BELATION OF CAPACITY TO SLOPE. 



115 



The adjusted values of capacity may be 

 found in Table 12, together with a variety of 

 other data. The values of i t are contained in 

 Table 17, having been computed for the range 

 of slopes covered by the observations. In 

 order to compare these with the indexes of 

 relative variation for the single grades from 

 which the mixtures were made, Table 18 has 

 been prepared, containing indexes correspond- 

 ing to the uniform slope of 1.2 per cent. An 



analysis of this table shows that 3 mixtures 

 are more sensitive to variation of slope than 

 are components, 13 are less sensitive, and 18 

 show sensitiveness of intermediate rank. A 

 general average shows the mixtures 1 1 per cent 

 less sensitive than the (means of) components, 

 but the contrast is much more pronounced for 

 natural river debris and for the most complex 

 artificial combination than for the simpler 

 mixtures. 



TABLE 18. Comparison of values of i^ for mixtures and their components. 

 [S-1.2; w-l.OO.J 



o For these, <r and iin. 



The general tenor of the evidence from mix- 

 tures is to show that in passing from the labora- 

 tory conditions (with graded debris) to natural 

 conditions it will be proper to reduce the esti- 

 mates of the sensitiveness of capacity to changes 

 of slope. 



RELATION OF INDEX TO MODE OF TRACTION. 



One of the possible ways of bridging the 

 chasm between laboratory conditions and river 

 conditions is through the consideration of 

 modes of traction. River discharges are enor- 

 mously greater than the experimental, river 

 slopes are relatively minute, and river channel 

 sections are very dissimilar in form and pro- 

 portions, but the three modes of traction have 

 the same sequence in rivers as in the labora- 

 tory troughs. This consideration has led to the 

 separation of certain data connected with a sin- 

 gle mode of traction the one characterized by 

 smooth surfaces of water and debris. Table 19 

 contains values of the index of relative varia- 

 tion, I,, associated with smooth traction and 

 grouped with reference to various conditions. 

 In the upper division of the table all the values 

 pertain to the same grade of debris (C) and 



are arranged according to discharges and chan- 

 nel widths; in the lower division all pertain to 

 the same width of channel and are arranged ac- 

 cording to discharges and grades of d6bris. 



TABLE 19. Values of \ associated with the smooth mode of 

 traction. 



I. Values pertaining to the same grade of debris, (C). 



II. Values pertaining to the same width of channel, 0.66 feet. 



