210 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



tion than for stream traction. Expressed as 

 an exponent, 7 3 , its average value for the range 

 of the experiments recorded in Table 72 is 1 .26. 

 Values of 7 3 were also obtained from data in 

 Table 68 by comparing the capacities for Q 

 0.363 ft. 3 /sec. with those for <> = 0.734 ft. 3 /sec.; 

 and the mean of such values is 0.97. These 

 mean values are not necessarily inconsistent, 

 for the synthetic index varies with the range in 

 discharge for which it is computed and is lower 

 as the discharges are higher; but a study of in- 

 dividual values shows that under identical con- 

 ditions the data of Table 68 give the lower esti- 

 mates of sensitiveness. The data as a whole 

 indicate that, for the range of conditions real- 

 ized in the experiments, the average value of 

 the exponent expressing sensitiveness to dis- 

 charge is 1.2. There can be no question that 

 for the larger discharges used it falls below 

 unity. 



As the sensitiveness of the duty of water, and 

 also the efficiency, to discharge is expressed by 



an exponent which is less by unity than the 

 corresponding exponent for the sensitiveness of 

 capacity to discharge, it follows that duty and 

 efficiency vary little with discharge. In general 

 they gain slightly with increase of discharge, 

 but they lose when the discharge or slope is 

 relatively large. This accords with a result 

 obtained by G. A. Overstrom/ who found from 

 experiments with launders that duty rose with 

 increase of depth to a limited extent only. 



CAPACITY AND FINENESS. 



In Tables 68 and 72 the values of capacity 

 standing in any horizontal line constitute a 

 series illustrating the variations which are re- 

 lated to grades of debris, and if those in the 

 columns for mixtures be excepted they illus- 

 trate the relations of capacity to fineness. 

 Table 73 contains a selection of data from those 

 tables, together with a single line taken from 

 Table 69. 



TABLE 73. -Values of capacity for flinne traction, illustrating the control of capacity by fineness of debris. 



As the tables are examined, one of the fea- 

 tures arresting attention is that in most of the 

 series the smallest value of capacity does not 

 appear at one end of the line but at some inter- 

 mediate point. The occurrence of a minimum 

 is in fact characteristic of all tested varieties 

 of flume traction except that in which the bed 

 is a pavement of pebbles. To give the feature 

 graphic expression the data of the last five 

 lines of Table 73 are plotted in figure 69, where 

 the horizontal scale is that of linear fineness, F. 

 The plotted points are far from regular, but 

 the general character of the representative 

 curves is unmistakable, and freehand lines have 

 been drawn. On another sheet, not reproduced, 

 the same data were plotted in relation to mean 

 diameter of particles the reciprocal of F 

 with similar result, except that the lower two 

 curves became concave upward. 



These curves illustrate the most important 

 difference between the laws of flume traction 

 and those of stream traction. In stream trac- 

 tion capacity increases continuously as fineness 

 increases. In flume traction capacity increases 

 with fineness when the grades of debris com- 

 pared are relatively fine but increases with 

 coarseness when the grades are relatively 

 coarse. So far as these experiments show, the 

 minimum of capacity corresponds to a coarse 

 sand, but its position on the scale of fineness 

 may be assumed to vary with slope, discharge, 

 and roughness of bed. 



The curve for flume traction over a bed 

 paved with gravel shows no minimum but is 

 of the same type as the curve for stream trac- 

 tion. This fact is confirmatory of an inference 



1 Quoted by R. H. Richards in Ore dressing. 



