216 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



while the observations of others pertain to 

 velocity. 



The experiments of Dubuat * (1783) have 

 been assumed, both by him and by others, to 

 pertain to stream traction, but his account of 

 apparatus and methods makes it probable that 

 what he really investigated was chiefly com- 

 petence for flume traction. He used a trough 

 of plank, with the grain lengthwise, and meas- 

 ured the velocity of the current by observing 

 the speed of balls slightly heavier than water 

 as they were swept along the bottom. In a 

 current of a particular velocity he placed 

 successively various kinds of debris and noted 

 their behavior, then changed the velocity and 

 repeated. His results are as follows: 



Competent bed velocity 



(ft./sec.). 

 Potter's clay Between 0.27 and 035 



0.7 



1.1 



0.35 

 0.62 

 1.1 



2.1 

 3.2 



0.53 



0.7 



1.55 



3.2 

 4.0 



Coarse angular sand 

 River gravel: 



Size of anise seed 



Size of peas 



Size of common beans 



Rounded pebbles, 1 inch in diame- 

 ter 



Angular flints, size of hen's eggs. . . 



J. W. Bazalgette, in discussing the flushing 

 of sewers and therefore presumably consider- 

 ing flume traction rather than stream traction, 

 quotes the following results of experiments by 

 Robison : 2 



Competent bed velocity 

 (ft./sec.). 



Fine sand 0. 5 



Sand coarse as linseed 67 



Fine gravel 1.0 



Round pebbles, 1 inch in diameter 2. 



Angular stones, size of eggs 3. 



In 1857 T. E. Blackwell 3 conducted elabo- 

 rate experiments to determine the vel >cities 

 necessary to move various materials in sewers. 

 His channel was of rough-sawn plank, 60 feet 

 long and 4 feet wide, with level bottom. Ve- 

 locities were measured by a tachometer, but the 

 relation of the velocity measurements to the 

 bed is not stated. The tests were applied to 

 natural d6bris of various kinds and also to types 

 of artificial objects likely to enter sewers. The 

 objects were treated singly and in aggregates, 

 with the general result that an aggregation re- 



1 Dubuat-Naneay, L. G., Principes d'hydraulique, vol. 1, p. 100; vol. 

 2, pp. 57, 79, 95, Paris, 1786. 



2 Inst. Civil Eng. Proc., vol. 24, pp. 289-290, 1865. 



3 Accounts and papers [London], Sess. 2, 1857; Metropolitan drainage, 

 vol. 36, Appendix IV, pp. 167-170, Pis. 1-5. 



quires higher velocity to move it than does a 

 single object. It is evident that the experi- 

 ments on single objects pertain to flume trac- 

 tion and some of those on aggregations to 

 stream traction. From his tabulated results 

 the subjoined data are selected as representing 

 or illustrating the velocities competent for 

 natural debris, the column of mean diameter 

 being added by me. He infers from the ex- 

 periments that (1) for objects of the same 

 character competent velocity increases with the 

 mass; (2) for objects of the same size and form 

 it increases with the specific gravity; (3) for 

 objects of different form it is greater in propor- 

 tion as they depart from the form of a sphere; 

 and (4) for objects in motion the rate of travel 

 increases with the velocity of the current. 



TABLE 78. Observations by Blackwell on velocity competent 

 for traction. 



WORK OF OVERSTROM AND BLUE. 



Certain experimental work on the capacity of 

 currents for flume traction has for its specific 

 purpose the determination of dimensions for 

 launders, the flumes in which pulverized ore is 

 conveyed. R. H. Richards's "Ore dressing" 4 

 contains an abstract of results obtained by G. 

 A. Overstrom, accompanied by the statement 

 that the experimental data are extensive but as 

 yet unpublished. The troughs employed were 

 flat-bottomed and probably of wood. For each 

 slope, width of flume, and grade of transported 

 material he found (1) that the duty of water 

 varies with the discharge and that some par- 

 ticular discharge is associated with a maximum 

 duty, so as to be the most economical; (2) that 



< Vol. 3, pp. 1592-1594, 1909. 



