table 65, the data of Table 1 being 

 repeated so far as necessary. The elements of 

 the table are defined at page 21. The material 

 of the coarse grades (I) and (J) differs from 

 that of the finer, being about 2 per cent less 

 dense. Its particles also are somewhat less 

 thoroughly rounded, their journey from the 

 parent rock bed having been short. 



TABLE 65. Grades of debris . 



APPARATUS AND METHODS. 



The experiment trough, a modification of 

 that represented in the frontispiece, was 60 

 feet long and 1.91 feet wide, with vertical 

 sides. The sides and bottom were of wood, 

 planed and painted. For a portion of the 

 experiments the bottom was covered by a 

 false bottom, specially prepared to present a 

 definite character of roughness, the sides re- 

 maining smooth. The trough was so arranged 

 that it could be given various determinate 

 slopes up to 3 per cent, and by means of an 

 inclined false bottom a slope of 4.5 per cent 

 was made. By means of a partition the width 

 was reduced, for the greater part of the work, 

 to 1.00 foot. 



The width of trough at the outfall end was 

 regulated by a contractor, as described on 

 page 25. The debris was delivered at the 

 outfall to a settling tank, which had two 

 divisions; and a deflecting apparatus was so 

 arranged that the delivery could be instanta- 

 neously diverted from one division to the other. 



Above the trough near its head was a sloping 

 platform on which measured units of debris 

 were dumped at regularintervals, determined by 

 a watch, and from which the debris was fed to 

 the current by hand, with the aid of a scraper. 

 The rate of eed was modified by changing the 

 interval between dumpings, and by successive 

 trials it was adjusted to the capacity of the 

 current. 



In accelerating the debris, as it fell into the 

 water, the current was retarded, so that close 

 to the feeding station it was slower than else- 

 where. When the load was approximately 

 adjusted to the general capacity of the current 

 it constituted an overload in this particular 

 tract, with the result that a portion was de- 

 posited. The load would then traverse an 

 upper division of its course on a bed of debris, 

 while in the lower and principal division it was 

 in direct contact witn the bottom of the trough. 

 A tendency of the stream of debris to clog near 

 the upper end of the trough, although moving 

 freely beyond, was the ordinary criterion of the 



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